ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-18-1171-2018The effects of isoprene and NOx on secondary
organic aerosols formed through reversible and irreversible uptake to
aerosol waterThe effects of isoprene and NOx on aqSOAEl-SayedMarwa M. H.Ortiz-MontalvoDiana L.HenniganChristopher J.hennigan@umbc.eduhttps://orcid.org/0000-0002-2454-2838Department of Chemical, Biochemical and Environmental Engineering,
University of Maryland, Baltimore County, Baltimore, MD, USANational Institute of Standards and Technology (NIST), Gaithersburg,
MD, USAChristopher J. Hennigan (hennigan@umbc.edu)30January20181821171118428July201717August20176December201710December2017This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://acp.copernicus.org/articles/18/1171/2018/acp-18-1171-2018.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/18/1171/2018/acp-18-1171-2018.pdf
Isoprene oxidation produces water-soluble organic gases capable of
partitioning to aerosol liquid water. The formation of secondary organic
aerosols through such aqueous pathways (aqSOA) can take place either
reversibly or irreversibly; however, the split between these fractions in the
atmosphere is highly uncertain. The aim of this study was to characterize the
reversibility of aqSOA formed from isoprene at a location in the eastern
United States under substantial influence from both anthropogenic and
biogenic emissions. The reversible and irreversible uptake of water-soluble
organic gases to aerosol water was characterized in Baltimore, Maryland, USA, using
measurements of particulate water-soluble organic carbon (WSOCp)
in alternating dry and ambient configurations. WSOCp evaporation
with drying was observed systematically throughout the late spring and
summer, indicating reversible aqSOA formation during these times. We show
through time lag analyses that WSOCp concentrations, including
the WSOCp that evaporates with drying, peak 6 to 11 h after
isoprene concentrations, with maxima at a time lag of 9 h. The absolute
reversible aqSOA concentrations, as well as the relative amount of reversible
aqSOA, increased with decreasing NOx/ isoprene ratios, suggesting
that isoprene epoxydiol (IEPOX) or other low-NOx oxidation products may
be responsible for these effects. The observed relationships with NOx
and isoprene suggest that this process occurs widely in the atmosphere, and
is likely more important in other locations characterized by higher isoprene
and/or lower NOx levels. This work underscores the importance of
accounting for both reversible and irreversible uptake of isoprene oxidation
products to aqueous particles.
Introduction
Isoprene (2-methyl-1,3-butadiene, C5H8) is the most abundant
non-methane organic compound emitted globally (Guenther et al., 2012).
Isoprene oxidation stimulates tropospheric ozone production and contributes
substantially to secondary organic aerosol (SOA) formation, thus impacting
air quality and climate (Henze and Seinfeld, 2006; Pfister et al., 2008). In
the southeastern United States, isoprene is likely the dominant SOA precursor
during summer (Ying et al., 2015; Kim et al., 2015). The oxidation products
of isoprene include compounds that partition to aerosol liquid water (ALW),
such as isoprene epoxydiol (IEPOX), glyoxal, and methylglyoxal. These species
do not partition to dry particles (Kroll et al., 2005; Nguyen et al., 2014),
so their condensed-phase products are called aqueous SOA (aqSOA; Ervens et
al., 2011). IEPOX uptake also depends on the inorganic composition and
acidity of the seed particles (Surratt et al., 2010; Gaston et al., 2014;
Budisulistiorini et al., 2017; Lin et al., 2012; Riedel et al., 2015). A body
of work indicates that the uptake of water-soluble organic gases into
atmospheric waters (clouds, fogs, and aerosol water) is an important pathway
for SOA formation (Ervens et al., 2011). Isoprene oxidation products can also
form SOA in the absence of aerosol water (Surratt et al., 2006; Nguyen et
al., 2014), though the majority of regional-scale isoprene SOA is currently
thought to form through aqueous pathways (Marais et al., 2016). Isoprene
emissions show strong seasonal variations in most locations (Guenther et al.,
2012), suggesting that aqSOA formation is similarly seasonal in nature.
Indeed, SOA formed from IEPOX shows a pronounced seasonal signature in the
southeastern USA that is consistent with isoprene emissions (Budisulistiorini
et al., 2016; Xu et al., 2015).
Although substantial evidence from laboratory, modeling, and ambient studies
indicates the importance of aqSOA formation, many uncertainties remain in
understanding this pathway on a mechanistic level (McNeill, 2015). A
significant uncertainty is the fate of aqSOA under conditions of water
evaporation, such as in a cloud cycle or with diurnal changes in ambient
relative humidity (RH). The formation of aqSOA is initiated by the
equilibrium (and thus reversible) partitioning of water-soluble organic
gases to liquid water (McNeill, 2015). In the aqueous phase, the dissolved
organics can undergo reversible reactions such as hydration and
oligomerization (De Haan et al., 2009) or irreversible reactions such as acid
catalysis, reaction with inorganics, or radical reactions (e.g., Ervens et
al., 2014; Ortiz-Montalvo et al., 2014; Lee et al., 2013). The former process
implies that at least some of the dissolved organics will repartition back to
the gas phase when water evaporates, while the latter process can form
low-volatility products that remain in the particle phase even after the
evaporation of water. Most clouds are non-precipitating (Pruppacher, 1986)
and ALW changes throughout the day with changing RH (Nguyen et al., 2014;
Khlystov et al., 2005). Thus, determining whether the uptake is reversible or
irreversible is critical in understanding the fate of many oxidized organics
in the atmosphere. While ambient studies provide evidence for both reversible
and irreversible aqSOA formation (El-Sayed et al., 2016, 2015), the reasons
underlying these differences are still unclear.
