Continuous aerosol and cloud condensation nuclei (CCN) measurements carried out at the ground
observational facility situated in the rain-shadow region of the Indian subcontinent are
illustrated. These observations were part of the Cloud Aerosol Interaction Precipitation
Enhancement Experiment (CAIPEEX) during the Indian summer monsoon season (June to September) of
2018. Observations are classified as dry–continental (monsoon break) and wet–marine (monsoon
active) according to the air mass history. CCN concentrations measured for a range of
supersaturations (0.2 %–1.2 %) are parameterized using Twomey's empirical relationship. CCN
concentrations at low (0.2 %) supersaturation (SS) were high (>1000cm-3) during
continental conditions and observed together with high black carbon
(BC∼2000ngm-3) and columnar aerosol loading. During the marine air
mass conditions, CCN concentrations diminished to ∼350cm-3 at 0.3 % SS and
low aerosol loading persisted (BC∼800ngm-3). High CCN activation
fraction (AF) of ≅0.55 (at 0.3 % SS) was observed before the monsoon rainfall, which
reduced to ≅0.15 during the marine air mass and enhanced to ≅0.32 after that. There was mostly
monomodal aerosol number size distribution (NSD) with a mean geometric mean diameter (GMD) of
≅85nm, with least (≅9 %) contribution from nucleation mode
(<30nm) particles persisted before the monsoon, while multimode NSD with
≅19 % of nucleation mode particles was found during the marine air mass. Critical
activation diameters (dcri) for 0.3 % SS were found to be about 72, 169, and
121 nm prior to, during, and after the marine conditions, respectively. The better
association of CCN with aerosol absorption, and the concurrent accumulation mode particles during
continental conditions, points to the possibility of aged (oxygenated) carbonaceous aerosols
enhancing the CCN activity prior to the marine conditions. An enhancement in CCN concentrations and
k values during the daytime along with absorption Ångström exponent was observed during the
marine conditions. Best closure obtained using measured critical diameter and ammonium sulfate
composition during continental conditions emphasizes the role of aged aerosols contributing to the
accumulation mode, enhancing the CCN efficiency. The overestimation of CCN and less
hygroscopicity of accumulation mode aerosols during the marine air mass indicate the role of
size-dependent aerosol composition in CCN activity during the period.
Introduction
Atmospheric aerosol particles (APs), emitted from both natural and anthropogenic sources, affect the
radiation budget as well as the hydrological cycle of Earth, mainly through its direct and indirect
effects. Those APs or condensation nuclei (CN) which act as the cloud condensation nuclei (CCN) at
a specific supersaturation (SS) can indirectly affect the climate by altering the cloud
microphysical properties. In general, an increase in APs increases the cloud droplet number
concentration and decreases the size of droplets (Twomey and Warner, 1967) for a fixed liquid water
content, which in turn increases the cloud albedo (Twomey, 1977) and cloud lifetime
(Albrecht, 1989). In the real atmosphere, the SS measurements are seldom possible, and
the large disagreements between the CCN and cloud droplet number concentration remain elusive (Moore
et al., 2013). All these effects eventually modify the precipitation pattern (Lohmann and Feichter,
2005; Rosenfeld et al., 2008). Some of these aerosol indirect effects are moderately understood,
while others are not, which contribute to significant uncertainty among all the climate forcing
mechanisms (IPCC, 2013). Characterization of the hygroscopic growth of APs, which is generally
addressed by the Köhler theory (Köhler, 1936), is the most fundamental aspect in assessing
the aerosol–cloud interactions (ACI) for reducing the uncertainties in indirect radiative forcing
estimation. However, Köhler theory is modified to accommodate the real atmospheric conditions
and applied for both laboratory and field measurements, as well as in the climate models (Shulman
et al., 1996; Laaksonen et al., 1998; Raymond and Pandis, 2003; McFiggans et al., 2006; Petters and
Kreidenweis, 2007; Rose et al., 2008; Mikhailov et al., 2009). For a given particle, the size and
composition determine its CCN activity at a specific SS, while the CCN spectrum (CCN at different
SS) depends on the median diameter and standard deviation, number concentration, and the mixing
state of the aerosol system (Quinn et al., 2008). In this regard, closure studies are necessary to
understand the role of each parameter in the activation of APs as CCN, which may improve the accuracy
of climate models to address the ACI (Fountakis and Nenes, 2005).
Large spatial and temporal heterogeneities are found in APs and CCN properties, and thus the
regional characterization of CCN in different meteorological settings is imminent. Temporal and
spatial heterogeneities of CCN and different mechanisms affecting CCN are investigated in several
studies (Hoppel et al., 1973; Hudson and Xie, 1999; Paramonov et al., 2015, Schmale et al., 2018;
Nair et al., 2020) over both the continental and marine environments. Over the land mass,
significant variability in CCN activation properties are reported due to urban and industrial
influences (Sotiropoulou et al., 2007; Asa-Awuku et al., 2011). Carbonaceous combustion APs are produced
mostly from urban and industrial activities, contributing more than half of the global CCN
concentrations (Spracklen et al., 2011). Though nascent black carbon (BC) APs are insoluble
(Weingartner et al., 1997), coating, condensation, and coagulation of organic and inorganic APs can
increase their hygroscopicity, thereby acting as CCN (Liu et al., 2013). However, the role of
organics, mostly from carbonaceous combustion sources, in determining the CCN activity is still
uncertain. Ervens et al. (2005) have reported a broad range (-86% to 110 %) of changes in
cloud droplet number concentration due to the organics. The reduction in surface tension by organic
APs can even perturb the first indirect (Twomey) effect (Nenes et al., 2002). The presence of water
soluble organic carbon can increase the CCN concentrations, especially in rural and urban settings
(Mircea et al., 2002). Recent studies (Singla et al., 2017; Nair et al., 2020) highlight the
dominance of organic APs and their significant role in CCN activation over the Indian subcontinent. The
increasing trend in aerosol loading (Babu et al., 2013) and the significant contribution of
carbonaceous APs from both fossil fuel and biomass burning over the Indian subcontinent (Nair
et al., 2007) highlight the necessity of the characterization of CCN and the role of carbonaceous APs
over distinct environments in India.
Even though the aerosol properties such as aerosol optical depth (Babu et al., 2013) and BC mass concentration (Manoj et al., 2019) have been studied across the Indian subcontinent
through a network of observatories (Moorthy et al., 2013) for decades, only a few CCN studies are
available for the last few years over specific regions. Year-round CCN measurements are reported from
the high-altitude observatory over the Western Ghats (Leena et al., 2016), Indo-Gangetic Plain (IGP;
Patidar et al., 2012), central Himalayas (Gogoi et al., 2015), and eastern Himalayas (Roy et al.,
2017). CCN characteristics for a specific season, including closure analysis, were reported by
Jayachandran et al. (2017, 2018) at peninsular India and by Bhattu and Tripathi (2015) at
IGP. Apart from these studies, Indian Ocean Experiment (INDOEX; Ramanathan et al., 2001),
Cloud Aerosol Interaction Precipitation Enhancement Experiment (CAIPEEX, Kulkarni et al., 2012), South-West Asian Aerosol–Monsoon Interaction and Regional Aerosol Warming Experiment (SWAAMI–RAWEX;
Jayachandran et al., 2020), and Integrated Campaign for Aerosols, gases and Radiation Budget 2018
(ICARB-2018, Nair et al., 2020) are other major multiplatform campaigns carried out over the
subcontinent and nearby marine environment to study the regional ACI. CAIPEEX has conducted both
aircraft- and ground-based observations of aerosols, clouds, and planetary boundary layer (PBL) since
2009 in a phased manner. Details of the CAIPEEX are available in Prabha et al. (2011) and Kulkarni
et al. (2012). Various studies have addressed the spatiotemporal distribution of APs (Padmakumari
et al., 2013; Varghese et al., 2019), cloud microphysics (Prabha et al., 2011, 2012; Padmakumari
et al., 2018), rainfall (Maheshkumar et al., 2014) properties, and the relationship between cloud
microphysics and thermodynamics (Bera et al., 2019), and ACI (Pandithurai et al., 2012; Prabha
et al., 2012; Konwar et al., 2012; Gayatri et al., 2017; Patade et al., 2019) from the unique data
obtained from the CAIPEEX. Varghese et al. (2016) investigated the linkages of surface and cloud
base CCN spectral characteristics over the rain-shadow region.
The assessment of the effects of APs on clouds and precipitation due to the changes in the
atmospheric composition by anthropogenic activities is very significant over India as the
agriculture and economy of the region mostly depend on the Indian summer monsoon (ISM) rainfall. The
western coast of India, which is the gateway of the ISM, receives almost 2.5 times the long-term
monsoon mean rainfall observed all over India (Parthasarathy et al., 1995). The mountain ranges
along the western coast of India, known as the Western Ghats (WG) mountains, play a pivotal role in
ISM rainfall due to orography (Grossman and Duran, 1984; Sijikumar et al., 2013). A few studies have
been carried out to date to understand the aerosol loading (Udayasoorian et al., 2014), CCN
characteristics (Leena et al., 2016; Jayachandran et al., 2018) and its influence on the aerosol
indirect effects (Anil Kumar et al., 2016) from different locations in the WG. However, the rain-shadow region (leeward side) is prone to drought conditions with predominant continental effects,
and the relevant studies are sparse.
