Introduction
Aerosols are well known to be important drivers of climate from regional to
global scales by serving as cloud condensation nuclei (CCN) and scattering
and absorbing radiation (IPCC, 2007, 2013). Though much progress has been
made in understanding the role of aerosols in the climate system, the
formation of secondary organic aerosols (SOAs) and the partitioning of
semivolatile organics are topics subject to considerable uncertainty (e.g.,
Kanakidou et al., 2005; Goldstein and Galbally, 2007; Kroll and Seinfeld,
2008; Donahue et al., 2009; Hallquist et al., 2009; Ervens et al., 2011).
Recent studies show that different estimation methods of SOA production
result in ranges from 140 to 910 Tg C yr-1 (Goldstein and
Galbally, 2007). The large uncertainty in these estimates (e.g., Pye and
Seinfeld, 2010; Hallquist et al., 2009; Jimenez et al., 2009; Kanakidou
et al., 2005) illustrates the need for better understanding of organic
aerosol composition, volatility, hygroscopicity, and CCN activity.
Few studies have investigated the link between aerosol hygroscopicity,
volatility, and level of aerosol oxidation (e.g., Jimenez et al., 2009;
Poulain et al., 2010; Tritscher et al., 2011; Hong et al., 2014), though
numerous studies have focused on the link between two of these parameters
(e.g., Kuwata et al., 2007; Asa-Awuku et al., 2009; Meyer et al., 2009;
Ristovski et al., 2010; Massoli et al., 2010; Chang et al., 2010; Lambe
et al., 2011; Lathem et al., 2013; Frosch et al., 2011, 2013; Villani et al.,
2013; Xu et al., 2014). Several studies have shown and proposed
parameterizations for the relationship between organic hygroscopicity and
degree of oxidation (e.g., Chang et al., 2010; Massoli et al., 2010; Lambe
et al., 2011; Duplissy et al., 2011; Frosch et al., 2013), with the
hygroscopicity parameter, κ, being either inferred from subsaturated
growth factor (GF) measurements or supersaturated CCN activation spectra
(Petters and Kreidenweis, 2007) and the degree of oxidation represented by
the oxygen-to-carbon ratio (O : C) or f44, the ratio of the m/z 44
peak to the total organics signal in an aerosol mass spectrometer (AMS) (Ng
et al., 2010; Aiken et al., 2008). Jimenez et al. (2009) showed results of
several studies correlating GF with O : C and proposed a 2-dimensional
framework for organic aerosols where hygroscopicity and oxidation level
increase with decreasing volatility. There are several other studies, though,
that have shown that the link between these properties is not always
straightforward (e.g., Meyer et al., 2009; Poulain et al., 2010; Frosch
et al., 2011; Tritscher et al., 2011; Lathem et al., 2013; Alfarra et al.,
2013; Villani et al., 2013). Frosch et al. (2011) saw weak sensitivity of
supersaturated κ to O : C and f44, respectively; Alfarra
et al. (2013) saw weak correlation for both subsaturated and supersaturated
hygroscopicity measurements with f44 in laboratory studies of α-pinene SOA. Tritscher et al. (2011) found in chamber studies of α-pinene SOA that volatility generally decreased while subsaturated
hygroscopicity and O : C remained fairly constant, and an additional study
of α-pinene SOA by Meyer et al. (2009) measured a decrease in
subsaturated hygroscopicity with increased volatility. Lathem et al. (2013)
found that for biomass burning aerosols sampled during the ARCTAS and ARCPAC
field campaigns, supersaturated organic hygroscopicity increased while
O : C remained fairly constant. In ambient measurements by Villani
et al. (2013), subsaturated hygroscopicity in several externally mixed air
masses was found to both increase or decrease after volatilization. Asa-Awuku
et al. (2009) observed that the most hygroscopic fraction of β-caryophyllene SOA was also the most volatile. Kuwata et al. (2007) found
for subsaturated measurements of ambient aerosols in Tokyo that, after
briefly heating to 400 ∘C, a less hygroscopic particle mode was also
less volatile while a more hygroscopic mode was more volatile. The results of
these studies illustrate the range of possible relationships between
hygroscopicity, volatility, and oxidation level and the need to better
understand why and when these interactions occur. An additional level of
complexity arises in interpreting these results as subsaturated GF and
supersaturated κ measurements can differ due to the difference in
water volume between measurement conditions, thus impacting the assumption of
an ideal solution and the importance of surface tension effects and partial solubility (Wex et al.,
2009; Petters et al., 2009; Ruehl et al., 2010). Furthermore, recent studies
have shown that average carbon oxidation state,
OS‾c, may be a better indicator of aerosol
oxidation than O : C as O : C may not capture oxidative changes due to
the breaking and forming of bonds (Kroll et al., 2009, 2011).
Instrument setup used during the SOAS campaign, combining a PILS,
thermodenuder, CCNc, AMS, and SMPS to measure the water-soluble fraction of
ambient aerosol (green line) and ambient aerosol (blue line).
Also important to SOA formation and processing are aqueous phase aerosols.
Aqueous particles can both absorb species from the gas phase and serve as
a medium for aqueous SOA production (e.g., Zuend and Seinfeld, 2012; Ervens
et al., 2011, 2013; Sareen et al., 2010). Hennigan et al. (2009) found that
water-soluble organic carbon represented a major portion of SOA in Atlanta.
It has been proposed that SOA produced in the aqueous phase may have higher
κ than SOA formed in the gas phase with an upper limit comparable to
ammonium sulfate (κ=0.6) (Ervens et al., 2011); few experiments,
however, have quantified the impact of SOA aqueous phase processes on total
κ in ambient aerosols.
The southeastern United States presents itself as a particularly interesting
location of study, as it has experienced an overall cooling trend in surface
temperature (1950–2006 average; Portmann et al., 2009) in contrast to the
largely warming trend seen elsewhere in the United States. Carlton and Turpin
(2013) found that aqueous SOA formation is particularly important in the
eastern United States. These findings demonstrate that the SOA processes
happening in this region are both complex and important. Motivated by these
findings, the Southern Oxidant and Aerosol Study (SOAS) was a collaborative
field mission which aimed to study the aerosols in the region. This study
focuses on aerosols collected at a rural field site in Centreville, Alabama,
and uses a variety of aerosol instrumentation to probe aerosol composition,
volatility, and CCN properties.