It is important to note that we define aqSOA as all organics present in the
condensed phase through partitioning to liquid water, regardless of whether
the uptake is reversible or irreversible. Although some definitions of aqSOA
only include the organic material that is taken up into liquid water and
remains in the particle phase after water evaporation (e.g., Ervens et al.,
2011), we favor a more comprehensive definition since the organics contribute
to aerosol effects on health and optical properties when they are in the
condensed phase. Our definition is consistent with the treatment of other
semi-volatile aerosol species such as ammonium nitrate. It is, however,
important to distinguish reversible and irreversible aqSOA since the
atmospheric lifetime of these compounds may differ significantly depending on
their phase (Nguyen et al., 2015). Therefore, we define the low-volatility
products that remain in the particle phase after the evaporation of liquid
water as “irreversible aqSOA”, and the organic compounds taken up in liquid
water that repartition back to the gas phase with water evaporation as
“reversible aqSOA”.
Nitrogen oxides (NOx≡ NO + NO2) may be one factor
affecting the reversibility of isoprene aqSOA. NOx plays a critical role
in the oxidation of volatile organic compounds (VOCs). This includes a major
effect on the chemical pathway of isoprene oxidation, and on the resulting
SOA yield (Kroll and Seinfeld, 2008; Ervens et al., 2008). NOx affects
the volatility, oxidation state, and aging of isoprene-derived SOA (Xu et
al., 2014). Recent modeling studies predict that isoprene oxidation in the
eastern USA is split almost equally between high- and low-NOx pathways
(Travis et al., 2016). Laboratory studies show significant evaporation of
aqueous isoprene SOA particles when dried, indicating reversible aqSOA (Wong
et al., 2015). This is consistent with the understanding of aqSOA formed from
individual isoprene oxidation products thought to be predominantly IEPOX and
glyoxal (Sareen et al., 2017). During the summer, model predictions suggest
that glyoxal production from isoprene occurs almost equally through low- and
high-NOx pathways in the eastern USA (Chan Miller et al., 2017). Glyoxal
is taken up to ALW reversibly and irreversibly (Ortiz-Montalvo et al., 2012;
Galloway et al., 2009). IEPOX is formed predominantly through the
low-NOx pathway (Paulot et al., 2009; Surratt et al., 2010), and its
uptake to ALW could be reversible or irreversible (Nguyen et al., 2014;
Riedel et al., 2015). Therefore, potential differences in reversible aqSOA
associated with NOx may be due to differences in IEPOX production under
these chemical regimes. The aim of this study was to characterize the effects
of isoprene and NOx on aqSOA formed reversibly and irreversibly at a
site in the eastern USA heavily impacted by biogenic and anthropogenic
emissions.
MethodsWSOC measurements
Ambient measurements were carried out across all four seasons in Baltimore,
Maryland (Table 1). The experimental setup has been described in detail elsewhere
(El-Sayed et al., 2016, 2015). Briefly, water-soluble organic carbon was
measured in the gas phase (WSOCg) using a mist chamber (MC), and in
the particle phase (WSOCp) using a particle into liquid sampler
(PILS; Brechtel Manufacturing), both coupled to a total organic carbon (TOC)
analyzer (model 900 Turbo, GE Analytical) operated in Turbo mode. The
WSOCp measurement was alternated between an ambient channel
(WSOCp) and a “dried” channel (WSOCp,dry) using an
automated three-way valve (Brechtel Manufacturing). The WSOCp sample
was at ambient RH while the WSOCp,dry sample passed through a
silica gel diffusion dryer (Table S1 in the Supplement). Both the
WSOCp and the WSOCp,dry samples pass through a
parallel-plate carbon denuder (Sunset Laboratories) prior to sampling in the
PILS. This reduces gas-phase interferences, which are minor in the PILS
(Sullivan et al., 2004), and prevents the re-condensation of volatilized
organic gases that evaporate in the dryer. Although some gas-phase organics
may be lost to the silica gel (Faust et al., 2017), potentially perturbing
the gas–particle equilibrium for the dry channel, sampling both channels
through the carbon denuder should minimize such differences. Further, based
upon the timescales of ambient organic aerosol (OA) equilibration
(minutes to hours; Saha et al., 2017), it is highly unlikely that stripping
gas-phase compounds would produce any appreciable OA evaporation with only
the 7 s residence time encountered in our system. The diffusion dryer does
not implement heating, so differences in the WSOCp concentrations
between the two channels are due to WSOCp evaporation that
results from ALW evaporation. Note that the WSOCp,dry channel has
not been designed to dry particles completely to efflorescence (El-Sayed et
al., 2016). WSOCp losses through the three-way valve and through the
dried channel are less than 1 % (mass concentration basis; El-Sayed et
al., 2016): no corrections to the data were applied. A ratio of
OM / OC = 2.1 was used to convert aerosol organic carbon (OC) into
organic mass (OM), based upon characterizations of WSOCp in the
eastern USA (Xu et al., 2017a).
Seasonal sampling periods in Baltimore, Maryland.
SeasonSampling periodFall3–30 September 2014Winter4 February–23 March 2015Early spring23 April–8 May 2015Late spring9 May–14 May 2015Summer6 July–14 August 2015
WSOCp is operationally defined based upon the solubilities of the
organics themselves and upon the level of dilution employed for the analysis
(Psichoudaki and Pandis, 2013). In the eastern USA, the WSOCp
measurement is often used as a surrogate for SOA, especially during summer
(Weber et al., 2007). The measurement includes SOA formed through absorptive
partitioning and through aqueous-mediated pathways (aqSOA). We consider any
WSOCp that evaporates with drying to be reversible aqSOA, since
this material exists in the condensed phase because of the aerosol water and
partitions back to the gas phase when the water evaporates.