CAIPEEX observations were conducted over the rain-shadow region to understand the cloud and
precipitation microphysics and AP properties to derive guidelines for the precipitation enhancement
over the region. CAIPEEX Phase IV was designed to address the major objectives for the science of
weather modification. The background observations of CCN were trivial for the design and validation
of the experiment, and the data presented in this study are aimed at understanding the aerosols and
its cloud-activation properties near the surface. The present study addresses the first reporting of
CCN and its characteristics under different air mass and meteorological conditions throughout the
ISM season (June to September) of 2018 over this region. The study focuses on the variations in CCN
characteristics within the ISM season, and the possible factors are investigated using the
concurrent and collocated aerosol size distribution and BC measurements. Another focus of the study
is the CCN closure analysis to assess the role of size and composition at different atmospheric
conditions.
Experiment detailsLocation, measurements, and database
As part of the ground segment of the CAIPEEX IV campaign, aerosol and PBL measurements have been
taken since May 2017 at N.B. Navale Sinhgad College of Engineering in Solapur (17.70∘ N,
75.85∘ E; ≅490ma.m.s.l.), which is at the center of the rain-shadow
region. The location is marked as a circle in Fig. 1 and is a semiarid region. The college site is
12 km away from Solapur, and the aerosol sampling laboratory is on the top (third) floor of the building, away from all local activities in the rural setting. Even though the sampling
site is well isolated from the urban contamination, Solapur consists of numerous sugar and textile
industries emissions, apart from the seasonal emissions from agricultural activities.
Details of instrumentation and data used for the present study are illustrated in Table 1. Aerosol
sampling was carried out through separate PM2.5 inlets, from about 2 m above the
rooftop, connected with conductive tubing. CCN concentrations were measured at every second, using
a continuous-flow streamwise temperature gradient CCN counter (CCN-100, DMT; Roberts and Nenes,
2005). Initially (June 2018) CCN were measured at five SSs (0.2 %, 0.4 %, 0.6 %,
0.8 %, and 1.0 %), and in July the CCN counter was calibrated again and the SS was set at
0.3 %, 0.5 %, 0.8 %, and 1.2 %. Calibrations were carried out both before and after
the experiments using ammonium sulfate APs following Rose et al. (2008). The instrument was also
factory calibrated before the campaign. During the calibration experiments, CCN efficiency spectra
were recorded for different ΔT values. The activation diameter corresponding to 50 % CCN
efficiency for each spectrum was taken as the critical dry diameter for the CCN activation of
ammonium sulfate particles. The corresponding critical supersaturation was calculated with the
activity parameterization Köhler model (AP3) mentioned in Rose et al. (2008). The calculated
critical supersaturation was taken as the effective supersaturation at the given ΔT value.
Details of aerosol measurements used in the current study.
The CCN counter uses the fundamental principle of the difference in the diffusion rate of heat and water
vapor. A fixed temperature gradient is maintained along the walls of the wetted cylindrical column
inside the instrument in which the desired SS is generated depending on the temperature gradient and
the flow rate. The APs are fed at a constant sheath to sample flow of (10:1) along the
center line of the column, and the total flow rate was maintained at 500 vccm. The details of
the working principle of the instrument are available in Roberts and Nenes (2005) and Lance et al.
(2006). During June, each SS was maintained for 5 min, except for 0.2 % which was for
10 min. About 2 min of data during the SS transition was discarded to avoid the uncertainty
in establishing the required SS during the transition. At 0.2 % SS (lowest set SS), about 4 min of initial data was discarded. Except for June, all the SSs were set for 7 min each,
except for 0.3 % SS, which was maintained for 9 min. Here also, the initial 3–4 min of
data were discarded to ensure the set SS conditions. Thus, one cycle of the complete set of SS took
30 min, and the cycle was repeated. APs were continuously exposed to the SS inside the column, and
those having their critical SS less than that of the set SS inside the column were activated as liquid
droplets and counted by the optical particle counter operated by a laser diode at 660 nm at
the exit of the column. Since the CCN concentrations have always been less than 6000 cm-3,
correction for water vapor depletion inside the column as suggested by Lathem and Nenes (2011) was
not applied.
Size-segregated aerosol number concentration (NSD) from about 15 nm to about
685 nm, distributed among 107 size bins, was measured every 3 min
using a scanning mobility particle sizer (SMPS; TSI Incorporated, model 3082). The setup
consists of an electrostatic classifier, including a long differential
mobility analyzer (LDMA; TSI Incorporated, model no. 3081), and a butanol-based condensation
particle counter (CPC; TSI Incorporated, model no. 3772). Before entering the LDMA the APs are
charged to a known charge distribution by a bipolar charger in the
electrostatic classifier, and the charged APs were size segregated according to their
electrical mobility (Wiedensohler, 1988; Wang and Flagan, 1990) in the DMA.
The APs classified according to their sizes were counted by the CPC. The
sheath and sample flow were maintained at 0.3 and 3 Lmin-1,
respectively. Multiple charge corrections and diffusion charge corrections
were applied to the aerosol NSD data during the data inversion. APs were
passed through a diffusion dryer before the classifier to prevent the high-humidity conditions.
Radiation absorption properties of APs at different wavelengths were measured
using a dual-spot Aethalometer (AE33; Magee Scientific) at every
minute. The Aethalometer operated at a flow rate of 2 LPM and measured the
attenuation of light due to the APs deposited on a filter tape (Hansen et al., 1984) at seven different wavelengths – 370, 470, 520, 590, 660, 880, and
950 nm. From this, the absorption coefficient (σabs) is
estimated from the rate of attenuation, filter spot area, and the flow rate
(Weingartener et al., 2003). The new-generation AE33 compensates the loading
effect and multiple scattering effects (Arnott et al., 2005) associated with
the filter-based optical attenuation techniques (Drinovec et al., 2015).
The wavelength dependence of the absorption coefficient of aerosols is
parameterized using the following equation:
σabsλ=β×λ-αabs,
where β is a constant and αabs is the
absorption Ångström exponent. The nature of the carbonaceous sources can be
inferred from the value of αabs. Humic-like
substance (HULIS) and brown carbon produced from biomass burning have higher
absorption at lower wavelength (ultraviolet and blue) regions (Gelencser
et al., 2003). Hence, αabs will be higher
(∼2) for biomass-dominant sources, while fossil-fuel-dominant
sources will have αabs close to unity
(Kirchstetter et al., 2004).
Ambient weather parameters such as temperature, pressure, wind speed, wind
direction, relative humidity, and rainfall were also used in the present
study from the automatic weather station (AWS) measurements located at the
site. All the instruments operated during CAIPEEX were calibrated
periodically, especially before and after the experiments. The uncertainty
associated with all the measurement techniques used in the present study is
<10%. All the measurements having different sampling
frequencies were averaged to hourly intervals for analysis and
interpretation. Air mass pathways were investigated using Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model (Draxler and
Rolph, 2014) available from the NOAA ARL READY website.
Meteorology
The air mass back trajectories for 5 d reaching 50 m above the site
were examined using back-trajectory analysis and found that two distinct air
masses reached the site during the observation period (1 June to 30
September 2018). These are classified as (a) continental (dry) and (b)
marine (wet) and are shown in Fig. 1. Those days on which air masses were
over the landmass and within 1 kma.g.l. for minimum 3 d before reaching
the site, and hence having a significant continental influence are
segregated as continental, while those from the nearby marine atmosphere are
classified as the marine and the corresponding period include the monsoon
rainfall period over the site. The continental air mass consistently
prevailed over the site during the first week of June (denoted as
continental-1) and from 15 to 30 September (denoted as continental-2) of
2018. Marine air mass days consist of 8–12 and 15–31 July (denoted as
marine-1) and 1–26 and 28–31 August (denoted as marine-2). Thus, the
observations and findings throughout this paper are examined on the
basis of this classification. From Fig. 1a it can also be seen that the
air mass history for the continental classification is mostly within 1 km
above the surface, indicating the chances for the influence of local aerosol
sources.
Air mass back trajectories with 5 d duration reaching 50 m
above the site segregated to (a) continental and (b) marine. The color of
the trajectories indicates the altitude of the air mass above ground
level (a.g.l.).
The meteorological parameters observed at the site from the AWS measurements
during these periods are shown in Fig. 2. Diurnal variation of temperature
and relative humidity (RH) are shown in Fig. 2a and b, respectively.
