ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus GmbHGöttingen, Germany10.5194/acp-15-3773-2015Atmospheric oxidation of isoprene and 1,3-butadiene: influence of aerosol
acidity and relative humidity on secondary organic aerosolLewandowskiM.lewandowski.michael@epa.govJaouiM.OffenbergJ. H.KrugJ. D.KleindienstT. E.US Environmental Protection Agency, National Exposure Research
Laboratory, Research Triangle Park, NC 27711, USAAlion Science and Technology, Inc., PO Box 12313, Research Triangle Park, NC 27709,
USAM. Lewandowski (lewandowski.michael@epa.gov)8April2015157377337835September201428November20145March201518March2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/15/3773/2015/acp-15-3773-2015.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/15/3773/2015/acp-15-3773-2015.pdf
The effects of acidic seed aerosols on the formation of secondary organic
aerosol (SOA) have been examined in a number of previous studies, several of
which have observed strong linear correlations between the aerosol acidity
(measured as nmol H+ m-3 air sample volume) and the percent change
in secondary organic carbon (SOC). The measurements have used several
precursor compounds representative of different classes of biogenic
hydrocarbons including isoprene, monoterpenes, and sesquiterpenes. To date,
isoprene has displayed the most pronounced increase in SOC, although few
measurements have been conducted with anthropogenic hydrocarbons. In the
present study, we examine several aspects of the effect of aerosol acidity on
the secondary organic carbon formation from the photooxidation of
1,3-butadiene, and extend the previous analysis of isoprene.
The photooxidation products measured in the absence and presence of acidic
sulfate aerosols were generated either through photochemical oxidation of
SO2 or by nebulizing mixtures of ammonium sulfate and sulfuric acid into
a 14.5 m3 smog chamber system. The results showed that, like isoprene
and β-caryophyllene, 1,3-butadiene SOC yields linearly correlate with
increasing acidic sulfate aerosol. The observed acid sensitivity of
0.11 % SOC increase per nmol m-3 increase in H+ was
approximately a factor of 3 less than that measured for isoprene. The results
also showed that the aerosol yield decreased with increasing humidity for
both isoprene and 1,3-butadiene, although to different degrees. Increasing
the absolute humidity from 2 to 12 g m-3 reduced the 1,3-butadiene
yield by 45 % and the isoprene yield by 85 %.
Introduction
The role of aerosol acidity in increasing the formation of secondary organic
aerosol (SOA) in the atmosphere continues to be a topic of considerable
debate. Field studies at ground level have indicated that increases in
ambient secondary organic carbon (SOC) due to ambient acidity are likely
subtle. Zhang et al. (2007) examined increases of SOA species in the
Pittsburgh area under acidic conditions, and found at most a 25 %
increase in ambient SOA from the Pittsburgh area that could be attributed to
acid catalyzed effects. In another study from the SEARCH network, Tanner et
al. (2009) report low apparent impacts on aerosol acidity at the rural sites
at Yorkville, GA, and Centreville, AL, where biogenic hydrocarbons and
anthropogenic oxidants from nearby urban centers might be expected to produce
relatively high levels of aerosol acidity in the presence of the oxidation
products of biogenic hydrocarbons.
Most laboratory studies that aimed at addressing the impact of aerosol
acidity on SOA concentrations have focused on isoprene. Emissions of isoprene
(C5H8) from vegetation constitute the greatest worldwide source of
nonmethane hydrocarbons (Guenther et al., 1995). SOC formation from isoprene
has been shown to increase in the presence of sulfate acidity in smog chamber
experiments (Edney et al., 2005; Surratt et al., 2007), with a variety of
organosulfate compounds detected in the aerosol phase (Surratt et al., 2008,
2010).
The effect of acidity to produce organosulfates has been studied mainly for
aerosols with strong biogenic inputs. Surratt et al. (2007) initially showed
that sulfate esters were formed in the aerosol products from photooxidations
of isoprene and α-pinene in the presence of acidic seed aerosol.
These products were then compared to those found in ambient aerosol collected
at ground sites in the southeastern US (i.e., the SEARCH network) and found
to be similar to the laboratory aerosol (Jaoui et al., 2008). Additional
studies (Froyd et al., 2010) showed that products of isoprene oxidation could
render a single organosulfate compound (IEPOX-sulfate), which comprised up to
3 % of the organic aerosol mass under some conditions in the free
troposphere.
On a broader basis, laboratory studies have readily shown that acidic sulfate
aerosol produces increased organic aerosol yields from the products of
biogenic and anthropogenic oxidation systems (e.g., Jang et al., 2002). Since
the initial studies, efforts have been undertaken to quantify the magnitude
of the aerosol acidity effect. Surratt et al. (2007) investigated the effect
of sulfate acidity on photooxidation products from the
isoprene/NOx system. They found that secondary organic carbon
increases linearly with aerosol acidity, [H+]air, an acidity
measure that gives its air concentration (nmol m-3) rather than an
aerosol pH. Offenberg et al. (2009) extended this same analysis to examine
the acidity effects on monoterpenes (α-pinene) and sesquiterpenes
(β-caryophyllene). For α-pinene aerosol products, the effect
of acidity was found to be independent of organic carbon mass present and was
a factor of 8 lower than the effect for isoprene. The β-caryophyllene
aerosol products, by contrast, showed an effect similar to that for isoprene
and a factor of 5 higher than that for α-pinene. Analysis by Chan et
al. (2011) confirmed the presence of organosulfate compounds in β-caryophyllene SOA formed under these conditions. Zhang et al. (2012)
performed acidity experiments for 2-methyl-3-buten-2-ol (MBO), a compound
structurally related to isoprene. MBO was shown to be less influenced by
acidity than isoprene or β-caryophyllene, but more affected than
α-pinene. However, this comparison is complicated by the fact that
the MBO experiments were conducted under dry conditions using the photolysis
of hydrogen peroxide to generate OH radicals; in contrast, Surratt et
al. (2007) and Offenberg et al. (2009) relied upon NOx
photochemistry conducted at 30 % relative humidity to generate their
data.
