Introduction
Sulfate aerosols are known to impact ecosystems and climate through their
deposition and radiative effects. The deposition of sulfate aerosols can
cause acidification of soils and lakes . Furthermore,
their direct and indirect radiative effects can change the radiative budget
at regional scales and alter climate .
Sulfate aerosols can be primary or secondary. Primary particles are emitted
directly from the surface to the atmosphere but secondary particles are
formed in the atmosphere through gas to particle conversion. The majority of
anthropogenic and natural sulfur is emitted as sulfur dioxide (SO2)
or oxidized to SO2 in the atmosphere
. and
estimated that around 50 % of the globally emitted
SO2 is oxidized to form sulfate and the remainder is lost by dry and
wet deposition.
Dry deposition is important and gives SO2 a lifetime of about 3 days
for a boundary layer with 1000 m depth . Wet
deposition is important intermittently for rainy days or days with fog. The
lifetime of SO2 in the atmosphere can vary greatly from hours to days
depending on measurement location, season, time of day, etc. As an example,
measured SO2 lifetime in the eastern US and found
values of 19 ± 7 h. GEOS-Chem simulations suggest a value of 13 h
during summer for the same location.
A detailed understanding of SO2 oxidation pathways and their relative
importance is critical for accurate representation of sulfate's spatial
distribution as well as its impact on climate through aerosol radiative
forcing.
The oil sands regions are of great interest because of the large quantities
of SO2 emissions .
Therefore, a comprehensive knowledge of SO2 oxidation pathways
important in this region is useful to identify where and how atmospheric
sulfur species are transported and contribute to aerosol formation, growth,
and acid deposition.
Oil sands extraction and upgrading processes can be a
source of sulfate aerosols, SO2, and oxidants. The major sources of
SO2 emissions in the Athabasca oil sands region are upgrading and
energy production operations .
observed SO2 enhancements over the oil sands region with a maximum
value of 39 parts per billion by volume (10-9 ppb) relative to a
background value of 102 parts per trillion by volume (10-12 ppt).
showed that both SO2 and sulfate contributions
from the Athabasca oil sands region are significant compared to estimates
for potential background sources of sulfur such as annual forest fire emissions in Canada.
by the use of positive matrix factorization (PMF)
modeling suggested that secondary sulfate is the second most important
contributor to PM2.5 mass in Fort MacKay (31 %).
Sulfur dioxide is converted to sulfate in homogeneous and
heterogeneous reactions. The oxidation pathway is a very important factor to
determine the effects of the sulfate formed on the environment. Gas phase
oxidation of SO2 by hydroxyl radicals (OH) produces sulfuric acid
(H2SO4) gas, which can nucleate in the atmosphere to form new
particles . These newly formed aerosol
particles are buoyant and can be dispersed far from the emission source.
Newly formed sulfate aerosols also impact direct radiative forcing by
scattering sunlight back to space. These particles can grow by the addition
of organics to create a large number of accumulation mode aerosols, which are
more easily deposited on local surfaces, increasing the potential for
acidification at regional to local scales. They also have the ability to form
cloud condensation nuclei (CCN; ).
After forming CCN they can increase the albedo and lifetime of clouds
. Homogeneous oxidation of SO2 in the
gas phase by OH is as follows :
SO2+OH+M→HOSO2+M,HOSO2+O2→HO2+SO3,SO3+H2O+M→H2SO4+M.
A range of 17 to 36 % of global sulfate production can be
attributed to this pathway .
Heterogeneous oxidation of SO2 primarily occurs in cloud droplets,
although oxidation on the surface of aerosols can be important regionally
. Heterogeneous oxidation prevents H2SO4 gas
production and new particle formation. Sulfate formed by this pathway can
modify the aerosol size distribution, which affects both direct and indirect
aerosol forcing. Scattering efficiency of the particle population can be
increased, which is responsible for direct scattering
. In addition, acidity of aerosols as well as
their CCN activity of the particle population can be modified and affect the
indirect radiative forcing .
showed various steps in SO2 dissolution before
oxidation by major oxidants, these are H2O2, O3, and
O2 catalyzed by transition metal ions (TMIs) such as Fe3+
or Mn2+ in a radical chain reaction pathway .
SO2(g)⇌SO2(aq)SO2(aq)+H2O⇌HSO3-+H+HSO3-+H+⇌H2SO3HSO3-⇌SO32-+H+2HSO3-⇌H2O+S2O52-
After the dissolution, S(IV) is oxidized to S(VI) by O3,
H2O2, and O2 in the presence of TMIs.
The oxidation of SO2 by O3 and O2 catalyzed by TMIs
is pH dependent and becomes faster as pH increases, whereas oxidation by
H2O2 within normal atmospheric pH ranges (2–7) does not depend on
pH .
Field studies suggested that TMI-catalyzed oxidation is the dominant sulfate
formation pathway in polluted environments in winter
. Oxygen isotope measurements of sulfate
aerosols collected at Alert, Canada (82.5∘ N, 62.3∘ W)
showed that TMI-catalyzed SO2 oxidation is significant during winter
. Recent studies have shown that the TMI-catalyzed
oxidation pathway is underestimated (more than an order of magnitude) in all
current atmospheric chemistry models . For
example, measured the sulfur isotopic composition of
SO2 upwind and downwind of clouds and used the difference to
calculate the fractionation that occurred for in-cloud SO2 oxidation.
They showed that SO2 oxidation catalyzed by natural TMIs on mineral
dust is the dominant in-cloud oxidation pathway and is underestimated by more
than an order of magnitude in current atmospheric models. To the best of our
knowledge there is no study to investigate the importance of the
TMI-catalyzed pathway in SO2 oxidation on the surface of aerosols in
highly polluted areas such as the Alberta oil sands region during summer.
Until recently, OH-radical-initiated oxidation of SO2 was considered
the only gas phase oxidation pathway important in the atmosphere. However,
recent measurements of the rate constants for oxidation of SO2 by
Criegee biradicals and model simulations of field observations have shown
this pathway is more significant than previously thought
. Criegee biradicals are
formed through ozonolysis of unsaturated hydrocarbons such as biogenic
terpenes . The rate constants of the reaction of
Criegee biradicals and SO2 are somewhat uncertain but researchers
agree that the reaction is faster than what has been previously thought
(e.g., 6 ×10-13 and
8 ×10-13 cm3 molecule -1 s-1 for Criegee
biradicals originating from the ozonolysis of α-pinene and limonene,
respectively, ). Several studies have linked biogenic
volatile organic compound (BVOC) environments to an increase in SO2
to sulfuric acid and/or sulfate conversion rates. For example,
reported the oxidation of SO2 by Criegee
biradicals faster than what has been thought before, during a field study in
a boreal forest and confirmed the results by laboratory and theoretical
studies.
In this study, we investigated the importance of the various SO2
oxidation pathways, including Criegee biradicals in a polluted region with
high volatile organic compound (VOC) emissions using measurements of sulfates, SO2
concentrations, and isotopic composition.