The WSOCp,dry measurement system employs a total drying time of
≈ 7 s. The residence time for equilibrium to take place in
evaporating water/organic droplets is dependent on the specific
organics as well as the aerosol inorganic chemical composition. Longer drying
times may increase the amount of evaporated aqSOA in our system, indicating
that our measurements provide a conservative (low) bound estimate on the
concentration of reversible aqSOA and on the
WSOCp,dry/ WSOCp ratio (El-Sayed et al., 2016).
The fully automated online system was housed in a temperature-controlled
environmental enclosure (EKTO, Inc.) placed on the rooftop of the Engineering
Building at the University of Maryland, Baltimore County (UMBC). The three
samples: WSOCg, WSOCp, and WSOCp,dry were
repeatedly measured in a 14 min cycle with sampling times of 4, 5, and
5 min, respectively. Dynamic blanks were measured regularly throughout each
ambient sampling period. Factory calibrations of the TOC analyzer were
regularly checked with sucrose solutions prepared to bracket the range of
concentrations observed during ambient sampling.
VOC and NOx measurements
Isoprene measurements from the Essex Photochemical Assessment Monitoring
Stations (PAMS; AQS ID# 240053001) were provided by Maryland Department
of the Environment (MDE). The Essex site represents the PAMS station closest
to UMBC (≈ 20 km distance). Isoprene was measured by MDE every 6
days from September to May, and hourly during the summer (June, July, and
August). The hourly isoprene measurements were automated following EPA method
142, using cryogenic preconcentration for sample collection followed by
analysis via gas chromatography with flame ionization detection (GC-FID,
Perkin Elmer Clarus 500). Hourly measurements of NOx were also carried
out by MDE at the Essex site following method 74 (chemiluminescence). Data
were acquired from the US Environmental Protection Agency
(https://aqs.epa.gov/api).
A key assumption employed in this analysis is that the WSOC measurements made
at UMBC are representative of conditions at Essex, the location of the
NOx and isoprene measurements. Aerosol concentrations in the
Baltimore–Washington, D.C. region are spatially uniform over tens of kilometers
(Beyersdorf et al., 2016). Further, WSOCp concentrations in the
eastern USA exhibit small spatial variations across urban-to-rural gradients
during the summertime (Weber et al., 2007). These prior analyses showed that
aerosol concentrations, and in particular WSOC, were not dependent on wind
direction. Isoprene emissions in the eastern USA are regional in nature, due
to the expansive coverage of broadleaf forests (Pye et al., 2013; Guenther et
al., 2012). NOx emissions are spatially segregated from those of
isoprene, and are far more localized. However, the isoprene–NOx chemical
regime (high- or low-NOx) in the eastern USA is generally
well represented with model resolution of 28 × 28 km, suggesting
that the chemistry occurring on small scales, such as in individual power
plant plumes, does not significantly affect the regional isoprene–NOx
regime (Yu et al., 2016). NOx concentrations at Essex (20 km ENE of
UMBC) and HU-Beltsville (35 km SSW of UMBC) are strongly correlated (R=0.89, Fig. S1 in the Supplement), likely due to the overwhelming
contribution of mobile source emissions along the heavily traveled I-95
highway corridor to the region (Anderson et al., 2014). Together, this supports our
analysis into the effects of isoprene and NOx on reversible aqSOA using
the measurements described above.
Results
An overview of the seasonal sampling periods is given in Table 1.
Measurements were taken from 3 to 4 weeks on average during each of the four
seasons. Note that the spring season has been divided into early (23 April to
8 May) and late (9 to 14 May) periods due to the differences in the
WSOCp results observed during these times. The WSOCp
measurements have been reported to be a good surrogate of the total SOA in
the atmosphere (Weber et al., 2007; Kondo et al., 2007), which includes aqSOA
as well as SOA formed through traditional gas-phase partitioning (Donahue et
al., 2009). The formation of aqSOA was observed throughout the year,
except for the early spring season. This observation was based on the
relationship between the fraction of total WSOC in the particle phase,
Fp (Fp= WSOCp/ (WSOCp+ WSOCg)) as a
function of RH in combination with seasonal ALW analyses (Hennigan et al.,
2008). The individual results for the fall and summer have been previously
reported (El-Sayed et al., 2016, 2015). A synthesis of aqSOA formation across
all seasons is the subject of ongoing analysis.
Reversibility of aqSOA formation by season
Previous studies conducted by our group have provided evidence for both
irreversible (El-Sayed et al., 2015) and reversible (El-Sayed et al., 2016)
aqSOA formation during the fall and summer seasons, respectively. Figure 1
shows the WSOCp,dry/ WSOCp ratio across all of
the seasons. A ratio of unity indicates that drying did not impact
WSOCp while a ratio less than unity indicates that particle
drying caused the evaporation of some WSOCp, and thus was
considered reversible aqSOA (El-Sayed et al., 2016). Figure 1 shows that the
WSOCp,dry/ WSOCp ratio was unity during the fall
and winter, indicating that the WSOCp remained in the condensed
phase upon drying. Therefore, the aqSOA formation that was observed occurred
irreversibly (El-Sayed et al., 2015). In the early spring, the
WSOCp,dry/ WSOCp ratio was also unity, but this
was expected since no Fp-RH enhancement was observed and no
significant aqSOA was observed during this period. Beginning in the late
spring and continuing into the summer, the
WSOCp,dry/ WSOCp ratio was systematically lower
than unity. During both seasons, we observed systematic evaporation of some
WSOCp as a result of the ALW evaporation. In the late spring, the
WSOCp,dry/ WSOCp ratio was 0.92, on average, and
decreased further during the summer when it reached an average of 0.87
(El-Sayed et al., 2016). This observation indicates that at least some of the
aqSOA formation occurring in the late spring and summer seasons was
reversible. WSOCp evaporation was higher during the night than
during the day (Fig. S2), likely due to higher RH levels and higher ALW at
night (Guo et al., 2015).