The temperature and RH values are distinctively different during
continental-1 compared to other periods. Marine-1 and 2 periods experienced
low temperatures and high RH throughout the day, while continental-2 had
higher temperature and lower RH during noon and afternoon hours. The monthly
mean temperature during the study periods of continental-1, marine-1,
marine-2, and continental-2 were 29.5±3.6, 25.9±2.6, 25.4±2.8, and 27.1±3.5∘C,
respectively. A dry spell existed during the continental-1 days when the
maximum hourly mean temperature recorded was ∼38∘C. The maximum temperature at all the periods was observed at 15:00 and
16:00 Indian standard time (IST), and the lowest temperatures were observed before sunrise
of the day. Intermittent rainfall happened during July and August, and a few
heavy rainfall events occurred during these months. The aerosol–CCN
measurements during heavy rainfall are not included in the analysis for
interpretation (missing days in Table 1). From the wind rose diagram (Fig. 2c), it can be noted that the strong winds were blowing mostly from the western
and southwestern part of the site during continental-1 and, in a few cases, winds
were blowing from the northeasterly direction. Westerly and southwesterly
winds were present during marine-1 and 2 conditions (Fig. 2d and e), while on
the continental-2 days (Fig. 2f) mostly northeasterly winds were observed.
Diurnal variation of (a) temperature and (b) relative humidity along
with the standard deviation of the mean values. The wind rose diagrams from
the co-located AWS measurements for (c) continental-1, (d) marine-1, (e)
marine-2, and (f) continental-2 conditions.
Results and discussions
CCN characteristics at the site are investigated with aerosol size
distribution and BC measurements.
Overview of aerosol loading
The frequency distribution of BC mass loading and the mean values (and its standard deviation)
during the observation days are shown in Fig. 3a with an aim to understand aerosol loading and the
influence of anthropogenic activities. The distinct atmospheric conditions and the air mass history
are evident in the BC mass loading at the site. Before the onset of monsoon, under the influence of
continental air mass, the mean BC values were above 2000 ngm-3, which decreased to very
low values (∼746ngm-3) during the marine conditions. BC concentration was even
higher than 4000 ngm-3 during the continental air mass, while in many cases values
were almost 100 ngm-3 under marine conditions.
Apart from the near-surface measurements, the columnar aerosol optical depth (AOD) is examined using
the Moderate Resolution Imaging Spectroradiometer (MODIS) on board the Aqua satellite at 550 nm. The AOD
observed from MODIS during the continental air mass conditions along with the site (white star) is
shown in panels (b) and (c) of Fig. 3. It can be seen that heavy aerosol loading
(AOD>0.5) persisted around the rain-shadow regions and the Mumbai coast (northwest of
the site) in addition to the high loading over the IGP. After the monsoon rainfall, the aerosol
loading has reduced all over India as seen in Fig. 3c. Still, high aerosol loading
(AOD>0.4) was observed around the observation site, IGP, and the northern part of the
east coast.
(a) Frequency distribution of BC mass concentration and its mean values for marine
and continental conditions. Aerosol optical depth (AOD) at 550 nm observed from the
Moderate Resolution Imaging Spectroradiometer (MODIS) for the (b) continental-1 and (c) continental-2 conditions. The Solapur site is indicated by the star in the spatial AOD maps.
BC can be considered as a proxy for the anthropogenic activities (Myhre et al., 2013; Lelieveld
et al., 2019), and the BC loading over Solapur indicates that the anthropogenic influence is
predominant during the continental air mass conditions. This observation is in line with the
columnar aerosol observation from MODIS. Apart from fossil fuel combustion, biomass burning may also
contribute to the carbonaceous APs that prevailed over the site. The fire counts observed from
the MODIS (Collection 6 product obtained from https://earthdata.nasa.gov/firms, last access: 23 July 2019) may support
this inference, which is given in the Appendix (Fig. A1). The high aerosol loading locations in
Fig. 3 are associated with the numerous fire events which can be seen in Fig. A1. From another site
in the rain-shadow region closer to the central part of India, namely Nagpur, Kompalli et al. (2014) reported a BC mass of ∼2000ngm-3 before the monsoon, which is similar to the
present study. From the long-term observations of BC from the northwestern part (Pune) of the current
study, Safai et al. (2013) reported a mean BC mass of ∼1200ngm-3 during the
monsoon period. Both the high surface BC and total column aerosol loading observed before the
monsoon indicate the significant anthropogenic influence on the total aerosol loading. The low BC
values (<1000ngm-3) during the marine conditions at Solapur represent a cleaner
atmosphere, while ∼1500ngm-3 BC was reported from a coastal location in
peninsular India (Babu and Moorthy, 2002) during monsoon. About 50 % reduction (∼10000
to 6000 ngm-3) in BC mass associated with the dominance of fossil fuel sources replacing
the biomass during monsoon compared to the premonsoon values was reported by Vaishya
et al. (2017) from a heavily polluted IGP site. The low BC loading during the marine conditions over
Solapur is due to the wet scavenging of aerosols and the distinct air mass reaching the site, as well
as due to the reduced local burning during the active monsoon conditions. The high AOD and BC
observations identify Solapur as a polluted continental environment which is cleaner during active
monsoon compared to the other periods.
CCN number concentrations and its variations
The mean CCN concentrations at different SS, known as the CCN–SS spectra, segregated according to the
air mass conditions, are shown in Fig. 4. It can be seen that the CCN concentrations at all SS are
higher during continental, compared to marine, conditions. The highest CCN concentration is observed
during continental-1, which is in line with the surface BC loading and the total columnar aerosol
loading. CCN spectra are similar for the marine conditions, except the slight difference at the
lowest SS. CCN concentrations before the monsoon ranged from ∼1600 to 3600 cm-3 for
0.2 % to 1.0 % SS. Meanwhile, the CCN concentration was only ∼900cm-3 during
marine air mass – even at 1.2 % SS. Thus, a clear distinction is seen in the CCN concentrations
between the marine and continental air mass conditions within the same ISM period.
The CCN concentration varies with SS, and its parameterization is very important for its
applicability in climate models (Khvorostyanov and Curry, 2006). The measured CCN spectra are
parameterized by Twomey's empirical fit relationship (Twomey, 1959, 1977), which is widely used
due to its simplicity (Cohard et al., 1998) and is given as follows:
CCNss=C×SSk,
where C and k are the empirical fit parameters characterizing the spectra.
It should be noted that the empirical fit parameter k is different from the
effective hygroscopicity parameter-κ discussed by Petters and Kreidenweis
(2007).
More than 90 % of cases in the current observations show a high
correlation coefficient (R>0.95) with Twomey's empirical
fit, except during continental-2 during which about 65 % of the cases
only had high (>0.95) correlation coefficient with Twomey's
fit. The spectra having a correlation coefficient of more than 0.95 with the
empirical fit are only considered in the present study.
Hygroscopic or bigger particles have flat CCN spectra and low k values,
while hydrophobic and ultrafine (UF) mode (<100nm) APs will have
steep CCN spectra and high k values (Hegg et al., 1991; Jefferson, 2010) as
those particles need higher SS to activate as CCN. Thus, the empirical fit
parameter “k” indicates the nature of the aerosol system towards CCN
activation, and “C” indicates the CCN concentrations at 1.0 % SS.
Generally, high C and k values are reported for the anthropogenic, while low
values are reported for the natural/marine APs (Seinfeld and Pandis, 2016;
Andreae, 2009). The highest k value (∼0.67) is observed
during marine-2 and the minimum (∼0.52) during continental-1.
As may be noted, bigger or hygroscopic (or both) particles which are CCN
active were abundant during continental-1 compared to the marine conditions
when fine or hydrophobic (or both) particles were predominant.
Mean CCN concentrations (± SD) for different
SS during continental and marine conditions. The power law fit k value of
each spectrum is also given.
CCN concentrations and the k values reported during the current study, along
with a few other studies, are given in Table 2. Generally, most of the
aerosol abundance measurements such as BC mass (Kompalli et al., 2014) and
aerosol number concentration (Babu et al., 2016) showed the lowest seasonal
mean value during ISM over the Indian region, mainly due to the wet removal
of APs. The CCN concentration at semiarid Solapur before the onset of ISM is
comparable to the values (∼2000cm-3) observed over the
arid northwestern region of India reported by Jayachandran et al. (2020).
Interestingly, the CCN concentration at Solapur during the active marine
conditions is the lowest among the values reported over the Indian
subcontinent. The current values (∼350cm-3) are
comparable to those reported from Ponmudi (∼400cm-3) at
the southern part of the WG, and another site at WG, namely Mahabaleshwar
(∼500cm-3) at 0.2 % SS. From Table 2, very high
values of CCN concentrations are reported from polluted urban environments.
Very low CCN concentrations (<300cm-3 at 1 % SS) are
reported from pristine environments like the Amazon (Pöhlker et al., 2016,
2018) and Alps (Juranyi et al., 2011). The mean CCN values observed at
Solapur during ISM are comparable to those classified as “polluted marine”
by Andreae (2009).
CCN and aerosol characteristics reported over various locations
along with the present results (at 0.3 % SS). CCN reported for Thumba@ 0.4 and Colorado# 0.36 % SS, respectively.