Sulfur isotope analysis is a powerful tool to investigate SO2
oxidation pathways in the atmosphere. As an example, used
high-sensitivity measurements of cosmogenic 35S in SO2 and
sulfate from the ambient boundary layer over coastal California and the Tibetan
Plateau to identify oxidation of SO2 to sulfate. The lifetime in
summer ranged from 1 to 2 days suggesting that there might be oxidation
pathways which are more important than previously thought.
In this study, stable sulfur isotope values for SO2 and size-segregated sulfate aerosols
were measured. δ34S values of potential
sources in the region and isotope fractionation data
were used to investigate atmospheric sulfur oxidation
pathways in the Athabasca oil sands region. The sulfur dioxide to
sulfate conversion ratio F(s)=[SO4][SO4]+[SO2] was also used as
a tool to investigate the possible SO2 oxidants in the region.
Although the data represent a short period of time and do not reflect the
variability on a seasonal timescale, showed that short-term
measurements are more suitable for source identification. They mentioned that
the source signals of NO2 and SO2 emissions are available in
hourly to daily timescales and long-term observation may cause a loss in
short term variation.
Sulfur isotopes
Stable sulfur isotopes can be used to investigate sulfur sources, transport,
and chemistry such as the relative importance of oxidation pathways
. Sulfur has four stable isotopes: 32S,
33S, 34S, and 36S with relative abundances of ∼ 95,
0.75, 4.21 and 0.015 %, respectively. The isotopic composition is
described using the delta notation:
δxS(‰)=nxSn32SSamplenxSn32SV-CDT-1×1000,
where n is the number of atoms, xS is the heavy isotope and V-CDT is the
international sulfur isotope standard, Vienna Canyon Diablo Troilite, with
the isotopic ratio of R34=34S32S=0.044163,
R33=33S32S=0.007877 and
R36=36S32S=1.05×10-4. For the purpose of this
paper we only analyze δ34S values and use δ33S values to
find enrichment of samples.
The isotopic composition (δ34S) of major sources of atmospheric
sulfur in the Athabasca oil sands region were quantified by
. They reported sulfur isotope values for bitumen,
(+4.3 ± 0.3 ‰), untreated oil sands,
(+6.4 ± 0.5 ‰), and the isotopic composition of products such
as (NH4)2SO4, which is produced in flue-gas desulfurization,
(+7.2 ‰), coke (+4.0 ± 0.2 ‰), and elemental
sulfur (+5.3 ± 0.5 ‰). Primary sulfate with diameter D< 2.5 µm are reported to have δ34S values between
+7.0 ‰ and +7.8 ‰ with an average of
+7.3 ± 0.3 ‰, and between +6.1 ‰ and
+11.5 ‰ with an average of +9.4 ± 2 ‰ for two of
the largest stacks in the region. These two stacks are 12.2 and 19.4 km
south and southeast of the measuring site, respectively.
In addition to sulfur emissions from oil sands processing, aerosols can
potentially be produced from vehicle exhaust. Combustion emissions from
vehicles showed a δ34S of +5 ‰ for SO2 from
engine exhaust in Alberta and British Columbia
. On average, diesel and gasoline contained
very low amounts of sulfur (0.008 %, ) and combustion
produces both primary sulfate as well as SO2. Other sulfur emissions
in the region may result from anoxic conditions in the environment or the
tailing ponds associated with sulfate-reducing bacteria. Biogenic emissions
of hydrogen sulfide (H2S) have negative δ34S values which
can be as negative as -30 ‰ . H2S is
oxidized to SO2 with a lifetime of 1 day and
the sulfur isotopic composition is not expected to change during oxidation of
H2S to SO2 .
Differing isotopic contributions from sulfur sources can drive variations in
aerosol sulfate δ34S values. Another reason for δ34S
variation can be isotopic fractionation. The oxidation of SO2 causes
isotope fractionation between the products and reactants as long as the
reaction is not complete. When the reactant is available as an infinite
reservoir, the fractionation factor is calculated as
α34=RProductsRReactants,
where R=34S32S. Following the definition for α used
by for both kinetic and equilibrium reactions, α<1
means that the light isotopes react faster, so products are isotopically
lighter than the reactant.
During this study, minute quantities of 34SO2 were emitted from a
chemical ionization mass spectrometer (CIMS) exhaust 50 m away from the
high-volume sampler near the ground for special periods. Here we refer to
these particular periods as CIMS-ON. The enrichment of 34SO2 was
sufficiently large that isotopic fractionation can be neglected during
CIMS-ON periods. However, sulfur sources and oxidation pathways can be
examined using δ34S values for the periods when CIMS was not
operational (CIMS-OFF). During SO2 oxidation to sulfate, isotope
fractionation occurs between reactants and products which is unique for each
oxidation pathway. Note that sulfur isotope fractionation resulting from
oxidation by Criegee biradicals is not currently known.
reported temperature dependent fractionation factors for different
SO2 oxidation pathways as follows: SO2
oxidation by OH radicals favors heavy isotopes and the fractionation
decreases slightly with temperature (Eq. 3).
(α-1)(‰)=(10.60±0.73)-(0.004±0.015)×T(∘C)
Aqueous phase oxidation can occur by H2O2 and O3, and
fractionation during this pathway (Eq. 4) also prefers heavy isotopes and
decreases with temperature slightly.
(α-1)(‰)=(16.51±0.15)-(0.085±0.004)×T(∘C).
The fractionation during the TMI-catalyzed oxidation pathway acts in the opposite
direction to the other two pathways. TMI-catalysis is the only known
oxidation pathway which favors lighter isotopes in the product sulfate and
the fractionation strongly depends on temperature (Eq. 5).
(α-1)(‰)=(-5.039±0.044)-(0.237±0.004)×T(∘C).
Methods
Field measurements
Temperature, relative humidity, and wind speed and direction time series are
shown in Fig. A1 in the Appendix. A diurnal cycle in relative humidity (RH) is evident for all days during the
campaign except 25 August which was a rainy period.
A high-volume sampler placed at ground level with a flow rate of
0.99 ± 0.05 m3 min-1 was used to collect aerosols and
SO2. The high-volume sampler was fitted with a five-stage cascade
impactor to collect size-segregated aerosols on glass fiber filters in five
ranges of aerodynamic diameter as A (>7.2 µm),
B (3.0–7.2 µm), C (1.5–3.0 µm),
D (0.95–1.5 µm), and E (0.49–0.95 µm). The final filter
for fraction F<0.49µm was a 20.3 cm ×25.4 cm glass
filter to collect aerosols with D<0.49 µm. An SO2
filter pretreated with potassium carbonate (K2CO3) and glycerol
solution was located beneath these six size-segregated aerosol filters
. The sampling interval was 12 h (daytime 05:00 to
17:00 MDT (Mountain Daylight Time)
and nighttime 17:00 to 05:00 the next
day) for the first 12 days except 20 and 27 August after which samples were
collected for 24 h (05:00 to 05:00). Field blanks were collected on three
separate occasions at the start, in the middle, and at the end of the
campaign. Filter blanks from the field were loaded and then unloaded, stored,
and analyzed using the same protocols as samples. The high-volume sampler was
turned off during field blank sampling. Filters were stored in ziplock bags
and kept at temperatures less than 4∘C and transferred to the lab for analysis.
