Concurrent measurements of the altitude profiles of the concentration of cloud
condensation nuclei (CCN), as a function of supersaturation
(ranging from 0.2 % to 1.0 %), and aerosol optical properties
(scattering and absorption coefficients) were carried out aboard an
instrumented aircraft across the Indo-Gangetic Plain (IGP) just prior to the
onset of the Indian summer monsoon (ISM) of 2016. The experiment was
conducted under the aegis of the combined South-West Asian Aerosol–Monsoon Interactions and Regional
Aerosol Warming Experiment (SWAAMI–RAWEX) campaign. The measurements
covered coastal, urban and arid environments. In general, the CCN
concentration was highest in the central IGP, decreasing spatially from
east to west above the planetary boundary layer (PBL), which is
∼1.5km for the IGP during pre-monsoon period. Despite this, the CCN
activation efficiency at 0.4 % supersaturation was, interestingly,
the highest over the eastern IGP (∼72 %), followed by that
in the west (∼61 %), and it was the least over the
central IGP (∼24 %) within the PBL. In general, higher
activation efficiency is noticed above the PBL than below it. The central
IGP showed remarkably low CCN activation efficiency at all altitudes, which
appears to be associated with high black carbon (BC) mass concentration
there, indicating the role of anthropogenic sources in suppressing the CCN
efficiency. These first-ever CCN measurements over the western IGP, encompassing
“the Great Indian Desert” also known as “the Thar Desert”, showed high CCN
efficiency, ∼61 % at 0.4 % supersaturation, indicating the
hygroscopic nature of the dust. The vertical structure of CCN properties is
found to be air mass dependent, with higher activation efficiency even over
the central IGP during the prevalence of marine air mass. Wet scavenging
associated with precipitation episodes seems to have reduced the CCN
activation efficiency below cloud level. An empirical relation has emerged
between the CCN concentration and the scattering aerosol index (AI), which
would facilitate the prediction of CCN from aerosol optical properties.
Introduction
The cloud-nucleating ability of aerosols is fundamental in understanding
aerosol–cloud interactions (ACIs) and associated feedback processes, which
are complex in nature and pose a major challenge in quantifying the indirect
climate forcing of aerosols (Boucher et al., 2013; IPCC, 2013). Cloud condensation nuclei (CCN) form a
sub-set of atmospheric aerosols (also known as condensation nuclei; CN) and
take part in cloud processes, accelerate the condensation of water vapour
leading to the formation of liquid cloud droplets and modify the
microphysical properties of clouds, depending on the number size
distribution, chemical composition and mixing state of aerosols (Dusek et al., 2006; Farmer et al., 2015; Zhang et al., 2017). Several
investigators have examined the temporal and spatial distribution of the CCN
properties and their processing by non-precipitating clouds over both
continental and marine environments (Hoppel et al., 1973; Hudson and Xie, 1999; Jurányi et al., 2011; Paramonov et al., 2015; Schmale et al., 2018). Significant variability in the CCN
activation efficiency has also been reported over regions influenced by
urban (Sotiropoulou et al., 2007) and industrial emissions (Asa-Awuku et al., 2011). Efforts have also been made to infer
or predict CCN properties based on optical
properties (for example, Jefferson, 2010; Liu and Li, 2014). However, due to the region-specific and
heterogeneous nature of the composition of aerosols, their chemical
interactions, vertical mixing, and advection to long distances, significant
uncertainties still persist in characterising the CCN activation efficiency,
especially its region-specific nature and altitude variation in the
real atmosphere (Zhang et al., 2017). The information on the vertical distribution of the
CCN number concentration, CCN efficiency and its variation with
supersaturation are some of the vital parameters needed in quantifying the
ACI. In situ measurements of the vertical distribution of the CCN activity
especially over polluted regions are very important in accounting for the
ACI in climate models (Li et al., 2016).
In the above context, the importance of the South Asian region is unequivocal.
Aerosol physicochemical properties show large spatiotemporal variation over
this region owing to the diverse source influence, both natural and
anthropogenic, which shows large seasonality (Jethwa et al., 2005) and dependence on large-scale
meteorology (Lawrence and Lelieveld, 2010; Babu et al., 2013; Nair et al., 2017). Even within South Asia, the Indo-Gangetic Plain (IGP) falls
under those regions in the globe where very high aerosol loading persists
almost throughout the year (Di Girolamo et al., 2004) and also depicts a steadily increasing trend in
aerosol optical depth (AOD) (Dey and Di Girolamo, 2011; Babu et al., 2013), increasing surface dimming (Padma Kumari et al., 2007; Badarinath et al., 2010) and enhanced mid-tropospheric warming (Satheesh et al., 2008). Through modelling efforts, Vinoj et al. (2014) have shown possible
linkages of western Asian dust loading over the Arabian Sea with the Indian
summer monsoon (ISM). The competing roles of natural (mostly mineral dust
and marine aerosols) and anthropogenic aerosols over this region and their
high seasonality, aided by the large-scale industrial and agricultural
activities in this region and its particular orography, makes the IGP one of
the best natural laboratories for investigating complex aerosol impacts
on clouds and precipitation (Moorthy et al., 2016). Despite these, the characterisation of the
vertical structure and the spatial variability of CCN characteristics
across the IGP remains quite limited, except for some recent efforts using
instrumented aircraft during the summer monsoon season under the Cloud
Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX)
(Prabha et al., 2012; Konwar et al., 2014; Padma Kumari et al., 2017). A few ground-based measurements also exist scattered across the
subcontinent (Bhattu and Tripathy, 2014; Gogoi et al., 2015; Jayachandran et al., 2017; Singla et al., 2017).