Data collection
Measurement site
Data were collected during the Southern Oxidant and Aerosol Study (http://soas2013.rutgers.edu), part of the Southeast Atmosphere Study
(SAS; http://www.eol.ucar.edu/projects/sas), in Centreville, Alabama
(+32∘54′11.81′′ N, 87∘14′59′′ W), a site highly
influenced by biogenic volatile organic compound emissions combined with varying levels of
anthropogenic influence (http://soas2013.rutgers.edu/). Measurements
were taken from 1 June through 15 July 2013 using a suite of aerosol
instrumentation, including a Droplet Measurement Technologies continuous-flow
streamwise thermal gradient CCN counter (referred to hereafter as the
CCNc; Roberts and Nenes, 2005; Lance et al., 2006), thermodenuder (TD;
Cerully et al., 2014), particle-into-liquid sampler (PILS; Weber et al.,
2001; Orsini et al., 2003), and high-resolution time-of-flight aerosol mass
spectrometer (HR-ToF-AMS, referred to hereafter as AMS; DeCarlo et al., 2006)
to quantify CCN activity and hygroscopicity, volatility, water-soluble
aerosol components, and aerosol composition and oxidation state,
respectively. These measurements are then used here to investigate the
relationship between hygroscopicity, volatility, and oxidation state for the
ambient and water-soluble fraction of the aerosols sampled.
Experimental setup and data collection
The instrument setup is shown in Fig. 1. Aerosols were collected using
a PM1.0 cyclone and either directly introduced into the aerosol
instrumentation or passed through the PILS to study the water-soluble
fraction of the aerosol (as the aerosol is not dried before entering the
PILS, any volatile aqueous phase material is also captured). The former flow
configuration is hereon called “ambient aerosol”, while the latter is
referred to as “PILS aerosol”. Prior to analysis, ambient aerosol is first
dried using a Nafion dryer and charge-neutralized using a Po-210 bipolar
charging source. The PILS liquid is then filtered (25 mm diameter,
0.45 µm pore syringe filter, Fisher Scientific, Fisherbrand*),
passed through a debubbler, nebulized (U-5000AT, Cetac Technologies Inc.,
Omaha, Nebraska, USA; Ohata et al., 2011), dried with a Nafion dryer, and
charge-neutralized. The polydisperse aerosol stream then passed through the
TD or through a bypass line before being sampled by the AMS, scanning
mobility particle sizer (SMPS; TSI Classifier model 3080, condensation
particle counter (CPC) model 3022A), and CCNc. To account for nebulizer
efficiency and any losses in the PILS–nebulizer system, sulfate measured by
the PILS–AMS system was compared to ambient AMS sulfate and used as a scaling
factor. Sulfate is used for this purpose because it is completely water soluble
and nonvolatile. The CCNc was operated in Scanning Flow CCN Analysis (SFCA)
mode (Moore and Nenes, 2009), scanning flow rate sinusoidally from 0.2 to
0.9 Lmin-1 then back to 0.2 Lmin-1 over 2 min,
resulting in a CCN spectrum between 0.10 and 0.40 % supersaturation, s
(see Sect. 3.1). The flow entering the CCNc was held constant at
1 Lmin-1 using a laminar flow box and introduced to the CCN
column as needed. The sheath-to-aerosol ratio in the CCNc was approximately
10 : 1. The top-to-bottom temperature difference in the CCN instrument was
6 ∘C. Temperature set points in the TD heating section were switched
between 60, 80, or 100 ∘C. The flow rate used for TD operation was
1.5 L min-1, corresponding to an average residence time of roughly
7 s. In this setup, the TD was operated without a cooling section, as
recondensation of vapors is minimal at low ambient mass loadings
characteristic of the conditions during this study (e.g., Cappa, 2010; Saleh
et al., 2011). The SMPS was operated with a sheath flow rate of
3.0 Lmin-1 and aerosol flow rate of 0.30 Lmin-1
(10 : 1 ratio), with the CPC 3022A in low flow mode. Aerosol size
distributions were collected every 3 min, scanning particle sizes from 14.6
to 685.4 nm. The AMS was operated in V mode (DeCarlo et al., 2006)
with a data averaging time of 3 min. AMS measures mass concentrations of
non-sea salt chloride, sulfate, nitrate, ammonium, and organics; refractory
species (at 600 ∘C), including sea-salt, black carbon, and crustal
material, are not detected. Separate from this setup was an additional PILS
coupled with an ion chromatograph (IC), referred to hereafter as the PILS–IC,
which measured the ambient water-soluble Na+, NH4+,
K+, Ca2+, Mg2+, Cl-, NO3-,
NO2-, SO42-, as well as acetate, formate, and oxalate
every 20 min (Orsini et al., 2003).
The sampling schedule shown in Fig. 2 was designed to allow for automated
measurements of ambient and water-soluble ambient aerosol. Thermally denuded
measurements were conducted from 20 June to 15 July. The temperature in the
TD heating section was switched between 60 and 80 ∘C from 20 June to
midday 30 June 2013 (Fig. 2, solid red line), and between 60 and
100 ∘C from evening 30 June through 15 July 2013 (Fig. 2, dashed
line). SMPS distributions and AMS spectra sampled right before or after each
switch between ambient/PILS sampling lines or between bypass and TD thermal
processing were discarded. This leaves three SMPS scans and AMS samples per
15 min measurement interval. CCN properties (e.g., CCN concentration as
a function of s) were averaged when more than one SFCA scan occurred during
a single SMPS sample. CCN spectra were smoothed by fitting CCN concentrations
as a function of s. These fits were filtered for cases where flow rate fit
parameter standard deviations (SDs) are greater than
25 cm3min-1, resulting in a supersaturation uncertainty of
approximately ±0.03 % (according to the calibration described in
Sect. 3.1). The resulting CCN concentrations were compared with activation
spectra predicted by applying Köhler theory (Köhler, 1936) to
SMPS-measured concentrations under the assumption that the distributions had
a κ of approximately 0.2–0.3, a general estimate of an aerosol
population composed of organics and ammonium sulfate with equal volume
fractions or slightly higher organic volume fraction than ammonium sulfate
(see Sect. 3.3), as a filter to help identify irregularities in the data. All
supersaturations in the instrument were corrected for supersaturation
depletion from condensation of water vapor onto the activated CCN (see
Sect. 3.1). Data were further filtered for days with measured precipitation
≥0.1 mm (5 min resolution precipitation data provided by
Atmospheric Research & Analysis, Inc.) and for any influences from
nonrepresentative local sources (e.g., diesel exhaust from truck deliveries
to the sampling site). As flow, and therefore supersaturation, is scanned in
the CCNc, spectra were divided into bins of s (%) ±0.005 % and
averaged within each bin.