Location (coordinates; a.m.s.l.)Type/conditionPeriodCN (cm-3)CCN (cm-3)kAF (%)GMD (nm)ReferenceSolapur (17.70∘ N, 75.85∘ E; ∼490m)Continental Monsoon Monsoon ContinentalJun 2018 Jul 2018 Aug 2018 Sep 20183427±1064 – 2356±9844381±18241946±594357±92322±1181497±5240.52±0.110.58±0.160.67±0.180.56±0.160.55±0.09 – 0.15±0.060.32±0.1085±10 – 69±1181±14Present studyMahabubnagar 17.70∘ N, 78.85∘ E; ∼490 mContinental (polluted)Oct 2011–∼5400 at 1 % SS∼0.45∼0.9–Varghese et al. (2016)Ponmudi (8.8∘ N, 77.1∘ E; ∼960m)Western Ghats monsoonJul–Sep 2016∼2000∼4000.65±0.28∼0.20–Jayachandran et al. (2018)Mahabaleshwar (17.56∘ N, 73.4∘ E; 1348 m)Western Ghats Premonsoon monsoonMar–May Jun–Aug 2012∼3100∼3200∼1200∼500 at 0.2 % SS∼0.5 ∼1∼0.35 ∼0.35∼90∼77Leena et al. (2016)Thumba@ (8.5∘ N, 76.9∘ E; 3 m)Coastal monsoonAug–Sep 2013∼49002096±8340.54±0.210.46±0.15∼103Jayachandran et al. (2017)Nainital (29.2∘ N, 79.3∘ E; 1960 m)CentralHimalayasJun 2011 Jul 2011 Aug 2011 Sep 20112425±11121874±7761606±4532304±904925±601881±500684±3961233±6770.57±0.110.45±0.080.45±0.040.39±0.030.38±0.110.47±0.110.42±0.180.54±0.12–Dumka et al. (2015)Darjeeling (27.02∘ N, 88.25∘ E; 2200 m)EasternHimalayasMar–May 20167220±1988∼16000.38±0.05∼0.25–Roy et al. (2017)Kanpur (26.5∘ N, 80.3∘ E; 142 m)Urban/pollutedMay–Jun (dry) Aug (wet)∼7110∼6450∼4570∼2360–∼0.64 ∼0.36–Bhattu and Tripathy (2014)South Korea (37.6∘ N, 127.04∘ E)Urban/pollutedMay–Jun 201610825±48633105±1521––44±14Kim et al. (2018)Guangzhou (23.07∘ N, 113.21∘ E)Clean pollutedSummer 8246±35957193±37753017±14502883±1158– 0.39±0.120.45±0.13–Duan et al. (2017)Colorado# (38.64∘ N,105.11∘ W; 2300 m)ForestJun Jul Aug∼1400∼1800∼1250∼500–∼0.30∼68∼80∼90Levin et al. (2012)Amazon (2.13∘ S, 59∘ W; 130 m)Forest Dry WetAug–Nov Feb–May1520±780330±130∼1469∼289 at 1% SS0.36±0.060.57±0.03––Pöhlker et al. (2016, 2018)
Generally, an enhancement in k values is observed during the monsoon period, which is seen only
during the marine-2 conditions in this study. In all other cases, the k values are
comparable. Jayachandran et al. (2017, 2018) reported similar results for the monsoon period both at
a coastal site and at a hill station in the WG. From the southern tip of India, Jayachandran
et al. (2017) have shown the enhancement of k values (∼0.7) associated with wet
scavenging and low k values (∼0.55) during no rainfall conditions within the same ISM
period. The enhancement in k values (2 times) associated with the monsoon rainfall can be seen
from Mahabaleshwar also (Table 2). Thus, the CCN concentrations at different SS at Solapur during
ISM are low compared to those reported from other environments in India, while the CCN spectra show common characteristics to those values reported from WG and peninsular India.
Significant diurnal variations are seen in the PBL AP properties over the Indian subcontinent (Nair
et al., 2007). Daytime high and nighttime low aerosol abundance characterized by anomalous high
values just after sunrise, known as the fumigation peak (Prakash et al., 1992), is generally
observed. This diurnal pattern is mostly due to the evolution of the PBL and due to local emissions
(Nair et al., 2007; Kumar et al., 2015). As the CCN activation and its properties are highly
heterogeneous, it is very important to know its variation in a day. The diurnal variations of CCN
concentration at 0.3 % SS, segregated to air mass, are shown in Fig. 5a and b, respectively. The
CCN variations in a day are similar during the marine conditions (Fig. 5a), while it differs before
and after the monsoon rainfall. In general, CCN concentrations show a slight enhancement (more
prominent during clean background–marine air mass) during daytime due to the anthropogenic
activities. A rapid increase is seen just after sunrise in all the conditions, though it is weak in
continental-2, due to the mixing of the nocturnal residual layer with the evolving PBL (fumigation
peak). There is no vivid diurnal variation in CCN during continental-2. The diurnal variations of CN
and BC concentrations for different periods are shown in Fig. 6. The diurnal variation of the CN
concentration and BC mass concentration is more vivid than that of the CCN concentration. A clear
bimodal variation is seen in both CN and BC diurnal variations during marine conditions. A sharp
peak is seen in both CN and BC after sunrise (06:00–07:00 IST) and the next peak starts
increasing from 15:00 IST and the maximum is at around 20:00 IST. The diurnal variations in CN and BC
are less prominent during the marine, compared to those during the continental conditions. In both
the conditions a small increase is seen in the CN concentration around the afternoon. The magnitude
of the fumigation peak seen in BC during the continental conditions was more than twice the
corresponding daytime average values. Another important observation is that the CN values were
consistently higher during continental-2 throughout the day than continental-1, while the daytime BC
mass was higher during continental-1 than 2. In contrast to the CCN, the BC had well-defined
multiple peaks (morning and evening), indicating the contrasting aerosol source characteristics
during the diurnal cycle. The well-mixed conditions during the late afternoon hours and the PBL
evolution have a well-defined role in the reduction of concentrations during the daytime until new
sources of aerosol are injected in to the atmosphere in the evening hours. The nighttime increase in
BC could be due to the stable conditions and less vertical mixing. The role of the PBL in modulating
regional aerosol characteristics will be dealt with in a separate study.
Diurnal variation of CCN concentrations at 0.3 % SS during (a)
marine and (b) continental conditions.
Diurnal variations of (a) CN and (b) BC concentrations during
marine and continental conditions.
A few studies reported the diurnal variations in CCN and its properties at
coastal (Jayachandran et al., 2017), WG (Leena et al., 2016; Jayachandran
et al., 2018), IGP (Patidar et al., 2012), central Himalayan (Gogoi et al.,
2015), and eastern Himalayan (Roy et al., 2017) environments of the Indian
subcontinent. Weak diurnal variations in CCN concentrations during ISM,
similar to the present study but with opposite pattern, were reported from
the southern coast by Jayachandran et al. (2017) and from WG by Leena et al.,
(2016). Day–night variations in the CCN concentrations can be due to the
changes in aerosol sources, PBL dynamics, or both. Since the sky has
generally been overcast during the ISM and hence a shallow moist PBL (Sandeep et al., 2014) prevails, the observed diurnal variations during marine
conditions are mainly due to the diurnal variations in the source and sink
processes. The bimodal diurnal pattern seen in BC mass concentration at
Solapur is seen similar to the observations reported by Safai et al. (2007)
over Pune. Apart from the fumigation process happening during sunrise,
vehicular and biomass emissions also have a role in the peaks observed in
a day. Thus, both local emissions and PBL dynamics contributed to the diurnal
variations observed in the AP characteristics. The diurnal variations during
the continental conditions indicate the consistently high AP background
conditions. While the diurnal variations during the marine conditions
indicate the significant presence of local AP sources.
CCN–CN association
The association of CCN concentration at 0.3 % SS with the concurrent
total AP number concentration, CN (∼15–685 nm), is
investigated separately for different conditions and is shown in Fig. 7.
CCN concentration at 0.3 % during continental-1 is estimated from the
measured CCN spectra. The role of the aerosol NSD is revealed through the
color of the scatter which represents the geometrical mean diameter (GMD) of
the corresponding AP system. It can be inferred from the figure that the
relationship between CCN and CN is different for different conditions.
A least-square linear fit forced through origin (as there is no CCN in the
absence of CN) is made through the scatter.
