While photooxidants are important in atmospheric condensed phases, there are
very few measurements in particulate matter (PM). Here we measure light
absorption and the concentrations of three photooxidants – hydroxyl radical
(⚫OH), singlet molecular oxygen (1O2*),
and oxidizing triplet excited states of organic matter (3C*) –
in illuminated aqueous extracts of wintertime particles from Davis,
California. 1O2* and 3C*, which are formed
from photoexcitation of brown carbon (BrC), have not been previously measured
in PM. In the extracts, mass absorption coefficients for dissolved organic
compounds (MACDOC) at 300 nm range between 13 000 and
30 000 cm2 (g C)-1 are approximately twice as
high as previous values in Davis fogs. The average (±1σ)⚫OH steady-state concentration in particle extracts is
4.4(±2.3)×10-16 M, which is very similar to previous values
in fog, cloud, and rain: although our particle extracts are more
concentrated, the resulting enhancement in the rate of ⚫OH
photoproduction is essentially canceled out by a corresponding enhancement in
concentrations of natural sinks for ⚫OH. In contrast,
concentrations of the two oxidants formed primarily from brown carbon (i.e.,
1O2* and 3C*) are both enhanced in the
particle extracts compared to Davis fogs, a result of higher concentrations
of dissolved organic carbon and faster rates of light absorption in the
extracts. The average 1O2* concentration in the PM extracts
is 1.6(±0.5)×10-12 M, 7 times higher than past fog
measurements, while the average concentration of oxidizing triplets is 1.0(±0.4)×10-13 M, nearly double the average Davis fog value.
Additionally, the rates of 1O2* and 3C*
photoproduction are both well correlated with the rate of sunlight
absorption.
Since we cannot experimentally measure photooxidants under ambient particle
water conditions, we measured the effect of PM dilution on oxidant
concentrations and then extrapolated to ambient particle conditions. As the
particle mass concentration in the extracts increases, measured
concentrations of ⚫OH remain relatively unchanged,
1O2* increases linearly, and 3C* concentrations increase less
than linearly, likely due to quenching by dissolved organics. Based on our
measurements, and accounting for additional sources and sinks that should be
important under PM conditions, we estimate that [⚫OH] in
particles is somewhat lower than in dilute cloud/fog drops, while [3C*]
is 30 to 2000 times higher in PM than in drops, and [1O2*] is
enhanced by a factor of roughly 2400 in PM compared to drops. Because of
these enhancements in 1O2* and 3C* concentrations,
the lifetimes of some highly soluble organics appear to be much shorter in
particle liquid water than under foggy/cloudy conditions. Based on
extrapolating our measured rates of formation in PM extracts, BrC-derived
singlet molecular oxygen and triplet excited states are overall the dominant
sinks for organic compounds in particle liquid water, with an aggregate rate
of reaction for each oxidant that is approximately 200–300 times higher
than the aggregate rate of reactions for organics with ⚫OH. For
individual, highly soluble reactive organic compounds it appears that
1O2* is often the major sink in particle water, which is a new
finding. Triplet excited states are likely also important in the fate of
individual particulate organics, but assessing this requires additional
measurements of triplet interactions with dissolved organic carbon in
natural samples.
Introduction
Photochemically generated oxidants largely drive atmospheric chemistry, both
in the gas phase (Thompson, 1992; Finlayson-Pitts and Pitts Jr., 1999;
Seinfeld and Pandis, 2012) and in aqueous drops, where they largely govern
the reactions and lifetimes of organic compounds (Lim et al., 2005, 2010;
Ervens et al., 2011; He et al., 2013; Herrmann et al., 2015; Blando and
Turpin, 2000). Similarly, photooxidants can be important for transformations
in water-containing particulate matter (PM): they make new PM mass by
functionalizing gaseous volatile organics to oxygenated lower-volatility
products and decrease PM mass by fragmenting large organics into smaller,
more volatile species (Jimenez et al., 2009). Oxidants in condensed phases
can come from the gas phase (e.g., the mass transport of hydroxyl radical,
⚫OH) or can be formed photochemically within the particle
or drop (Herrmann et al., 2010b). Our focus in this paper is on the latter
pathway.
Of the photooxidants formed in airborne particles, hydroxyl radical
(⚫OH) is the most widely studied. While its concentrations have
been measured in cloud/fog drops, rain, and dew (Arakaki and Faust, 1998;
Arakaki et al., 1999; Anastasio and McGregor, 2001; Kaur and Anastasio,
2017), there are only four known measurements of ⚫OH
photoproduction rates, lifetimes, and steady-state concentrations in ambient
particles, all from coastal or marine locations (Anastasio and Jordan,
2004; Arakaki et al., 2006, 2013; Anastasio and Newberg, 2007). Based on these and other measurements (e.g., Tong et al., 2017)
and complementary modeling work (Herrmann et al., 2010b, 2015), the major sources of ⚫OH include photolysis of
nitrate, nitrite, and hydrogen peroxide (HOOH) as well as reactions of
Fe(II) with HOOH or organic peroxides. The major sinks of ⚫OH
are organic molecules since these reactions typically have nearly
diffusion controlled rate constants (Arakaki et al., 2013; Herrmann et
al., 2010a, 2015).
Photoexcitation of organic chromophores, i.e., light-absorbing brown carbon
(BrC), can also form oxidants in particles and drops. For example, sunlight
absorption by organic chromophores can promote the molecules from their
ground states to reactive triplet excited states (McNeill and Canonica,
2016; Kaur and Anastasio, 2018b). Triplets can both directly oxidize
organics via electron-transfer reactions and form other photooxidants,
including singlet molecular oxygen (1O2*) (Zepp et al.,
1985) and hydrogen peroxide (Anastasio et al., 1997). In this work we
examine oxidizing triplets, which we refer to as 3C* or simply
“triplets” for simplicity. Such species are important in surface waters,
where they rapidly oxidize several classes of compounds including phenols,
anilines, phenylurea herbicides, and sulfonamide antibiotics (Canonica et
al., 1995, 2006; Canonica and Hoigné, 1995; Boreen et al., 2005; Bahnmüller et al., 2014).
There has been growing interest in the role and reactivity of triplets
formed from particulate brown carbon, especially their role in forming
aqueous secondary organic aerosol (SOA(aq))(Smith et al., 2014, 2015; Yu
et al., 2014, 2016; Laskin et al., 2015). There is evidence that
triplet-forming, light-absorbing species, e.g., imidazoles and pyrazines,
are formed in drops and particles (De Haan et al., 2009, 2010; Hawkins et
al., 2018), and a few laboratory studies have examined how illuminated
imidazole particles can oxidize isoprene or other alkenes to increase PM
mass (Aregahegn et al., 2013; Rossignol et al., 2014). But the
formation of SOA(aq) from such reactions appears not to be significant under
environmentally relevant conditions where concentrations of triplet
precursors are much lower (Tsui et al., 2017). While we
recently made the first measurements of triplet concentrations in fog waters
(Kaur and Anastasio, 2018b), there are no measurements of
3C* in particles, making it difficult to assess their significance.
This is doubly difficult because triplets are not a single oxidant but
rather a suite of species with a wide range of reactivities (McNeill
and Canonica, 2016).