WBEA SO2 data were used with a sampling interval of 5 min. Ozone
and NO2 mixing ratios were measured by UV absorption using a Thermo
49i O3 monitor every 10 s and a blue diode laser cavity ring-down
spectrometer every 1 s, respectively (data were averaged to 1 min; ).
The slope uncertainties in these
measurements were ±1 and ±10 %, respectively. Radiometer
measurements using a pair of spectral radiometers (one facing the zenith, the
other the nadir direction) were used to determine actinic flux and to
calculate photolysis frequencies (j values; ). Iron (Fe)
and Manganese (Mn) were measured by semi-continuous X-ray fluorescence
measurements of metals taken every hour on a filter tape with a measurement
uncertainty of ±10 % .
Monoterpenes were measured hourly by gas chromatography ion-trap mass
spectrometry (GC-IT-MS; ). VOCs and C2-C12
were sampled in canisters over a period spanning 09:30
to 08:30 of the next day, and analyzed using gas chromatography mass
spectrometery (GCMS). Detection limits for VOC measurements can be found in
the online JOSM database
(ftp://arqpftp:research@ftp.tor.ec.gc.ca/OS/AMS13, last
access: 10 October 2017).
A chemical ionization mass spectrometer (CIMS) similar to the one described
by was used to measure OH reactivity at a distance of
10 m horizontally from the high-volume sampler. Enriched 34SO2
was emitted from an exhaust pipe at ground level less than 50 m to the east in an
unused area containing shrubs. Enriched 34SO2 affected a portion
of our samples during CIMS-ON periods; these periods were used to trace the
fate of local 34SO2 emitted from the CIMS exhaust near the ground.
Analysis of high-volume filter samples
Filter papers were shredded and sonicated for 30 min in distilled deionized
water in the laboratory (200 mL for SO2 filters and filters to
collect particles in the size range F<0.49µm and 75 mL for
slotted filters to collect particles in sizes larger than 0.49 µm).
For SO2 filters, 1 mL of 30 % w/w hydrogen peroxide
(from BDH) was added to oxidize the SO2 to sulfate before sonication.
Filter paper fibers were removed by 0.45 mm Millipore filtration, and 10 mL
of the filtrate samples was analyzed using a Dionex ICS-1000 ion
chromatography (IC) system with a Dionex IonPac AS14 column and electric
conductivity detector to determine the concentration of sulfate with an
uncertainty of 5 %. Prior to treatment, the pH of the remaining filtrate
was measured and found to be ∼ 6.0. The remaining filtrate was treated
with 0.5 mL of 10 % BaCl2 (dihydrate 99 %, from EMD), and dilute
(0.5 normal) OmniTrace HCl (34–37 %, from EMD) was added to samples until a pH
of 3 was achieved. Approximately 100 µL of 0.5 normal
HCl was used for
aerosol filters. The samples were then heated to facilitate precipitation of
BaSO4. Barium sulfate was isolated by Millipore filtration, and dried
samples were packed into tin cups and analyzed with a PRISM II continuous-flow
isotope ratio mass spectrometer (CF-IRMS) to obtain δ34S
values (relative to V-CDT; ). The precision in measuring
δ34S is ±0.3 ‰ which is determined as the standard
deviation (1σ) of δ34S for several standard runs.
δ34S measurements were blank corrected using the sulfur
concentration and δ34S values for field blanks. Insufficient
sulfate was present for some samples after concentration blank correction.
Although the concentration of sulfate was too small to perform blank
correction for some samples, they displayed the same range for δ34S
values as those which were blank corrected. This suggests little to no bias
was introduced by blank correction. Therefore, δ34S values are
reported from some samples which were not isotopically blank corrected. These
samples are indicated with a * in Tables 1 and 2.
The PRISM II continuous flow isotope ratio mass spectrometer measures
δ34S and δ33S simultaneously and the values for
non-enriched samples were expected to be related according to the mass
dependent fractionation (MDF) relation (δ33S∼0.51δ34S).
For this experiment, some of the samples were enriched in 34S and they
were identified by the use of the MDF relation between δ34S and
δ33S of the standards for the same run. δ33S/δ34S
was averaged for standards for each run and
δ33Sδ34S-2σ was used as a cutoff
criterion and data falling below this criterion were tagged as enriched.
Care was taken to analyze sufficient standards and blanks between enriched
samples (CIMS-ON periods) to ensure carryover was minimal. Little to no
deviation in standards and blanks was apparent after enriched δ34S
values from CIMS-ON periods were analyzed. In this paper uncertainties are
reported as 1σ standard deviation.
Natural tracer experiment
Sulfur 34S release
The CIMS was operated between 12 August 12:00 to 14 August 12:00 and
20 August 12:00 to 7 September 09:45 MDT. Ten standard cubic
centimeters of 0.9 % 34SO2 was diluted in 30 SLPM N2
to obtain a mixing ratio of 3 ppm for 34SO2 in the sample flow.
34SO2 reacts with OH to form H234SO4 which is ionized
by NO3- to form H234SO4- and SO42- ions
that are detected at m/z= 99 and m/z= 49 in the negative
ion spectrum of the mass spectrometer. An excess amount of 34SO2
compared to the required 34SO2 to complete titration of OH in the
sample flow was used for ambient air OH reactivity measurements. Almost all
of the flow entering was exhausted by the instrument which contained excess
34SO2 and formed H234SO4. In 1 min, n34SO2=(7.4×106)nH234SO4. Some of the
formed H234SO4 is also lost by wall loss in the instrument so the
majority of the exhaust is in the form of 34SO2. For the periods
when the CIMS was operational (CIMS-ON), significant 34S isotope
enrichment was observed; therefore, samples were divided into two sets,
CIMS-ON and CIMS-OFF.
The first set is for samples collected during the shutdown periods of the
CIMS (CIMS-OFF). These CIMS-OFF periods were used to investigate the isotopic
composition of size-segregated sulfate aerosols and SO2 in the region
and the possible sources and formation pathways of sulfate aerosols. The
second set (CIMS-ON) is for samples affected by enriched 34S and is not
used as an indicator of sulfur isotopic composition of sulfate aerosols in the
region. Instead, the enriched 34SO2 is used as a natural tracer to
follow the fate of SO2 emitted from a local ground-based source and
its oxidation.
Sulfur conversion ratio
In this paper we use the sulfur conversion ratio, which is defined as the
portion of SO2 which is converted to particulate sulfate:
F(s)=[SO4][SO4]+[SO2].