Box plot of the overall seasonal
WSOCp,dry/ WSOCp ratios. For each bin, mean (red
marker), median (horizontal black line), 25th and 75th percentiles (lower and
upper box values), and 5th and 95th percentiles (vertical lines) are
shown. The dotted green line at unity is shown for visual reference. Numbers
at the top represent the number of paired
WSOCp,dry/ WSOCp measurements within each season.
Annual climatology of isoprene concentrations in Essex, Maryland (2011 to
2015). Symbols represent average concentrations (in ppbC or
nmol C mol-1) while error bars represent ±1σ.
We attribute the observed WSOCp evaporation during the late
spring and summer seasons to aqSOA that partitions reversibly to ALW. The
physical properties that affect SOA formed through absorptive partitioning
(what Ervens et al., 2011, call gasSOA) and SOA formed through an aqueous
mediated pathway (aqSOA) are fundamentally different (vapor pressure and gas
solubility in water, respectively). Note that ALW can affect SOA formed
through traditional absorptive partitioning by increasing the total
concentration and decreasing the average molecular weight of the absorbing OM
phase (Seinfeld and Pankow, 2003). Models predict that this phenomenon
enhances SOA concentrations in the eastern USA (Pankow et al., 2015; Jathar et
al., 2016) and that drying the particles will result in the evaporation of
some semi-volatile SOA compounds in response to this perturbation (Pankow,
2010). However, the effect of ALW on gas–particle partitioning is more
pronounced at low organic concentrations (1 to 2 µg m-3), and
its sensitivity becomes less profound at higher OA levels (Pankow, 2010).
Previous results from our group showed the opposite effect: evaporated
WSOCp concentrations increased significantly with an increase in
OA concentrations (El-Sayed et al., 2016). Further, the semi-volatile organic
compounds most influenced by this water effect are predicted to be the less-oxidized, fresh SOA (Pankow, 2010). WSOCp is more strongly
correlated with the LV-OOA (low-volatility oxygenated organic aerosol) factor
identified by the Aerodyne aerosol mass spectrometer (AMS) compared to the
SV-OOA (semi-volatile OOA) factor (Sun et al., 2011; Xu et al., 2017b; Kondo
et al., 2007). This suggests that the evaporation of WSOCp was
not due to the overall effects on OA partitioning (Jathar et al., 2016), but
was due to the reversible partitioning of water-soluble organic gases to
aerosol water. In the following sections, we characterize the reasons
underlying the seasonal differences in
WSOCp,dry/ WSOCp shown in Fig. 1.
Climatology of isoprene
Isoprene oxidation products are thought to be the most important precursors
to aqSOA formation (Marais et al., 2016). Figure 2 shows the average annual
climatology of isoprene in Baltimore, Maryland. These measurements were made at the
MDE Essex site, a location ≈ 20 km from UMBC where the WSOC
measurements were conducted. In the eastern USA, isoprene emissions are
regional (Palmer et al., 2003); therefore, data from the Essex site will show
consistent trends with those at UMBC. Isoprene concentrations in Baltimore
tend to be very low in the winter and early spring seasons, with average
monthly values of ≈ 0.2 ppbC (nmol mol-1), but they start
to rise sharply at the beginning of May, and remain elevated (though
variable) during the summer season. This is highly consistent with previously
measured seasonal isoprene emissions in other parts of the eastern USA
(Goldstein et al., 1998). Isoprene concentrations decrease dramatically in
September (average decrease of 70 % from 1 to 30 September), and then
remain low through the winter.
Effect of isoprene on reversible aqSOA
During the late spring, the onset of reversible aqSOA formation corresponds
to the dramatic increase in isoprene concentrations (Fig. 2). Observations of
the AMS IEPOX factor (Budisulistiorini et al., 2016) and chemical markers for
isoprene SOA (Kleindienst et al., 2007) show similarly sharp transitions in
the spring and fall in the southeastern USA. The highest reversible aqSOA
levels were observed during the summer when isoprene emissions were at their
maximum. Other VOCs, such as monoterpenes, also contribute to SOA in the
eastern USA (Xu et al., 2015), but monoterpene and isoprene SOA tracers show
distinctly different temporal patterns in the eastern USA. Isoprene SOA peaks
during the summer, but monoterpene SOA tracers exhibit similar (or lower)
concentrations in the summer compared to other seasons (Kleindienst et al.,
2007; Ding et al., 2008). Further, monoterpene SOA is typically associated
with semi-volatile and less-oxidized OA factors in the AMS analysis (Xu et
al., 2015; Jimenez et al., 2009) but WSOCp is poorly correlated
with these factors (Timonen et al., 2013; Xu et al., 2016). On the basis of
these prior studies and the results in Figs. 1 and 2, we attribute the
reversible aqSOA in Baltimore to isoprene.