Another important photooxidant in atmospheric and surface waters is singlet
molecular oxygen (1O2*), which is formed by energy transfer from a
triplet excited state to dissolved oxygen and lost via deactivation by
water (Zepp et al., 1977; Haag and Hoigné, 1986; Haag and Gassman,
1984; Faust and Allen, 1992). Similar to triplets, singlet oxygen has been
studied widely in surface waters (Zepp et al., 1977; Haag and Gassman,
1984; Haag and Hoigné, 1986; Tratnyek and Hoigné, 1994) and reacts
rapidly with electron-rich organics such as phenols, polycyclic aromatic
hydrocarbons, amino acids, and reduced sulfur species (Wilkinson et al.,
1995). However, there are only four measurements of 1O2*
concentrations in atmospheric waters (Anastasio and McGregor, 2001; Kaur
and Anastasio, 2017; Albinet et al., 2010; Faust and Allen, 1992) and none
in aqueous particles.
To address this gap, we measured ⚫OH, 1O2*, and
3C* in illuminated aqueous extracts of fine particles collected from
the Central Valley of California during winter, a period of heavy
residential wood burning. The goals of this study are to (1) quantify
⚫OH, 1O2*, and 3C* kinetics and concentrations in
particle extracts; (2) compare light absorption and photooxidant kinetics
with previous measurements made in fog; (3) measure the dependence of oxidant
concentrations on particle dilution to predict photooxidant concentrations
in ambient particle liquid water; and (4) assess the importance of particle
photooxidants in processing organic compounds in the atmosphere.
ExperimentalChemicals
All chemicals were used as received. Furfuryl alcohol (98 %), syringol
(99 %), methyl jasmonate (95 %), benzene (≥99.9 %),
2-methyl-3-buten-2-ol (98 %), deuterium oxide (99.9 % atom D), and
2-nitrobenzaldehyde (98 %) were from Sigma-Aldrich, and sulfuric acid
(trace metal grade) was from Fisher. All chemical solutions and particulate
matter extracts were prepared using purified water (Milli-Q water) from a
Milli-Q Advantage A10 system (Millipore; ≥18.2 MΩ cm) with an
upstream Barnstead activated carbon cartridge; total organic carbon
concentrations were below 10 ppb C.
Particle collection and extraction
Wintertime particles were collected in a residential neighborhood in Davis,
California, (38.5539∘ N, 121.7381∘ W; 16 m a.s.l.) during
December 2015 and January 2016, a period with significant wood burning.
PM2.5 was collected on 20.3 cm × 25.4 cm (8 in. × 10 in.) Teflon-coated quartz filters (Pall Corporation,
EmFab™ filters, type TX40HI20-WW) using a
high-volume sampler with a PM10 inlet (Graseby
Andersen) followed by two offset, slotted impactor plates (Tisch
Environmental, Inc., 230 series) to remove particles greater than
2.5 µm. Due to technical difficulties, the air flow rate was
variable and typically ranged between 1130 and 1560 L min-1,
corresponding to particle cut points of 2.5 to 1.6 µm. Particles
were generally collected over two to three consecutive nights between 17:30
and 07:30 local time, but one sample (number
3) was collected continuously (day and night) for 72 h (Table S1 in the
Supplement).
Immediately upon collection, samples were wrapped in aluminum foil
(previously baked at 500 ∘C for 8 h), sealed in Ziplock™
bags, and stored at -20∘C. On the day of extraction, several 2 cm × 2 cm pieces were cut (using stainless-steel tools) from the
same filter, each was put into a separate pre-cleaned 10 mL amber glass
vial, Milli-Q water was added (see below), and the vial was sealed and
shaken for 3 h in the dark. The extracts were filtered (0.22 µm
PTFE; Pall Corporation), combined, and labeled as particulate matter extract (PME). The
standard condition was to use 1.0 mL of Milli-Q to extract each filter
square, but in our initial work we used 2.5 mL of Milli-Q per filter square;
these latter “dilute extracts” are indicated by an asterisk and footnotes
in the figures and tables. We switched from dilute to standard conditions
after PME1–3, but we include both results in this work to compare the two
types of extracts.
In addition, to study the effect of PM mass concentration, separate portions
of filter number 3 were extracted using five different extraction volumes
between 0.5 and 10 mL (discussed later). Those extracts are labeled as
PME3Dx, where “x” is the extraction volume (e.g., PME3D1.3 for filter
squares extracted in 1.3 mL of Milli-Q). Upon extraction, each PME was
stored in the refrigerator (5 ∘C) until the day of the illumination
experiments. All illumination experiments and analyses on a PME sample were
completed within a week of its extraction.
Sample illumination and chemical analysis
For all illumination experiments except ⚫OH measurements
using benzene (discussed in Sect. 2.5.1), on the day of the experiment a
1.0 mL aliquot of an air-saturated particle extract was first acidified to
pH 4.2±0.2 using 10 mM sulfuric acid (with sample dilution ≤10 %) to mimic the particle water acidity in wintertime PM in
California's Central Valley (Parworth et al., 2017). The pH of the sample was
measured using a pH microelectrode (MI-414 series, protected tip, 16 gauge
needle, 6 cm length; Microelectrodes, Inc.). The acidified extract was then
spiked with a single photooxidant probe and put into a silicone-plugged,
fully filled GE021 quartz tube (4 mm inner diameter, 6 cm length, 1.0 mL
volume) and illuminated with a 1000 W xenon arc lamp filtered with a water
filter (to reduce sample heating), an AM 1.0 air mass filter (AM1D-3L,
Sciencetech), and a 295 nm long-pass filter (20CGA-295, Thorlabs) to mimic
tropospheric solar light (Kaur and Anastasio, 2017). Because of the small
tube size, samples were not stirred, but the entire sample was illuminated in
a chamber held at 20 ∘C. 100 µL aliquots of illuminated
(and parallel dark) samples were periodically removed and analyzed for the
concentration of photooxidant probe (see below) using HPLC (high-performance
liquid chromatography; Shimadzu LC-10AT pump,
ThermoScientific BetaBasic-18 C18 column (250×33 mm,
5 µM bead), and Shimadzu-10AT UV–Vis detector). The photon flux in
the sample was measured on each experiment day using a 10 µM
solution of 2-nitrobenzaldehyde (2NB) in the same type of quartz tube as the
sample (Galbavy et al., 2010).
Major anions and cations in the extracts (Table S2) were quantified using
two Metrohm ion chromatographs (881 Compact IC Pro) equipped with
conductivity detectors (Ge et al., 2014; Kaur and Anastasio, 2017).
Dissolved organic carbon (DOC) in the filtered extracts was measured using a
Shimadzu TOC-VCPH analyzer (Yu et al., 2014).
Light absorbance
Light absorbance was measured immediately after extraction using a Shimadzu
UV-2501PC spectrophotometer with 1 cm quartz cuvettes and a baseline of
Milli-Q water. Absorbance (Aλ) was converted to light absorption
coefficients using
αλ=Aλl,
where l is the path length in centimeters. The rate of sunlight absorption
(Rabs, mol photons L-1 s-1) in each extract was calculated as
Rabs=2.303×103NA×∑300nm450nm(αλ×Iλ×Δλ),
where 2.303 is for base conversion, 103 is for units conversion
(cm3 L-1), NA is Avogadro's number, Iλ is
the Davis winter-solstice actinic flux
(photons cm-2 s-1 nm-1) from the Tropospheric Ultraviolet
and Visible (TUV) Radiation Model version 4.1 (Madronich et al., 2002), and
Δλ is the interval between adjacent wavelengths in the TUV
output (nm).