Example sampling schedule illustrating the valve switching
between both PILS and ambient sampling lines and TD and bypass
sampling lines. TD sampling temperatures from 20 June to midday
30 June are indicated by the red solid line, and sampling
temperatures from evening 30 June to 15 July are indicated by the
dashed line.
Methodology
Instrument calibration and supersaturation
depletion
The relationship between supersaturation and instantaneous flow rate was
calibrated using the procedures of Moore and Nenes (2009). Briefly, ammonium
sulfate solution is atomized, dried using two silica gel diffusion dryers, and
charge-neutralized using Po-210. The dried aerosol is then classified by
a differential mobility analyzer (DMA; TSI model 3081 and split between
a CPC, giving the total number of condensation nuclei, and the CCNc. The
activation ratio, or ratio of CCN to CN concentration, is plotted against
instantaneous flow rate and fit to a sigmoid function. The point where half
of the total particles act as CCN corresponds to a “critical flow rate” and
the instantaneous value of supersaturation corresponds to the known critical
supersaturation, sc, of the classified ammonium sulfate (Moore
and Nenes, 2009). This relationship is determined for a range of classified
ammonium sulfate particles, resulting in a calibration curve, in this case
ranging from 0.10 to 0.40 %.
The calibration method described above was conducted with ammonium sulfate
aerosol concentrations below 700 cm-3 in order to avoid water
vapor depletion in the instrument (Lathem and Nenes, 2011). For measurements
made throughout the study, supersaturation depletion in the CCNc column was
accounted for using the correction found in Raatikainen et al. (2014); this
typically has a negligible effect when sampling low ambient number
concentrations but can be important when sampling from the PILS–nebulizer
system (Fig. 1, green dashed line), where total aerosol number concentrations
can reach as high as approximately 2.5×105 cm-3.
Determining total aerosol hygroscopicity
Aerosols activate in the CCNc when their critical supersaturation,
sc, is greater than the instantaneous supersaturation (i.e., flow
rate), s, in the CCNc column. This sc corresponds to a critical
diameter, dp, c, above which all particles activate. These
parameters are used to determine the aerosol hygroscopicity parameter,
κ (Petters and Kreidenweis, 2007):
κ=4A327dp, c3sc2,
where A=(4Mwσw)/(RTρw), and
Mw, σw, and ρw are the molar
mass, surface tension, and density, respectively, of water at the average
mid-column temperature, T, in the CCNc (305 K). R is the universal gas
constant. The dp, c is obtained by matching the concentration of
CCN activated at a given s (where s=sc) with the backwards
integrated SMPS number distribution (thus, the corresponding size bin and
dp, c; Moore et al., 2011). This analysis method operates under
the assumption that the aerosols are internally mixed (i.e., particles of
a given size have similar composition) and that the size-dependent
hygroscopicity does not vary enough to inhibit activation at larger sizes. In
cases where ambient or thermally denuded measurements are inhomogeneous,
κ is most representative of particles with sizes near dp,
c.
Uncertainty in measured hygroscopicity
The uncertainty in the measured κ can be mainly attributed to
uncertainties in the measured particle diameter and instrument
supersaturation and can be described by
Δκ=∂κ∂sΔs2+∂κ∂dpΔdp2,
where Δs and Δdp are the instrument uncertainties
in CCNc-measured supersaturation and DMA-measured diameter, respectively.
Absolute uncertainty from CCNc supersaturation is estimated at
±0.04 % (Moore et al., 2011) while DMA sizing uncertainty, based on
the width of the DMA transfer function (Wang and Flagan, 1990) and the
10 : 1 sheath-to-aerosol flow ratio used in this study, is approximately
10 %. Average critical diameters of ambient aerosol of 83, 95, and
116 nm and measured at supersaturations of 0.40, 0.30, and
0.20 %, respectively, are reported and discussed in Sect. 4. Applying
these values to Eq. () gives a Δκ of 0.033, 0.053, and
0.097 for 0.40, 0.30, and 0.20 % supersaturation, respectively.
Inferring organic aerosol hygroscopicity
Total aerosol hygroscopicity can be expressed as a sum of
contributions from each aerosol component
κ=∑jεjκj,
where εj and κj are the volume fraction and
hygroscopicity of species j, respectively (Petters and Kreidenweis, 2007).
Using this rule, aerosols can be separated into its organic (org) and
inorganic (inorg) contributions to the total measured hygroscopicity, where
κ=εorgκorg+εinorgκinorg.
Measurements of particle composition, in this case, can come from either AMS
or PILS–IC measurements. Using the five aerosol components measured by AMS,
aerosols can be separated into its primarily inorganic ([NH4+],
[SO42-], [Cl-], and [NO3-]) and organic ([Org])
mass concentrations. A typical organic density of 1.4 gcm-3 is
assumed for volume calculations (e.g., Moore et al., 2011; Lathem et al.,
2013). In order to better assess the properties of the inorganic aerosols,
the partitioning of aerosols between sulfate species is evaluated (Nenes
et al., 1998; Moore et al., 2011; Lathem et al., 2013). Using the molar ratio
of ammonium ions to sulfate ions, RSO4, sulfate is determined
to exist as a mixture of ammonium bisulfate and sulfuric acid for
RSO4<1, as ammonium sulfate and ammonium bisulfate for 1<RSO4<2, or as ammonium sulfate for RSO4>2.