In light of the above, and with a view to understand the ACI and its linkage
to the ISM, an experimental campaign was undertaken under the aegis of
SWAAMI (South-West Asian Aerosol–Monsoon Interactions) and RAWEX (Regional
Aerosol Warming Experiment), executed jointly by the Indian Space Research
Organisation (ISRO) and the Ministry of Earth Sciences (MoES) of India and
the Natural Environment Research Council (NERC) of the UK. Under this,
concurrent and co-located airborne measurements of the vertical structure of
the CCN characteristics and aerosol scattering and absorption coefficients
were carried out across the IGP, just prior to the onset of the ISM. The
campaign was planned to quantify the vertical distribution of total aerosols
(CN) and CCN concentrations at different supersaturations and its spatial
variation across the IGP, just prior to the onset of the ISM, when different
aerosol types are known to co-exist over this region. The data are analysed
to understand the altitude distribution of CCN characteristics, its
activation efficiency and its relationship with scattering and absorption
properties of aerosols as well as the variation of those from west to east across
the IGP. The campaign details along with the measurement protocols are given
below, followed by the results and discussions.
The strength and direction of winds at (a) 975 hPa and (b) 700 hPa
over the Indian subcontinent during June 2016. White dots indicate the base
stations. Wind data are from the ERA-Interim reanalysis.
Details of the sorties, including the dates, instruments used and rain
events for the campaign period.
RegionCoordinatesHeight, mPeriodRemarksInstruments(base station)(∘ N, ∘ E)(a.m.s.l.)(2016)Eastern IGP20.24, 85.81421–5 JuneRain on 3 and 4 June after the sortiesCCN counter (model CCN-100, DMT) Condensation particle counter (model 3776, TSI) Aethalometer (model AE33, Magee Scientific) Nephelometer (model 3563, TSI)(BBR)Central IGP25.45, 82.85818–13 JuneRain on 7 June evening(VNS)Western IGP26.25, 73.0421917–20 JuneNo rain(JDR)Experiment details, data and analysisCampaign
Airborne measurements of the CCN number concentration as a function of
supersaturation (0.2 %, 0.3 %, 0.4 %, 0.7 % and 1.0 %) along with the scattering and the
absorption coefficients were carried out across the IGP from 1 June until 20 June 2016, prior to onset of the ISM over central and northern India, using
the instrumented research aircraft of the National Remote Sensing Centre
(NRSC) of the ISRO. The details of the sorties, base stations and instruments
used are listed in Table 1. Monthly mean synoptic wind conditions, using the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim (ECMWF Reanalysis) product during June 2016 at two altitude levels, (a) 975 hPa (near to the surface) and (b) 700 hPa (free-tropospheric altitude), are
shown in Fig. 1. The near-surface advection of a marine air mass is seen over the
peninsula and regions south of ∼25∘ N, while to the
north of it and at higher levels, a dry continental air mass is advected from
the northwest. As per the India Meteorological Department (IMD), the onset of the ISM during 2016 was on 8 June 2016 at the southern peninsular coast of the state of
Kerala which advanced to the eastern IGP by 10 June and reached
the central IGP by 19 June.
Aircraft measurements were carried out from three base stations, each
representing distinct regions of the IGP: (i) Bhubaneswar (BBR,
20.24∘ N, 85.81∘ E, 42 m a.m.s.l.) – a
semi-urban coastal location at the eastern end of the IGP, (ii) Varanasi (VNS,
25.45∘ N, 82.85∘ E, 81 m a.m.s.l.)
representing the aerosol-laden (polluted) central IGP and (iii) Jodhpur (JDR,
26.25∘ N, 73.04∘ E, 219 m a.m.s.l.)
representing a semi-arid location on the western IGP, which receives a large
amount of mineral dust, lofted from the adjoining deserts as well as
advected from western Asia and eastern Africa. The base stations
along with the direction of sorties for different days of the campaign are
shown in Fig. 2a. The instruments aboard and the local weather conditions
are listed in Table 1. As seen from the table, pre-monsoon showers occurred
on 2 d at BBR and on 1 d at VNS. The campaign was executed just
prior to the onset of the ISM at each of the base station.
(a) Base stations for the aircraft sorties with the track of each
sortie superimposed. (b) A typical sortie pattern (staircase) which
represents all the sorties carried out during the experiment and the
photograph of NRSC aircraft.
All aircraft sorties were carried out late in the morning and early
afternoon (10:00–14:00 IST; IST stands for Indian standard time,
which is 5.5 h ahead of UTC) to ensure that the planetary boundary
layer (PBL) is fully evolved and aerosols are well mixed within the PBL.
During this period, which is summer over the Indian region, the PBL would be
quite deep as the thermal convections would be strong providing a thorough
vertical mixing. Estimated from the NCEP/NCAR (National Centers for Atmospheric Prediction/National Center for Atmospheric Research) global reanalysis product at 0.25∘× 0.25∘ grid resolution data, mean PBL heights at local noon over the IGP regions for the flight sortie days
were 1.4±0.2, 2.3±0.5 and 1.3±0.5km for BBR,
VNS and JDR, respectively (Vaishya et al., 2018). Due to the unpressurised
mode of operation of the aircraft, the ceiling altitude of airborne
measurements was ∼4km a.m.s.l. In all, 14 sorties were undertaken, with
five from each base station, except from JDR, where only four sorties were completed.
Each sortie was undertaken for a period of ∼3.5h, during which the
measurements were made at six altitude levels – ∼500, 1000,
1500, 2000, 2500 and 3000 m above the ground level (a.g.l.), following the
“staircase pattern” shown in Fig. 2b (Babu et al., 2016). Accordingly, after takeoff, the
aircraft climbed to the first level (500 m a.g.l.), stabilised its attitude
and flew at that level for ∼30min during which it
covered a horizontal distance of ∼150km before climbing up
to the next higher level and retracing the path. This procedure was repeated
until the highest level (ceiling altitude) was reached, after which the
aircraft descended to the base. The sorties were repeated on consecutive
days, except that on each day the aircraft proceeded to a different radial
direction from the base, as shown in Fig. 2a, so that the five sorties
together provided a gross picture of the aerosol properties around the base
station within a radius of about 150 km.