Due to the magnitude of regional isoprene emissions and its predicted
contribution to SOA, we would expect relationships between isoprene and both
WSOCg and WSOCp concentrations. However, simple
correlations between isoprene and WSOC are not expected due to dramatic
differences in their atmospheric lifetimes. Under typical summertime
conditions, the oxidation of isoprene to form WSOCg will take a few
hours (Hodzic et al., 2014). These oxidation products can undergo further
reactions to form lower-volatility compounds that partition to the aerosol
phase contributing to WSOCp, a process that is expected to take
several hours (Ng et al., 2006; Atkinson and Arey, 2003).
The relationship between isoprene and WSOC (both WSOCp and
WSOCg) was characterized for the summer, when hourly isoprene data
were available. To account for the differences in the expected time frame for
transformation of isoprene into WSOCg and WSOCp, we
analyzed the WSOC concentrations as a function of isoprene with a variable
time lag. We investigated the relationship between the isoprene
concentrations at time t and the WSOC concentrations at t+n, where n
is the time lag, which was systematically varied from 0 to 13 h. For
example, a 1 h time lag indicates that the isoprene concentrations at t
are compared to the WSOC concentrations measured t+1 h after those of
isoprene. An offset of zero indicates that the timing of the WSOC
measurements is aligned with the timing of the isoprene measurements. During
each hour, there were four to five WSOCp and WSOCg
measurements corresponding to one isoprene sample; therefore, hourly averages
of WSOC were calculated to provide a consistent basis for analysis.
First, the isoprene–WSOCg relationship was analyzed for time lags
in the range of 0 to 6 h (Fig. 3). The WSOCg data were binned
based on the corresponding isoprene concentrations; each marker represents
the median of the WSOCg concentration within each isoprene
concentration bin. At 0 to 2 h time lags, no relationship was observed
between isoprene and WSOCg. This was anticipated because isoprene
has a typical atmospheric lifetime of 1 to 2 h against oxidation by OH
(Atkinson and Arey, 2003). However, with a time lag of 3 h, an increase in
isoprene concentrations was coincident with an increase in WSOCg
concentrations. This effect was observed for time lags up to 5 h, as
illustrated by the solid blue lines in Fig. 3. Across the entire summer, a
5 ppbC (nmol mol-1) increase in isoprene concentrations was
associated with a median increase of 2.0 µg C m-3 in
WSOCg. When the time lag between isoprene and WSOCg was
more than 5 h, there was no longer a relationship between isoprene and
WSOCg concentrations. This observation highlights the effect of
isoprene on the formation of water-soluble organic gases. Isoprene and
WSOCg showed similar diurnal profiles during the summer, especially
when the time lag was considered (Fig. S3). Overall, this suggests that fresh
isoprene emissions take about 3 to 5 h to form WSOCg in an urban
environment during typical summertime conditions. Note that the measurement
of WSOCg only includes compounds with effective Henry's law
constants above ≈ 103 M atm-1 (≈ 101 mol m-3 Pa-1) (Spaulding et al., 2002), so the MC
does not efficiently sample many first-generation isoprene oxidation
products, such as methacrolein (KH=4×100 M atm-1, or
4×10-2 mol m-3 Pa-1) or methyl vinyl ketone (KH=4×101 M atm-1, or 4×10-1 mol m-3 Pa-1; Sander, 2015).
Median WSOCg concentrations as a function of isoprene
concentrations (in ppbC or nmol C mol-1) at different WSOCg
time lags during the summer. The following isoprene concentrations bins were
defined as follows: < 1 ppbC, 1 to 2 ppbC, 2 to 3 ppbC, 3 to 4 ppbC, 4 to
5 ppbC, and > 5 ppbC. Scatter and box plots showing individual data are
presented in Fig. S6.
The relationship between isoprene and evaporated WSOCp (i.e.,
reversible aqSOA) was characterized using the same time lag analysis,
extended from n= 0 to 13 h. At time lags less than 5 h, there was
no relationship between isoprene and evaporated WSOCp
concentrations (red dotted lines in Fig. 4). However, the amount of
evaporated WSOCp increased with increasing isoprene
concentrations when the evaporated WSOCp time lag was in the range
of 6 to 11 h (green solid lines in Fig. 4). The highest response of
evaporated WSOCp to isoprene was found for a time lag of 9 h. At
this time lag, an increase of 5 ppbC (nmol C mol-1) in isoprene
concentrations led to a median increase of 0.7 µg m-3 in
evaporated WSOCp. Beyond the 11 h time lag, no relationship was
observed between isoprene and evaporated WSOCp levels (blue dotted
lines in Fig. 4). The average evaporated WSOCp concentrations
showed a similar increase with increasing isoprene, but were even higher than
the median levels (Fig. S4). For example, at a 9 h time lag, a
5 ppbC (nmol C mol-1) increase in isoprene corresponded to an
average increase in evaporated WSOCp of
1.6 µg m-3. The 6 to 11 h time lag between isoprene and
the evaporated WSOCp is consistent with the predicted kinetics of
isoprene epoxydiol-derived secondary organic aerosol (IEPOX-SOA) formation in the eastern USA (Budisulistiorini et al., 2017). This
observed 6 to 11 h time lag between isoprene and the evaporated
WSOCp is likely due to multi-generational oxidation (Carlton et
al., 2009; Hodzic et al., 2014; Paulot et al., 2009). Alternately, it could
be that the isoprene oxidation products that partition reversibly to liquid
water were formed relatively quickly (< 6 h), but responded to the
diurnal cycle in ALW, which peaks in the eastern USA in the early morning
hours (Guo et al., 2015). The observed delay time could also be the
combination of these factors. Consistent with Figs. 3 and 4, there was also a
strong relationship between the WSOCg concentration and the
time-offset evaporated WSOCp concentration (Fig. S5). The above
observations suggest that isoprene is strongly linked with the formation of
reversible aqSOA in the eastern USA. Based on this relationship, we next
consider the effect of NOx on reversible aqSOA formation since NOx
is critical to isoprene oxidation chemistry (Kroll et al., 2006).