Wavelength-dependent mass absorption coefficients for DOC (MACDOC;
cm2 (g C)-1) were estimated by subtracting the contributions of
nitrite and nitrate from the measured absorbance at each wavelength (which
were small, ≤7 % of the total absorbance) and then dividing the
remainder by the DOC concentration:
MACDOC,λ=αDOC,λ×ln10×103×103[DOC],
where αDOC,λ(cm-1) is the sample absorbance
coefficient at wavelength λ due to DOC (Kaur and Anastasio, 2017),
ln(10) is a base conversion factor, the two 103 factors are for unit
conversion (cm3 L-1 and mg g-1), and the DOC concentration
is in milligrams of carbon per liter (mg C L-1).
Since the average organic-matter-to-organic carbon (OM/OC) ratio in
California Central Valley particles is approximately 1.7 (Young et al.,
2016), the absorption coefficients normalized by OM mass will be
approximately 60 % of the MACDOC values.
Measurement of photooxidantsHydroxyl radical (•OH)
We quantified ⚫OH kinetics using a benzene probe (Zhou and
Mopper, 1990; Anastasio and McGregor, 2001; Kaur and Anastasio, 2017).
Briefly, four aliquots of each extract were spiked with varying
concentrations of benzene to trap ⚫OH and form phenol (yield:
73 %), which is quantified (Fig. S1 in the Supplement). Each benzene stock was made a day
before the illumination experiment. Similar to the other photooxidant
experiments, all aliquots were air-saturated, acidified to an initial pH of
4.2(±0.2), capped, and then constantly stirred during illumination
in airtight 5.0 mL, 1 cm path length, rectangular quartz cuvettes with no
initial headspace. For all ⚫OH measurements where benzene is
used as a probe, we used this larger sample volume (5 mL instead of 1 mL) to
minimize the headspace in the cuvette and prevent benzene loss due to
volatilization. Throughout the illumination period, 100 µL aliquots
were collected through the cap septum and analyzed for phenol using HPLC-UV
(eluent of 30 % acetonitrile: 70 % Milli-Q, flow rate of 0.6 mL min-1,
detection wavelength of 210 nm, and column temperature of 35 ∘C).
As described in Kaur and Anastasio (2017), we use these results to
determine three experimental quantities for ⚫OH: the rate of
photoproduction (POH,EXP), the rate constant for ⚫OH loss
due to natural sinks (kOH′), and the steady-state concentration
([⚫OH]EXP). Measured rates of ⚫OH
formation and steady-state concentrations were normalized to values
expected under midday, Davis winter-solstice sunlight and were corrected for
the small amount of internal light screening due to light absorption by dissolved
organic matter (DOM):
[⚫OH]=[⚫OH]EXPSλ×j2NB,EXP×j2NB,WIN.
In this equation, Sλ is the internal light screening factor
(Table S1), j2NB,WIN is the rate constant for loss of
2-nitrobenzaldehyde at midday near the winter solstice in Davis (solar
zenith angle = 62∘, j2NB,WIN=0.0070 s-1; Anastasio and
McGregor, 2001), and j2NB,EXP is the measured rate
constant for loss of 2NB on the day of the experiment. ⚫OH
results are in Tables S3–S6.
We also measured ⚫OH steady-state concentrations in squares of
particle filter number 3 using five different dilutions with water (discussed
later). Because these sample volumes were too small to use the benzene
technique, we determined the steady-state concentration of ⚫OH
by measuring the loss of 2-methyl-3-buten-2-ol (MBO) (Sect. S1). We then
measured POH in a 1 cm cuvette using a high benzene concentration (1.5 mM) and determined the rate constant for ⚫OH loss due to natural
sinks by dividing the rate of photoproduction by the steady-state
concentration, kOH′=POH/[⚫OH] (Sect. S1.3). In
contrast to the benzene technique, there was some quenching of ⚫OH by the probe MBO in our PME3 samples; this quenching was most
significant in the most dilute extract, PME3D10. We corrected measured
⚫OH concentrations for quenching by MBO in the PME3 samples
(Sect. S1), and the final, corrected values are given in the Tables mentioned
above.
Singlet molecular oxygen (1O2*)
Singlet oxygen was quantified by measuring the loss of a furfuryl alcohol
(FFA) probe and using heavy water (D2O) as a diagnostic tool (Kaur
and Anastasio, 2017; Anastasio and McGregor, 2001). Briefly, each extract
was divided into two aliquots, acidified to pH 4.2 (±0.2), and
diluted 50:50 using H2O or D2O. Both aliquots were spiked to 10 µM FFA and illuminated in 1 mL quartz tubes. (At this concentration,
FFA should decrease the steady-state concentration of 1O2* in
air-saturated solutions by less than 1 %.) FFA loss was detected using
HPLC-UV (eluent of 10 % acetonitrile: 90 % Milli-Q water, flow rate of
0.6 mL min-1, detection wavelength of 210 nm, and column temperature of
35 ∘C). The loss of FFA followed pseudo-first-order kinetics and
the slope of the plot of ln([FFA]t/[FFA]0) versus time is the
negative of the pseudo-first-order rate constant for loss of FFA
(illustrated in Fig. S2). Loss of FFA in the D2O-diluted aliquot is
faster than in H2O because H2O is the dominant sink for
1O2*, which reacts less quickly with D2O (Bilski et
al., 1997). The differences in the pseudo-first-order rate constants for
loss of FFA between the two aliquots of sample were used to calculate the
steady-state concentration of 1O2* and the rate of singlet oxygen
photoproduction (Anastasio and McGregor, 2001). These were normalized to
values expected in Davis winter-solstice sunlight (i.e.,
[1O2*] and P1O2*) and corrected for internal light
screening using an equation analogous to Eq. (4). 1O2*
measurements are in Table S7.
Oxidizing triplet excited states of organic matter (3C*)
Triplets were measured using the dual-probe technique we developed recently
for fog waters (Kaur and Anastasio, 2018b): two 1.0 mL, pH 4.2
aliquots of each extract were spiked to 10 µM of either syringol (SYR)
or methyl jasmonate (MeJA), and the loss of each probe was measured during
illumination in plugged quartz tubes (Sect. 2.3). The measured
pseudo-first-order rate constant for probe loss (kProbe,EXP′) was
determined as the negative of the slope of the plot of
ln([Probe]/[Probe]0) versus illumination time. Values of
kProbe,EXP′ were normalized to Davis winter-solstice sunlight and
corrected for internal light screening using an analog of Eq. (4); the
resulting rate constants are termed kProbe′ (s-1) (Tables S8 and S9 of
the SI). This pseudo-first-order rate constant for loss of probe represents
the sum of all loss pathways:
kProbe′=kProbe+OH⚫OH+kProbe+1O2*1O2*+ΣkProbe+3Ci*3Ci*+jProbe5+ΣkProbe+OtherOther,
where the first two terms are the contributions of ⚫OH and
1O2* to probe loss; Σ(kProbe+3C*[3C*]) represents the sum of
all triplet contributions to probe loss; jProbe is the
first-order rate constant for direct photodegradation of the probe, which is
negligible for our illumination times (< 4.3×10-6s-1 and 4.8×10-7 s-1 for SYR and MeJA,
respectively, under Davis winter conditions); and Σ(kProbe+Other[Other]) is the sum of contributions from all other
oxidants. As described in Sect. S3, we estimate that these other oxidants
(hydroperoxyl radical/superoxide radical anion, ozone, carbonate radical,
hydrogen ion/aquated electron) contribute 12 % or less of the average
measured syringol loss (Sect. S3) and so are ignored. We can then simplify
and rearrange Eq. (5) to determine the triplet contribution to probe
loss:
kProbe,3C*′=ΣkProbe+3Ci*3Ci*=kProbe′6-kProbe+OH⚫OH+kProbe+1O2*1O2*.