This method assumes that the relative contribution of nitrate (and will
therefore exist mainly in the gas phase) and other inorganic cations (such as
sodium) to the aerosol is minimal, which is the case here. Inorganic cations
are not measured by the AMS, while nitrates are not present owing to the high
acidity of the aerosol sampled (Guo et al., 2015). Once the species are
determined, volume fractions are calculated using AMS mass fractions and the
species densities and hygroscopicities listed in Padró et al. (2010). For
mixtures of more than one compound (i.e., for the RSO4<1 or
1<RSO4<2 cases), inorganic properties are calculated as the
average of the individual component properties.
PILS–IC measurements can also be used to determine the inorganic and organic
contributions to the aerosol. In this case, as measurements of Na+
are included (Guo et al., 2015), it can be assessed whether the presence of
these species can impact the predicted aerosol hygroscopicity. The speciation
of the inorganic fraction of the aerosol becomes more complex, and the
ISORROPIA II aerosol thermodynamic equilibrium model (Nenes et al., 1998;
Fountoukis and Nenes, 2007) was employed to determine the mixture of
inorganic salts present in the aerosol from the
PILS-IC measurements. κ and density for each
of the components were taken from Padró et al. (2010) when available.
Otherwise, estimated intrinsic κ of Sullivan et al. (2009) were used
with densities from Perry and Green (1997). As PILS–IC does not measure
organic compounds, the AMS organic mass is used for calculation of volume
fractions.
Average (standard deviation) of κ for
the non-denuded and thermally denuded ambient and
water-soluble ambient aerosol.
Ambient
PILS
s (%)
0.40
0.30
0.20
0.40
0.30
0.20
Non-denuded
0.18±0.05
0.21±0.05
0.25±0.08
0.25±0.06
0.25±0.07
0.23±0.09
TD at 60 ∘C
0.17±0.05
0.20±0.06
0.24±0.08
0.26±0.07
0.26±0.08
0.25±0.09
TD at 80 ∘C
0.19±0.08
0.22±0.10
0.25±0.10
0.26±0.06
0.27±0.07
0.27±0.12
TD at 100 ∘C
0.19±0.04
0.23±0.04
0.30±0.05
0.31±0.10
0.27±0.10
0.31±0.13
Temporal ambient (top) and PILS (bottom) κ and AMS inorganic
mass fractions for the entire study. Non-denuded and thermally denuded
measurements are indicated by color. Each point represents an average of all
κ values measured over each 15 min sampling period.
Results
The temporal variation of κ and AMS inorganic mass fraction for
ambient and PILS water-soluble aerosols at s=0.40 % are shown in
Fig. 3. All κ values for this study, as shown in Fig. 3, represent the
average of all κ values measured within a given 15 min sampling
period. Thermally denuded measurements are indicated by the set point
temperature which the aerosols were exposed to in the TD. Throughout the
study, the trends in ambient and PILS κ are similar (not shown),
though the magnitude can vary at different supersaturations as indicated in
Table 1. Note that the SDs in κ in Table 1 for all conditions are
typically very close to the Δκ values calculated in Sect. 3.2.1;
thus, it is expected that changes in the average reported κ are
robust. PILS aerosols show a slightly larger average hygroscopicity than the
ambient aerosol measured at both s=0.30 % and s=0.40 %. The
increase in hygroscopicity of ambient aerosol with decreasing supersaturation
indicates that ambient particles have increasing hygroscopicity with size;
average dp, c for ambient non-denuded aerosol at 0.40, 0.30, and
0.20 % s are 83±9, 95±9, and 116±11 nm,
respectively. As the mixing in the PILS system results in a completely
chemically homogeneous aerosol, as opposed to the ambient measurements, PILS
aerosol hygroscopicity is relatively invariant with supersaturation
(Table 1). Approximately 80 % of the ambient
aerosol measured throughout this study was found to be water-soluble and PILS
aerosol composition and hygroscopicity is dominated by the mass and
composition of the larger sampled aerosol sizes (Guo et al., 2015)); it is
thus expected that the PILS aerosol is also
representative of the bulk ambient aerosol. As such, the ambient non-denuded
aerosol hygroscopicity at the largest diameters (i.e., measured at s=0.20 %) is similar to that of the measured PILS aerosol. The previous
discussion makes it reasonable to assume that any additional aqueous
processes taking place in the PILS sampler have a negligible impact on the
overall aerosol hygroscopicity. This assumption will be tested throughout the
following analysis by comparing the properties of ambient (undenuded/denuded)
aerosols measured at s=0.20 %, which is expected to be most
representative of the bulk aerosol and AMS measurements, and PILS aerosol
measured at s=0.40 %. Also, potential artifacts in the observations
from semivolatile re-condensation after the thermodenuder are not expected to
be important because our measurements occurred under both low aerosol mass
loadings and residence times in the TD (Cappa, 2010; Saleh et al., 2011). The
presence of soluble (including volatilized aerosols) gases in the CCN
instrument could affect the observed aerosol hygroscopicity; very high levels
of such vapors, however, are required as soluble gases tend to be lost to the
wetted instrument walls before affecting CCN activity (Romakkaniemi et al.,
2014). It is, however, possible that there may be some additional
volatilization in the CCN instrument, especially when sampling the undenuded
ambient/PILS aerosols. A future study will focus on these issues.
κ of thermally denuded and non-denuded aerosols
Comparisons of the hygroscopicities of thermally denuded, referred to
hereafter simply as denuded, and non-denuded aerosol sampled directly from
ambient and by PILS are shown in Fig. 4. The points shown are averages of
measurements taken within each 15 min sampling period and represent only
those points where denuded and non-denuded samples were collected directly
before or after one another. It is expected that denuding aerosols would
volatilize organics; thus the remaining aerosols would have an increased
inorganic fraction and display higher hygroscopicity than its non-denuded
counterpart. Denuded PILS aerosols show slightly higher hygroscopicity than
the non-denuded PILS aerosols, though these changes in κ are within
10 %. Average thermally denuded ambient aerosols, however, display hygroscopicity similar to that
of the non-denuded ambient aerosols, with some scatter that could be from the
size-dependent variability of composition (that is not present in the PILS
aerosol).