Concentrations of the isoprene SOA tracer products, 2-methylthreitol and
2-methylerythritol, have also been found to rise with increased aerosol
sulfate acidity. These results suggested that particle-phase reactions could
contribute to the increased isoprene aerosol yields and compound
concentrations. Mechanisms for C5 and C10 organosulfate formation
in the atmosphere have been proposed (Surratt et al., 2008). Subsequent
studies by Paulot et al. (2009) gave strong evidence that the atmospheric
formation of isoprene sulfates under conditions of low nitrogen oxides
involved a stable gas-phase C5-hydroperoxide epoxide. Once uptake of the
epoxide into acidified aerosol occurs, inorganic sulfate nucleophiles were
able to convert the epoxide to organosulfates, and hydrolysis led to the
formation of the 2-methyl tetrols, depending on the competitive rates of
different nucleophiles in the aerosol. However, a recent study by Lin et
al. (2013) reports measurements made in Chapel Hill, NC, an area impacted by
anthropogenic oxidant emissions, that show epoxide formation also occurs
through NOx channel reactions. In these reactions,
methylacryloylperoxy nitrate (MPAN), an intermediate stable product from
isoprene oxidation, reacts with OH radicals, leading to methyl acrylic
epoxide (MAE).
While considerable effort has been expended studying acidic effects of
biogenic precursors, far less effort has been made to examine such effects on
hydrocarbons having an anthropogenic origin. An interesting anthropogenic
compound for consideration is 1,3-butadiene (C4H6). The main source
of this compound is from automotive exhaust emissions, although additional
sources from cigarette smoke, evaporative emissions of gasoline, and from
biomass combustion have been reported (Anttinen-Klemetti et al., 2006;
Dollard et al., 2001; Eatough et al., 1990; Hurst, 2007; Pankow et al., 2004;
Penn and Snyder, 1996; Sorsa et al., 1996; Thornton-Manning et al., 1997; Ye
et al., 1998). It has been classified as a hazardous compound in the 1990
Clean Air Act Amendments (US EPA, 1996), a carcinogenic and toxic pollutant,
and a genotoxic chemical in humans and other mammals (Acquavella, 1996; US
EPA, 2002). With respect to aerosol formation, 1,3-butadiene is also of
interest as a structural analog for isoprene. SOA formation from
1,3-butadiene has been examined in a number of recent studies (Angove at al.,
2006; Sato, 2008; Sato et al., 2011; Jaoui et al., 2014), although with only
limited consideration of the effects of aerosol acidity.
The main focus of the present study is to explore some additional aspects of
the role of acidic sulfate aerosol in the formation of SOA from isoprene and
1,3-butadiene. For isoprene, we examine the increase in SOA using acidic
sulfate derived from the photooxidation of SO2 to see if the results are
consistent with those using nebulized acidic sulfate seed aerosol. In
addition, we have measured the extent to which the isoprene analog –
1,3-butadiene – also shows an increase in SOA formation in the presence of
acidic aerosol. The results are then compared to biogenic compounds
previously studied to determine the relative magnitudes of the effect. In
addition, this study attempts to extend the analysis over a broader range of
humidities in an effort to assess the impact of aerosol water content on
acidity-influenced SOA formation. In the previous studies by Surratt et
al. (2007) and Offenberg et al. (2009), all measurements were conducted at a
single humidity level (30 % relative humidity), while Zhang et al. (2012)
examined only dry conditions. Extending these studies to a wider range of
hydrocarbons and across a more realistic range of humidities should provide
data of greater atmospheric relevance and contribute to further development
of acidity-influenced SOA chemistry in air quality models.
Experimental
Secondary organic aerosol was generated in a 14.5 m3 fixed-volume,
Teflon-coated reaction chamber. The chamber used a combination of
UV-fluorescent bulbs that provided radiation from 300 to 400 nm with a
distribution similar to that of solar radiation to the extent that can be
achieved with UV bulbs (Kleindienst et al., 2006). The reaction chamber was
operated as a continuous stirred tank reactor having a residence time of
4 h, to produce a constant, steady-state aerosol distribution that could be
repeatedly sampled at different seed aerosol acidities.