In other probe techniques, the equivalent of Eq. (6) is rearranged so that
∑[3Ci*] can be determined based on the measured value of
kProbe,3C*′ and the literature value of the second-order rate
constant kProbe+3Ci*. However, because triplets represent a suite of
unidentified compounds, there is no one value of kProbe+3Ci*. To estimate
this second-order rate constant in each sample, we used a combination of
rate constants from four model triplets – 2-acetonaphthone (32AN*),
3'-methoxyacetophenone (33MAP*), 3,4-dimethoxybenzaldehyde
(3DMB*), and benzophenone (3BP*) – that roughly span the range of
triplet reactivities in natural samples. We first identified the “best
match triplets”, i.e., the one or two model triplets that match the average
oxidizing triplet reactivity in a given extract. To do this, we determined
the model triplets whose mole-fraction-weighted ratio of second-order rate
constants (i.e., kSYR+3C*/kMeJA+3C*) matches the ratio of
the measured first-order probe loss rate constants due to triplets
(kSYR,3C*′/kMeJA,3C*′) in each extract (for more details,
see Kaur and Anastasio, 2018b). Ratios of the second-order
rate constants (kSYR,3C*/kMeJA+3C*) of the model triplets
range from 1.7 for the most reactive species (3BP*) to 100 for the
least reactive, 32AN* (Table S10). For each extract, we calculated two
mole-fraction-weighted second-order rate constants for triplets (one for
each probe) and used them to estimate the triplet steady-state
concentration:
Σ3Ci*Probe=7kProbe,3C*′χ3C1*×kProbe+3C1*+χ3C2*×kProbe+3C2*,
where χ3C1* and χ3C2* are the mole fractions
of the two best match triplets (3C1* and 3C2*), and
kProbe+3C1* and kProbe+3C2* are the second-order
reaction rate constants of the best model triplet matches. Equation (7) gives us
two estimates of the triplet steady-state concentration, one from each
probe, i.e., ∑[3Ci*]SYR and ∑[3Ci*]MeJA. We averaged the two to obtain the best value for
the triplet steady-state concentration in each extract, ∑[3Ci*].
We next estimated the rate of triplet photoformation (P3C*):
P3C*=Σ[3Ci*]×(k3C*+O2[O2]+(krxn+kQ)[DOC]),
where k3C*+O2 is the average bimolecular rate constant for
quenching of the model triplets by O2 (=2.8×109 M-1 s-1: Table S11 and Canonica et al., 2000),
[O2] is the dissolved oxygen concentration of 284 µM at
20 ∘C (USGS, 2018), krxn+kQ is the overall reaction
and quenching rate constant for triplets by DOC (9.3×107 L (mol C)-1 s-1; see below), and [DOC]
values are in Table S2. At the concentrations we used (10 µM), SYR
and MeJA are negligible sinks for triplets. Measurements for triplets are in
Tables S12 and S13.
For all three photooxidants, the quantum yield of formation was calculated
as
ΦOx=POxRabs,
where POx is the Davis winter-solstice-normalized rate of oxidant
photoproduction and Rabs is the rate of sunlight absorption by the
extract.
PM mass concentration factor (CF)
Due to the volume required for our probe techniques, we extract particles
into Milli-Q water, resulting in extracts that are approximately 1000 times
more dilute than ambient particles. To examine the impact of dilution on
photooxidant concentrations, we extracted sample number 3 in five different
volumes of Milli-Q water (0.5 to 10 mL) and measured ⚫OH,
1O2*, and 3C* steady-state concentrations in the five
extracts. We define the PM mass concentration factor (CF) as the ratio of
(PM mass) / (water mass) in a given extract relative to the most
concentrated extract that we can make:
CF=VMINVEXT+VP,
where VMIN is the minimum experimentally feasible volume of
Milli-Q needed for extraction of one filter square (0.5 mL),
VEXT is the volume of Milli-Q used to extract a given filter
square (0.5 to 10 mL), and VP is the volume of probe stock
solution added (typically 20 µL). Values of CF for the PME3D
extracts ranged from 0.05 (least concentrated) to 0.96 (most concentrated)
and are listed in Table S14.
Uncertainties
In figures, error bars represent ±1 standard error (SE) calculated by
propagating the uncertainties in each term used to calculate the plotted
value.
Results and discussionGeneral extract characteristics
Similar to Davis fogs collected in 1997–1998 (Anastasio and McGregor, 2001)
and 2011 (Kaur and Anastasio, 2017), the most abundant ions in the particle
extracts are ammonium (NH4+, 280–2600 µM) and nitrate
(NO3-, 380–3300 µM) (Table S2). This is expected
since ammonium nitrate is the most significant inorganic component of
wintertime particles in the Central Valley (Herner et al., 2006; Heald et
al., 2012; Young et al., 2016). The average values of NO3- and
NH4+ are not statistically different (p>0.5) between the
current particle extracts (PME) and previous fogs, although the ranges are
much wider in the particle extracts (Table S2). Similar to nitrate, nitrite
is another important source of hydroxyl radical in the aqueous phase
(Anastasio and McGregor, 2001), with an average concentration of 6.9(±2.9)µM in the particle extracts, again statistically similar to
the 2011 fog average. On the other hand, the average concentration of
potassium – commonly used as a tracer for biomass burning (Silva et al.,
1999; Parworth et al., 2017) – is nearly 40 times higher in the particles
than in the 2011 Davis fog samples (p=0.019), suggesting PME enrichment
by residential wintertime wood-burning. This is reflected in the dilute PM
extracts as well: even though most characteristics in the dilute extracts are
similar to fog, the average K+ (38±7µM) in the
dilute PMEs is 10 times higher than the fog value. Dissolved organic carbon
(DOC) in the standard extracts (mean: 3400(±760)µM C) is, on average,
3 times higher than both the dilute extracts and fog.
We employed two field blanks in this study, one each for dilute and standard
extraction conditions. Ions and DOC in both field blanks are lower than
10 % of the corresponding PME sample averages, with a few exceptions
(Table S2).