Median evaporated WSOCp concentrations as a function of
isoprene concentrations (ppbC or nmol C mol-1) at different evaporated
WSOCp time lags during the summer. Scatter and box plots showing
individual data are presented in Fig. S6.
Note that we assume that the WSOCp measurement is a surrogate for
SOA (Weber et al., 2007). However, WSOCp is weakly correlated
with lightly oxygenated components in OA, such as the SV-OOA factor often
resolved by the AMS (Timonen et al., 2013). Thus, our analysis would likely
be a poor method for some SOA systems, for example α-pinene
ozonolysis (Jimenez et al., 2009). As discussed above, the WSOCg
measurement does not efficiently sample compounds with low Henry's law
constants, including some first-generation isoprene oxidation products
(Hodzic et al., 2014). These measurement limitations contribute to the 6 to
11 h and 3 to 5 h time lags for the isoprene associations with
evaporated WSOCp and WSOCg, respectively. For many
compounds, multi-generation oxidation contributes significantly to SOA
formation (Ng et al., 2006), and this is almost certainly the case for
atmospheric SOA (Jimenez et al., 2009). However, shorter lag times may be
observed with other instruments sensitive to early generation oxidation
products.
Effect of NOx on reversible aqSOA
Figure 5 shows the relationship between evaporated WSOCp and the
NOx/ isoprene ratio during the summer. For this analysis, hourly
NOx/ isoprene ratios and the hourly evaporated WSOCp
concentrations with a 9 h time lag were used, since this timing corresponded
to the maximum evaporated WSOCp. Figure 5 shows that the amount of evaporated
WSOCp decreased substantially with an increase in the
NOx/ isoprene ratio. At low NOx/ isoprene ratios (less
than 0.5 ppb ppbC-1, or
0.5 mol (mol C)-1), the amount of evaporated WSOCp was at
its maximum (average of 1.4 µg m-3); however at
NOx/ isoprene ratios more than
15 ppb ppbC-1 (mol (mol C)-1), the evaporated WSOCp
was as low as 0.2 µg m-3. Generally, the evaporated
WSOCp decreased with the increase in NOx/ isoprene
ratios, but flattened out beyond NOx/ isoprene ratios of ≈ 5 ppb ppbC-1 (mol (mol C)-1).
Scatter and box plots of evaporated WSOCp (9 h time
lag) as a function of NOx/ isoprene ratio (ppb ppbC-1 or
mol (mol C)-1 in the summer. Bins were chosen to include at least 50
data points. For each bin, mean (blue marker), median (horizontal black line),
25th and 75th percentiles (lower and upper box values), and 5th and
95th percentiles (vertical lines) are shown.
Similarly, the effect of NOx/ isoprene ratios on WSOCp
concentrations during the summer is shown in Fig. 6. As in Fig. 5, the hourly
NOx/ isoprene ratios were compared against the hourly
WSOCp concentrations at a time lag of 9 h. At
NOx/ isoprene ratios of less than
0.5 ppb ppbC-1 (mol (mol C)-1), the average WSOCp
concentration was ≈ 5 µg m-3, but it decreased
substantially to ≈ 1.5 µg m-3 (almost summertime
WSOCp background levels) at NOx/ isoprene ratios above
15 ppb ppbC-1 (mol (mol C)-1).
Scatter plot of WSOCp as a function of
NOx/ isoprene ratio (ppb ppbC-1 or mol (mol C)-1) in the
summer. Symbols and bins are consistent with those defined in Fig. 5.
If isoprene is indeed associated with the evaporated WSOCp that
we observed during the late spring and summer, then a logical question is why
we did not observe this phenomenon during measurements throughout September
(Fig. 1, El-Sayed et al., 2015). Although isoprene emissions decrease
dramatically during September, there are still periods with elevated
concentrations. Here, we analyze the effects of NOx and isoprene on the
reversibility of isoprene aqSOA by considering the average daily
NOx/ isoprene ratios during the late spring, summer, and fall. For
this analysis, daily averages were used due to the lack of hourly isoprene
measurements during the late spring and fall. The relationship between the
WSOCp,dry/ WSOCp and NOx/ isoprene
ratios across all three seasons is shown in Fig. 7. Figures 5 and 6 show that
the relationships of the NOx/ isoprene ratio with WSOCp
and evaporated WSOCp are qualitatively similar. However, it is
clear from Fig. 7 that WSOCp and the evaporated WSOCp
are affected differently by NOx/ isoprene. The days in which
average NOx/ isoprene ratios were higher than
5 ppb ppbC-1 (mol (mol C)-1) were characterized by
WSOCp,dry/ WSOCp ratios very close to unity,
indicating irreversible aqSOA. On the other hand, the days in which
NOx/ isoprene ratios were lower than
5 ppb ppbC-1 (mol (mol C)-1) were all characterized by
WSOCp,dry/ WSOCp ratios lower than unity,
indicating some reversible aqSOA on these days. Further, the
WSOCp,dry/ WSOCp ratio decreased with decreasing
NOx/ isoprene ratios under these conditions. IEPOX is produced under
low-NOx conditions with very limited formation in NOx rich
environments (Zhang et al., 2017), whereas glyoxal can be produced from both
low-and high-NOx pathways with higher yields at high-NOx conditions
(Chan Miller et al., 2017). Based on our observations, this suggests that
IEPOX was more abundant during the late spring and summer and was responsible
for the reversible aqSOA formed under the lower NOx/ isoprene
conditions. These results provide an explanation for the variability in the
seasonal occurrence of reversible aqSOA in the eastern USA.