The best linear relationship between the CCN at 0.3 % and CN concentrations is seen during
continental-1, and the corresponding slope and correlation coefficient (R) of the fit are
∼0.56 and 0.98, respectively. The linear association weakens during the marine condition when
the slope and correlation coefficient values of the fit reduce to ∼0.12 and 0.90,
respectively. Even though under continental conditions the slope of the linear fit during
continental-2 (∼0.28) reduces to the half of that measured during continental-1, the
correlation coefficient value (R=0.94) also reduces. The relationship between CCN and CN during
marine-2 is weaker than the continental, and only a few APs are activating as CCN. It can be seen
that most of the scatter points which lie below the linear fit line and correspond to the higher
(than the monthly mean) CN values have GMD less than 50 nm. Even though the number of
cases is less, similar observations can be seen during the continental case too. The two points
(black circle) corresponding to CN concentrations higher than 13 000 cm-3 have GMD
less than 50 nm. These cases which reduce the CCN activation indicate the presence of a UF
mode, probably due to the new particle formation (NPF) events. However, the presence of UF particles
is not the only reason for less activation of CN as CCN in marine conditions as the scatter and the
linear fit excluding the UF particles also have low correlation and slope values.
Association between CCN (at 0.3 % SS) and CN concentrations
segregated to (a) continental and (b) marine conditions. The color of the
scatter indicates the concurrent geometrical mean diameter of the aerosol
system. The least-square linear fit is also shown along with the fit
parameters.
During marine-2 (Fig. 7b), the CCN concentration at 0.3 % was not increasing beyond
600 cm-3 despite CN concentration increasing to ∼7500cm-3. This is
indicative of a significant number of UF APs that require high SS for activation. Similar behavior of AP
system towards CCN activation is observed at the eastern Himalayas (Roy et al., 2017). The drastic
difference in CCN–CN association, similar to the present study, is also reported by Asmi
et al. (2012) between winter and summer months at a high-altitude site in France. They have
attributed the predominance of accumulation mode particles and fine-mode particles during winter and
summer months, respectively, to the distinct CCN–CN association. From the central Himalayas, Dumka et al. (2015) have shown an increase
in CCN–CN scatter during ISM due to the change in the aerosol physicochemical properties. The
spread of the scatter between CN and CCN increased for polluted conditions (Jayachandran et al.,
2020), which was attributed to the associated complex aerosol size distribution and mixing
state. Thus, the CCN dependence on the CN population during the ISM shows a complex dependence on
the aerosol size and mixing state.
CCN activation fraction
The fraction of APs acting as CCN at a given SS is known as the CCN
activation fraction/ratio (AF) and is an important parameter for characterizing
the CCN activity (Dusek et al., 2006; Andreae, 2009; Deng et al., 2013). The
CCN AF values for all the SS segregated to different air mass history are
shown in Fig. 8. During continental-1, more than 40 % of the APs are
being activated as CCN at 0.2 % SS. Nevertheless, during marine-2,
about 40 % of the particles only are activating as CCN even at 1.2 %
SS, revealing the highly CCN inactive nature of the APs. During
continental-2, the CCN AF values at all SS are between those of
continental-1 and marine-2.
Variation of CCN activation fraction at different SS during
continental and marine (conditions). Vertical error bars indicate the
standard error.
The diurnal variations of k and AF (0.3 % SS) values are shown in Fig. 9. Unlike the continental conditions, the k values show a clear diurnal
pattern during the marine conditions, similar to the case of CN and BC
concentrations. The k values almost double after sunrise and
decrease thereafter to reach the nighttime values by evening hours (16:00
IST) during marine conditions. Again, the k values peak at around 21:00 h
IST. The enhancement in CCN and k values during daytime in marine airmass
conditions indicates the influence of local anthropogenic aerosol sources in
determining the CCN activation. As discussed in Fig. 8, the CCN AF values
are very low throughout the day, with a slight increase during noon hours in
marine-2 when the k values are low. In contrast, the AF values are
consistently high throughout the day during continental-1. During
continental-2, an increase in CCN AF (from ∼0.3 to 0.4) can
be seen during the daytime.
It is well understood that the CCN characteristics are a function of aerosol
size and composition. Hence it will be interesting to know the diurnal
variations of the concurrent aerosol size and composition. In the absence of
continuous aerosol composition measurements, the absorption Ångström
exponent (αabs), which is a proxy to identify
the nature of the carbonaceous aerosols, is estimated, and its diurnal
variation for different conditions is shown in Fig. 10a and b. The
diurnal variation of the GMD for the corresponding periods is shown in
Fig. 10c and d. The diurnal variations of αabs are similar for the marine conditions, with the values
peaking by sunrise (06:00–09:00 IST) and late evening hours (18:00–20:00 IST).
Almost the same pattern is seen during continental-2 also but of different
magnitudes. Meanwhile, there are no clear diurnal variations in
αabs during continental-1 observations similar
to the diurnal variations of k and AF.
Diurnal variations of k values (a and b), and CCN activation
fraction (c and d), during marine and continental conditions. Vertical bars
indicate the standard error values.
Diurnal variation of absorption Ångström exponent (a and b) and
geometrical mean diameter (c and d) during continental and marine
conditions. Error bars indicate the standard error associated with the mean
values.
The mean GMD values during the marine conditions decrease from 12:00 IST onwards and reach the
minimum value (<60nm) from 16:00 to 18:00 IST, which increases back to
∼72nm by the midnight hours. Interestingly, a small dip in the GMD is observed during
the fumigation peak (∼07:00 IST), which is associated with the sharp increase in αabs
values. The GMD values were consistently higher throughout the day during continental-1, in that
the lowest mean value (∼79nm) observed at the 14:00 IST is higher than the maximum
mean GMD (∼78nm) observed in a day (03:00 and 04:00 IST) during marine-2. The GMD
during continental-2 depicts a clear diurnal variation, which is opposite to that observed during
continental-1, with distinct high values during the daytime. Similar to marine-2, GMD decreases
during the fumigation peak associated with a sharp increase in the αabs values.
Even though the mean αabs values are almost similar in all periods with
comparatively higher (1.29±0.09) during continental-1 and lower during marine-2
(1.19±0.14), the values show diurnal variations systematic with aerosol abundance diurnal
variations. The sudden sharp increase in the k values (Fig. 9a) during sunrise hours of marine-2
is associated with a similar enhancement in the αabs values. The high k values
(>0.8) during these hours are due to the organic aerosols inferred from αabs
values. Chung et al. (2012) have reported αabs values above 1.6 for organic
aerosols while Gyawali et al. (2009) have reported αabs values above 1.4 for
biomass smoke. The daytime enhancement (∼2 times) in CCN AF during continental-2 is exactly
according to the daytime enhancement seen in the aerosol GMD. Interestingly, a similar association
is not seen in other periods. Jayachandran et al. (2017) have reported a similar association between
CCN AF and aerosol GMD diurnal pattern during ISM from a coastal site in southern peninsular
India. Thus, the aerosol composition, especially the organic aerosols inferred from the high
αabs values, is playing a role in determining the CCN activation during the marine
conditions, while the aerosol size is determining the CCN activation during the continental
conditions.
In general, high AF is found for aged background aerosols, while freshly emitted polluted urban
aerosols have low CCN efficiency (Andreae and Rosenfeld, 2008). CCN AF values reported from India
and some relevant studies reported across the globe are mentioned in Table 2. The similarity seen in
CCN concentration and k values is seen in CCN AF also between Solapur and Ponmudi during the
ISM. At both the places, only a small fraction (15 %–20 %) of the ambient APs are activating as CCN
at 0.3 % SS. As seen in the Table, high AF values are reported from the coastal location and
the central Himalayas. The high CCN AF during the continental conditions at Solapur is similar to
those reported during dry conditions in Nainital (Gogoi et al., 2015), where the high CCN AF was
attributed to biomass burning. The low CCN AF observed at Solapur during marine conditions resulting
from the wet scavenging is consistent with the values reported over the subcontinent during
similar conditions, while the high CCN AF before and after monsoon rainfall is observed by several
studies to resemble a biomass-burning-dominant polluted environment (Andreae, 2009).
As mentioned earlier and reported by several studies (Dusek et al., 2006;
McFiggans et al., 2006), aerosol size plays a major role in determining the
CCN activation ability of aerosols. It has been found that the UF particles
were present during the monsoon conditions when CCN AF was very low (Fig. 8). Meanwhile, the presence of bigger particles is enhancing the CCN
activation in other cases. To investigate the role of aerosol size in the
observed CCN activity, the aerosol NSD during each condition is examined in
detail.
Aerosol size distribution and critical activation diameter
The simultaneous and co-located aerosol size distribution observations and
critical diameter are examined. The fraction of particles in the nucleation
mode, Aitken mode, and accumulation mode is estimated and shown (in %) in
Fig. 11a. Nucleation mode particles are those observed below 30 nm,
Aitken mode particles are those from 30 to 100 nm, and accumulation mode
particles are those beyond 100 nm (Ueda et al., 2016; Willis et al., 2016).
The corresponding mean NSDs of APs along with the standard deviations for the
study period are shown in panels (b), (c), and (d) of Fig. 11. The frequency
of occurrence of the GMD for each period, along with the mean GMD values, is
shown in Fig. 11e. As seen in the CCN characteristics, aerosol NSD also
depicts distinct features prior to the marine (Fig. 11b), during marine
(Fig. 11c), and after the marine air mass (Fig. 11d).