Light absorption in particle extracts
As shown in Fig. 1a and Table S1, the path-length-normalized absorbance
(α, cm-1) declines exponentially with wavelength, with values
at 300 nm (α300) between 0.27 and 0.58 cm-1 for the
standard extracts PME3–6. The average α300 value is nearly
5 times higher in standard extracts than values in Davis fog samples
(Table S1, Fig. S3, data available in Kaur and Anastasio,
2018a), while the dilute extracts (PME1*, PME2*, and PME3D2.5*) have
absorbances very similar to fog samples. Values of the absorption Ångström
exponent (AAE) for all PM extracts range between 6.2 and 7.9 (Table S1),
similar to those reported previously for water-soluble particulate BrC from
biomass burning (Hecobian et al., 2010; Kirchstetter and Thatcher,
2012). For both the fog and PM extracts the calculated rate of sunlight
absorption between 300 and 450 nm (Rabs) is well correlated with
dissolved organic carbon (DOC) (R2=0.89 and 0.67, respectively; Fig. S4), suggesting that BrC is mainly responsible for light absorption. The
Rabs values for the standard extracts are high, with an average value of
9.1(±4.1)×10-6 mol photons L-1 s-1, 5 times
higher than the dilute extracts and past Davis fogs (Table S1).
Similar to fog (Kaur and Anastasio, 2018b), the average rate of
sunlight absorbance in the standard particle extracts is 17 times higher
than the total formation rates of the three photooxidants (discussed later),
indicating that most of the (photo) energy absorbed is either dissipated via
non-reactive pathways or leads to formation of other products.
(a) Light absorption coefficients, αλ, in
particulate matter extracts (PME) (Eq. 1) and field blanks (FB). The legend
shows the sample identities, arranged from the highest absorbing (top) to
lowest absorbing (bottom) at 300 nm. Solid and dotted lines represent
standard and dilute extracts, respectively (with the latter indicated with an
asterisk; Sect. 2.2). (b) Mass absorption coefficients of DOC in the
particle extracts (Eq. 3).
We next calculated mass absorption coefficients for the organics
(MACDOC) by subtracting the absorbance contributions by nitrite and
nitrate from α and dividing by the DOC concentration (Eq. 3).
Across both standard and dilute extracts, the average (±σ)
MACDOC value at 300 nm is 2.2(±0.7)×104 cm2 (g C)-1,
1.7 times higher than the fog sample average (Figs. 1b
and S3; data available at Kaur and Anastasio, 2018a). Both
α and MACDOC in the PME are generally higher than in fog,
especially at shorter sunlight wavelengths (Fig. S5), although AAE values
are similar in the extracts and fog (Table S1). Since MACDOC accounts
for dilution (Eq. 3), the higher values in PM extracts indicate that
water-soluble organics in particles are either more strongly light-absorbing
(on a per-carbon basis) and/or less diluted with non-absorbing DOC,
compared to those in fog. Our PME mass absorption coefficients at 300 nm are
very similar to values reported for the humic-like fraction of
biomass-burning aerosols in the Amazon basin (Hoffer et al., 2006)
and for the water-soluble organic fractions of rural aerosols (Varga et
al., 2001; Sun et al., 2007).
Compared to the samples, light absorption in the field blanks is negligible,
representing 0.7 % and 3 % of the average α300 in the
standard and dilute extracts, respectively (Table S1).
Hydroxyl radical
The average Davis winter-solstice-normalized rate of ⚫OH
photoproduction (POH) in the standard extracts is 1.2(±0.5)×10-9 M s-1 (i.e., 4.2±1.7µM h-1),
3.3 times faster than the average of previous Davis fogs (Table S3). In
Davis fog, the main sources of ⚫OH were nitrite and nitrate
photolysis, accounting for 70 %–90 % of measured POH (Anastasio
and McGregor, 2001; Kaur and Anastasio, 2017). However, in the standard PM
extracts, nitrite and nitrate together account for an average of only (34±14) % of POH (Table S4), while other unidentified species
account for the remaining (66±14) %. While NO2- and
NO3- concentrations in PME and fog are similar, measured
⚫OH photoproduction rates are much higher in the particle
extracts. The additional sources of ⚫OH likely include
photo-Fenton processes (Arakaki and Faust, 1998) and organic
peroxides (Tong et al., 2016, 2017; Lim and Turpin, 2015),
although there is only a modest correlation between DOC and POH due to
unidentified sources (Fig. S6).
While organic compounds are potentially important sources of ⚫OH
in the particle extracts, they are almost certainly the main ⚫OH
sink, as found previously for atmospheric and surface waters (Brezonik
and Fulkerson-Brekken, 1998; Dong et al., 2010; Arakaki et al., 2013). The
average (±1σ) rate constant for ⚫OH destruction,
kOH′, in the standard extracts is 2.5(±1.1)×106 s-1,
3 times higher than in dilute extracts and fog (Table S3); DOC
concentrations in the standard PM extracts are similarly enhanced, ranging
between 2350 and 4090 µM C (Table S2). Based on our calculations,
inorganic species together account for no more than 10 % of kOH′ in
the PM extracts except for PME3D10, which is the most dilute sample and has
the largest uncertainty (Tables S5 and S6). The rate constant for ⚫OH destruction due to organics, i.e., kOH,org′, obtained by
subtracting contributions of the inorganic sinks from kOH′, is
well correlated with DOC concentrations (R2=0.73) (Fig. S6).
Arakaki et al. (2013) showed that the ratio
kOH,org′/[DOC] is relatively constant in atmospheric waters, with an
average (±1σ) value of 3.8(±1.9)×108 L (mol C)-1 s-1. Our average (±1σ) measured
ratio in all particle extracts is nearly twice as high, 7.1(±2.7)×108 L (mol C)-1 s-1 but not statistically
different (Table S3).
Davis winter-solstice-normalized ⚫OH steady-state concentrations
in all extracts are in the range of (1.7-7.9)×10-16 M,
with an average (±1σ) value of 5.1(±2.4)×10-16 M in the standard extracts (Fig. 2a, Table S3). While both the
⚫OH photoproduction rate and rate constant for ⚫OH
loss are approximately 3 times higher in the standard PM extracts
compared to the dilute extracts and fog, the two enhancements cancel out to
give ⚫OH steady-state concentrations that are similar across all
three sample types. This relative consistency of ⚫OH
concentrations has been reported for a wide variety of atmospheric waters
(Arakaki et al., 2013); our average
concentration is similar to most of these past results (Fig. S7). As we
discuss in Sect. 3.6, transport of ⚫OH from the gas phase is
also an important source to drops and particles, but its importance
decreases with decreasing particle size.
Measured steady-state concentrations of (a) hydroxyl
radical, (b) singlet molecular oxygen, and (c) oxidizing
triplet excited states of organic matter in particle extracts, along with
previous measurements made in Davis fogs collected between 1997–1998 and
2011–2012 (Anastasio and McGregor, 2001; Kaur and Anastasio, 2017, 2018b).
All concentrations are normalized to Davis midday, winter-solstice sunlight.
Dilute particle extracts are indicated with an asterisk. Dashed lines
represent sample averages.
We also calculated the quantum yield of hydroxyl radical formation, i.e.,
the fraction of absorbed photons that result in ⚫OH formation
(Eq. 9). The average (±1σ) value of ΦOH in all
particle extracts is (0.014±0.010) %, which is statistically
similar to the average fog result (Table S3): while photoformation rates of
⚫OH increase from fog to standard particle extracts (Table S3),
light absorption shows a similar trend (Table S1).