To supply isoprene and 1,3-butadiene, high-concentration gas mixtures were
produced in high-pressure cylinders diluted with nitrogen (N2). Tank
concentrations were approximately 2000 ppm for isoprene and 4500 ppm for
1,3-butadiene. The hydrocarbons, NO, and SO2 (when used) were added
through flow controllers into the inlet manifold, where they were diluted and
mixed prior to introduction into the chamber. Inorganic aerosol was added to
the chamber by nebulizing dilute aqueous solutions of ammonium sulfate and/or
sulfuric acid (TSI, model 9302, Shoreville, MN), with the total sulfate
concentration of the combined solution held constant in order to maintain
stable inorganic concentrations in the chamber. The seed aerosol stream then
passed through a 85Kr neutralizer (TSI, Model 3077, Shoreville, MN) and
equilibrated to the computer-controlled relative humidity designated for a
particular experiment. To change the acidity of the seed aerosol, the ratio
of the ammonium sulfate and sulfuric acid solutions was changed to produce a
constant aerosol sulfate concentration (typically
∼ 30 µg m-3) across the range of acidities used.
Concentrations of isoprene and 1,3-butadiene in the inlet manifold and
chamber were measured using a gas chromatograph with flame ionization
detection (Hewlett-Packard, model 5890 GC). NO and NOy were
measured with a TECO model 42C (Franklin, MA) oxides of nitrogen
chemiluminescent analyzer, SO2 was monitored by pulsed fluorescence
detection (TECO, model 43A), and O3 was measured with a chemiluminescent
ozone monitor (Bendix model 8002, Lewisburg, WV). Temperature and relative
humidity were measured with an Omega digital thermo-hydrometer (model RH411,
Omega Engineering, Inc., Stamford, CT).
Aerosol samples were collected on 47 mm Teflon membrane filters (Pall
Corporation, Ann Arbor, MI) for determination of the particulate sulfate
concentration (in select experiments) and the aerosol hydrogen ion
concentration per unit volume of air sampled, or [H+]air,
expressed as nmol H+ m-3. Aerosol produced in the chamber was
collected at a rate of 10 to 20 L min-1 over a period of approximately
4 h. Filters were extracted by sonication for 30 min using 10 mL of
distilled, deionized water in a 50 mL polypropylene vial. Extracts were
analyzed for sulfate (SO42-) ions using a Dionex DX500 ion
chromatography system equipped with an electrical conductivity detector.
Anion analysis was conducted using a Dionex IonPac AS14A column and an
isocratic 4 mM sodium carbonate / 0.5 mM sodium bicarbonate eluent.
For [H+]air determination, the extracts were allowed to cool
to room temperature and the pH of each extract was measured with a
temperature-compensated Oakton 300 series pH/conductivity meter (OAKTON
Instruments, Vernon Hills, IL). The [H+]air was calculated
by dividing the measured aqueous concentration of hydrogen ions by the volume
of air collected, as described by Surratt et al. (2007). While this method
provides a simple, easily repeatable measure of bulk acidity, it does not
fully capture the actual acidity of individual aerosol particles, which is
more likely to be of physical significance in these chemical systems. It is
also of limited value in experiments where the relative humidity is varied,
as the extraction of the collected aerosol effectively masks the effects of
changing particulate liquid water concentrations. Further limitations of the
[H+]air measurement techniques have been described in detail
in Hennigan et al. (2015). While a number of methods have been developed to
measure aerosol liquid water content directly or estimate it through the use
of thermodynamic models such as ISORROPIA (Fountoukis and Nenes, 2007) or AIM
(Wexler and Clegg, 2002), liquid water measurements were not available for
this study, and insufficient aerosol compositional information was collected
for accurate use of thermodynamic modeling. Nevertheless, in the absence of a
true aerosol pH measurement, the [H+]air approach appears to
provide a useful, if limited, surrogate measure under sufficiently
constrained experimental conditions.
Measurements of particulate organic carbon were performed with an on-line
thermal optical transmittance carbon analyzer using a parallel plate, carbon
strip denuder (Sunset Laboratories, Tigard, OR; Birch and Cary,
1996) prior to aerosol collection on the quartz
filter within the instrument. Other details of operation for the carbon
analyzer on the photochemical reaction chamber are described elsewhere
(Offenberg et al., 2007). The duty cycle for this
measurement was 0.75 h (i.e., 0.5 h sampling and 0.25 h analysis times,
respectively). All particulate carbon concentrations measured during the
interval of aerosol acidity filter collections were averaged for comparison
with the integrated measurements of aerosol acidity.
Four different sets of experiments were performed, each involving multiple
stages: (1) an isoprene/NO experiment in which different concentrations of
SO2 were used to generate varied levels of aerosol acidity, (2) a
1,3-butadiene/NO experiment in which different nebulizer solutions were used
to generate varied levels of aerosol acidity, (3) a pair of isoprene/NO
experiments, one using a low-concentration ammonium sulfate seed and the
other using an acidic inorganic component, in which the inorganic
compositions were held constant while the humidity levels were varied, and
(4) a comparable pair of 1,3-butadiene/NO experiments in which humidity
levels were systematically varied.
In the isoprene/NO experiment (ER370), the initial mixture of isoprene, NO,
and SO2 was irradiated in the chamber until the reaction mixture reached
a steady-state concentration. For each of the three successive stages, the
SO2 concentration was progressively reduced and the reaction mixture was
allowed to equilibrate. In the final stage, SO2 was turned off to
generate a “base case” aerosol from the isoprene/NOx reaction
alone. In all cases, filter measurements were conducted only after the
steady-state condition was achieved.