Daily average WSOCp,dry/ WSOCp ratios
as a function of daily average NOx/ isoprene ratios
(ppb ppbC-1 or mol (mol C)-1). Gray dotted line is representative
of the transition zone from reversible to irreversible aqSOA conditions.
Error bars represent ±1σ, and are shown for one-third of the
summertime data for clarity.
There is uncertainty in the absolute NOx/ isoprene ratio that
represents the transition to reversible aqSOA. Although NOx
concentrations at Essex are strongly correlated with those at a site 50 km
away (HU-Beltsville), the absolute NOx concentrations are approximately
2 times higher at Essex (Fig. S1), due to its closer proximity to downtown
Baltimore. Therefore, although Fig. 7 suggests that reversible aqSOA
formation occurs at NOx/ isoprene ratios below
5 ppb ppbC-1 (mol (mol C)-1), transitions at lower ratios may be
observed in other areas.
Atmospheric implications
These results represent the first observations to characterize the seasonal
occurrence of reversible aqSOA formation. The results suggest an important
effect on aerosol measurements that implement drying, which may not measure
(or may incompletely measure) reversible aqSOA. Our results suggest that this
is especially relevant in areas with high isoprene emissions. For example,
Zhang et al. (2012) observed substantial loss of WSOCp (≈ 30 % on average) from federal reference method (FRM) filters in the
southeastern USA. It is likely that reversible aqSOA contributed to this
measurement artifact, although direct comparisons to our
WSOCp,dry measurement would be needed to test this hypothesis.
These compounds are important, since they contribute to aerosol effects –
visibility, aerosol optical depth (AOD), health, climate – when they are in
the condensed phase.
We hypothesize that the evaporation of WSOCp observed with drying
during the late spring and summer is due to the reversible partitioning of
IEPOX to aerosol water, or to other low-NOx isoprene oxidation products
such as multifunctional hydroperoxides (Y. Liu et al., 2016; Krechmer et al.,
2015; Riva et al., 2016; J. Liu et al., 2016). This is supported by strong
associations between the evaporated WSOCp and isoprene
concentrations using the time lag analysis. It is further supported by the
decreasing WSOCp,dry/ WSOCp ratios with decreasing
NOx/ isoprene ratios. Note that Sareen et al. (2017) predict very
low dissolved IEPOX in the eastern USA during summer
(< 0.01 µg m-3), suggesting reversibly formed reaction
products are the dominant contributors to reversible aqSOA.
Laboratory studies have found reversible and irreversible uptake of IEPOX to
aqueous particles (Nguyen et al., 2014; Riedel et al., 2015). However,
ambient studies generally suggest that IEPOX-SOA has very low volatility
(Lopez-Hilfiker et al., 2016; Hu et al., 2016). This could be due to
challenges measuring the reversible aqSOA by the methods used to derive
volatilities. For example, it is unclear how the instruments employed by
Lopez-Hilfiker et al. (2016) and Hu et al. (2016) respond to reversible IEPOX
reaction products present in the aqueous phase. It could also be that the
evaporated WSOCp we observe is contributed by other low-NOx
isoprene oxidation products. Approximately 30 % of isoprene-derived SOA generated
under NOx-free conditions partitioned reversibly to aerosol water, but
the molecular identities of the reversible aqSOA were not determined (Wong et
al., 2015). Although the experiments of Wong et al. (2015) were performed in
a chemical regime where IEPOX formation is favored, it did not contribute to
the SOA in their experiments due to high OH levels. Given the absence of
IEPOX-SOA in the experiments of Wong et al. (2015), the atmospheric relevance
of their results needs further review. The uptake of other, non-IEPOX,
low-NOx oxidation products may explain such observations (J. Liu et al.,
2016; Y. Liu et al., 2016; Riva et al., 2016; Krechmer et al., 2015).
Overall, identifying the molecular composition of the reversible aqSOA that
is associated with low-NOx isoprene oxidation will require targeted
field measurements.
The effect of water evaporation on WSOCp also has important
implications for the representation of SOA formation in models. The results
in Fig. 1 show that ≈ 10 to 15 % of the total WSOCp
evaporates with drying during the late spring and summer, on average. This
suggests that the fraction of aqSOA that is formed reversibly is much higher
than 15 %, since the measurement of WSOCp includes aqSOA and
compounds formed through traditional SOA partitioning (e.g., Donahue et al.,
2009). Further, the fraction of WSOCp that evaporates with drying
is variable, with values of up to 60 % for individual measurements
(El-Sayed et al., 2016). Models that include aqSOA and aerosol multi-phase
chemistry can improve predictions of OA (e.g., Carlton et al., 2008; Marais
et al., 2016). A complication of model evaluations is that comparisons of
modeled OA concentrations to ambient measurements may be problematic if the
measurements themselves are subject to the bias discussed above. For this
reason, accounting for both reversible and irreversible uptake of
water-soluble organic gases to liquid water is critical (McNeill, 2015). Our
observations, supported by laboratory studies (Faust et al., 2017), suggest
that treatment of aqSOA as an irreversible uptake process is not consistent
with actual phenomena occurring in the atmosphere, especially in the eastern
USA. Although likely due to a different mechanism, Y. Liu et al. (2016) and
Riva et al. (2017) also showed that isoprene oxidation forms semi-volatile
compounds that re-partition back to the gas phase after forming SOA.