The rate of ⚫OH photoproduction in the field blanks is
negligible, representing 1 % and 6 % of the average rate in standard
and dilute extracts, respectively. The rate constants for ⚫OH
destruction (kOH′) in the standard (FB2) and dilute (FB1) field blanks
represent 10 % and 43 % of the corresponding PME averages. The latter
result is puzzling, since the concentrations of ⚫OH sinks
measured in FB1 (i.e., DOC and NO2-; Table S2) are much lower
relative to the extract. We discuss measurements of kOH′ in the blanks in
more detail in Sect. S2. We do not subtract the field blank results for
kOH′ from the corresponding PM extract values and thus our sample results
are upper bounds.
Singlet molecular oxygen
The average (±1σ) Davis winter-solstice-normalized
1O2* concentration in the dilute extracts (2.4(±0.7)×10-13 M)
is very similar to the previous fog average (Fig. 2b). This is likely because brown carbon is the source of 1O2*
(Faust and Allen, 1992; Zepp et al., 1977) and the DOC concentrations in
the fog and dilute extracts are very similar (Table S2). On the other hand,
the average [1O2*] in the more concentrated, standard PM extracts
(PME3–6) is 1.6(±0.5)×10-12 M, nearly
7 times higher than the averages in Davis fog and dilute extracts (Fig. 2b,
Table S7). This is because the standard extracts have higher DOC
concentrations but the same major 1O2* sink, i.e., water. Across
all fog and particle extracts, the rate of singlet oxygen formation
(P1O2*) is strongly correlated with the rate of sunlight absorption
(Rabs) (R2=0.94; Fig. 3a), although this correlation is not
evident in only the fog samples (Kaur and Anastasio, 2017). As seen for
⚫OH, quantum yields of 1O2* are similar in the
extracts (standard and dilute) and fog (Table S7); the slope of the
P1O2* versus Rabs correlation line (Fig. 3a) gives an overall
quantum yield of 1O2* of (3.8±0.2) %; i.e., across all
samples roughly 4 % of the photons absorbed lead to the formation of
singlet oxygen. This is nearly 260 times higher than the average quantum
yield of ⚫OH. Our quantum yields for singlet oxygen formation in
PM extracts are similar to values previously reported for surface water
organics (e.g., 2 %–5 % in Zhou et al. (2019).
Correlations between (a) the rate of singlet oxygen
photoproduction normalized to Davis winter-solstice sunlight
(P1O2*), (b) the rate of triplet
photoproduction normalized to Davis winter-solstice sunlight
(P3C*), and the rate of light absorption (Rabs)
between 300 and 450 nm. Triplet rates for the fog samples were adjusted to
account for the small DOC sink for triplets (Eq. 8). The P/Rabs
ratios (±1 SE) listed are unitless and represent the quantum yields.
Triplet excited states of organic matter (3C*)
We also determined the kinetics and concentrations of oxidizing
triplets by measuring the loss of two probes, syringol (SYR) and methyl
jasmonate (MeJA) (Fig. S8). In the standard extracts, the average (±σ) Davis winter-normalized rate constants for loss of SYR and MeJA
(kProbe′) are (4.3±1.7)×10-4 s-1 and (2.6±0.7)×10-5 s-1, which are equivalent to average
lifetimes of 0.70(±0.20) and 11(±3) h, respectively (Tables S8 and S9). Triplet probe lifetimes in
the dilute extracts are approximately
3 times longer and are very similar to fog values, indicating that the
main source of triplet precursors to fog drops is the BrC present in the fog
condensation nuclei rather than mass transport from the gas phase.
We correct the loss of triplet probes for oxidation by hydroxyl radical and
singlet molecular oxygen (Eq. 6). In the standard extracts, 1O2*
and ⚫OH account for an average of 13 % and 3 % of SYR
loss, respectively (Table S8, Fig. S9); for methyl jasmonate, the
corresponding contributions are 37 % and 13 %.
Next we use the ratio of the pseudo-first-order rate constants for probe
losses by triplets, i.e., kSYR,3C*′/kMeJA,3C*′, to
characterize the average reactivity of the triplet species in each sample: a
ratio close to 1 indicates higher reactivity, while a higher ratio indicates
lower reactivity. The kProbe,3C*′ ratio (i.e., kSYR,3C*′/kMeJA,3C*′)
in all extracts ranges between 7.9 and 37 (Table S12),
which is a narrower range than in Davis fog samples (7.5 to 110)
(Kaur and Anastasio, 2018b). Based on the kProbe,3C*′
ratios, triplets in the PM extracts generally have an average reactivity
similar to model aromatic triplets 3'-methoxyacetophenone (33MAP*) and
3,4-dimethoxybenzaldehyde (3DMB*) (Fig. 2c, Table S12). The average
(±σ) triplet steady-state concentration in the standard
extracts is 1.0(±0.4)×10-13 M (Fig. 2c, Table S13), which is nearly twice the
fog average but not statistically
significantly different. If we consider only the PM and fog samples that
have triplet reactivities similar to 33MAP* and 3DMB* (i.e., the
green average lines in Fig. 2c), the average triplet concentration in the
standard PM extracts is nearly 4 times greater than in fog (Table S2),
similar to the ratio of DOC concentrations.
Effect of change in
aqueous particle mass concentration (i.e., sample dilution) for sample PME3
on (a) rate of light absorption and the steady-state concentrations
of (b) hydroxyl radical, (c) singlet molecular oxygen, and
(d) oxidizing triplet excited states of organic matter. The last
panel shows both linear (dotted) and hyperbolic (dashed) fits to the data. In
each plot the x axis is a measure of sample dilution, with higher
concentration factors corresponding to more concentrated particle extracts
(Eq. 10).
In the standard extracts the average concentration of oxidizing triplets is
16 times lower than [1O2*] but nearly 200 times higher than
[⚫OH] from in situ sources. Our measurements of oxidizing triplet
concentrations lie at the higher end of measured and estimated
concentrations of total (i.e., oxidizing and energy transfer) triplets in
surface waters, 10-15–10-13 M (Zepp et al., 1985; Grebel
et al., 2011). The average (±1σ) rate of triplet
photoformation, P3C*, is 2.0(±1.0)×10-7 M s-1 (i.e., 720(±360)µM h-1) in the standard extracts
(Table S13). Thus the ratios of the average production rates for
1O2*, 3C*, and ⚫OH are 290:170:1. There is a
fair correlation between P3C* and Rabs(Fig. 3b), similar to
the case for P1O2* (Fig. 3a), which is consistent with BrC as the source of
triplets. Sample-to-sample variability in the fraction of the total triplet
pool that can oxidize organics likely causes the P3C* correlation
(R2=0.81) to be weaker than that of P1O2* (R2=0.94).
The average (±1σ) oxidizing triplet quantum yield in standard
extracts is (2.4±1.0) % (Table S13), approximately 2 times
lower than the value for 1O2* (Table S7) but 150 times higher than
for ⚫OH (Table S3). Our triplet quantum yields are within the
wide range of values that has been reported for surface waters,
approximately 0.4 %–7 % (Zepp et al., 1985; Grebel et al., 2011; Zhou
et al., 2019).