Measurements
Ambient air was aspirated to the instruments using a solid diffuser inlet
(University of Hawai`i) maintained at isokinetic flow conditions, as detailed
in Babu et al. (2016), with a volumetric flow rate of 70 Lmin-1 (litres per minute), for the aircraft's
average cruising speed of 300 kmh-1. The efficacy of
the inlet to sample aerosols below 4 µm, under such conditions, was demonstrated during the DC-8 Inlet Characterization Experiment
(McNaughton et al., 2007). Further details of the experiment setup are explained in Babu et al. (2016) and Vaishya et al. (2018). The air,
aspirated through this inlet, is then fed to different instruments through a
manifold. Aerosol instruments onboard were calibrated prior to and after the
campaign to ensure consistency in the measurements. Concurrent time and
space coordinates were logged continuously using a high-resolution global
positioning system (GPS).
The CCN concentration at different supersaturations was measured every second
using a continuous-flow CCN counter (model CCN-100, Droplet Measurement
Technologies) by feeding the aspirated air continuously to the cylindrical
column of the counter at a constant flow rate of 0.5 Lmin-1, where it is
exposed to the desired supersaturations. Details of the principle of operation
of the CCN counter are available elsewhere (Roberts and Nenes, 2005; Lance et al., 2006). Aerosols, according to their
composition and size, having a critical supersaturation less than the
effective supersaturation inside the column, will spontaneously grow into a
droplet as they exit the column. These droplets are counted with an optical
counter using a laser with a 650 nm wavelength. During each set of measurements,
the supersaturation was varied through 0.2 %, 0.3 %, 0.4 %, 0.7 % and 1.0 % over
a cycle of 30 min, and the cycle is repeated at each altitude level so
that a complete CCN spectra (of CCN vs. supersaturation) is available at
every altitude level. In the present study, the CCN concentrations never
exceeded 5000 cm-3, and hence the correction for water vapour
depletion (Lathem and Nenes, 2011) is not applied. Pressure correction was done to the set
supersaturation at each altitude layer depending upon the change in pressure
between ambient and calibration pressure (Lance et al., 2009). Data points during the
supersaturation transition are excluded due to the inherent ambiguity in the
stability of the attained supersaturation. The measured CCN concentration
has a maximum uncertainty of 10 % (Rose et al., 2008).
The total aerosol number concentration was measured using an ultrafine
condensation particle counter (model 3776, TSI), developed by Stolzenburg and McMurry (1991). It measures
CN of a diameter of 2.5 nm and above, with a time base of 1 min. The aspirated
air is continuously fed at 1.5 Lmin-1 and mixed with clean sheath air, which is
saturated with butanol vapour while passing through a saturator. The
resultant flow is passed through a condenser where a sudden cooling result
in the condensation of butanol vapour onto aerosols occurs due to supersaturation,
and the droplets are counted using a counter working with a laser diode at
650 nm. Further details of the instrument and its adaptability for
aircraft-based experiments are explained by Takegawa et al. (2017).
Aerosol absorption measurements at seven different wavelengths (370, 470, 520,
590, 660, 880 and 950 nm) were carried out using a DualSpot AE33 Aethalometer
(Magee Scientific) (Drinovec et al., 2015) which works on the principle of a
filter-based optical attenuation technique (Hansen et al., 1984). A filter loading artefact of
the instrument is corrected in real time as explained by Drinovec et al. (2015). Absorption
measurements were corrected for a change in flow rate at high altitudes
following Moorthy et al. (2004). Optical attenuation at 880 nm is used to estimate the black
carbon (BC) mass concentration using the specific absorption cross section
value (7.77 m2g-1). The Integrating Nephelometer (model 3563, TSI) measured the scattering coefficient (σsca) at 450, 550 and 700 nm wavelengths. Scattering measurements were corrected for
non-linearity in the angular truncation error following Anderson and Ogren (1998).
For the CCN data analysis, the initial 5 min of data at each altitude
level were discarded considering the stability of the measurements, and the
data were averaged for every minute. Hence a minimum of 20 min of usable
data comprising five supersaturations is available for each altitude level. Measurements of CN,
spectral scattering and spectral absorption were also
synchronised to the 1 min averaged CCN data. Thus, for each region (eastern,
western and central IGP), five vertical profiles of CCN and CN concentration
and scattering and absorption coefficients were obtained.
Vertical distribution of regionally averaged (a) aerosol number
concentration (CN) and (b) CCN number concentration across the IGP. The circle, triangle and square represent the BBR, VNS, and JDR IGP regions, respectively. Error bars represent the standard deviation around the mean
values.
Results and discussionVertical distribution of CN and CCN
Vertical profiles of CN and CCN concentrations (at 0.4 % supersaturation)
for the three subregions of the IGP are shown in Fig. 3. Each profile is
an average of all the sorties carried out from the base station. Significant
differences are seen below ∼1.5km, which represents the
well-mixed region within the PBL, and are attributed to the subregional
scale emissions. As such, the CN concentrations are up by nearly a factor of
2 at the central IGP (VNS) compared to the eastern or western ends of the
IGP, owing to the large-scale anthropogenic activities in the central IGP.
Beyond ∼2 km altitude, the CN concentrations remain quite
comparable in magnitude across the entire IGP with similar vertical
variations.
In contrast to this, there is a significant difference in the aerosol type
across the IGP (attributable to the source heterogeneity), as revealed by
the CCN concentration as revealed by the CCN concentration in Fig. 3b, especially in
the free troposphere (above 2 km). Near to the surface, where the local
source impacts dominate, the CCN concentration is the least over the arid
western IGP (JDR), followed by the industrialised eastern IGP (BBR), with
the VNS depicting the highest concentration. At all the subregions, CCN
concentrations decrease towards higher altitudes. However, there is a sharp
difference in the decreasing pattern, with the concentrations over the VNS
falling off very rapidly and almost merging with the profile over the arid
region (JDR). The decrease is rather inconspicuous over BBR. The CCN
concentration, though decreasing initially with heights up to 1 km, was
more or less steady above 1 km, suggesting the prevalence of more hygroscopic
particles aloft.