The lifetime of organic compounds in the atmosphere is strongly dependent on
their phase (Pye et al., 2017). Oxygenated organic compounds in the gas-phase
often have much shorter lifetimes than particle-phase organics due to
significantly higher dry deposition velocities (Nguyen et al., 2015) and
photolysis rates (Fu et al., 2008). The reversible uptake of
WSOCg to aerosol water may effectively shield these species from
such loss processes, resulting in enhanced transport. Thus, accounting for
the reversible partitioning of water-soluble organic gases to aerosol water
would likely improve model predictions of these compounds.
NOx plays a critical role in the oxidation of VOCs, including effects on
the composition and quantity of SOA produced. Herein, we show that NOx
strongly affects the amount and nature of SOA produced in an urban area that
is under substantial influence from biogenic emissions. Higher concentrations
of WSOCp were associated with decreasing NOx/ isoprene
ratios. The fraction of WSOCp that evaporated with drying was
also inversely related to NOx/ isoprene. In the future, isoprene
concentrations are predicted to increase in response to changes in
temperature and land use associated with climate change (Heald et al., 2008;
Sanderson et al., 2003). The eastern USA is currently undergoing a transition
from high- to low-NOx chemical regimes (Travis et al., 2016; Edwards et
al., 2017), and NOx levels are likely to continue decreasing (He et al.,
2013). This suggests future NOx/ isoprene ratios will generally
decrease across the eastern USA, as well, resulting in increased production of
reversible aqSOA. The current results are from the greater Baltimore
metropolitan area; although we observe a range of NOx concentrations and
NOx/ isoprene ratios, these measurements are representative of an
urban environment. Thus, we may expect
WSOCp,dry/ WSOCp ratios to be even lower in more
rural environments impacted by isoprene emissions.
In addition to NOx, sulfate also strongly affects SOA formation from
isoprene through its separate contributions to ALW, particle acidity, and
aqueous chemistry (Xu et al., 2015; Nguyen et al., 2014; Surratt et al.,
2010). Laboratory studies have not yet elucidated the role of each factor in
the reversibility of isoprene SOA, and we do not have sufficient sulfate data
to characterize such effects with our analysis. However, it is worth noting
that particle acidity is not likely a factor in the relative split between
reversible and irreversible aqSOA formed from isoprene. Studies predict that
particles in the eastern USA are highly acidic throughout the year (Weber et
al., 2016; Battaglia et al., 2017; Guo et al., 2016, 2015), and acidity is
not a limiting factor in isoprene SOA formation during the summer
(Budisulistiorini et al., 2016; Xu et al., 2015). The implication from our
observations is that reversible aqSOA from isoprene forms even in the
presence of such persistently acidic particles. This further questions the
treatment of isoprene SOA as an irreversible uptake process in models.
Conclusions
The eastern USA is undergoing a transition from a high- to low-NOx
chemical regime, which has broad implications for nighttime chemistry, ozone
production, and SOA formation (Travis et al., 2016; Marais et al., 2016;
Edwards et al., 2017). Using a time lag analysis, we show that
NOx/ isoprene strongly affects concentrations of SOA in the eastern
USA, including SOA formed through the reversible uptake of water-soluble
organic gases to aqueous particles. Lower NOx leads to a higher fraction
of aqueous SOA formed reversibly. Our measurements from an urban area suggest
that this process is even more important in other, more rural environments.
Predictions of future NOx and isoprene emissions in response to
regulations, technology, and climate change also suggest that this process
may increase in importance going forward. Such an inference is complicated by
concurrent reductions in SO2 emissions in the USA and other developed
nations. The consequent decreases in sulfate may offset the effects of
NOx reductions on isoprene SOA (de Sá et al., 2017). However, we
stress that prior studies into the NOx–sulfate–isoprene system have not
systematically determined how these species affect the reversibility of
isoprene SOA. Therefore, while we hypothesize that future decreases in
NOx and increases in isoprene will increase reversible isoprene SOA (or
at least the reversible fraction), the role of changing sulfate will also
need to be considered. Future laboratory and modeling studies will be needed
to address this question directly.
We hypothesize that IEPOX uptake to aqueous particles is responsible for the
reversible aqSOA, but other low-NOx isoprene oxidation products are
possible as well (Wong et al., 2015). We quantify reversible aqSOA through
observations of WSOCp evaporation that results from drying.
Ultimately, molecular composition measurements made concurrently with our
WSOC system are required to identify the chemical species responsible for
this phenomenon. The evaporation of WSOCp with drying occurred
systematically during the late spring and summer, and was linked to isoprene
and NOx. This has importance for a wide range of aerosol measurements
that implement drying. It also has importance for modeling multi-phase SOA
formation, as simplified treatment of irreversible uptake does not represent
actual atmospheric processes.
The NOx and isoprene data are available from the EPA (https://aqs.epa.gov/api). The WSOC data are available upon request.
The supplement related to this article is available online at: https://doi.org/10.5194/acp-18-1171-2018-supplement.
The authors declare that they have no conflict of
interest.
Acknowledgements
Certain commercial equipment, instruments or materials are identified in this
paper to foster understanding. Such identification does not imply
recommendation or endorsement by the National Institute of Standards and
Technology.
This work was
supported by the National Science Foundation through award
CHE-1454763. Edited by: Jason
Surratt Reviewed by: three anonymous referees
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