Non-denuded vs. thermally denuded κ at 60, 80, and
100 ∘C TD sampling temperatures for ambient aerosol at s=0.20 % (left) and PILS aerosol at s=0.40 % (right). The
solid line represents 1 : 1 agreement while dashed lines represent
deviations of ±10 %. All points shown are for periods where
non-denuded measurements are directly followed by denuded
measurements and vice versa.
Average and SD in mass
fractions remaining (MFR) and relative change of thermally denuded vs.
non-denuded aerosol for ambient κ measured at s=0.20 %
and PILS κ measured at s=0.40 %.
Ambient
PILS
MFR
κTD-κoκo
MFR
κTD-κoκo
TD at 60 ∘C
0.90±0.10
-0.02±0.20
0.73±0.19
0.01±0.12
TD at 80 ∘C
0.78±0.05
0.11±0.33
0.65±0.11
0.10±0.10
TD at 100 ∘C
0.65±0.08
0.12±0.16
0.45±0.13
0.11±0.07
An unexpected result is that the ambient aerosol hygroscopicity does not
change much after passing through the TD, even when significant
volatilization occurs. There are several potential reasons why this may
occur. Here, it is discussed how changes in κ are related to possible
composition changes taking place in the aerosol, such as a negligible loss of
mass, volatilization of inorganic material, or higher volatility compounds
having higher hygroscopicity, and to the chosen measurement method. Regarding
the loss of mass in the aerosol, if the aerosol is largely nonvolatile at these
temperatures, a change in hygroscopicity after being thermally denuded would
not be expected; this is not the case here. Average ambient aerosol mass
fractions remaining calculated from AMS data and the average relative
change in thermally denuded hygroscopicity, κTD, vs.
non-denuded hygroscopicity, κo, for ambient measurements
at s=0.20 % and PILS measurements at s=0.40 % are shown in
Table 2. Volatilization (by mass) reaches as high as approximately 35 %
in the ambient aerosol and 55 % in the PILS aerosol. Note that the
measured mass fractions remaining are not expected to be the same for the PILS
and ambient aerosols due to general shifts in composition with size expected
for ambient aerosol; however, the expected changes in κ, if ambient
aerosol at s=0.20 %, and PILS aerosol should be relatively similar.
Indeed, relative changes in hygroscopicity are, on average, only 12 % for
ambient mass losses of approximately 35 %. Villani et al. (2013) also
found small changes in hygroscopicity for thermally denuded aerosol,
measuring changes in subsaturated hygroscopicity generally less than 5 %
for thermally denuded (from 70 to 100 ∘C) aerosols measured at four
unique ambient environments (Puy de Dôme, France; Clermont-Ferrand,
France; Mace Head, Ireland; Leipzig, Germany). They also determined that
denuded particles could display increased or decreased hygroscopicity, as
seen here.
The magnitude of changes in κ is dependent on how mass is lost
in the aerosol particles after passing through the TD. Assuming that volatilization is mainly
associated with organics, the change in hygroscopicity expected for
the data, particularly at 80 and 100 ∘C for PILS and ambient
data, should be notable. For example, according to the mixing rule in
Eq. (), for a given particle composed of equal volumes of
inorganic, assuming inorganic κ equals that of ammonium sulfate
of 0.6 (Petters and Kreidenweis, 2007), and organic, assuming organic
κ of 0.2 (organic κ typically ranges from 0.1 to 0.3;
Petters and Kreidenweis, 2007), measured κ would equal 0.4. If
the same particles were denuded and the loss of organic mass resulted
in a particle that is 80 % by volume inorganic and 20 % by
volume organic, the resulting total κ would equal 0.52
(a 30 % increase in measured κ). One issue that makes this
argument more complex is that, while it might be expected that
a majority of the mass fraction lost would be from organics, it is
also known that inorganic compounds (such as ammonium sulfate) can
volatilize at the temperatures set in the thermodenuder. For example,
ammonium sulfate aerosol has been found to volatilize at as low as
75 ∘C (Clarke, 1991; Burtscher et al., 2001; An et al.,
2007), potentially decreasing the expected change in hygroscopicity
after denuding. In our data, the fraction of the inorganic mass lost
in the denuder did not exhibit any temperature dependence between 30
and 100 ∘C, indicating that volatilization is not the main
cause of this phenomenon.
An additional possibility for changes in κ to be suppressed even after
loss of organic and inorganic materials (after passing through the TD) is
a decrease in the remaining organic hygroscopicity after denuding.
Conventional thought suggests that the more volatile the organic compounds,
the less aged and less hygroscopic they are (e.g., Jimenez et al., 2009).
This implies that if the lowest hygroscopicity compounds are volatilized
first, the remaining organic compounds should be of higher hygroscopicity. If
volatilized organic material is actually of higher κ than the
remaining organics (e.g., as seen by Asa-Awuku et al., 2009), a decrease in
the hygroscopicity of the remaining organic could suppress changes seen in
the total κ after denuding.
One final and necessary component in evaluating these results and their
consequences on CCN activity is to consider the effects of ambient
size-resolved composition. Regarding measurement methods, as described in
Sect. 3.2, the critical dry diameter, dp, c, is determined by
matching the CCN concentration with the backwards-integrated SMPS size
distribution. Thus, κ is expected to be most representative of
aerosols
with similar diameter to dp, c. According to Eq. (),
dp, c at a given s will change if κ changes, as might be
expected when volatilizing material in the denuder. For a mixture that is
chemically homogeneous with size, as is the PILS aerosol, a change in
dp, c should not have an effect on κ (see Table 1). As the
ambient aerosol display size-dependent composition (Table 1), changes in
dp, c could potentially affect κ. Thus, it is difficult to
probe the overall changes in chemistry of a given particle, but it is
possible to look at how changes impact CCN activation at a given
supersaturation. These changes in hygroscopicity for a given supersaturation
are a measure of the importance of volatilization on CCN activity. Results
reported in Table 2 clearly show that even with ambient mass losses of ∼35 %, the effect of volatility on κ, and thus CCN activity, is
at most 10 %.