(a) Fraction (in %) of nucleation mode, Aitken mode, and
accumulation mode particles during the observation periods. Aerosol mean
number size distribution (with standard deviations) during (b)
continental-1, (c) marine-2, and (d) continental-2 conditions. The bars in
the same plots indicate the frequency distribution of critical activation
diameters at 0.3 % SS of the corresponding conditions (right axis). (e) Frequency distribution of the geometric mean diameter of the aerosol system
during the observation periods.
During continental-1, most of the distributions are monomodal, peaking
around 80 nm, and the mean GMD during this period is ∼85nm.
In this period, the majority (∼55 %) of the APs were
present in the Aitken mode and least (∼7 %) in the
nucleation mode. During the continental-2 air mass conditions, a prominent
presence of nucleation mode particles (15 %) was also seen (Fig. 11d).
The consistent presence of such particles is seen as the spread of the
distribution of the GMD in Fig. 11e. Unlike the continental air mass
conditions, the aerosol size distributions are entirely different during the
marine air mass under the monsoon rainfall as seen in Fig. 11c. Three
modes are distinctly observed in Fig. 11c with two peaks below 100 nm.
The mean GMD during this period is ∼69nm with the frequency
distribution spreading towards the lower size range. About 19 % of the
total aerosols were found in the nucleation mode (<30nm) during
marine-2, and this feature continues in continental-2 also – even though the
air mass history changes. The accumulation mode (30–100 nm) AP concentration
diminished (only 31 %) during the marine air mass.
For a given aerosol NSD, the critical activation diameter (dcri) serves as an important
parameter for representing the CCN activity, along with the CCN activation fraction and the
empirical fit parameter, namely k values. Assuming homogeneous composition, dcri for
a specific SS can be estimated by integrating the aerosol NSD from the higher to lower size until
the integration becomes equal to the measured CCN number concentration at that SS (Furutani et al.,
2008; Kammermann et al., 2010; Deng et al., 2011; Fang et al., 2016). The lower limit of the
integration can be considered as the “apparent” critical activation diameter, as the ambient
aerosol system can have both internal and external mixing states, and size-dependent
composition. Comparing the dcri for different conditions, the values were always less
than 100 nm during continental-1; in contrast, the values were always greater than
100 nm during marine-2. The dcri values were around 100 nm during
continental-2. The mean (± SD) values of dcri were 72±12nm,
169±38nm, and 121±20nm for continental-1, marine-2, and continental-2
conditions, respectively.
Different factors such as heterogeneous sources (Kim et al., 2002; Morawska, 2002), local
meteorology (Wehner and Wiedensohler, 2003; Du et al., 2018), long-range transport (Birmili et al.,
2001), and cloud processing (Noble and Hudson, 2019) can influence and modify the NSD. The
predominant (69 %) fine-particle (<100nm) size distribution (bimodal) during
marine air mass is similar to the two modes observed in the fine size range at the
urban site, Kanpur (Bhattu and Tripathy, 2014), during the ISM. The GMD values and the corresponding
CCN properties from the present study and other relevant studies are listed in Table 2. Lower AP
concentration with low GMD (74 nm) was observed during the monsoon at an urban site by
Kanawade et al. (2014) and at a background Himalayan site (86 nm) by Kompulla et al. (2009)
over the Indian subcontinent. Similar to the present study, GMD was high before and after the
monsoon period in both studies. From a high-altitude site in WG, Leena et al. (2016) have reported
the lowest seasonal mean GMD of ∼77nm during ISM. The enhancement of the smaller
particles in the total aerosol system, causing the reduction of the GMD especially during monsoon as
seen in the present study, is consistent with the previous studies (Babu et al., 2016). The mean GMD
value observed over Solapur during ISM is the lowest (69 nm) reported value during a
similar period over the Indian region.
Similar to the present observations, comparable accumulation and Aitken modes and a dominant
accumulation mode were reported over the Amazon for wet and dry months, respectively by Pöhlker
et al. (2016). The accumulation mode particles are associated with either aged biomass burning
particles (Kalvitis et al., 2015) or condensation or coagulation of secondary organics and
inorganic particles (Seinfeld and Pandis, 2016). Interestingly, low aerosol GMD values
(<70nm), similar to the present observations during marine air mass, were consistently
observed near anthropogenic sources by Quinn et al. (2008). In the same study, they found bigger APs
(GMD>70nm) for observations carried out away from anthropogenic sources, which
is similar to the present observations during the continental conditions. From the concurrent
aerosol mass spectrometer measurements, they found that hydrocarbon-like organic aerosols (HOA)
having mass spectrum characteristic of long-chain hydrocarbons from fresh diesel exhausts were
responsible for the fine mode, while oxygenated organic aerosols and sulfates are responsible for
the higher GMD. Hence the presence of freshly produced local fossil fuel combustion aerosols in the
UF mode can be a reason for the low CCN activity during marine-2 in the present study. During the
marine condition, most of the bigger APs, which are potential CCN, are either washed out by wet
scavenging or already activated as cloud droplets. Hence the measured APs are devoid of CCN active
particles which are clearly seen from the aerosol NSD during the relevant period (Fig. 11). Thus,
the low CCN activation fraction during the marine conditions is due to those CCN active particles missing near the surface.
Since the CCN activity depends mainly on the aerosol size and chemical
composition (Dusek et al., 2006; McFiggans et al., 2006),
dcri estimated from concurrent aerosol NSD and CCN
measurements can be considered as a proxy for the variations in the chemical
composition of the aerosol system. As the aerosol size distribution and
chemical composition are intrinsically associated with each other, any shift
in the physical size distribution is mostly associated with the change in
the aerosol composition arising mainly due to the change in the sources or
due to different processes such as aging, coating, or scavenging, except for
externally mixed systems (Crosbie et al., 2015). Quinn et al. (2008) have
correlated the dcri with the HOA mass and
found that HOA can explain about 40 % of the variance in the
dcri. They have reported 70–90 nm and higher values
(>90nm) as the dcri for marine
and inland regions, respectively, at 0.44 % SS. For anthropogenic and
marine environments, Furutani et al. (2008) have reported
dcri values of 70–110 and 50–60 nm,
respectively, at 0.6 % SS. The dcri values
observed during continental conditions at Solapur are similar to the values
observed at a tropical monsoon climate region by Fang et al. (2016) under urban influence. The dcri values during marine-2 are
higher than the corresponding values reported from the polluted North China
Plain by Deng et al. (2011). From an urban site, Burkart et al. (2011)
have reported an average value of ∼169nm for
dcri at 0.5 % SS. Freshly emitted
carbonaceous combustion particles can have large
dcri values up to ∼350nm, even
at a high SS (0.7 %; Hitzenberger et al., 2003). The
dcri can exhibit much lower values than
theoretically estimated ones in the presence of partially or fully soluble
particles as their slight presence can greatly enhance the CCN activity of
insoluble particles such as BC and dust (Dusek et al., 2006; Begue et al.,
2015). The sharp distinction in the dcri values and aerosol NSD between different atmospheric/air mass conditions
within the same season in the present study indicates the difference in the
aerosol composition.
Relationship between aerosol absorption and CCN properties
The aerosol composition plays an important role associated with the changes
in the aerosol NSD due to the meteorological processes and active source and
sink mechanisms prevailing during the monsoon conditions. The significant
influence of aerosol composition in determining the CCN activity at lower SS
which is more probable in the real atmosphere is demonstrated in many
studies (Cubison et al., 2008; Kammermann et al., 2010; Bhattu et al., 2015;
Jayachandran et al., 2017). In this aspect, the role of carbonaceous
aerosols in determining the CCN activation is investigated.
The association of CCN concentration at 0.3 % SS and absorption
coefficient at 550 nm, segregated for different periods, is shown in Fig. 12. The association between aerosol absorption and CCN concentration is
generally low but moderate during the continental air mass and very weak
during the marine air mass. The better association between the CCN
concentration and absorption properties may be due to (i) absorbing APs
acting as CCN or (ii) aerosol species co-emitted with the absorbing
APs activating as CCN. The higher slope and better association observed
during continental-1 indicate that the low k, high AF values, and the high
association between CCN and CN during the period are due to the major
contribution of carbonaceous aerosols towards CCN activation. It can be due
to the co-emitted organics enhancing the CCN efficiency of the aerosol
system, due to the aged carbonaceous aerosols activating as CCN, or
due to the combination of both. The enhancement in the accumulation mode
aerosols supports this observation as oxygenated organic aerosols and
sulfates are found in the accumulation mode (O'Dowd et al., 1997; Quinn et al., 2008). During marine air mass, there is no clear association between
aerosol absorption and CCN and fewer APs are activated as CCN. It indicates
that there is a change in the source/sink and nature of CCN during
marine/wet and continental/dry conditions.