In this formula, [SO4] is the concentration of sulfate aerosols
with D< 0.49 µm. In this study the sulfate is dominantly
secondary , corroborated here by the absence of soil
indicators (Sect. 5.1). F(s) can be affected by dry deposition. Since
little is known about the appropriate dry deposition velocities in this
region, potential variations between SO2 and sulfate dry deposition
rates are neglected in the analysis.
Since F(s) is a measure of SO2 to sulfate conversion, it is a
measure of oxidant loading. Therefore, significant positive correlation
between F(s) and other compounds may be an indicator of the importance of
that compound as a tracer for SO2 oxidation. This formula can be used
for both CIMS-ON and CIMS-OFF periods since the number of enriched molecules
reaching the high-volume sampler is very small and cannot change F(s). The
number of enriched molecules reaching the high volume sampler is calculated
using equations described in Sect. 4.3.3 and the fraction of enriched
molecules in comparison to the total sulfur concentration is reported in
Table A1.
Concentration of 34S enriched molecules
The concentration of enriched molecules as 34SO2 and
34SO4 were calculated using the following equations during CIMS-ON
periods. Isotope ratio (R) values show the ratio of sulfur isotopes to the
most abundant isotope, which is 32S for sulfur.
R34=n34S/n32S,R33=n33S/n32S,R36=n36S/n32S,Renriched34=(n34S+n34S*)/n32S,n32S+n33S+n34S+n36S+n34S*=Stotal,
in which n34S* is the number of 34S atoms reaching the filter from
the CIMS exhaust and Stotal is the total number of sulfur atoms
on the filter. The R34 value is calculated as the average of R34 values
for samples without enrichment. There were R33 data available from the
IRMS but the uncertainty was high (±3 ‰) and we used the value
for the international standard for sulfur V-CDT. 36S is included in
calculations since the amount of 34S* from the CIMS exhaust is on the
same order of magnitude. Renriched34 values were available for
each sample. The concentration of sulfate for each sample was available from
IC and the number of sulfur atoms as SO2 or sulfate can be
calculated. Then the number of 34S from CIMS was calculated and divided
by the volume of total sampled air and the number of 34SO2* and
34SO4* molecules cm-3 was calculated (Table A1).
F(s) (CIMS-ON and CIMS-OFF) versus relative
humidity (RH). (a) correlation during daytime (AM) and nighttime (PM)
and (b) correlation for daytime, nighttime, and daily data.
P value < 0.05.
Results
Sulfur conversion ratio (F(s))
The sulfur conversion ratio (F(s), Eq. 6) was calculated for the smallest
size fraction of measured sulfate (F<0.49µm). Absence of Ca
and Mg in this size fraction (all concentrations were below the IC detection
limit of 0.1 mg L-1) indicates that primary soil particles were not
present in this size fraction. suggested that less than
10 % of total sulfur emissions from two major stacks in the region in
PM2.5 were primary sulfate. Therefore, primary sulfate from stacks
do not form a significant portion of sulfate aerosols in the D< 0.49 µm size range. Based on these two pieces of information, it
is expected that sulfate particles on this size fraction are mostly
(> 90 %) secondary. As a result, F(s) gives valuable information
about which pathways dominate SO2 oxidation and formation of sulfate
aerosols.
F(s) is not affected by enriched sulfate emissions during CIMS-ON periods
(because the amount of 34SO2 emitted was relatively small, Table A1).
Hence, F(s) reflects the conversion of SO2 to sulfate for
the entire measuring period. This implies negligible changes to F(s) values
because of the CIMS emissions.
F(s) (CIMS-ON and CIMS-OFF) is plotted versus relative humidity in Fig. 2.
Positive correlations were observed for daytime (AM) and nighttime (PM) and daily
samples (r = 0.88, r = 0.59, r = 0.58, respectively) with
the same slope (≃ 0.01).
F(s) values were usually higher during the daytime in comparison to
nighttime values (Tables 1 and 2), which was what we expected for OH-driven
oxidation during daylight. In the troposphere, the OH radical is produced
mainly from photolysis of O3 to O(1D) and subsequent
reaction with water vapor. If a steady state in O(1D) is assumed
with respect to its production and loss, the (instantaneous) daytime OH
production rate is proportional to jO(1D)×[H2O]×[O3]. A negative correlation was observed between
F(s) and this (integrated) OH production rate during the daytime
(r = -0.72, P value < 0.05; Fig. A2). However, two data points
with the highest RH (25 and 26 August) drive this correlation, and no
correlation was observed for the remainder of the samples. This suggests that
there may be SO2 oxidation pathways in addition to OH during the day
in this region.
The time series for SO2 during the campaign is shown in Fig. 3a. The
time series was dominated by spikes in the SO2 mixing ratio.
used PMF to determine
concentration time series for five factors during the campaign. This analysis
showed that on the 14, 23, and 24 August and 3 and 4 September were periods that the site
was impacted by upgrader emissions. Concentrations of SO2, Fe, and
Mn (measured in PM2.5) were markedly higher during these periods
(Fig. 3).
(a) SO2 time series with a sampling interval of
5 min (there is a gap in 14 August data) and (b) hourly data for Mn
(left axis) and Fe (right axis). Shaded areas indicate polluted periods.
It is interesting to note that F(s) for daytime was higher than nighttime
for all samples except periods when the site was impacted by plumes from
major oil sands upgrading facilities (polluted periods; ).
A comparison between AM and PM values for F(s) for 23 and 24 August showed
that nighttime values were almost double the daytime values. F(s) data were
not available for the daytime of 14 August to compare with the nighttime
value, but 14 August PM showed the highest value for F(s) (0.77) during the entire
campaign (Tables 1 and 2). At night, aqueous phase oxidation is believed to
be the dominant SO2 transformation pathway as OH is absent
. No correlation was observed between the F(s) and O3
mixing ratio for daytime, nighttime, or daily samples (Fig. A2). Therefore,
it is expected that SO2 oxidation occurs by the H2O2 and/or
the TMI-catalyzed pathways.
Fe and Mn concentrations in PM2.5 aerosols, averaged over the
nighttime high-volume sampling periods, are shown in Fig. A3 . The
averaged nighttime concentrations of Fe and Mn were higher during polluted
periods (average values of 57 ± 20 ng m-3 and 1.5 ± 0.5 ng m-3,
respectively) in comparison to other periods (average values
of 9 ± 3 ng m-3 and 0.13 ± 0.06 ng m-3,
respectively; Fig. A3). The data collected on 21 August PM were excluded from
this analysis because the PMF analysis by showed this
period to be distinct (discussed further below). To check if the
TMI-catalyzed pathway played a role in SO2 oxidation during
nighttime, averaged concentrations of Fe and Mn were added and
[Fe + Mn] × [H2O] values were
calculated and shown in Fig. 4a. F(s) is also shown for nighttime samples
(Fig. 4b). When [Fe + Mn] × [H2O]
values are high, F(s) is also high.
(a) ([Fe] + [Mn]) × [H2O] values for nighttime (PM) samples as an indicator of the
TMI-catalyzed SO2 oxidation pathway (Fe and Mn concentrations were
averaged over the running periods of the high-volume sampler)
and (b) F(s) values for the nighttime samples. Polluted periods
and the soil episode are shown by gray and yellow shaded areas, respectively.