Average and SD in κorg of non-denuded and denuded
ambient aerosol at s=0.20 % and water-soluble ambient aerosol
at s=0.40 % using bulk AMS composition and PILS–IC with
ISORROPIA II composition.
AMS
PILS–IC
Ambient (s=0.20 %)
PILS (s=0.40 %)
PILS (s=0.40 %)
Non-denuded
0.14±0.09
0.14±0.06
0.11±0.07
TD at 60 ∘C
0.12±0.08
0.12±0.06
–
TD at 80 ∘C
0.12±0.11
0.09±0.04
–
TD at 100 ∘C
0.08±0.07
0.08±0.06
–
Variation in κorg with O : C for ambient
aerosol at s=0.20 % and PILS aerosol at s=0.40 % for
non-denuded and thermally denuded conditions. Small colored dots
indicate all measured points while larger circles and squares
indicate the averages. Errors bars indicate 1 standard deviation in
measured values for ambient and PILS aerosols. Also
shown are dashed lines indicating the parameterizations of
κorg with O : C from Lambe et al. (2011) and
Chang et al. (2010).
Inferred organic hygroscopicity
Organic hygroscopicity, κorg, was inferred using
composition measurements from PM1 AMS and PILS–IC with ISORROPIA II
and the methods described in Sect. 3.3 (Table 3). PILS–IC compositions were
averaged over 1 h (i.e., spanning two periods of non-denuded PILS measurements
in the PILS–AMS–CCNc setup). Using AMS composition and the total measured
κ to infer κorg shows that, the less volatile organic
aerosol is also less hygroscopic. Though the SDs in κorg
are nearly as large as the average κorg values (Table 3),
the consistent trend between PILS and ambient aerosols, with the possible
exception of ambient κorg at 80 ∘C (where the
highest SD is also seen), suggests that the most volatile fraction in the
aerosol actually is of higher hygroscopicity than the less volatile material.
Though this contradicts the conventional view that the most volatile aerosol
components are the least hygroscopic, the change in hygroscopicity of only
the organic fraction of the aerosol with volatility has not been studied in
the ambient environment to the knowledge of these authors. Such behavior has
been seen in chamber-generated aerosols (e.g., Asa-Awuku et al., 2009; Meyer
et al., 2009). It is possible that the least volatile fraction is also the
least hygroscopic due to the presence of oligomers: high molecular weight
compounds with low volatility and solubility – hence hygroscopicity (e.g.,
Ervens et al., 2011; Li et al., 2011; Sareen et al., 2010; Asa-Awuku et al.,
2009; Reynolds et al., 2006; Varutbangkul et al., 2006; Dommen et al., 2006;
Baltensperger et al., 2005; VanReken et al., 2005; Kalberer et al., 2004). As
temperature is increased in the thermodenuder, oligomers may partially
dissociate into more volatile and hygroscopic fragments upon heating,
contributing to the decreased hygroscopicity at increased temperatures in the
TD (Table 3).
To evaluate the importance of assumptions of the inorganic composition in
inferring the organic hygroscopicity, κorg was additionally
determined for non-denuded PILS aerosols using data from a PILS–IC system
concurrently sampling non-denuded ambient aerosols and located separately from
the main PILS–CCNc–AMS setup. Data were input into the ISORROPIA II aerosol
equilibrium model to determine speciated composition of the inorganics,
investigating how realistic the assumptions are of treating the inorganic
hygroscopicity as an ammonium and sulfate system only (Sect. 3.3). ISORROPIA
II is able to resolve sodium, ammonium, magnesium, calcium, and potassium
species. In Centreville, however, sodium, magnesium, calcium, and potassium
components were found to typically be negligible. For this reason, there
should be little difference in the assumed inorganic hygroscopicity using the
AMS or PILS–IC analysis methods. The slight decrease in ambient PILS κorg (∼0.03; Table 3) resulting from PILS–IC vs. AMS analysis
confirms the validity of using simplified assumptions of aerosol speciation to
calculate inorganic hygroscopicity for this study and does not change the
trend of decreased κorg with increased volatility seen here.
Organic aerosol hygroscopicity and degree of
oxidation
The potential link between organic hygroscopicity and the level of aerosol
oxidation is investigated using AMS measurements of the bulk O : C ratio
with the κorg determined in Sect. 4.2 with. O : C
throughout the study and for different TD sampling temperatures varied
slightly with ambient average O : C values of 0.58±0.06 for
non-denuded aerosols; no variation in average O : C was seen for denuded
aerosols, being 0.59±0.05, 0.57±0.04, and 0.59±0.06 for TD
temps of 60, 80, and 100 ∘C, respectively (Fig. 5, left). PILS-generated aerosol showed similar O : C values of 0.55±0.04, 0.57±0.05, 0.56±0.03, and 0.53±0.07 for non-denuded and denuded
aerosol at 60, 80, and 100 ∘C, respectively (Fig. 5, right). There
is no clear relationship between κorg with O : C for any
of these data as the correlation between κorg and O : C in
all cases is low (R2<0.23 for ambient and <0.53 for PILS aerosol).
Variation in κorg with
OS‾c for ambient aerosol at
s=0.20 % for the total study period (left) and for 80 and
100 ∘C measurement periods only (right top and bottom,
respectively). Small colored dots indicate all measured points while
larger circles indicate measurement averages and errors bars
indicate a single SD in measured values.