For the 1,3-butadiene/NO experiment (ER444), an ammonium sulfate solution was
used to generate approximately 35 µg m-3 of inorganic aerosol
to provide a base case. In subsequent stages, the seed aerosol was made
progressively more acidic by reducing the proportion of ammonium sulfate and
adding increasing fractions of sulfuric acid to the solution. This approach
offers two main advantages over the SO2 oxidation method described
above. First, it provides a consistent level of inorganic sulfate aerosol at
all stages; in contrast, the SO2 oxidation produces variable inorganic
concentrations, and effectively no inorganic content in the base case without
SO2 addition. Second, the addition of the seed aerosol should have a
negligible effect on the gas-phase radical chemistry, which may otherwise be
affected by the conversion of SO2 to sulfuric acid.
For the humidity studies, each hydrocarbon was examined with two different
experiments. First, each hydrocarbon/NO system was tested at multiple
humidity levels using only a low concentration (1 µg m-3)
ammonium sulfate seed aerosol (ER666 for 1,3-butadiene, ER667 for isoprene).
This provided a base case for exploring the changes in SOC formation and
aerosol yield in the absence of significant aerosol acidity. Relative
humidities were varied between roughly 10 and 60 %, which corresponded to
absolute humidities of approximately 2 to 14 g m-3. For isoprene, this
base case experiment was then repeated in the presence of a moderately acidic
sulfate aerosol, which was held constant across the full range of humidities
examined (ER662). For 1,3-butadiene, a more acidic inorganic aerosol,
generated using a solution incorporating a higher fraction of sulfuric acid
solution to ammonium sulfate solution, was employed (ER444).
Results and discussion
The experiments presented here support previous studies suggesting that
acidic aerosol can lead to increased SOA formation from the photooxidation of
isoprene under laboratory conditions. Changing the source of the acidity from
nebulized inorganic aerosol to a more atmospherically relevant photochemical
conversion of SO2 into acidic sulfate aerosol produced only a minor
change in the resulting percent increase in SOC per unit increase in
[H+]air. In addition, 1,3-butadiene, a chemically similar
compound released from primarily anthropogenic sources, was also demonstrated
to produce higher concentrations of SOA under acidic conditions. The humidity
experiments further suggest that humidity level, and likely aerosol liquid
water content, can have a substantial effect on SOA formation from isoprene
and 1,3-butadiene. Increasing humidity produces a notable reduction in SOC
formation in both the isoprene and 1,3-butadiene photochemical systems.
However, this reduction is more pronounced in both systems in the presence of
acidic inorganic aerosols, and was most pronounced for the isoprene/NO
system.
Isoprene acidity variation
Data for the isoprene/SO2 acidity experiment are provided in Table 1.
For this experiment, the initial isoprene concentration was 8.4 ppmC, the
initial NO was 0.37 ppm, and the relative humidity averaged 30 % at
25 ∘C (6.5 g m-3 absolute humidity, on average). SO2
ranged from near background to 0.23 ppm. Residual SO2 might have
contributed to the background [H+]air of 54 nmol m-3,
although this value is more likely due to aerosol-phase organic products of
isoprene oxidation, particularly organic acids. However, in terms of the
relative changes in percent OC increase, this background value is of little
consequence. Generating the acidity with SO2 allows the
[H+]air to achieve values in excess of 1500 nmol m-3,
a value much greater than can be reliably maintained using nebulized
solutions. However, unlike nebulized aerosol, the concentrations of inorganic
sulfate in the product aerosol vary at each stage of the experiment, as shown
in Table 1. Sulfate concentrations were measured by ion chromatography at
each stage of this experiment.
With no added SO2 (stage ER370-9), the organic carbon from the isoprene
reaction resulted in 5.3 µgC m-3 of SOC formed (corrected for
chamber losses). Percent increases over this base case value ranged from 62
to 459 % at the highest acidity level (1524 nmol m-3). Figure 1
provides a plot of the percent change in OC against the aerosol acidity.
Error bars for [H+]air are derived from the rated
variability of the pH probe used, converted into nmol m-3 using the
sampling and extraction volumes employed. Error bars on the OC concentrations
are based on the variability in the replicate semi-continuous OC measurements
conducted during each sampling period (typically, n > 20). As
seen in the figure, the relative increase in organic carbon correlated well
with increasing acidity, with an R2 of 0.985. The negative intercept
resulted from the small amount of acidity measured under the condition
without SO2, and the slope indicates a 0.31 % SOC increase per
nmol m-3 of increased [H+]air.
Isoprene SOA as a function of sulfate acidity from the
photooxidation of SO2.
∗ Measurement subject to possible HC interference. No
SO2 was added in stage ER370-9.
A comparison of the sulfate acidity effect for isoprene SOA. For
Surratt et al. (2007) (open circles), the acidity was derived from nebulized
sulfate aerosol. In the present study (closed circles), the acidity was
derived from the photooxidation of SO2.