Concentrations of Fe and Mn were associated with upgrader, soil, and haul
road dust factors during polluted nighttime periods (14, 23, 24 August:
). Although 21 August PM was not a polluted period, it showed high
[Fe + Mn] × [H2O] values but F(s)
was not high (Fig. 4b). For 21 August, the analysis by
showed that there was a peak for the soil factor but not upgrader and haul
road dust. F(s) on this night was markedly lower than during periods when
upgrader and haul road dust factors were high. F(s) for 25 August was also
high since this was a rainy period (Sect. 4.1).
δ34S values for size-segregated sulfate aerosols and SO2 during CIMS-OFF
periods
During CIMS-OFF periods 34SO2 emissions were absent, so
δ34S values reflect the sulfur isotopic composition of the sulfur
compounds in the region and/or fractionation as the SO2 is oxidized
and transported to the AMS 13 site. δ34S values during CIMS-OFF
periods for SO2 and size-segregated sulfate in size ranges F<0.49µm , E0.49-0.95µm, D0.95-1.5µm, C1.5-3.0µm, B3.0-7.2µm, and A>7.2µm are shown
in Table 1. Possible oxidation pathways of SO2 to sulfate were
investigated using these δ34S values.
Blank corrected δ34S values for SO2 were +5.1 and
+10.8 ‰. No negative δ34S values were observed for
SO2. If it is assumed that no fractionation occurred during formation
of primary sulfate in major stacks, then it is expected that δ34S
values for SO2 would be the same as primary sulfate (with an average
of +7.3 ± 0.3 ‰ and +9.4 ± 2.0 ‰). The
δ34S values of SO2 ranged from +5.1 to +11.1 ‰
(Table 1) and are consistent with this assumption. The lowest value
(+5.1 ‰) is consistent with a δ34S value for SO2
from vehicle exhaust (Table 1).
δ34S values for size F<0.49µm particles ranged
between +1.8 and +15.1 ‰ with an average of
+7.4 ± 4.2 ‰. Although this average overlaps with values
given by for primary sulfate from the stack emissions
(+7.3 ± 0.3 and +9.4 ± 2.0 ‰), there were
δ34S values lighter and heavier than what was expected from
potential sulfur sources in the region in this size range. Therefore,
δ34S of sulfate cannot be used as a quantitative indicator for
industrial SO2 emissions as isotope fractionation may have occurred
as the stack emissions (SO2) were transported to the AMS 13 site. As
shown in Sect. 5.1 sulfate particles in this size range are predominantly
secondary; therefore, these data can be used to investigate the importance of
different SO2 oxidation pathways during transport.
δ34S (‰) values for SO2, and
sulfate aerosols in size ranges F<0.49µm, E0.49-0.95µm, D0.95-1.5µm, C1.5-3.0µm, B3.0-7.2µm,
and A>7.2µm during CIMS-OFF periods. Not blank corrected
samples have an uncertainty of ± 0.3 ‰, and the uncertainty
for blank corrected samples are shown in parentheses.
Date
SO2
F
E
D
C
B
A
F(s)
Error in F(s)
14 Aug, pm
+10.8
+4.6 (0.4)
+2.8 (3.5)
–
–
+2.1*
–
0.77
0.09
15 Aug, am
+11.1*
+6.5 (0.8)
+6.5 (3.8)
+0.4*
-0.38*
-0.24*
–
0.47
0.06
15 Aug, pm
–
+12.9 (2.0)
+2.2*
+1.5*
–
-0.33*
-4.1 (2.1)
0.22
0.04
16 Aug, am
+5.1
+1.8 (0.8)
+6.5 (2.2)
+3.2*
-2.9*
-1.1*
-4.5 (1.4)
0.13
0.03
16 Aug, pm
+7.4*
+8.9 (1.9)
-0.2*
–
-0.4*
+3.5*
-2.1 (1.1)
0.16
0.02
17 Aug, am
–
+15.1 (2.1)
+6.3*
-1.1*
+1.5*
+1.8*
+0.74*
0.13
0.01
17 Aug, pm
–
+9.0*
+1.3*
+0.1*
+3.3*
+2.1*
-1.2*
0.03
0.01
18 Aug, am
+10.2*
+6.5 (1.7)
-1.7 (1.9)
-0.89*
-0.88*
-5.3 (2.2)
-0.56*
0.13
0.01
18 Aug, pm
–
+6.1 (2.7)
+2.3 (5.0)
+2.1*
-1.0*
-1.8 (2.4)
-2.9*
0.10
0.01
19 Aug, pm
–
+8.2 (1.1)
-0.6*
+2.6*
+5.4*
+1.6*
-0.67*
0.16
0.05
* not blank corrected samples.
Particles in larger size ranges (E0.49-0.95µm,
D0.95-1.5µm, C1.5-3.0µm, B3.0-7.2µm, and A>7.2µm) are expected to contain
more primary sulfate and have lower δ34S values in comparison to the
F<0.49µm size range. There were no negative values for
sulfate particles in the size fraction F<0.49µm, but
negative values were observed for the size fraction E0.49-0.95µm. There was a tendency to lighter δ34S values for
larger sulfate particles as shown in Fig. 5.
δ34S ranges for F<0.49µm, E0.49-0.95µm, D0.95-1.5µm, C1.5-3.0µm, B3.0-7.2µm, and A>7.2µm size ranges during
CIMS-OFF periods. As the particles become larger, δ34S becomes more
negative.
Correlation between Fe and Mn and sulfate concentration and δ34S values during CIMS-OFF periods
Sulfur dioxide can be oxidized in the aqueous phase by O2 in the
presence of TMIs predominantly by Fe3+ and
Mn2+. If this is an important oxidation
pathway, more secondary sulfate is expected to be produced when the
concentrations of catalysts are higher. Since concentrations of Fe and Mn
were measured in PM2.5 particles, the concentration of sulfate in
impactor size fractions (F<0.49µm, E0.49-0.95µm, D0.95-1.5µm, and C1.5-3.0µm) were added to find
the concentration of sulfate in particles with D< 3 µm. The
sulfate concentration in the impactor size range D< 3 µm is almost
the same as the concentration in PM2.5 since the concentration in
size fraction C1.5-3.0µm was very low (zero for all
periods except polluted periods, which ranged between 0.58 and 1.76 µg m-3).
The concentration for particles from the impactor with D< 3 µm is plotted versus Fe and Mn concentrations and the sum of
Fe and Mn in Fig. 6. Positive correlations were observed for all three cases
(r = 0.86, r = 0.89, r = 0.86, respectively; Fig. 6).
Positive correlations were also observed when the concentrations of sulfate
in the aerosol size fractions F<0.49µm and E0.49-0.95µm were plotted against the concentrations of Fe and Mn and
the sum of Fe and Mn (Fig. A4). There were not enough sulfate concentration
data for size fractions C1.5-3.0µm and D0.95-1.5µm to show the individual correlations with Fe, Mn, and the sum
of Fe and Mn.