The relationship between κorg with average carbon oxidation
state OS‾c, calculated as
2 × O : C–H : C (where H : C is the hydrogen-to-carbon ratio
of the aerosol measured by the AMS) is further investigated for the ambient
aerosol (Fig. 6, left panel). While there is no clear relationship between
κorg and oxidation in terms of
OS‾c, it appears that denuded aerosols at
100 ∘C are more oxidized than at other temperatures, as might be
expected, but this is not indicated by O : C. Measurements at 80 ∘C,
though, still appear to be less oxidized than thermally denuded at
60 ∘C. Although at first this seems counterintuitive (Sect. 2.2 and
Figs. 2 and 3), 60 ∘C measurements were taken throughout the
measurement period while 80 ∘C were taken only during the beginning
portion of the study and 100 ∘C only during the latter portion of
the study. Therefore, it is more appropriate to focus on the relationship
between κorg and OS‾c for
each separate measurement period (Fig. 6, right). In this case,
OS‾c is consistent with expectation that
oxidation increases as more material is volatilized in the denuder as
indicated by the change in OS‾c between
non-denuded, 60, and 100 ∘C measurements (Fig. 6, right, bottom)
with comparable increases in OS‾c for 60 and
80 ∘C measurements compared to non-denuded measurements (Fig. 6,
right, top). This is not, however, the case for O : C, as O : C of
60 ∘C measurements remains greater than that of 100 ∘C
measurements (not shown). O : C of 60, 80 ∘C, and non-denuded
measurements are comparable (not shown). Overall,
OS‾c appears to be more consistent with the
expectation of the least volatile fraction being the most oxidized while
O : C appears to show no correlation with volatilization. Even
OS‾c, however, may appear counter to
expectation during portions of the sampling period (Fig. 6, left); this is
a result of changing air masses which can change the relationship between
hygroscopicity and volatility with oxidation. Furthermore, the increase in
OS‾c with volatility is still consistent
with the potential presence of oligomers discussed in Sect. 4.2.
Attributing organic hygroscopicity to AMS
factors
Positive matrix factorization (PMF; Lanz et al., 2007) analysis is performed
on high-resolution organic mass spectra for source apportionment. A detailed
discussion of the PMF results can be found in Xu et al. (2015). In brief,
four subtypes of OA are identified in the ambient aerosol after carefully
examining the scaled residual, solution rotational ambiguity, and factor
correlations with external tracers. Two oxygenated OA factors with high but
differing O : C ratios are termed more-oxidized oxygenated organic aerosols
(MO-OOA, O : C = 0.8) and less-oxidized OOA (LO-OOA, O : C =0.46). MO-OOA contributes 39 % of total OA. LO-OOA, accounting for
32 % of OA, is found to be mainly from β-pinene oxidation by
NO3 radical. Biomass burning OA (BBOA) factor, characterized by ions
at m/z 60 (C2H4O2+) and 73 (C3H5O2+) in the
mass spectrum, shows good correlation (R=0.8) in time series with brown
carbon. The fourth factor is interpreted as isoprene-derived OA
(isoprene-OA). This factor exhibits a similar mass spectrum to
laboratory-generated isoprene SOA via the reactive uptake of epoxydiols
(prominent signals at C4H5+ and C5H6O+) (Li
et al., 2012; Nguyen et al., 2014). Additionally, it shows a good correlation
with particle-phase methyltetrols (tracer for isoprene OA). PMF on the PILS
aerosol resulted in three factors corresponding to LO-OOA, MO-OOA, and
isoprene-OA; this is because during the period of measurements with the PILS,
the concentration of BBOA was too low to expect a PMF factor to emerge from
the analysis. PMF factors of the total PILS aerosol measured by AMS were used
to perform linear regression on the PILS non-denuded κorg
(at s=0.40 %) to infer the hygroscopicity of each PMF factor, and its
contribution to organic aerosol hygroscopicity:
κorg=εLO-OOAκLO-OOA+εMO-OOAκMO-OOA+εisoprene-OAκisoprene-OA,
where properties are representative of AMS mass spectra for LO-OOA,
MO-OOA, and isoprene-OA. A system of equations is
determined using corresponding measurements of the PILS non-denuded
κorg and PMF factor mass fractions. A bootstrapped
resampling of the regression indicates that while the average
κMO-OOA and κisoprene-OA are similar at
0.16±0.02 and 0.20±0.02, respectively, they are at least
twice as large as the average κLO-OOA of 0.08±0.02 (Fig. 7).
(top) Examples of binned κorg
solutions from a bootstrapped resampling of the linear regression of
the CCN activity of PILS bypass aerosol, where the solid green and
red lines represent the average and 1 SD, respectively, in each
factor. (bottom) κorg found for each PMF
factor through linear regression vs. O : C for non-denuded PILS
aerosol at s=0.40 %, where error bars represent the SD
obtained from the bootstrap analysis.
(a) Average mass fraction diurnal profile of the three
characteristic aerosol factors identified in the AMS spectra.
(b) The corresponding diurnal contribution of each aerosol factor to
the κorg, computed by multiplying the mass fraction by the
corresponding hygroscopicity parameter (Fig. 7) and the predicted diurnal
profile of the total κorg in the ambient aerosol.
As a consistency check, the values of hygroscopicity parameter determined in
Fig. 7 are compared against those retrieved from non-denuded ambient
κorg measurements. Given that non-denuded PILS-derived
κorg does not contain biomass burning influence and the
method works best for an aerosol that is chemically uniform with size (e.g.,
Cerully et al., 2011), we analyze ambient data during periods where PILS data
are available and for which the biomass burning factor in the ambient aerosol
was less than 1 % of the total organic aerosol. This constraint filtered
out periods of data when the aerosol was an external mixture, as the aerosol
was unimodal (not shown) and more consistent with the requirement of chemical
uniformity. The filtered data were then processed to infer the
κorg by application of Eq. (5). A subsequent bootstrap
analysis led to values of the hygroscopicity parameter that were very similar
to those shown in Fig. 7: κisoprene-OA=0.26±0.07;κMO-OOA=0.17±0.05;κLO-OOA=0.1±0.03. The broader
SDs of the ambient factor analysis are a consequence of the inherently larger
uncertainty associated with ambient aerosol analysis; nevertheless the
analysis indicates that isoprene-OA is the most hygroscopic factor, followed
by MO-OOA and LO-OOA. Furthermore, the PMF factor spectra remain largely
unaltered by the volatilization (E. Kostenidou, personal communication,
2015); hence the hygroscopicities in the ambient and PILS TD aerosols are not
expected to be affected by the volatilization process.