Carbonaceous aerosols form a major source of CCN concentration and thereby contribute to the
indirect effect of aerosols (Novakov and Penner, 1993). Anthropogenic carbonaceous aerosols cause
an indirect effect of -0.9Wm-2, while sulfates cause only -0.4Wm-2
(Lohmann et al., 2000). Spracklen et al. (2011) have shown through simulations that about 60 %
of the global CCN concentration is from carbonaceous sources. Various processes such as aging,
coating, and mixing can enhance the water activity of BC (Lammel and Novakov, 1995; Kuwata et al.,
2009), which is hydrophobic when freshly emitted. Mixing with hydrophilic substances like inorganic
salts can also enhance the CCN activity of carbonaceous aerosols (Dusek et al., 2006). Thus, the
better correlation observed between CCN and aerosol absorption and the associated high CCN
efficiency during the continental conditions indicates the significant role of carbonaceous aerosols
in CCN activation at Solapur. Jayachandran et al. (2018) illustrated the close association of CCN
with aerosol absorption properties (better than the present study) from WG and the lack of association
between the parameters at a coastal site during the monsoon conditions. Enhancement in CCN
concentration along with an increase in the aerosol absorption coefficient was observed at the central
Himalayas (Gogoi et al., 2015). In general, carbonaceous aerosols have a significant role in CCN
concentration during continental conditions.
Association between CCN (at 0.3 % SS) and absorption
coefficient (at 550 nm) for (a) continental-1 (b) continental-2, (c)
marine-1, and (d) marine-2 conditions. The color of the scatter indicates
the concurrent absorption Ångström exponent values. The least-square linear
fit is also shown along with the fit parameters.
As the very low AF is observed along with the enhancement in the nucleation mode particles and
depletion of Aitken particles after the onset of ISM over the study region, the chemical
characteristics of Aitken, and accumulation mode particles, are investigated. Most of the
studies from different parts of the globe have reported about the high hygroscopicity of
accumulation mode particles and lower hygroscopicity for the Aitken mode particles (Paramonov et al.,
2013, 2015). From the long-term observations from the Amazon, Pöhlker et al. (2016) have concluded that
organics predominantly present in the Aitken mode reduce the hygroscopicity, while the dominance of
inorganics in the accumulation mode enhances the aerosol hygroscopicity – which was in line with other
studies (Gunthe et al., 2009; Pöhlker et al., 2012).
The linear fit parameters between BC mass and Aitken/accumulation mode particle concentration under different loading and air mass conditions.
The CCN concentration exhibited an association with the absorption coefficient during the
continental air mass compared with the marine air mass conditions. The reduced CCN efficiency due to
the presence of Aitken or UF mode has already been discussed. To ascertain the contribution of the
carbonaceous APs to the NSD, the association of BC with Aitken mode particles and accumulation mode
particles are examined under high and low aerosol loading conditions and shown in Fig. 13. Since the
BC loading was mostly (>99% cases) less than 2500 ngm-3 during marine-2, cases
above this value are taken as polluted (p, high loading) in the continental air mass. A least-square
linear fit is also made and the corresponding parameters are shown in Table 3. Under similar BC
loading (low), Aitken mode APs are better associated with BC with a higher slope (m=0.68) during
marine-2, while the accumulation mode APs showed a similar correlation with BC during low loading and
the lowest slope during marine-2. The Aitken mode APs showed better association with BC during
continental-2 under the high BC loading conditions. Except during continental-2, the accumulation mode APs
were more associated with BC compared to Aitken mode APs. These all indicate that BC or the
co-emitted APs from carbonaceous combustion sources were predominant in Aitken mode during marine-2
and polluted continental-2 conditions. In all other cases, the carbonaceous combustion APs
contributed to the accumulation mode.
Association of BC mass concentration with Aitken (a, b) and
accumulation (c, d) mode particles segregated for different air mass and
loading conditions. The least-square linear fit lines are also shown.
The association of BC with the accumulation mode particles and the association of CCN with absorption
coefficients during continental-1 along with high CCN AF indicate that the aged, large hygroscopic
particles from carbonaceous combustion sources were present prior to the monsoon at the location. The
strong convective conditions existing during this period over the region can take the APs to high
altitudes where they can absorb radiation and may lead to semidirect effects. The association of BC
with accumulation mode APs during continental conditions suggests that the carbonaceous APs existing
in this size range can act as CCN. From the eastern coast of India, Kompalli et al. (2020) have
reported highly coated larger BC particles (>110nm) in dry conditions under the
continental influence, while nascent BC particles (∼80nm) with less coating were
found during ISM due to wet scavenging. This finding is in line with the current marine-2
observations. The enhanced fine-particle concentration having better association with BC mass
concentration during continental-2 underlines the possibility of freshly emitted carbonaceous
aerosols reducing the CCN AF. The association of BC mass and accumulation mode aerosol number
concentration also points to the possibility of inorganic aerosols like sulfate, co-emitted along
with BC from carbonaceous combustion sources, enhancing the CCN activity of APs in continental
conditions. Hence, the role of carbonaceous APs in modulating both cloud microphysics and dynamics
needs to be investigated in detail. However, the current investigation could not address these
probable aspects.
CCN closure
To understand the role of the size and composition of aerosols in CCN activity, CCN closure studies have
been carried out by many investigators (Brokehuizen et al., 2006; Lance et al., 2009; Juranyi
et al., 2011; Bhattu and Tripathy, 2015; Crosbie et al., 2015; Jayachandran et al., 2017) at
different environments, leading to the better understanding of the CCN activation from APs. CCN
concentration at the rain-shadow region under different air mass conditions is estimated and
validated with the measured CCN concentrations. The CCN concentrations are estimated by (i)
assuming accumulation mode aerosols are activating as CCN, (ii) applying the mean “apparent” critical
diameter, and (iii) assuming the aerosol composition as ammonium sulfate ((NH4)2SO4) and comparing with the observations. The scatter between the CCN concentration at 0.3 % SS
estimated and the corresponding measured CCN concentrations segregated according to marine and
continental conditions are shown in Fig. 14.
CCN are generally approximated as APs above 100 nm (accumulation mode) in many studies when
there are no concurrent CCN measurements (e.g., Andreae, 2009). Still, it is a rough approximation due
to the nonlinear dependence of CCN activation of APs, and this assumption is examined in Fig. 14a
and b. The variations in accumulation mode APs are correlated well (R>0.95) with the CCN
concentration in all the conditions. Interestingly, under the same conditions (continental) the
accumulation mode APs have different CCN activation efficiency and are underestimated (m=0.64)
during continental-1 and overestimated (m=1.26) during continental-2. But during marine-2 the
linear fit of the scatter shows that the estimated CCN concentration is almost twice that of the
measured concentration when the accumulation mode particles are considered to be CCN active.
Rather than taking an assumed value (100 nm) as the critical activation diameter, the mean
of the measured critical diameter (dcri) is used to estimate the CCN concentration and is
shown in panels (c) and (d) of Fig. 14. The mean dcri for 0.3% SS of ∼70,
∼165, and ∼120nm is used for estimating CCN during continental-1, marine-2,
and continental-2 conditions, respectively. The estimated and the measured CCN concentration
correlate well during the continental conditions with a high correlation coefficient (R=0.97) and
almost unit slope. From Fig. 14c, it can be seen that most of the points lie along the diagonal
1:1 line irrespective of the number concentrations and GMD values.
The CCN concentration at 0.3 % SS at different conditions is estimated (Fig. 14e and f) by
assuming an inorganic composition of (NH4)2SO4 as an ideal scenario. The methodology
used for the estimation is given in the Appendix. In all the conditions, CCN concentrations were
overestimated by this approach, indicating that the ambient aerosol system has a lesser
hygroscopicity (κ) than that of (NH4)2SO4. The highest overestimation
(m=3.38) is observed during marine-2. During continental-1, the maximum correlation coefficient
(R=0.98) is obtained, and the slope is also nearer to the unity (m=1.13). However, for the
continental-2 conditions, the CCN concentrations are overestimated (almost twice) when
(NH4)2SO4 composition is assumed. These observations confirm that the nearly
monomodal aerosol NSD observed during continental-1 is more similar to an aged continental aerosol
system that has similar hygroscopicity to sulfate aerosols. This observation has to be considered
along with the association of BC with accumulation mode aerosols (Fig. 13c). The AP system observed
during marine conditions is much less hygroscopic, and the multiple size modes observed in the
smaller size range indicate a heterogeneous composition in a complex mixing state during the wet
conditions.
Scatter between estimated and measured CCN concentration at 0.3 % SS for continental and marine conditions with the color indicating the
GMD. CCN estimated (a, b) as particles above 100 nm, (c, d) from
critical activation diameter, and (e, f) using aerosol NSD and ammonium
sulfate composition. Linear fit and the parameters are also shown. The broken
lines indicate the unit slope (m=1) line.