Sum of concentrations of sulfate in size ranges F<0.49µm, E0.49-0.95µm, D0.95-1.5µm, and C1.5-3.0µm versus the concentration
of (a) Fe, (b) Mn, and (c) Fe + Mn.
When SO2 is oxidized by the TMI-catalyzed pathway, the sulfur
isotopic composition of the sulfate formed is lighter than the isotopic
composition of the reactant SO2 . Significant
anti-correlations were apparent for sulfate δ34S values in the size
fraction F<0.49µm when plotted against Fe and Mn
concentrations (r = -0.80 and r = -0.76, respectively;
Fig. 7). This suggests that lighter δ34S values occur in secondary
sulfate in the presence of higher concentrations of Fe and Mn. Insufficient
isotope data were available to create similar plots for other size fractions.
δ34S values of size FD<0.49µm
sulfate aerosols versus the concentrations of (a) Fe and (b) Mn.
Positive correlations were also observed between concentrations of Fe and Mn
and the concentration of SO2 (r = 0.67 and r = 0.65,
respectively; Fig. A5), which may indicate that they originate from the same
source, or were transported together to the sampling site.
δ34S values of SO2 and size-segregated sulfate aerosols during CIMS-ON periods
The release of 34SO2 from the CIMS allowed for an examination of
SO2 oxidation to sulfate under field conditions. An unexpected result
was found: δ34S values for SO2 and sulfate samples with D< 0.49 µm during the periods when the CIMS was operated (CIMS-ON)
are shown in Table 2. The blank corrected data show that δ34S
values for enriched SO2 samples were only as high as
+35.6 ‰, and there were values without enrichment ranging
between +4.8 and +10.9 ‰ with an average value
of +8.3 ± 1.8 ‰. All sulfate samples in the size range
F<0.49µm representing SO2 oxidation during the
CIMS-ON periods were blank corrected, and all AM and PM samples were highly
enriched in 34S; the δ34S values were as high as +913 ‰
(Table 2).
A comparison between the isotopic composition of sulfate aerosols in the size
range F<0.49µm and SO2 samples
(RSO4/RSO2) showed that the sulfate particles with D< 0.49 µm were much more enriched in 34S from the
34SO2 tracer released by the CIMS. The concentration of enriched
sulfur as 34SO2 and 34SO4 molecules cm-3
is also calculated as described in Sect. 4.3.3 and the
data are reported in Table A1.
δ34S values (‰) for SO2 and sulfate
with diameter D<0.49 µm, ratio of sulfate to SO2 isotope during
CIMS-ON periods, F(s) values, and the error in F(s). Enriched samples were
selected by comparing the mass-dependent fractionation relation between
δ34S and δ33S for the sample and standards at the same
run. Average uncertainty for δ34S values is ±0.5 ‰.
Date
δ34SSO2
δ34SSO4
RSO4RSO2
F(s)
Error in F(s)
13 Aug, am
+18.6*,a
+155.8
–
–
–
14 Aug, am
–
+47.1
–
–
–
20 Aug, daily
+7.2*
+181.9
–
0.23
0.04
21 Aug, pm
–
+441.6
–
0.13
0.02
22 Aug, am
–
+409.5
–
0.23
0.02
22 Aug, pm
+12.2a
+572.6
1.553
0.13
0.02
23 Aug, am
+4.8
+27.8
1.022
0.20
0.01
23 Aug, pm
+18.4a
+15.5
0.997
0.38
0.01
24 Aug, am
+10.9
+26.8
1.015
0.26
0.01
24 Aug, pm
+19.6a
+88.7
1.068
0.43
0.06
25 Aug, am
+8.6
+33.1
1.024
0.65
0.07
25 Aug, pm
+8.4
+74.4
1.066
0.33
0.08
26 Aug, am
+7.7
+21.5
1.014
0.45
0.01
26 Aug, pm
+21.2a
+48.2
1.026
0.44
0.02
27 Aug, daily
+8.4
+21.5
1.012
0.47
0.01
28 Aug, am
+13.0*
–
–
0.36
0.03
28 Aug, pm
+10.2*
+298.1
–
0.23
0.04
29 Aug, daily
+10.0
+56.9
1.046
0.25
0.04
30 Aug, daily
+7.8*
+364.9
–
0.13
0.02
31 Aug, daily
+35.6a
+312.2
1.267
0.48
0.05
1 Sep, daily
–
+913.3
–
0.17
0.04
2 Sep, daily
+6.9*
+735.9
–
0.15
0.03
3 Sep, daily
+7.8
+24.6
1.016
0.20
0.01
4 Sep, daily
+26.94a
–
–
–
–
a tagged as enriched. * not blank corrected samples.
These are only shown for comparison, no calculation has been done using these
values.
F(s) versus the concentration of α-pinene, β-pinene, and limonene during daytime.
Sulfate aerosols during CIMS-ON periods in the size ranges E0.49-0.95µm,
D0.95-1.5µm, C1.5-3.0µm,
B3.0-7.2µm, and A>7.2µm also showed
enrichment for most of the samples (85 out of 100 samples showed enrichment; Table 3).
δ34S (‰) values for sulfate in size ranges
E0.49-0.95µm, D0.95-1.5µm, C1.5-3.0µm, B3.0-7.2µm, and A>7.2µm during CIMS-ON periods.
Average uncertainty for δ34S values is ±0.5 ‰.
Date
δ34SSO4(E)
δ34SSO4(D)
δ34SSO4(C)
δ34SSO4(B)
δ34SSO4(A)
13 Aug, am
+25.7*
+31.5*
+45.2*
+54.1
+23.1
14 Aug, am
+28.5*
+59.1
+75.1
+65.5
+49.0
20 Aug, daily
+66.2
+28.5*
+35.2*
+30.3*
+30.8*
21 Aug, pm
+31.0
+34.5*
+32.8*
+33.3*
+31.5
22 Aug, am
+76.2
–
+111.4*
+110.3*
+46.1*
22 Aug, pm
+55.8*
–
+61.4*
+67.5
+83.9*
23 Aug, am
+37.4
+29.9
+31.1
+23.7
+28.7
23 Aug, pm
+15.9
+12.5
+11.8*
+9.3*
+6.8
24 Aug, am
+8.0
–
+29.2
+18.5
+10.2
24 Aug, pm
+55.6
+15.9*
+21.6*
+22.1*
+39.5
25 Aug, am
+12.0*
+15.5*
+12.2*
+16.9
+22.9*
25 Aug, pm
+27.7*
+21.3*
+18.5*
+16.9*
+32.0*
26 Aug, am
+15.8
+19.7
+41.4
+31.9
+28.9
26 Aug, pm
+11.5b
–
+19.7*
+15.3*
+26.1*
27 Aug, daily
+25.6
+19.9
+20.9*
+31.8
+32.6
28 Aug, am
–
+217.1*
+201.3*
+212.1*
–
28 Aug, pm
+224.8*
+310*
+211.2*
+240*
–
29 Aug, daily
+80.3*
+98.5*
+85.1*
+71.9*
–
30 Aug, daily
+188.9*
–
+194.9*
+176.5*
+217*
31 Aug, daily
+166.8*
–
+537.7*
+341.8*
+339.5*
1 Sep, daily
+372.2*
+132.9*
+825.1*
–
–
2 Sep, daily
–
–
+483.4*
–
+274*
3 Sep, daily
+45.8
+33.5*
+38.9*
+30.8*
+14.6*
* not blank corrected samples
Sum of α-pinene, β-pinene, and Limonene versus ozone mixing ratio for daytime and nighttime and all data.