O : C values of the MO-OOA, LO-OOA, and isoprene-OA factors were determined
to be, respectively, 0.73, 0.47, and 0.41 (Fig. 7) by following the procedure
in Aiken et al. (2008). MO-OOA displays a higher κorg and
O : C compared to LO-OOA, but κorg does not clearly
correlate with O : C for all three factors. It is difficult to determine
why isoprene-OA displays the lowest O : C and the highest value of κorg, though this topic presents itself as an interesting area of
future study. The results of O : C with κorg of each PMF
factor are comparable to that of OS‾c with
each factor κorg (not shown).
From the average diurnal profile of the AMS mass factors (Fig. 8a) and the
hygroscopicity parameter of each, one can then attribute the contribution of
each factor to the overall κorg. This is important, because
it indicates the origin of aerosol hygroscopicity throughout the day. Given
that the biomass burning factor constituted roughly 10 % of the mass of
the aerosol during the period sampled by the PILS, its contribution to total
κorg is small but not negligible. We therefore determine an
optimal estimate of its hygroscopicity, κBBOA, by least
squares fitting of the ambient κorg for the whole sampling
period to predictions using the PMF factors mass fractions and the mean
values of κisoprene-OA,κMO-OOA, and
κLO-OOA shown in Fig. 7; we find that
κBBOA=0.31, which is at the upper limit of hygroscopicity
observed from biomass burning samples (e.g., Asa-Awuku et al., 2008). Based
on this set of hygroscopicity parameters and the mass fractions presented in
Fig. 8a, we then can determine the contribution of each mass factor to
κorg, and the resulting diurnal profile of
κorg all of which is shown in Fig. 8b. From the results it
is clear that 40–60 % of the organic hygroscopicity originates from
LO-OOA and another 20–30 % from isoprene-OA. The relatively low
hygroscopicity of LO-OOA and its opposing diurnal cycle with respect to
MO-OOA work synergistically with LO-OOA and isoprene-OA to yield the relative
invariance of κorg with time of day. Biomass burning has
a minor 10–15 % contribution to κorg; given that the
value of κBB-OOA is highly unlikely
to go above 0.3, this constitutes an upper limit of BBOA contribution to
hygroscopicity.
Conclusions
The volatility, hygroscopicity, and oxidation state of ambient and
water-soluble ambient aerosols collected in a rural site in the southeastern
United States during the SOAS field campaign were investigated. κ
measured at 0.40, 0.30, and 0.20 % supersaturation indicated that ambient
aerosol exhibited size-dependent composition. Average κ at s=0.20 % for thermally denuded and non-denuded ambient aerosol display
a strong similarity with PILS-generated aerosol at all supersaturations;
κ of PILS aerosol at s=0.40 % and ambient aerosol at s=0.20 % were compared and found to be remarkably similar throughout the
study period, suggesting that the PILS aerosols were representative of the
PM1 average composition.
κ of thermally denuded aerosol at 60, 80, and 100 ∘C showed
similar results for both ambient and PILS-generated aerosol with κ
increasing slightly with temperature and showing a much weaker dependence on
volatilization than expected. Even after volatilizing ∼35 % of the
ambient aerosol mass, relative changes in κ for PILS-generated and
ambient aerosols are only approximately 12 %. If this finding is
representative of other locations, this could mean that changes in volatility
may be of minor significance in the case of bulk aerosol hygroscopic
properties.
The hygroscopicity of the organic fraction, κorg, remaining
in the aerosol after volatilization was found to decrease slightly, which is
against conventional thinking that the highest volatility compounds are the
least hygroscopic. This trend has been observed previously in the laboratory
(e.g., Asa-Awuku et al., 2009) and could be attributed to the presence of
oligomers in the aerosol that dissociate into more volatile and more
hygroscopic fragments upon heating. The least volatile aerosol did appear to
be the most oxidized, as expected. This, however, was only indicated by
OS‾c, while no clear correlation was seen
between O : C with volatility or hygroscopicity, consistent with the notion
that OS‾c is a preferred indicator of oxidation (Kroll et al., 2011). Results of AMS
three-factor PMF analysis for the PILS aerosol were used to attribute the
organic hygroscopicity of each PMF factor to the total κorg.
κMO-OOA and κisoprene-OA showed the largest
hygroscopicities of 0.16±0.02 and 0.20±0.02, respectively, and
κLO-OOA showed a 2-fold lower hygroscopicity of 0.08±0.02. These parameters were verified against independent hygroscopicity
retrievals for the ambient aerosol during periods when the contribution of
biomass burning was negligible. No clear relationship between organic
hygroscopicity and O : C was found for all factors, particularly as O : C
in this study varies only slightly (average O : C of non-denuded and
thermally denuded ambient measurements range from only 0.59±0.05 to
0.57±0.04). The hygroscopicity and O : C of MO-OOA, however, were
greater than that of LO-OOA, consistent with expectations. However,
isoprene-OA was found to be the most hygroscopic factor and was also the
least oxidized (and likely the most volatile), which goes against
expectations. Similar results were found when compared to
OS‾c as an indicator of aerosol oxidation.
Some important implications arise from this study. First, for the range of
hygroscopicities measured, although still within the reported ranges
(0.08–0.2) for organic aerosol, the most hygroscopic components are likely
the most and least volatile features of the aerosol. This leads to a relative
invariance in organic aerosol properties, as both local production and
long-range transport of organics can equally contribute to water uptake;
hence the climate forcing associated with organic aerosol. Wet processing of
the aerosol by generating new particles from the ambient soluble material
collected in the PILS does not result in aerosol properties fundamentally
different from those determined in the ambient aerosol, which means that
deliquescence/efflorescence of aerosols in clouds does not alter the
hygroscopic properties of each
organic aerosol factor. Volatilization of the aerosol also does not
appreciably affect its hygroscopicity, implying that semivolatile
partitioning during dilution from boundary layer expansion, transport away
from source region, and entrainment into the free troposphere primarily
affect organic aerosol mass and not its water uptake properties. Considering
the diurnal variation of each PMF factor and its associated hygroscopicity,
isoprene-OA and MO-OOA are the prime contributors to hygroscopicity and
co-vary with LO-OOA in a way that induces the observed diurnal cycle in
organic hygroscopicity. BBOA contributes here a minor role in aerosol
hygroscopicity, which is expected since there was little biomass burning
activity during the sampling period examined.