Despite employing different mechanisms for generating the acidic aerosol, the
agreement in the data between this study and Surratt et al. (2007) is
excellent. The % SOC increase appears to be quite consistent (0.31 for
SO2 photooxidation vs. 0.32 via nebulization), suggesting that both
pathways lead to comparable acid enhancements. The results also suggest that
variations in the inorganic aerosol loading do not strongly impact the
observed % SOC increase, at least under the range of conditions
considered, which is consistent with the results previously reported by
Offenberg et al. (2009) for α-pinene/NO acidity experiments conducted
at different SOC concentrations.
Attempting to expand the SO2 experiment to incorporate additional
humidity conditions revealed a further challenge for the use of SO2 vs.
nebulization of sulfate aerosols in these acidity experiments. Changes to the
chamber humidification also resulted in changes in the amount of SO2
converted to aerosol-phase acidic sulfate, with higher humidity resulting in
lower aerosol sulfate concentrations. Nebulized sulfate aerosols, in
contrast, appear to retain stable aerosol sulfate concentrations and
[H+]air levels under variable humidity conditions. This
limitation could potentially be overcome through the use of a direct measure
of acidity in aerosol particles. However, given the inherent limitations of
the [H+]air measurement, the nebulization approach provides
a cleaner evaluation of the effects of humidity on SOC formation. For this
reason, the remainder of the experiments presented will focus on nebulized
inorganic sulfate for the generation of aerosol acidity.
1,3-butadiene acidity variation
Data for the 1,3-butadiene acidity experiment are provided in Table 2. For
these experiments, the initial 1,3-butadiene and NO concentrations were
6.8 ppmC and 0.34 ppm, respectively. The first acidity condition once the
reaction started was the base case of pure ammonium sulfate, which rendered
an [H+]air of 48 nmol m-3. The next condition used a
nebulizer solution of nominally one-third sulfuric acid and two-thirds
ammonium sulfate to give an [H+]air of 259 nmol m-3;
the third case was a nominal one-third ammonium sulfate and two-thirds
sulfuric acid solution giving an [H+]air of
666 nmol m-3; and the last case used a sulfuric acid solution only for
an [H+]air of 963 nmol m-3. The aerosol sulfate
concentration was measured as approximately 35 µg m-3 for the
ammonium sulfate nebulization prior to the start of photochemistry; previous
measurements have shown that sulfate concentrations remain stable as the
ammonium sulfate/sulfuric acid ratio of the nebulizer solution is varied. The
1,3-butadiene consumed by the reaction ranged from 4.9 to 5.2 ppmC and
averaged 5.03 ppmC.
Organic carbon concentrations increased with increasing acidity at the fixed
relative humidity of 30 % (at an average temperature of 22 ∘C)
from the base case of 22.6 to 44.7 µgC m-3 at the highest
acidity condition. SOC concentrations and percent increases from the base
case (ammonium sulfate) for the four stages are given in Table 2. The %
SOC increases monotonically with sulfate acidity up to nearly a 100 %
increase at the highest acidity condition. The yield determined as
[SOC] /Δ[1,3-butadienecarbon] was calculated for
each condition and found to increase from 0.009 at the lowest acidity
condition to 0.019 at the highest. Since ΔHC remained nearly constant
over the entire experiment, the increase in yield was essentially equivalent
to the increase in SOC, that is, a factor of 2.
Conditions and OC data for 1,3-butadiene photooxidation with the
nebulized inorganic aerosol. For each stage, the initial 1,3-butadiene was
6.8 ppmC, initial NO was 0.34 ppm, and relative humidity was 30 %
(6.1 g m-3 absolute humidity).
Aerosol acidity effect for 1,3-butadiene/NO, relative to
previously published data (Surratt et al., 2007; Offenberg et al., 2009).
All experiments were conducted with nebulized sulfate aerosol at 30 %
relative humidity.
Figure 2 provides a plot of the percent change in organic carbon vs. the
[H+]air for 1,3-butadiene SOA at 30 % relative humidity.
As seen in the figure, the relative increase in organic carbon correlated
well with increasing acidity, with an R2 of 0.967. The negative
intercept resulted from the small amount of acidity measured in the base case
with the ammonium sulfate nebulizer solution. The plot shows an increase of
0.112 % SOC for each nmol m-3 increase in [H+]air.
Figure 2 also compares the results from the 1,3-butadiene system with
similar acidity measurements from this laboratory. Superimposed on the
sulfate acidity effect from 1,3-butadiene SOA products are measurements made
for three biogenic hydrocarbons previously studied: isoprene (Surratt et
al., 2007), α-pinene, and β-caryophyllene (Offenberg et al.,
2009). In those studies, SOA formation from isoprene, β-caryophyllene, and α-pinene was found to correlate with aerosol
acidity as linear relationships with different slopes. From the present
work, the 1,3-butadiene case also follows the same trend with a slope larger
than that of α-pinene and smaller than that of β-caryophyllene. In all five of these studies, a relative humidity of 30 %
was used.
Table 3 further summarizes all the data from these [H+]air
variation experiments. [H+]air and absolute OC
concentrations are given as ranges for the individual studies. For most of
the experiments, Fig. 2 shows the relationship between the percent change in
SOC concentration compared to the “neutral” base case. All data from
experiments with isoprene, α-pinene, and β-caryophyllene are
from prior studies in this laboratory (Surratt et al., 2007; Offenberg et
al., 2009) and use a chamber relative humidity of 30 %. Table 3 also
includes data for the MBO experiment described by Zhang et al. (2012), where
SOA was produced under conditions of low NOx with the aerosol
generated through RO2+ HO2 and RO2+ RO2 reactions.