Since the CIMS exhaust was located to the southeast
of the high-volume sampler, wind
direction was considered as a potential factor in the analysis. No
correlation (r = 0.16) was observed between the percent of time the
high-volume sampler was downwind of the CIMS exhaust and the concentrations
of 34SO2 or 34SO4.
The role of Criegee biradicals in SO2 oxidation
As mentioned in Sect. 5.1, F(s) was higher during the daytime in comparison
to nighttime except for polluted periods. No correlation (r = -0.36,
excluding 25 and 26 August with the highest RH) was observed between F(s)
and the integrated OH production rate, suggesting that another oxidation
pathway for SO2 was active during daytime. One likely pathway is
oxidation of SO2 by Criegee biradicals.
Criegee biradicals are formed from ozonolysis of alkenes and may oxidize
SO2 to sulfate increasing F(s) . Therefore,
it is expected that correlations may exist between F(s) and precursors to
Criegee biradicals. Positive correlations between F(s) and the
concentration of α-pinene (r = 0.85), β-pinene
(r = 0.87), and limonene (r = 0.82) were observed during daytime
(Fig. 8). However, no correlations were observed between F(s) and
monoterpenes during nighttime.
The concentration of monoterpenes showed a negative correlation with the
mixing ratio of O3. There was a power law relationship between
monoterpenes and O3 mixing ratio during the daytime and a linear
dependency at night (r = -0.60; Fig. 9).
Concentrations of other VOCs were only available as 24 h averages. Most of
the alkenes measured were found to be below the detection limit. Alkenes with
concentrations higher than the detection limit except isoprene showed
significant positive correlations with secondary sulfate aerosols (D< 0.49 µm) and all of them except isoprene and tetrachloroethene
showed significant correlations with SO2 (Table A2).
Correlations with aromatic compounds generally fell into two categories. The
first set includes compounds which were highly correlated with SO2
and sulfate for impactor D< 0.49 µm (e.g., benzene). The second
set contains the compounds which show no such correlations but were correlated
with F(s) (Table A3). Styrene and p-cymene are two compounds with no
correlation with SO2 and sulfate but significant correlations with
F(s) (r = 0.58 for both, and r = 0.66 and r = 0.71,
respectively, when the rainy day data are omitted, P value <0.05; Table A3,
Fig. A6). They also show a positive correlation together (r = 0.71,
P value < 0.05; Fig. A6).
Discussion
Potential TMI-catalyzed SO2 oxidation
Sulfur conversion ratios (F(s)) and sulfur isotope data for SO2 and
size-segregated sulfate aerosols were used to investigate the role of
TMI-catalyzed SO2 oxidation in the region.
Figure 2 exhibits the
expected correlation between F(s) and OH in daytime but not at night when
aqueous phase reactions are important. The similar slopes for nighttime and
daytime F(s) versus RH plots suggests that SO2 aqueous phase
oxidation may be an important oxidation pathway for both day and night and
the offset (intercept that is higher for daytime than nighttime) suggests
there is additional gas phase SO2 oxidation that takes place during
the day.
Known aqueous phase oxidants for SO2 are H2O2, O3,
and O2 in the presence of TMIs . No correlation
was observed between F(s) and O3 mixing ratios, which suggests that
O3 is of minor importance as an oxidant in the aqueous phase. The pH
dependency of aqueous O3 oxidation of SO2 makes this reaction
very slow at low pH (pH < 5.5). This reaction is also self-limiting and
production of sulfate lowers the pH and slows down further reaction
. Therefore, aqueous phase oxidation of SO2
occurs mostly by H2O2 and/or O2 in the presence of TMIs.
The conversion ratio of SO2 to sulfate (F(s)) was higher during the
day than at night except during polluted periods (14, 23, and 24 August). This is
consistent with gas phase contributions to SO2 oxidation in addition
to aqueous phase oxidation that occurred both during the day and at night
(Sect. 5.1). On polluted nights, the SO2 to sulfate conversion ratio
was twice as high as during the day and on 14 August at night the highest (0.77)
conversion ratio for the entire campaign was observed (Tables 1 and 2).
Averaged Fe and Mn concentrations on these polluted nights coincided with the
highest values for SO2 to sulfate conversion (F(s); Fig. A3).
Whenever both RH and the sum of Fe and Mn concentrations were high at night,
the proportion of SO2 that was converted to sulfate (F(s)) was higher
as well (Fig. 4). These conditions of coincident high RH and Fe + Mn
concentrations was met on polluted nights during the campaign. On these
nights the ratios of Fe / Mn were around 40 (38, 40, 42 for 14, 23, 24 August,
respectively). This specific ratio may be a useful indicator for the source
of Fe and Mn in aerosols. A particular night that was not classified as
polluted (21 August) was identified as having Fe and Mn from soil
. The ratio of Fe / Mn on that night was 76, and the
SO2 to sulfate conversion ratio was indistinguishable from the
remainder of the non-polluted nighttime samples (Fig. 4). Therefore, it is
reasonable to suggest that a Fe / Mn value around 40 is associated with a non-soil
source. The two remaining sources are upgrader emissions and haul road dust.
This interpretation of Fe and Mn on polluted nights as originating from
anthropogenic emissions (Fe / Mn ∼ 40) rather than soil is consistent
with the higher solubility of anthropogenic TMIs relative to soil
.
Sulfur isotope measurements can provide the means to distinguish TMI from
H2O2 aqueous oxidation. Isotope fractionation will be evident in
sulfate when a large reservoir of SO2 (e.g., from stack emissions)
mixes with oxidants during transport and produces accumulated sulfate product
captured over 12 or 24 h. So long as the fraction of reaction is low
(< 30 %) the difference in δ34S values for SO2 and
sulfate will reflect the magnitude and direction of the fractionation
process. For the TMI-catalyzed pathway this direction is negative and
produces lighter sulfate than SO2. This directly contrasts with
fractionation for O3, H2O2, and OH oxidation pathways.