Triplet excited states have two main reaction pathways: energy transfer
(e.g., to make 1O2*) and electron transfer (e.g., to oxidize a
phenol) (Zepp et al., 1985; McNeill and Canonica, 2016; Kaur and
Anastasio, 2018b). Essentially all triplets possess enough energy to form
1O2* (McNeill and Canonica, 2016), but only a subset of the
triplet pool can oxidize organics via electron transfer. Thus the quantum
yield of 1O2* can be used to estimate the total triplet quantum
yield, while our measurements of Φ3C* constrain the smaller
subset of oxidizing triplets (assuming energy transfer from triplets is the
only source of 1O2*). The quantum yield for all triplets can be
estimated as Φ1O2*/fΔ, where fΔ, the
fraction of 3C* interactions with dissolved O2 that yield
1O2*, is approximately 0.5 (McNeill and Canonica, 2016; Kaur
and Anastasio, 2018b). For our standard extracts, the average value of Φ1O2*/fΔ is 0.078±0.019;
i.e., approximately 8 % of the photons absorbed by brown carbon chromophores make a triplet
excited state. Next we use the ratio Φ3C*/(Φ1O2*/fΔ) to estimate the fraction of all triplets that can
participate in electron-transfer (oxidation) reactions. The average value of
this fraction is 0.35±0.12 for all the PM extracts; i.e., on
average, approximately a third of all triplets are oxidizing (range = 18 %–50 %; Table S13).
Predicting photooxidant concentrations in ambient particle water
Since our particle extracts are approximately 1000 times more dilute than
ambient Davis particles during winter, we want to be able to estimate oxidant
concentrations under ambient conditions. To do this we first measured
photooxidant concentrations as a function of dilution for the same sample and
then extrapolated our results to ambient particle conditions. For the first
step, we extracted squares of filter number 3 using five different volumes of
Milli-Q water, from 10 to 0.50 mL (Sect. 2.5.4), corresponding to aqueous PM
mass concentration factors (CF) of 0.05 (most dilute) to 0.96 (most
concentrated) (Eq. 10). For this sample, these are equivalent to PM solute
mass/water mass ratios typical for dilute to very concentrated cloud or fog
drops, i.e., (0.35-8.4)×10-4µg PM/µg H2O; in comparison, ambient
particles have ratios of approximately
1 µg PM/µg H2O and higher (Table S14). The rate
of light absorption increases linearly with CF (Fig. 4a), indicating that BrC
and other chromophores are efficiently extracted for all Milli-Q volumes
employed.
The change in photooxidant concentration with CF depends on how the ratio of
sources and sinks varies with dilution. In the case of hydroxyl radical,
POH and kOH′ both increase as extracts get more concentrated (i.e.,
as CF increases), resulting in an ⚫OH concentration that is
noisy but essentially unchanged over the 20-fold increase in concentration
factor (Fig. 4b). This result is consistent with the relatively constant
[⚫OH] in our particle extracts relative to fog (Fig. 3a, dashed
black lines) and with prior results showing very similar concentrations for
rain, cloud, fog, and marine PM extracts (Fig. S7 and Arakaki et al., 2013).
To estimate [⚫OH] in particle liquid water, we use the
measured linear dependences of the rate of ⚫OH
photoproduction (POH) and loss rate constant (kOH′)
on concentration factor, which corresponds to a measured PM mass/water mass
ratio (Fig. S10). Under a typical wintertime, Central Valley ambient particle
water condition (1 µg PM/µg H2O), the in situ
POH and kOH′ are estimated to be 4.2×10-6 M s-1 and 5.5×109 s-1, respectively
(Fig. S10). This extrapolation of only aqueous processes gives an
⚫OH concentration in particle water of 7.6×10-16 M, which is similar to the average of the measurements in Fig. 4b.
However, this estimate does not include the contribution of mass transport of
gas-phase ⚫OH to the particles. As detailed in Sect. S4, we
estimate that the rate of ⚫OH gas-to-particle transport
under particle conditions is 4.2×10-7M s-1, which is
approximately 10 % of the ⚫OH photoformation rate from
aqueous sources. Figure 5 shows estimated ⚫OH steady-state
concentrations considering both aqueous reactions and gas-phase mass
transport across a wide range of drop-to-particle conditions:
[⚫OH] decreases from 5.4×10-15 M under dilute
drop conditions (3×10-5µg PM/µg H2O) to 8.4×10-16 M under the much more concentrated particle conditions
(1 µg PM/µg H2O). The calculated
[⚫OH] values (orange line in Fig. 5) are higher than our
measured values (orange points in Fig. 5) because of the gas-phase mass
transport source. Changes in this source are also responsible for the slow
decrease in calculated [⚫OH] as conditions become more
concentrated (i.e., as µg PM/µg H2O increases).
In the case of singlet oxygen, steady-state concentrations increase
proportionally with PM mass concentration factor (Fig. 4c). Our
interpretation of this result is that the concentrations of
1O2* sources (i.e., BrC) increase proportionally with
concentration factor, while the concentration of the main sink for
1O2* (i.e., water) is essentially unchanged. At higher PM
mass/water mass ratios, we calculate that organic compounds become a
significant sink for singlet oxygen (Sect. S4), leading to a plateau in
[1O2*] under the more concentrated conditions of particles
(Fig. 5). This extrapolation for ambient PM conditions
(1 µg PM/µg H2O) predicts an
1O2* concentration in particle water of 1.6×10-10 M (Table S15, Fig. 5), which is 2400 times higher than our
prediction for dilute fog/cloud drops. While there are no other measurements
of 1O2* in particles, similar enhancements in
1O2* concentrations (up to a factor of roughly 104)
have been found in cases where 1O2* precursors become
highly concentrated, e.g., in liquid-like regions of ice (Bower and
Anastasio, 2013) and in regions of hydrophobic chromophoric dissolved organic matte (CDOM) in solution (Latch and
McNeill, 2006).
Dependence of photooxidant concentrations on particle mass/water
mass ratio (i.e., aqueous particle concentration) in extracts of sample PME3.
Solid diamonds are measured values under experimental dilution conditions
(typical for clouds or fogs), while the open circles are values expected in
more concentrated particle liquid water based on the dashed line
extrapolations. For the solid symbols, error bars (±1σ) are often
smaller than the symbols. Data labels on the diamonds (e.g., D10) represent
the water volume used to extract the PME3 filter square (Sect. 2.5.4). The
dashed line extrapolations include the contributions from both aqueous
processes and interactions with the gas phase (Sect. S4). For oxidizing
triplets, two extrapolation scenarios are shown: a best estimate (lower line)
and a high estimate (upper line), as described in Sect. S4 and Table S15.
An increase in extract concentration (i.e., CF) also increases the triplet
steady-state concentration (Fig. 4d), but there is greater uncertainty in
this trend, in part because there is more uncertainty in measurements of
Σ[3Ci*]. As described in Sect. S4, we fit the data in
Fig. 4d with a hyperbolic regression under two cases: (1) a best fit, where
parameters were adjusted to minimize the regression error; and (2) a
high-estimate fit, where parameters were adjusted so that the regression line
passed near the upper portion of the error bar for the CF 0.96 data point.