Irrespective of all these, the CCN concentrations remain high (1000 to
>2000cm-3 at 0.4 % supersaturation), even at 3 km
altitude, which is above the base of monsoon clouds (Das et al., 2017). This will have
strong implications in cloud modification, as has been established elsewhere
(Andreae et al., 2004; Rosenfeld et al., 2008); however, their influence on the monsoon rainfall over the study region
has not yet been quantified. Based on the aircraft observations during
CAIPEEX over Hyderabad (17.45∘ N, 78.38∘ E) in southern
India, Padma Kumari et al. (2017) reported the suppression of warm rain process due to the presence of
high CCN concentration. During the collaborative Regional Aerosol Warming
Experiment and the Ganges Valley Aerosol Experiment (GVAX), Gogoi et al. (2015) have
reported CN and CCN (at 0.46 % supersaturation) concentrations of
∼2500 and ∼1100cm-3, respectively, for
June 2011, from a high-altitude station (∼2km a.m.s.l.),
Nainital in the central Himalayas. The high CN and CCN concentrations observed
in this study is in line with values reported from Nainital, which is an
optimal high-altitude site to study regional (IGP) as well as transported
aerosol characteristics over the IGP. In another study over the Loess
plateau in China during July–August months, Li et al. (2015) reported high
concentrations of CN and CCN, peaking within the PBL and decreasing with
increasing altitude. Lance et al. (2009) reported a CCN number concentration varying from
∼200 to more than 10 000 cm-3 during the Gulf of Mexico
Atmospheric Composition and Climate Study (GoMACCS) aircraft campaign, over
a heavily polluted region with power plants and ship channels of Houston.
Local aerosol sources play a significant role in determining the vertical
structure during the period when high convective mixing prevails, while
advection has a strong influence on the spatial variation of altitudinal
distribution above the PBL. From Fig. 1, it is clear that there is an
advection of marine air mass near the ground level (975 hPa) in both the eastern
and western regions of the IGP and intruding into the central IGP. However,
northwesterlies from the continental region pass through the
free-tropospheric heights (700 hPa) of the central IGP before reaching the eastern
coast. In short, the CCN concentration at cloud-forming heights, which is a
critical parameter in deciding the cloud droplet number concentration, is
quite abundant over the IGP, decreasing spatially from the eastern IGP to
the western IGP especially above the PBL.
Association between the total CN and CCN number concentration at
0.4 % supersaturation for the (a) eastern IGP (BBR), (b) central IGP (VNS) and
(c) western IGP (JDR) for each day of observation at all the observation
heights. Colour codes indicate the altitude above ground level, while symbols
represent the day of observation. Error bars represent the standard
deviation around the mean values.
Altitudinal dependence of CCN–CN association
Aerosol number size distribution and composition are known to show vertical
variations (Zhang et al., 2011; Li et al., 2015). Hence it is imperative to examine the altitudinal dependency of
CCN on CN and its region-specific nature. In Fig. 4, the altitude
variation of the CCN–CN relationship is presented for a constant
supersaturation (0.4 %) as a scatter plot of CCN vs. CN for the eastern
(top panel), central (middle panel) and western IGP (bottom panel) regions,
respectively. Each point in the figure corresponds to the mean concentration
at a particular altitude level above the ground (identified by the colour)
for each day of observation (identified by the shape of the point). The
striking linear relationship over the entire altitude range at the eastern IGP
(BBR; top panel) clearly indicates the vertical homogeneity in aerosol
composition in this region. The deviation of a couple of points at the highest
altitude from this relationship indicates the presence of different aerosol
types aloft. This is supported by the air mass back trajectories which are
examined in Sect. 3.4.
The CCN–CN relationship is quite nebulous over the central IGP, which is a
hotspot of anthropogenic activities, as revealed by the large scatter of the
points in Fig. 4b. The scatter at the lower altitudes indicates the
influence of local source impacts, which also leads to a large variation in
the concentration as revealed by the large standard deviations. The
association becomes better and stronger again as we move to the western IGP
(JDR), where mineral dust is the most dominant constituent. The linear
association between CCN and CN for low-to-moderate CCN concentrations (up to
∼4000cm-3) becomes non-linear for higher concentrations
(CCN>5000cm-3). The CCN concentration tends to saturate at
about 4000 cm-3 (for 0.4 % supersaturation), even though the CN
concentration increases beyond 10 000 cm-3. A similar saturation of the CCN
concentration associated with large CN concentrations was reported by Roy et
al. (2017) at ∼2.2km a.m.s.l. in the eastern Himalayas.
Association of CCN number concentration at 0.4 % supersaturation
with BC mass concentration over the eastern (BBR; circle), central (VNS; triangle) and western (JDR; square) IGP regions. Colour codes indicate the
altitude of observation.
To further investigate the above hypothesis of the role of local emissions
in weakening the relationship between CCN and CN over the central IGP, the
variation of the CCN number concentration at 0.4 % supersaturation with BC
mass concentration are examined in Fig. 5. For this, the concurrent BC mass
concentration measurements carried out from the same platform are used.
The central IGP showed the highest absorption coefficient (column averaged) of 26±9Mm-1, followed by the western (16±2Mm-1) and eastern
(15±3Mm-1) IGP (Vaishya et al., 2018). It is interesting to note that the linear
relationship is maintained for low-to-moderate concentrations of BC (up to
around 1000 ngm-3, which occurs mostly above the PBL), while significant
scatter occurs for higher values of BC (exceeding 2000 ngm-3), which
occurs mostly in the lower altitudes, supporting the hypothesis. Similar
deviations in the CCN–CN relationship with respect to altitude have also been
reported by Srivastava et al. (2013) over the central IGP region using aircraft measurements, where
they attributed it to the impact of local anthropogenic emissions.
Mean CCN spectra at six altitudes levels over different
subregions of the IGP. The error bars indicate the standard deviation
around the mean. The points correspond to measurements, while the lines are
the empirical fits. The circle, triangle and square represent
the eastern (BBR), central (VNS) and western (JDR) IGP subregions, respectively.