Unlike the other experiments presented in Table 3, the MBO experiment was
conducted under dry conditions (less than 3 % relative humidity).
Overall, the sulfate acidity effect follows the order (from greatest to least
effect) isoprene, β-caryophyllene, MBO, 1,3-butadiene, and α-pinene. However, the exact placement of MBO in this range is somewhat
questionable given the dramatic differences in experimental conditions used
in that study (low NOx chemistry and dry conditions) compared to
the others. In comparing the relative sensitivity of isoprene and
1,3-butadiene to sulfate acidity, there is about a factor of 3 difference in
the % SOC response to increasing [H+]air despite the
general structural similarity of the compounds. This could represent a
substituent effect that influences the sensitivity of the gas-phase
precursors to reaction by the acidic sulfate nucleophile, but further organic
analysis of the aerosol-phase constituents would be required to examine this
possibility in detail. Although recent studies have compared the reaction
pathways and products formed for 1,3-butadiene oxidation vs. isoprene
oxidation (Jaoui et al., 2014), these studies did not focus on
acid-influenced reactions or organosulfate formation.
Summary of the normalized yields for sulfate acidity effect for
precursor hydrocarbons studied to date. SOA formed in the presence of
NOx at 30 % relative humidity, except where indicated.
SOA precursor[H+]air[OC]NormalizedReference(nmol m-3)(µgC m-3)OC changea1,3-butadiene48–96322.6–44.70.11This workIsopreneb54–15245.3–29.80.31This workIsoprene32–51712.2–31.10.32Surratt et al. (2007)α-pinene (low OC)68–12298.0–11.60.044Offenberg et al. (2009)α-pinene (high OC)153–101440.5–55.30.039Offenberg et al. (2009)β-caryophyllene112–114710.0–34.00.22Offenberg et al. (2009)2-methyl-3-butene-2-ol (MBO)c125–15906.5–21.90.14Zhang et al. (2012)
a % SOC change per [H+]air;
b acidity generated from SO2 photooxidation; and
c experiment conducted in the absence of NOx under
dry conditions.
Reaction conditions for humidity variation experiments.
Table 4 provides the initial conditions for the two isoprene/NO experiments
designed to examine changes in SOC formation and yields resulting from
changes in humidity. In the base case experiment (ER667), the reaction was
conducted in the presence of only a low concentration
(∼ 1 µg m-3) of inorganic aerosol produced through the
nebulization of a 10 mg L-1 ammonium sulfate solution. The relative
humidity was then changed in stages from 9 to 49 % in ∼ 10 %
increments (at an overall average temperature of 28 ∘C). At each
stage, the chamber was allowed to equilibrate before a complete set of
[H+]air, SOC, and ΔHC measurements was made.
Measured [H+]air values averaged 54 nmol m-3 over the
course of the experiment, a level consistent with previous non-acidified
isoprene/NO systems (both Surratt et al., 2007, and ER370 reported above). In
addition, a comparable experiment (ER662) was conducted using a moderately
acidic inorganic aerosol generated via nebulization of a mixed ammonium
sulfate and sulfuric acid solution. In this experiment, duplicate
measurements were made at steady-state relative humidity levels of 8, 28, 44,
and 18 %. The overall average temperature over the course of the
experiment was 27 ∘C. In this experiment, the measured
[H+]air values averaged 275 nmol m-3. Based upon
previous isoprene acidity experiments, this modest level of sulfate acidity
would be expected to produce an increase in SOC of approximately 50–75 %
at a relative humidity of 30 %.
Figure 3 provides a plot of the measured SOC levels as a function of humidity
for these two isoprene systems. Due to temperature differences between these
experiments (and, more importantly, the 1,3-butadiene experiments described
below), measures of chamber relative humidity have been converted into
absolute humidity (g H2O m-3) to provide a common basis for all
four experiments. Error bars on the humidity axis are determined from the
variability in absolute humidities calculated on a 5 min basis throughout
the sampling periods. It is unclear whether relative humidity or absolute
humidity is of greater physical significance in the systems under
consideration. A direct measure of aerosol liquid water content would likely
be a more appropriate metric than either relative or absolute humidity for
this study. However, no method for the analysis of aerosol liquid water
content was available for these experiments.
A comparison of the effects of humidity variation on isoprene/NO SOC
formation. In ER667 (open circles), only a low-concentration ammonium sulfate
seed aerosol was present. In ER662 (closed circles), a moderately acidic
sulfate aerosol was generated via nebulization.
For the base case experiment, the SOC values range from a high of
13.3 µgC m-3 at the lowest humidity level
(2.6 g H2O m-3) to just over 3 µgC m-3 at the
higher humidities (10.4 to 13.1 g m-3). The reason for this reduction
in SOC formation is not entirely clear. Gas-phase NOx and O3
concentrations do not appear to change significantly as a function of
humidity level, as does the concentration of isoprene consumed in the
reactions. This suggests that early generation gas-phase oxidation reactions
are probably not altered significantly by changing humidification. Changes in
the aerosol liquid water content may affect the gas-particle partitioning of
later-generation isoprene oxidation products, or increased water content may
affect particle-phase organosulfate formation or the formation of oligomeric
species (Pye et al., 2013). Further analysis of gas- and particle-phase
organic constituents is required to further investigate this effect.