Evidence that SO2 released from tall stacks is transported high above
the ground and mixes down toward the surface at AMS 13 has been demonstrated
by and should provide conditions meeting the requirement
for the fraction of reaction less than 30 % described here. The observed
δ34S values for size-segregated sulfate aerosols in this study were
consistent with aqueous TMI rather than H2O2 oxidation. Light
δ34S values for sulfate aerosols were observed in the region in
comparison to other potential atmospheric sulfur sources. An alternate
explanation for isotopically light δ34S values in sulfate was
proposed by . Isotopically light δ34S values
(-3.9 and +0.3 ‰) were reported by this group for
sulfate from bulk and throughfall deposition (deposition of excess water onto
the ground surface from wet leaves) in the Athabasca oil sands region,
consistent with the observations in this study. Since these values were
lighter than the potential sources in the region, this suggests a
contribution of sulfate from a 34S depleted source. Proemse et al. (2012b) suggested that
the low δ34S values observed for atmospheric sulfate collected at
two sites were due to H2S emitted from tailing ponds. Tailing ponds
were in close proximity to the two sites where low δ34S values were
found. suggested that H2S was oxidized to
SO2 and subsequently formed sulfate that then contributed to local
sulfate deposition. The average value for δ34S of SO2
during CIMS-ON and CIMS-OFF (non enriched values) periods was
+7.9 ± 2.1 ‰. This value is in the range of
δ34S of primary sulfate from two major stacks .
No negative values were observed for δ34S of SO2. If
H2S was the main source of atmospheric sulfur, the opposite pattern
to that observed in Fig. 5, is expected. The reason is that isotopically
light SO2 from H2S oxidation is expected to produce secondary
sulfate aerosols (from both homogeneous and heterogeneous reactions) in the
smaller size fractions (F<0.49µm and E0.49-0.95µm) with isotopically light δ34S values. The larger A>7.2µm and B3.0-7.2µm size aerosols contain primary sulfate from
soil and would reflect δ34S values for untreated oil sand
(+6.4 ‰; ) in addition to sulfate from
H2S oxidation so they would have progressively more positive
δ34S values. Therefore, a discernable contribution of H2S
to isotopically light samples through an SO2 oxidation pathway is
ruled out.
Primary sulfate and SO2 can originate from haul road dust or diesel
exhaust. δ34S values for these two sources are +5 ‰
and higher . Therefore, if haul road dust and
diesel primary sulfate were transported with Fe and Mn, then δ34S
values should converge to +5 ‰ or higher. This should be
particularly evident for the larger size aerosols (A>7.2µm
and B3.0-7.2µm). In fact the opposite is observed in
Fig. 5. Isotopically light δ34S values for sulfate aerosols in size
ranges E0.49-0.95µm, D0.95-1.5µm,
C1.5-3.0µm, B3.0-7.2µm, and A>7.2µm were observed during CIMS-OFF periods. These values indicate that
there was no, or only a very small, contribution of primary sulfate from
major stacks. This leaves SO2 from upgrader emissions as the most
probable source of sulfate both for F<0.49µm size aerosols
and for secondary sulfate formed on larger aerosol size fractions.
δ34S values reflect isotope fractionation during oxidation of
SO2 rather than source signatures. This is supported by a positive
correlation between the sum of sulfate in size fractions F<0.49µm, E0.49-0.95µm, D0.95-1.5µm, and C1.5-3.0µm and the concentrations
of Fe and Mn and sum of Fe and Mn (r = 0.86, r = 0.89, and
r = 0.86, respectively). The concentration of sulfate in size fractions
F<0.49µm and E0.49-0.95µm also showed
positive correlations with the concentration of Fe and Mn. This indicates
that when Fe and Mn were prevalent in aerosols, either more sulfate
can be formed or Fe and Mn were transported to AMS 13 with SO2 from a
common emission source, likely upgrader emissions. There were also
anti-correlations between δ34S values of sulfate in size fraction
F<0.49µm and the concentrations of Fe and Mn. This shows
that lighter δ34S values were associated with secondary sulfate
formation and higher concentrations of Fe and Mn. One possible explanation
for these observations may be the TMI-catalyzed SO2 oxidation pathway
during transport to the AMS 13 site.
CIMS-ON
Little 34SO2 reached the SO2 filter in the high-volume
sampler since high sulfur isotope enrichment was not observed for SO2
samples (max δ34S=+35.6 ‰). Instead 34SO2
was oxidized to sulfate either as it moved in the atmosphere or on the
filters in the high-volume sampler. This result was unexpected since previous
studies of δ34S for sulfate and SO2 showed no evidence of
oxidation when SO2 passed through the filters under marine or
continental conditions . The lack of
34SO2 and the predominance of 34S molecules on sulfate
aerosols demonstrates an oxidation pathway that is rapid and specific to the
conditions at the ground level of the the AMS 13 site.
Potential oxidation of SO2 by Criegee biradicals
The proportion of sulfate from SO2 oxidation, F(s), during daytime
is generally larger than F(s) at night (Tables 1 and 2). Greater vertical
mixing is expected during the day than at night. Stack emissions high above
ground undergo oxidation during transport to the AMS 13
site. Aloft, conventional oxidation pathways (i.e., OH-driven oxidation) are
likely more important than near the surface. At the same time precursors to
Criegee biradicals will be released and mixed upward. A larger F(s) during
the day than at night suggests that during daytime gas phase SO2
oxidation occurs in addition to aqueous phase oxidation. Typically, OH is
expected to dominate gas phase SO2 oxidation during the day. However,
a correlation between F(s) and integrated OH production rate was not
observed. Instead, positive correlations between F(s) and α-pinene,
β-pinene and limonene were observed during the day but not at night
(Fig. 8). This, combined with the loss of monoterpenes as daytime O3
mixing ratio increased, suggests Criegee biradicals may be an important
factor in SO2 oxidation close to the surface during daytime.
Monoterpenes are oxidized by O3 to form Criegee biradicals which can
be stabilized and oxidize SO2 to form secondary sulfate. This pathway
is potentially more important during the day but less so at night. At night,
the emissions of monoterpenes continue into a shallow nocturnal boundary
layer that is decoupled from the residual layer above it. The terpenes then
titrate O3 at the surface, leading to the observed anti-correlation
and low surface O3 mixing ratio which limits Criegee biradical
production.
Reaction between O3 and anthropogenic alkenes may also generate
Criegee biradicals, potentially leading to higher SO2 to sulfate
conversion ratios (F(s)). Many anthropogenic alkenes and aromatics likely
have sources in common with SO2 since a correlation (P value < 0.05)
was observed between them (Tables A2 and A3). Their emissions are likely
injected into (and transported within) layers above the measurement site and
only sporadically entrain to the surface during daytime. When this happens,
relationships between F(s) and anthropogenic alkenes may be observed. As an
example, styrene and p-cymene did not correlate with SO2 or secondary
sulfate but they were correlated with F(s) (r = 0.66, r = 0.71,
respectively). Styrene and p-cymene were also highly correlated with each
other (r = 0.71) suggesting they originated from the same source or
sources. It is likely that styrene and p-cymene are indicators of other
anthropogenic alkenes that facilitate SO2 oxidation (for instance,
tetrachloroethene).