CCN spectra and parameterisation for different altitudes
Using the measurements of CCN number concentration as a function of
supersaturation, the mean CCN spectra are constructed for different
subregions of the IGP and shown in Fig. 6 for different altitudes. In
addition to the regional distinctiveness in the CCN number concentrations
seen in Fig. 3, it is interesting to note the rapid levelling off of the
spectra with increasing supersaturation at the eastern IGP (represented by
BBR, blue lines in Fig. 6), especially above 1 km. This is in contrast to the
other two regions, where the CCN concentrations keep on increasing with
increasing supersaturation at all heights. This clearly demonstrates a
change in the hygroscopicity of aerosols across the IGP, especially in the
free troposphere. To quantify this, the CCN spectra are parameterised by
evolving a least-squares fit with Twomey's relation (Twomey, 1959),
CCN (ss)=C(ss)k,
where CCN (ss) is the number concentration of CCN at a particular
supersaturation (ss) and C and k are empirical coefficients. Lower k values are
reported more frequently for marine air mass compared to continental air mass
(Twomey and Wojciechowski, 1969; Khain, 2009). The fine-mode anthropogenic aerosols exhibit high k values, while
hygroscopic and larger aerosols like sea salt have low k values (Hegg et al., 1991; Jefferson, 2010). The shape
of the CCN spectra, represented by the k values, showed significant
altitudinal variations. The altitude variations in the CCN spectra, which
can be due to the variations in aerosol number size distribution, will have
an impact on the droplet size distribution of the warm cloud formation
(Raga and Jonas, 1995).
Altitude variation of k (Twomey's empirical fit) over the eastern (BBR; circle), central (VNS; triangle) and western (JDR; square) IGP regions. Error
bars represent the standard deviation of the fit.
The vertical variation of the k values for each region is shown in Fig. 7,
which reveals a distinct transformation of the CCN properties of aerosol
across the IGP. Over the eastern IGP (which is industrialised and near the
coast), k is the least, with a small vertical variation that
shows a weak decrease initially and then a weak increase. The arid western
IGP shows a very similar vertical variation of k, but the values remain
consistently higher than those seen for the eastern IGP at all heights. The
highest values of k are seen over the central IGP, with a steady increase with
altitude. Across the entire IGP, k increases with altitude, indicating a
decrease in the hygroscopicity with altitude or a rapid change in the number
size distribution. As the CCN concentration at higher supersaturations
(>0.4 %) are mainly governed by the concentration of small
particles (<∼70nm) (Lance et al., 2009), the corresponding high CCN
concentration suggests the presence of a prominent fine-mode aerosol system,
which is clearly seen over the entire IGP, especially over the central IGP.
The near-flat CCN spectra at BBR (above 0.4 % supersaturation) indicate
the presence of highly soluble or coarse-mode aerosols, such that almost all
aerosols are activated at 0.4 % supersaturation itself. Similar
observations of low k values (∼0.2) are reported by
Jayachandran et al. (2017) from a coastal location in peninsular India during the sea breeze regime of the
monsoon season, when both the local mesoscale and synoptic circulations
bring marine (sea salt) aerosols to the region.
Reported k values and supersaturation (ss) range used for the
estimation, along with the CCN (at 0.4 % ss) concentrations, for high
altitudes above the Indian subcontinent.
Serial numberLocationTypeAltitudePeriodCCN0.4kReference(Latitude, Longitude)(mode)(a.m.s.l., km)(cm-3)(ss range)1Eastern IGPPolluted marine2June 2016∼22000.25 (0.2–1.0)Present study2Central IGPPolluted∼15000.51 (0.2–1.0)3Western IGPSemi-arid∼12500.31 (0.2–1.0)4Eastern Himalayas (27∘ N, 88.2∘ E)Urban (fixed)2.2March–May 2016∼18000.38±0.1 (0.1–1.0)Roy et al. (2017)5Central Himalayas (29.4∘ N, 79.5∘ E)Background (fixed)2June 2011∼10000.57±0.11 (0.17–0.75)Dumka et al. (2015)6Eastern IGPPolluted marine1.5June 2016∼23000.16 (0.2–1.0)Present study7Central IGPPolluted∼18000.45 (0.2–1.0)8Western IGPSemi-arid∼15000.24 (0.2–1.0)9HyderabadPolluted (aircraft)1.5October 2011∼11000.72 (0.2–0.8)Varghese et al. (2016)10Clean (aircraft)∼5000.25 (0.2–0.8)11Mahabaleshwar (17.6∘ N, 73.4∘ E)Western Ghats1.4March–May 2013∼15000.5 (0.2–1.0)Leena et al. (2016)
The similarity in the vertical profiles of the k value over the western and eastern
regions of the IGP show the presence of the similar nature of CCN active aerosols
over both regions. The reported CCN spectra and k values over the Indian
subcontinent at higher altitudes are listed in Table 2 for different
aerosol types using both ground-based and aircraft-based platforms. It
should be noted that the k values depend on the supersaturation range used
for its estimation, and hence the supersaturation range is also mentioned in
the table. From the table, it can be seen that the values reported from the
central Himalayas (2 km a.m.s.l.) are similar to the present observations
over the central IGP at similar altitudes. The central Himalayas experience an air mass
from the IGP as well as semi-arid regions of western Asia during pre-monsoon periods, and
Dumka et al. (2015) reported a mean k value of ∼0.58 for June 2011 during the
RAWEX–GVAX campaign. The current observations show k values above 0.51 for
altitudes above 2 km over the central IGP. In the present study, the k values
estimated for the altitude 2–3 km a.g.l. are in the range 0.25–0.32 and
0.31–0.46 above the eastern and western IGP, respectively. Roy et al. (2017) reported a mean k value
of ∼0.38 during the pre-monsoon period over the eastern Himalayas when
the air mass reached the site from the IGP as well as semi-arid regions of western Asia.
Examining the CCN spectra at the cloud base (∼1600m) during the
CAIPEEX campaign (October 2011) over peninsular India, Varghese et al. (2016) reported high k values (0.72) associated with polluted conditions and low k values (0.25)
during clean conditions. Flat CCN spectra with low k values observed in
this study over the eastern and western IGP indicate the high CCN-active nature of the
aerosols.