For the acidified experiment, SOC declined from above
30 µgC m-3 at the lowest humidity level (2.2 g m-3) to
around 4 µgC m-3 under the highest humidity condition
(11.3 g m-3). Although the absolute humidities considered in the two
experiments do not correspond precisely, the percent increase in SOC for the
acidic experiment vs. the base case ranges from approximately 140 % at
the lowest humidity levels, to approximately 65–75 % in the mid-range
(where these experiments best overlap with the previous SOC vs.
[H+]air studies), to virtually no statistical difference
between SOC levels above approximately 11 g H2O m-3. Figure 4
provides SOC yield curves for these two isoprene/NO scenarios. Error bars on
the SOC yields incorporate variability in the replicate measurements of both
the inlet and chamber hydrocarbon concentration as well as in the
semi-continuous OC measurements throughout each sampling period. As in the
experiments described in the previous sections, the humidity changes
performed here had a minimal impact on the measured ΔHC. As a result,
the isoprene/NO yield plots follow essentially the same pattern as that seen
for SOC formation in Fig. 3.
These results suggest that humidity can have a profound effect on the
acid-derived enhancement of SOC formation from isoprene. Although the range
of conditions explored is limited (only a single bulk acidity level; only a
partial range of relative humidities; and only a comparatively narrow
temperature range, by atmospheric standards), the data imply that, under some
circumstances, high humidity (or perhaps a high aerosol water content) can
essentially suppress enhanced SOC formation from isoprene photochemistry.
These results also reinforce the fundamental weakness of the
[H+]air measurement as a surrogate for acidity levels in
actual aerosol particles. Although the bulk acidic potential of the systems,
as measured by [H+]air, does not change significantly over
the range of humidities considered, the resulting changes in the SOC
concentrations suggest that the pH in aerosol particles may be changing
significantly due to variations in aerosol liquid water content, solution
ionic strength, or other factors not effectively captured by the
[H+]air measurement.
SOC yields for isoprene/NO and 1,3-butadiene/NO as a function of
absolute humidity. In ER667 (isoprene, open circles) and ER666
(1,3-butadiene, open diamonds), only a low-concentration ammonium sulfate
seed aerosol was present. In ER662 (isoprene, closed circles) and ER444
(1,3-butadiene, closed diamonds), an acidic sulfate aerosol was present.
1,3-butadiene humidity variation
Conditions for the two 1,3-butadiene/NO experiments for examining changes due
to humidity variations are presented in Table 4. As described above, the base
case experiment (ER666) was conducted in the presence of
∼ 1 µg m-3 of ammonium sulfate aerosol. The relative
humidity was then changed in stages from 10 to 60 % in increments of
roughly 10 % each, at an overall average temperature of 22 ∘C.
This was compared with an additional experiment (ER444) employing an acidic
inorganic aerosol nebulized from solution, with measurements made at
steady-state relative humidity levels of 31, 50, 10, and 62 %, at an
overall average temperature of 25 ∘C. The nebulizer solutions used
in ER444 used higher levels of sulfuric acid relative to ammonium sulfate
that the isoprene experiment described above (ER662). This produced a more
acidic inorganic aerosol, with measured [H+]air values of
718 nmol m-3 on average observed for the 1,3-butadiene acidic aerosol
experiment.
Figure 5 provides a plot of the measured SOC levels as a function of humidity
for these two 1,3-butadiene systems. For the base case experiment, the SOC
values range from a high of 45.1 µgC m-3 at the lowest
humidity level (2.5 g H2O m-3) to 24.7 µgC m-3 at
the higher humidity (13.6 g m-3). For the acidified experiment, SOC
declined from 60.3 µgC m-3 at the lowest humidity level
(1.9 g m-3) to 31.1 µgC m-3 under the highest humidity
(12.3 g m-3). The range in SOC enhancement from the base case to the
acidified case is far lower than that observed in the isoprene system,
ranging from approximately 35 % at low humidity to 25 % at high
humidity. These enhancements are somewhat lower than would be expected for
this level of acidity based on the data presented in Fig. 2. SOC yield
curves, provided in Fig. 4, follow this same trend, as the ΔHC shows
only minimal variation with humidity.
A comparison of the effects of humidity variation on isoprene/NO SOC
formation. In ER666 (open diamonds), only a low-concentration ammonium
sulfate seed aerosol was present. In ER444 (closed diamonds), an acidic
sulfate aerosol was generated via nebulization.
These results are markedly different from those seen for isoprene/NO, both in
terms of the level of SOC enhancement under the acidic condition and the
extent to which the SOC enhancement declines with increasing humidification.
It is not clear which factors are driving this difference in behavior. Part
of the difference likely derives from structural differences between the two
molecules, as was described above with respect to the SOC vs.