Comparing the three approaches used to estimate the CCN concentrations, using a sharp size cut for estimating CCN concentrations, suits all the cases well. However, there is a variation in the AP
composition in the continental air mass before and after the monsoon rainfall that is reflected in the
distinct CCN efficiency of the accumulation mode between these periods. The aged aerosol system
prior to the monsoon resembles a sulfate aerosol composition with a very high CCN activation
efficiency and low k values. During continental-2, all the accumulation mode APs are not
participating in the CCN activation (Fig. 14a), and the assumption of (NH4)2SO4
composition nearly doubles the estimated CCN concentrations. When comparing the aerosol NSD in continental
conditions, there is a depletion of Aitken mode particles and an enhancement in nucleation mode
particles during continental-2. Similar to the different AF between continental 1 and 2, Pöhlker
et al. (2016) have shown high CCN AF in the absence of nucleation mode.
This study is similar to the CCN closure reported by Crosbie et al. (2015) for North American
monsoon conditions. The complex meteorological pattern, including the monsoon showers and regional
aerosol production (both primary and secondary), causes large variability in the aerosol NSD as seen
in Fig. 11. The lower hygroscopicity of the accumulation mode aerosols during the marine air mass is
revealed in Fig. 14b. The least closure is obtained while assuming a uniform internal mixture of
hygroscopic inorganic composition. These all point to the highly complex mixture of the
size-dependent composition of the prevailing aerosol system during monsoon conditions. Studies
(Cubison et al., 2008; Ervens et al., 2010) have highlighted the need for size-resolved composition
information for estimating the CCN concentration for freshly emitted APs near to the sources. Thus
even though the nucleation mode APs present during the period hinder the CCN activity, the presence
of bigger particles in the same period is not supporting the CCN activation. It indicates that, apart
from the size of the aerosols, the composition/mixing state of the aerosol system during marine-2
also influences the CCN efficiency. From the aerosol optical properties (Fig. 10a), it is seen that
the low CCN AF and high k value is associated with the enhancement of the organics at the
site. These organics observed after sunrise hours during marine conditions may be limiting the CCN
activation of the aerosols. The quantification and classification of these species are essential for addressing the effect of aerosols on clouds in the rain-shadow region, especially during the monsoon.
Summary and conclusions
CCN characteristics at a rain-shadow region during the Indian summer monsoon (ISM) are studied with
respect to the different air mass and meteorological conditions that prevailed over the region. It
is found that the polluted-continental conditions transform into a polluted marine condition by the
onset of the ISM with a significant change in aerosol size distribution and composition affecting the
cloud nucleating properties. The important findings are listed below.
Comparatively high BC (∼2000ngm-3) loading and AOD (>0.5) prevailed over
Solapur before and after the marine air mass, which reduced to very low values
(BC∼800ngm-3) during the marine clean-background conditions.
The lowest CCN concentrations at any SS (∼900cm-3 at 1.2 % SS) are observed at
Solapur, compared to the values reported during the ISM over the Indian subcontinent. However, the k
values (∼0.6) during ISM are high and similar to those reported over the Western Ghats (WG) and
peninsular India under similar conditions.
A slight daytime enhancement in CCN is seen due to the influence of anthropogenic activities, while
a significant enhancement in k values (2 times) was observed during the daytime of the monsoon period with
concurrent high absorption Ångström exponent values.
Significant diurnal variations in CN, BC concentrations, and properties like “k”, CCN AF, and
αabs during the marine air mass indicate the dominant presence of local aerosol
sources, while the weak diurnal variations of the same parameters during the continental air mass
indicate the consistent polluted background conditions at Solapur.
The aerosol system prior to the onset of ISM that has a monomodal number size distribution (NSD) with
a geometrical mean diameter (GMD) of 85±10nm depicted high CCN activation fraction
(AF) of ∼55 % at 0.3 % SS. During the marine air mass, multiple modes were observed in
aerosol NSD with a predominant nucleation mode fraction (∼19 %) resulting in the lowest
CCN AF of ∼15 %. Just after the monsoon rainfall, aerosols were significantly present both
in nucleation (∼15 %) and accumulation mode (∼40 %) and the CCN AF enhanced to
∼32% only even though the corresponding aerosol GMD was 81±14nm.
The mean critical activation diameters (dcri) estimated for 0.3 % SS from concurrent
CCN and aerosol NSD measurements were highest during the marine air mass (∼165nm) and
lowest just prior to (∼70nm) and in between both after (∼120nm) the marine conditions.
Better association of absorbing-type aerosols with CCN and accumulation mode aerosols during
continental air mass conditions indicate the aged, bigger particles from carbonaceous
combustion sources possibly enhanced the CCN activity prior to the marine air mass. Aerosol absorption correlated well with Aitken mode particles during and after the marine air mass, which resulted in low CCN activation.
The closure study indicates the size dependency of CCN activation, especially during the
dry–continental conditions. Most of the CCN-active APs were removed from the atmosphere by activation
or wet removal, and the remaining particles were inherently CCN inactive as seen in the aerosol NSD
during the marine air mass. However, the CCN activation efficiency of the accumulation mode
particles reduced during and after the marine air mass.
The very low aerosol loading (towards an aerosol limited regime) with low CCN efficiency during the
ISM rainfall adds the significance of CCN in cloud droplet concentrations. The regional CCN
concentration can be estimated from the aerosol size distribution alone, indicating the size
dependency of CCN activation during the continental airmass conditions. But the distinct aerosol NSD
and CCN properties during the monsoon, due to the change in the aerosol source and sink mechanisms,
suggest the dependence of CCN activation on the composition of Aitken mode aerosols and their mixing
state. However, the predominance of ultrafine particles in the boundary layer and the corresponding
very low CCN efficiency demand further studies using the simultaneous cloud base observations to
understand the ACI affecting the precipitation pattern over the rain-shadow region against the
backdrop of cold-phase invigoration (Rosenfeld et al., 2008; Gayatri et al., 2017) and
condensational heating (Fan et al., 2018) mechanisms of tropical convective clouds.
Estimation of CCN concentration
The saturation ratio is given as follows:
s=awexpADp
and
aw=4σMwRTρw,
where aw is the water activity of the solution droplet and σ is the surface tension of the
solution. Mw and ρw are the molecular mass and density of water. R is the universal
gas constant and Dp is the size (Seinfeld and Pandis, 1998). The critical diameter for a given
aerosol system can be estimated from the Köhler theory based on its size distribution, chemical
composition, and hygroscopic growth information. The critical diameter derived from the Köhler
equation is as follows (Lance et al., 2009):
dcri=274logSS100+12ρwRT4σMw3MwρwρsϑϵMw-1/3,
where ρs, Ms, and ϵ denote density, molecular mass, and volume fraction of
the solute, respectively. ϑ is the effective van 't Hoff factor.
Spatial distribution of the Moderate Resolution Imaging
Spectroradiometer (MODIS) fire radiative power (Collection 6 product
obtained from https://earthdata.nasa.gov/firms, last access: 23 July 2019) for the measurement periods,
along with the observation site marked as a black star. Data points having
confidence values higher than 30, which come under the classification of
“nominal” and “high”, are only used.
Assuming a pure internally mixed aerosol system with uniform composition, CCN concentration can be
predicted using the following equation based on the measured aerosol NSD and estimated critical
diameter (Juranyi et al., 2011):
NCCNSSset=-∫DmaxdcridNdlogDDdlogD.
In the present study, the aerosol composition is assumed to be of ammonium
sulfate as an ideal case (Covert et al., 1998) in order to examine the
deviation of CCN activation from the ideal scenario. Hence the insoluble
fraction was taken as zero. The density of (NH4)2SO4 is taken as
1.76 gcm-3 (Hinds, 1999).
Data availability
Data used in the present study can be obtained by making a request through
http://www.tropmet.res.in/~caipeex/registrationform.php (last access: 17 June 2020) or
contacting Thara V. Prabha (thara@tropmet.res.in).
Author contributions
TVP conceptualized the experiment. TVP and VNJ designed the study. TVP, PM,
KST, SPB, GD, NM, JR, MK, SD, MV, and PDS were responsible for conducting the
campaign and data collection. VNJ carried out the scientific analysis of the
data and drafted the paper. TVP carried out the review and editing of
the paper.
Competing interests
The authors declare that they have no conflict of interest.
Special issue statement
This article is part of the special issue “Interactions between aerosols and the South West Asian monsoon”. It is not associated with a conference.
Acknowledgements
The CAIPEEX project is funded by the Ministry of Earth Sciences (MoES),
government of India. The authors acknowledge the team effort and dedication
of all CAIPEEX team members at the ground observatory. The authors are
thankful to the Director and Principal of N.B. Navale Sinhgad College of
Engineering for his support. Venugopalan Jayachandran acknowledges AVM (Rtd) K. Suresh Babu for
the scientific discussions. We acknowledge NOAA ARL for the providing the
Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) transport
and dispersion model used in this study. We are thankful to the MODIS
team for making AOD data freely available. The authors acknowledge the use
of data from LANCE/FIRMS operated by NASA's Earth Science Data and
Information System (ESDIS) with funding provided by NASA Headquarters
(http://earthdata.nasa.gov/firms).
Review statement
This paper was edited by Armin Sorooshian and reviewed by two anonymous referees.
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