Vertical distribution of the (a) mean CCN activation efficiency at 0.4 % supersaturation and (b) variation of k values with the corresponding
CCN efficiency at 0.4 % supersaturation over the eastern (BBR; circle),
central (VNS; triangle) and western (JDR; square) IGP regions. Error bars
represent the standard error of the mean.
CCN activation efficiency: vertical structure and variation across the
IGP
CCN activation efficiency is the ratio of the CCN number concentration at a
particular supersaturation to the total CN concentration. This ratio has
been estimated as a function of altitude for each of the sorties, and the
mean vertical profiles are shown in Fig. 8a for 0.4 % supersaturation.
Similar to the altitude variation of k shown in Fig. 7 over distinct
regions of the IGP, the activation efficiency is the least over the central IGP
(VNS) and the highest in the eastern IGP (BBR), with that over JDR coming
in between. At all the stations, the efficiency remains low within the PBL
(below 1.5 km) where the local-source impacts are rather substantial. Above
the PBL, it either increases or remains steady with altitude before
decreasing again above 2.5 km, probably due to different aerosol types (less
hygroscopic, finer particles) at the higher levels. The low CCN efficiency
over VNS is associated with the presence of a higher concentration of BC
(>4000ngm-3) and CN number (>10000cm-3), indicating a pollution surrogate from anthropogenic sources
modifying the CCN activation.
The variation of k with CCN activation efficiency at 0.4 %
supersaturation for the eastern IGP (BBR, circle), central IGP (VNS, triangle) and
western IGP (JDR, square) are shown in Fig. 8b. High values of k are
observed with a low CCN activation efficiency and vice versa, showing an
inverse relationship between the two parameters. CCN efficiency and k over
the desert region vary from ∼20 % to 65 % and
∼0.2 to 0.7, respectively. A similar inverse association
between CCN efficiency and k is reported by Hegg et al. (1991) and Jayachandran et al. (2017). High k values are due to
the dominant presence of small or less soluble particles in the aerosol
system, which in turn reduce the CCN efficiency. However, over the central IGP,
very low CCN efficiency (<20 %) values were observed with low k values
(∼0.4), which is not in line with the general inverse
relationship. These cases were observed within the PBL, indicating a
CCN-inactive aerosol system even at high (>0.8 %)
supersaturations. At high altitudes (>3km) over the IGP,
Srivastava et al. (2013) have reported an aerosol size distribution peaking below ∼40nm
due to new particle formation (NPF) events and cloud processing. Rose et al. (2017) have
reported the significant role of NPF in CCN activation above PBL especially
during the wet season at Chacaltaya (5240 m a.m.s.l.), Bolivia. In the present
study, the role of cloud processing or in-cloud scavenging for low CCN
efficiency and flat CCN spectra (low k) at cloud forming heights cannot be
neglected.
Based on measurements at the mean sea level and at 1 km above ground level,
Jayachandran et al. (2018) have shown the vertical heterogeneity in the CCN efficiency and CCN
spectra during the ISM on the southern coast of India. Li et al. (2015) have shown that the
anthropogenic influences can cause a strong variation in CCN efficiency from
10 % to 70 % from the near-ground level to about 4.5 km over China during
the Asian summer monsoon season. More than 50 % of the aerosols are CCN active
over the regions other than central IGP, which indicates the dominant role
of natural aerosols in warm cloud droplet activation over the subcontinent just prior to the ISM season. The air mass traversing through the
polluted continental regions is responsible for the lowering of CCN
activation efficiency at free-tropospheric heights over the eastern IGP. The
back-trajectory analysis of an air mass reaching 500 and 3000 m over BBR
(figure not included) clearly showed that the particles reaching 3000 m have
a purely continental history of passing across the IGP from the arid regions of
western India and western Asia, whereas those reaching 500 m pass over
the oceanic region of the Bay of Bengal before arriving at the location. This
distinctiveness in the air mass history at higher altitudes also causes
the scatter in the CCN–CN association seen in Fig. 4. The significant
influence of the nature of the air mass on CCN activation over the Indian region
is illustrated by the closure studies carried out by Srivastava et al. (2013) at various altitudes.
Jayachandran et al. (2017) have reported a higher CCN activation efficiency for marine air masses than
continental air masses from ground-based observations from peninsular India during the
ISM. Within the PBL, including near the ground level, CCN efficiency is
very high over the eastern IGP (coast), which will support the cloud droplet
formation with a sharp droplet size distribution.
Air mass back trajectories for 5 d at 500 m (black), 1500 m
(blue) and 3000 m (red) a.m.s.l. over the (a) eastern IGP (BBR), (b) central IGP
(VNS) and (c) western IGP (JDR) during the campaign period.
One of the striking features emerging from this study is the high CCN
efficiency over the arid region of the western IGP, which is reported for the
first time. This region is known for its dust dominance (both locally
generated and advected from the Middle East and eastern Africa). Though pure
dust is water inactive, its CCN efficiency will enhance when coated or mixed
with soluble salts like sulfates and nitrates (Kelly et al., 2007). Though Feingold et al. (1999) have shown that
coarse-mode dust aerosols can act as a giant CCN and initiate drizzle
formation, their number concentration is far less numerous, especially at
high altitudes (Padma Kumari et al., 2013). Thus, the observations of moderately high CCN activation
efficiency, lower values of k and a higher concentration of CCN over the desert region are
interesting and in need of discussion. Figure 9 shows air mass back trajectories
for 5 d and arriving at 500, 1500 and 3000 m a.m.s.l. above the (a) eastern IGP (BBR), (b) central IGP (VNS) and (c) western IGP (JDR). In Fig. 9c, it can be seen that the air mass reaching JDR (conducive for
dust advection) has a significant history over the northwestern Arabian Sea,
and hence it would also carry significant moisture. It is known that the
presence of hygroscopic salt aerosols can catalyse the reaction of dust with
acidic gases (Tobo et al., 2010), changing its hygroscopicity. Thus, the air mass reaching the
desert region, having a strong marine component, could enhance the activation
efficiency of the aerosols. A strong convection in the lower atmosphere will
also take salt aerosols to the atmosphere from the regional dry salt lakes.