[H+]air studies. Additionally, the higher level of
[H+]air used for the 1,3-butadiene experiment may be
partially offsetting the impact of increasing humidity, as more aerosol
liquid water would be needed to reduce actual particle acidity under these
conditions. Other factors, such as the relative hygroscopicity of isoprene
and 1,3-butadiene SOA, may also be contributing. Additionally, the
temperature difference between the two experiments, although relatively small
(approximately 3 ∘C on average) may be sufficient to introduce
differences in the gas-particle partitioning between the two experiments.
Further experimentation is needed to attempt to better understand which
aspects of these aerosol systems are physically significant for activation or
deactivation of these acid-influenced reaction pathways, in order to
determine if these pathways are ultimately important to SOA formation in the
ambient atmosphere.
Summary
These experiments support previous studies suggesting that acidic aerosol can
lead to increased SOA formation from the photooxidation of isoprene under
laboratory conditions. Changing the source of the acidity from nebulized
inorganic aerosol to a more atmospherically relevant photochemical conversion
of SO2 into acidic sulfate aerosol gives nearly identical results as
previous nebulized sulfate aerosol experiments. In addition, 1,3-butadiene, a
chemically similar compound released from primarily anthropogenic sources,
was also demonstrated to produce higher concentrations of SOA under acidic
conditions, albeit to a lesser extent than was seen with isoprene. The
humidity experiments further suggest that aerosol liquid water content can
have a substantial effect on SOA formation from isoprene and 1,3-butadiene.
Increasing humidity produces a notable reduction in SOC formation in both the
isoprene and 1,3-butadiene photochemical systems, which is more pronounced in
both systems in the presence of acidic inorganic aerosols, and was most
pronounced for the acidified isoprene/NO system.
In the isoprene/NO photochemical systems examined in this study, SOC
enhancement due to the presence of acidic inorganic aerosol was observed to
be negligible at absolute humidity levels above approximately
11 g H2O m-3. This lower SOC enhancement at elevated humidities
may explain, in part, the difficulties in detecting increased SOA formation
under acidic conditions in field studies of ambient air masses, particularly
in humid climates like the southeastern US. This work suggests that a more
detailed understanding of the role of humidity and of aerosol liquid water
content is likely required in order to accurately predict the impact of
acidity-influenced oxidation chemistry on overall SOA yields. While the data
presented here may suggest that enhanced SOA formation via acid-influenced
pathways is more constrained than previous studies may have suggested, it
does still appear to represent a viable pathway for additional SOA formation
from a number of precursor hydrocarbons, which may need to be incorporated
into air quality models in order to accurately estimate secondary PM
concentrations in certain locations.
While these experiments are suggestive, they also include a number of
shortcomings that need to be addressed in future work. Perhaps the most
significant is the use of absolute humidity and [H+]air as
surrogate measures of aerosol liquid water content and aerosol pH.
Determination of the effective pH in the aerosol particles through the
application of thermodynamic models, such as ISORROPIA or AIM, should provide
a more realistic assessment of actual acidity than the
[H+]air approach, provided adequate gas and particle
composition data are obtained experimentally. However, even these models
generally account for only the influence of inorganic species, while the
presence of isoprene SOA products has been reported as also contributing
significantly to water uptake on ambient aerosols (Guo et al., 2014). This
suggests that, for laboratory experiments with high organic aerosol
concentrations, particularly from isoprene-related parent hydrocarbons,
direct measurements of aerosol liquid water content may be required.
Additionally, the relative humidity experiments presented here consider
predominantly systematic increases in relative humidity. As some phenomena
related to aerosol liquid water content are known to display hysteresis, such
as deliquescence of sulfate aerosols, further testing is needed to determine
if the overall trends in OC vs. humidity are subject to hysteresis as well.
Experiments incorporating descending relative humidities and larger humidity
steps are warranted, particularly along with the inclusion of liquid water
content measurements. Further examination of the mixing state and phase of
the generated aerosols would also likely be of value, given that the
available interfacial area could affect interactions between H+,
sulfate, and SOA, which may in turn affect organosulfate formation or other
pathways to increased SOA yield.
Finally, further research is needed to examine changes in organic composition
triggered by the effects considered in this work. In particular, the
mechanism through which humidity level affects OC production from isoprene,
even under non-acidified conditions, could be important to the selection of
appropriate SOC yields in air quality models. A further examination of the
similarities and differences between acid-influenced OC formation in the
isoprene and 1,3-butadiene systems would also likely be valuable. A more
rigorous organic analysis of product distributions may help reveal why
isoprene appears to be significantly more sensitive to acidic conditions than
1,3-butadiene at low humidity levels, and whether this difference is due to
structural effects related to the additional methyl group affecting gas-phase
chemistry, particle-phase organosulfate formation, or due to other phenomena,
such as differences in volatility and partitioning of oxidized intermediates.
A more detailed comparison of isoprene and 1,3-butadiene organic chemistry
under acidic conditions may help with the development of more accurate
mechanisms for inclusion in air quality models.
Acknowledgements
The US Environmental Protection Agency through its Office of Research and
Development funded and collaborated in the research described here under
contract EP-D-10-070 to Alion Science and Technology. It has been subject to
agency review and approved for publication. Mention of trade names or
commercial products does not constitute an endorsement or recommendation for
use. Edited by: F. Keutsch
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