Bègue et al. (2015) have reported a CCN efficiency of ∼70 % for 0.2 %
supersaturation over the Netherlands during a dust transport event due to
the accumulation of solute particles on dust. The present study shows that
about 66 % of the total aerosols in the PBL of the western IGP (JDR) were
activated as CCN at 1 % supersaturation.
Altitude variation of CCN efficiency at 0.4 % supersaturation
showing the effect of (a) distinct air mass (continental – solid line; marine – broken line) at the central IGP (VNS) and (b) rainfall (before – solid line; after – broken line) at the eastern IGP (BBR).
The coastal location BBR (Fig. 9a) is strongly under the influence of a marine
air mass. It travels considerably across the Indian mainland
initially, enters the Bay of Bengal, turns and then arrives at BBR; thus it
would be moisture laden and contain sea salt particles. On the other hand,
at the central IGP, irrespective of the history of the air masses, they have
to travel considerable distances across the mainland and are thus conducive
for the advection of anthropogenic aerosols. Additionally they lose a significant amount
of moisture they have acquired from the ocean. Thus, VNS is under the influence
of local emissions, which includes hydrophobic particles such as BC, which is
also in the fine-size range, with all of this resulting in the highest values of
k and lowest values of CCN activation efficiency of the three IGP
subregions. Vertical profiles of CCN efficiency over VNS for the first day
of observation (8 June) when the air mass was from the marine region (Bay of
Bengal) and the mean picture for the other days (when the air mass was
continental) are shown in Fig. 10a, respectively, with dotted
and continuous bold lines. The significant increase in the activation
efficiency during marine-air-mass conditions is very conspicuous.
At BBR, there have been two episodes of pre-monsoon precipitation on 4 and 5 June 2018 (before the sorties), with an accumulated rainfall of 58
and 8 mm. The vertical profile of CCN activation efficiency over BBR
averaged for measurements before and after rainfall is shown in bold and
dotted lines, respectively, in Fig. 10b. There is a decrease (though
weak) in the activation efficiency (especially below the cloud level at 2 km)
after the precipitation, probably due to the removal of hygroscopic aerosols by
the precipitation. Even though the CCN efficiency is found to be slightly
reduced below 2 km, the CCN activation efficiency is found to be higher
above 2 km compared to observations before the rainfall. Near the
ground level, CCN concentration (mean±standard deviation) reduced
from 3431±572 to 1320±454cm-3 and from 1755±105 to 460±209cm-3 at ∼3km a.g.l. After the
rainfall, a reduction (<10 %) is seen in the CCN efficiency over
BBR; meanwhile, there is a large diminution in the number concentration of
CN and CCN.
The theoretical framework of the wet-scavenging process accounts for nucleation,
gravitational and inertial impacts, and turbulence scavenging mechanisms
(Pruppacher and Klett, 1980). However, uncertainties and difficulties still exist in attributing the
observational evidence of the wet scavenging of aerosols to different
scavenging mechanisms, especially in the case of moving air parcels.
The efficiency of below-cloud scavenging (wash out) mainly depends on the number
size distribution of both aerosols and raindrops, while the in-cloud
scavenging (rain out) depends mainly on the solubility of the aerosols
(Garrett et al., 2006). The decrease in CCN concentration over BBR after the rainfall and the
high CCN efficiency seen in the present study indicate the highly soluble
nature of the aerosol system prevailing over the region. The difference in CCN
activation efficiency at different altitude levels before and after rainfall
reinstates the difference in the aerosol types at different altitudes. One
of the possibilities for the observed CCN efficiency is that the rainfall
has removed coarser and hygroscopic particles by wet scavenging, resulting
in the reduction of the CCN activation efficiency below 2 km. Cloud
processing broadening the aerosol distribution as reported by Flossmann et al. (1987) may be
enhancing the CCN activation efficiency above 2 km. However, the effect of
cloud formation and further rainfall on CCN characteristics needs further
investigation. The modification in CCN efficiency over VNS and BBR
underlines the role of the specific type of the air mass and rainfall in determining the
vertical structure of CCN activation in a short duration.
Association between the total scattering AI at 450 nm and CCN
number concentration at 0.4 % supersaturation for the (a) eastern (BBR), (b) central (VNS) and (c) western (JDR) IGP regions. The colour indicates the
altitude of measurement. Dashed lines represent the linear least-squares fit to
the points for each region. Regression slopes and squared correlation
coefficients are written in each panel.
Association between the extinction AI at 450 nm and CCN number
concentration at 0.4 % supersaturation for the (a) eastern (BBR), (b) central
(VNS) and (c) western (JDR) IGP regions. The colour indicates the altitude of
measurement. The solid lines represent the linear least-squares fit to the
points for each region. Regression slopes and squared correlation
coefficients are written in each panel.
CCN and aerosol optical properties
Concurrent measurements of aerosol scattering and absorption coefficients
during the campaign provided an opportunity to examine possible links
between CCN and the optical properties of aerosols. Liu and Li (2014) and Jefferson (2010) have illustrated
the potential of using aerosol optical properties as a proxy and prognostic
variable for studying the CCN properties. Liu and Li (2014) have used the scattering aerosol
index (AI), which is the product of the scattering coefficient (at 450 nm) and
scattering Ångström exponent, to link the aerosol scattering properties to a
CCN concentration. Following their approach, we have estimated AI as AI=σsca450×αsca, where σsca450 is the scattering coefficient at 450 nm, estimated
from the Nephelometer data, and αsca is the Ångström exponent,
estimated over the wavelength range 450, 550 and 700 nm by adapting a
least-squares fit to the relation
σscaλ=σ0λ-αsca.
The scatter plots of CCN concentration at 0.4 % supersaturation against
scattering AI are shown in Fig. 11, with panels from left to right
representing the eastern, central and western IGP, along with the corresponding
altitudes of measurement, indicated by the colour code. Linear least-squares
fits to the points through the origin (implying that all the scattering
aerosols contribute to CCN concentration) are also shown in the figure
along with the fit parameters. Very good linear dependencies emerge from the
figure for all the stations across the IGP, though the slope appears to be
region specific.
The highest slope is observed at the least anthropogenically impacted and dust-dominated western IGP, while the slope values are comparable over the
anthropogenically influenced eastern and central IGP. As scattering AI is a
product of the scattering coefficient and scattering Ångström exponent, it
carried signatures of total particulate loading and the size distribution.
A scatter between the extinction aerosol index and CCN concentration at 0.4 % supersaturation is generated and shown in Fig. 12. If absorption
contributed insignificantly to the extinction, then this plot would not
differ significantly from Fig. 11. However, it can be seen in Fig. 12
that there is a significant reduction in the slope over the western and central
IGP (JDR and VNS). This indicates the reduction in CCN activation due to
absorbing aerosols, probably dust. However, there is no remarkable change in
the slope over BBR, which might be due to the reduced concentration of dust
(as most of it get removed as dust is advected across the IGP and also due
to the mixing of dust with other more hygroscopic aerosol species as it ages in the atmosphere). There is an increase in the correlation coefficient
over the eastern IGP also when we consider aerosol absorption, which might be
indicative of the contribution of these aerosols to CCN activation,
probably due to co-emitted or co-existing soluble inorganic particles.
Examining Fig. 11 along with the CN profile shown in Fig. 3a, it can
be seen that the higher slope (21.8) at JDR is due to the large-size dust
particles there, even though the CN concentrations at JDR and BBR are
comparable, except at the lowest altitude. The coarse-size distribution
would lead to the smaller scattering Ångström exponent resulting in low
scattering AI values. It is interesting to note that scattering AI values at
JDR are low, though the scattering coefficient values are higher than at BBR
(Vaishya et al., 2018). However, the slope at BBR is nearly half of that
seen at JDR, despite it having the highest activation efficiency. On similar lines, it appears that the size distribution of aerosols over VNS
has more fine particles (higher Ångström exponent but lower activation
efficiency). Thus, the size distribution and chemistry of the aerosol
influence the relationship between the scattering aerosol index and CCN
concentration. This dependency is useful in developing an empirical
relationship connecting CCN and light scattering properties at least on a
region-specific scale. The number concentration of Aitken mode aerosols,
especially the aerosols in the 60–100 nm range, and its composition is the main
factor in governing the variability in CCN properties, while the relative
dominance of accumulation mode aerosols will be determining the scattering
properties. Figure 11 demonstrates the strong relationship that exists
between the aerosol scattering properties and CCN concentration in the
vertical column over the IGP. The relationship between CCN and aerosol
optical properties further implied the use of satellite-retrieved AOD
products in the region, which are now matured and fairly accurate, and a
model-generated aerosol profile aided by ground- and space-based lidar in
predicting CCN.
Conclusions
The extensive characterisation of the CCN altitude distribution and its
spatial variation across the IGP has been carried out for the first time
using in situ measurements aboard an instrumented aircraft just prior to the
onset of the Indian summer monsoon. The results concluded below form a
significant step towards the characterisation and understanding of the ACI during the
Indian summer monsoon, though the impact on cloud microphysics needs further
investigation.
Spatial heterogeneity in total aerosol concentration exists over the IGP with
high concentrations (>13000cm-3), over the central IGP
(near to the ground level) and the least over the western IGP, while its
vertical variation remains the same above the planetary boundary layer (PBL)
at all regions.
A high CCN concentration (above 1000 cm-3 at 0.4 % supersaturation) is
observed up to 2.5 km across the IGP, indicating the significant possibility of
aerosol indirect effects.
The central IGP shows a higher CCN activation efficiency above the PBL
(>1.5km) than within it, despite the latter having high CN and
CCN concentrations indicating the activation of aerosols as CCN is suppressed by
freshly emitted aerosols, mostly from anthropogenic sources.
The high CCN activation efficiency, ∼61 % at 0.4 %
supersaturation, at ∼1.5km above the ground level is
observed over the dust-dominated western IGP. This high CCN activation
efficiency of dust aerosols can modify the cloud microphysics over the
region, hence affecting the precipitation pattern as well as the regional
radiation balance.
It is seen that while precipitation reduces the CCN activation efficiency
below cloud level, the advection of the marine air mass enhances CCN efficiency, even
over arid regions.
An empirical relationship between the CCN activation and optical properties
of aerosols has further implied the use of satellite-retrieved AOD products
and the model-generated aerosol profile aided by ground- and space-based lidar in
predicting CCN over the region.
Data availability
Data are available upon request from the contact author, Surendran Nair Suresh Babu
(s_sureshbabu@vssc.gov.in).
Author contributions
SNSB, SKS and KKM conceptualised the experiment and finalised the
methodology. SNSB, VNJ, AV and MMG were responsible for the data collection
on board the aircraft. VNJ carried out the scientific analysis of the data,
supported by SNSB, VSN and AV. VNJ drafted the paper. SNSB, KKM and SKS
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
This study was carried out as part of the combined South-West Asian Aerosol–Monsoon Interactions and Regional
Aerosol Warming Experiment (SWAAMI–RAWEX)
campaign. We thank the director of the National Remote Sensing Centre (NRSC) in
Hyderabad and the Aerial Services and Digital Mapping Area (AS & DMA) for
providing the aircraft support for this experiment. Aditya Vaishya was
supported by the Department of Science and Technology of the government of India,
through its INSPIRE Faculty Scheme. Details of the aircraft data used in
the present study and the point of contact are available at
http://spl.gov.in (last access: 14 January 2020) under “Research Themes” and “Aerosol, Trace gases and Radiative
Forcing Branch”. The RAWEX project is supported by ISRO (Indian Space
Research Organisation) and the SWAAMI project is supported by MoES (Ministry
of Earth Science).
Review statement
This paper was edited by B. V. Krishna Murthy and reviewed by two anonymous referees.
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