These are the dashed and dotted lines in Fig. 4d, respectively. In both cases
the triplet concentration initially rises more quickly with CF but then
approaches a plateau at higher CF values. Our interpretation of this behavior
is that as CF increases, [DOM] and P3C* increase linearly but
the dominant triplet sink switches from dissolved O2 at low CF to DOM at
high CF. Wenk et al. (2011, 2013) have shown that surface water
DOM can quench triplets when DOM concentrations are greater than 20 mg C L-1; in the PME3D extracts of Fig. 4, DOM ranges from 4.3 to
86 mg C L-1 (Table S2). Based on our previous work, we believe that phenols
from wood combustion are reacting with (and physically quenching) triplets in
our PM extracts (Smith et al., 2014, 2015). As described in Sect. S5, by fitting a kinetic model to our triplet dilution data we estimate that
the total (reaction and quenching) rate constant for triplets with DOC in the
PME3 extracts is 9.3(±1.3)×107 L (mol C)-1 s-1.
These two extrapolations result in oxidizing triplet concentrations under PM
conditions (1 µg PM/µg H2O) of 2.3×10-13 M (best fit) and 1.3×10-11 M (high
estimate). Taken together with the other oxidant measurements, we estimate
that the ratio of 1O2*:3C*:⚫OH concentrations
in ambient particle water is approximately 105:104-102:1.
Implications
Our dilution experiments suggest that ⚫OH, 1O2*, and
3C* behave very differently as the PM/water ratio increases from cloud
and fog drop conditions to water-containing particles (Fig. 5). To
understand what this implies for the fate of organic compounds, we estimated
the gas–aqueous partitioning and lifetimes of five model organic compounds
for both fog and aqueous aerosol (Fig. 6). We consider reactions with two
gas-phase oxidants (⚫OH, O3) and four aqueous-phase
oxidants (⚫OH, O3, 1O2*, 3C*) (Table S16).
Our model organics represent two groups in terms of gas–aqueous
partitioning: one group with modest Henry's law constants (KH∼104 M atm-1) and one with much higher
values (KH=109-1011 M atm-1) (Fig. 6 and
Table S17).
Fate of five model organic compounds – (1) syringol, (2) methyl
jasmonate, (3) tyrosine, (4) 1,2,4-butanetriol, and
(5) 3-hydroxy-2,5-bis(hydroxymethyl)furan – under liquid water content
conditions for fog (left of vertical dashed line;
1 g H2O/m3 air) and PM (right of line;
20 µg H2O/m3 air). Estimated Henry's law constants
for the compounds (in units of M atm-1) are in parentheses beneath each
structure. In panel (a) the columns represent overall lifetimes of
the organics and the open circles represent the fractions in the aqueous
phase. Panel (b) shows the fraction of each compound lost via
various gas and aqueous pathways. The triplet contribution in PM is estimated
using the lower-bound triplet concentration extrapolation, i.e., 1.3×10-13 M (Fig. 5). Oxidant concentrations and rate constants are in
Tables S16 and S17.
Figure 6a shows the overall lifetimes of the five model organics and the
fraction of each present in fog and PM. For the organics with the lowest
KH values, approximately 10 %–20 % is present in the aqueous phase
under fog conditions, but almost none is present in the particle liquid
water. Consequently, gas-phase reactions dominate their overall lifetimes,
which are approximately 2 to 3 h for both fog and PM conditions. In
contrast, the compounds with high KH values are partitioned strongly to
the aqueous phase for both the fog and PM scenarios (Fig. 6a). But due to
the overall higher oxidant concentrations in PM, the lifetimes of these
organics are predicted to be shorter – sometimes by large factors – in PM
than in fog (Fig. 6a, Table S17). Additionally, their main sinks change from
fog to PM, shifting from aqueous ⚫OH, O3, and
1O2* in fog to being generally dominated by 1O2* in PM
water (Fig. 6b). For example, for tyrosine (compound 3), the predominant
sink changes from aqueous O3 in fog to 1O2* in
water-containing particles, while its lifetime decreases from 1.6 to 0.04 h (Fig. 6b and Table S17).
While triplets are negligible oxidants for individual organics in particles
under the conditions of Fig. 6, the picture changes if we move from the Fig. 6 triplet concentration of 2.3×10-13 M to
the high-estimate concentration (1.3×10-11 M; Fig. 5). Under
this condition aqueous oxidation still dominates the loss of the
high-KH compounds, but 3C* becomes a much more important oxidant in
PM and organic lifetimes get shorter by factors of 3 to 180 compared to fog
(Fig. S11). While there is large uncertainty in the triplet concentrations
in PM, Figs. 6 and S11 both indicate that aqueous oxidants can control the
fate of highly soluble species in aerosols and that organic lifetimes can be
shorter in PM because of an enhancement in oxidant concentrations.
Finally, despite the uncertainty in triplet concentration under particle
conditions, the formation rate of 3C* is fast enough – and the
fraction of triplets lost via reaction with organics is high enough – that
triplets represent, in aggregate, a significant sink for organic compounds
in particles. While these two ideas might seem contradictory, we propose
that the suite of reactive organic compounds is suppressing the triplet
concentrations enough that 3C* are small sinks for individual organic
compounds but are significant sinks when integrated over all of the
reactive organics. As described in Sect. 3.5, the formation rates for
1O2*, 3C*, and ⚫OH have a ratio of 290:170:1,
respectively, in the PM extracts; based on our dilution experiments (Fig. 4), we expect similar ratios in ambient particle liquid water. Since organic
compounds appear to be the major sinks for all three oxidants under ambient
particle conditions, and since each oxidant is at a steady state, the ratio of
formation rates is approximately the same as the ratio of total rates of
organic compound oxidation by each oxidant. Thus, while the steady-state
concentration of 3C* might be significantly lower than that of
1O2* in particle water, both oxidants appear to be similarly
important in the overall processing of particulate organics. In contrast,
the total rate of oxidation of organics by ⚫OH appears to be
200–300 times slower, although ⚫OH will be relatively more
important for less reactive organics. This comparison suggests that both
singlet molecular oxygen and triplet excited states are important for the
processing of organic compounds in particle liquid water.
Conclusions and uncertainties
We have made the first measurements of singlet molecular oxygen and
oxidizing triplet states in aqueous extracts of particles, in addition to
measuring hydroxyl radical. Under our standard condition, the particle
extracts are approximately 3 times more concentrated than wintertime
Davis fog waters. The extracts contain significant amounts of brown carbon,
with DOC-normalized mass absorption coefficients between roughly 15 000 and
30 000 cm2 (g C)-1 and absorption Ångström exponents of 6.2 to 7.9.
Upon absorbing light, BrC and other chromophores in the samples form
significant amounts of ⚫OH, 1O2*, and 3C*. While
concentrations of ⚫OH in the PM extracts are in the same range
as found in fog waters, concentrations of the oxidants derived primarily
from BrC – i.e., 1O2* and 3C* – are higher in the
extracts compared to in fog by factors of approximately 7 and 2,
respectively.
Dilution experiments indicate that the ⚫OH concentration is
essentially independent of the PM mass concentration in solution, consistent
with previous results, while 1O2* and 3C* increase
with increasing aqueous PM concentration. Extrapolating our findings to the
much more concentrated conditions expected in ambient particle water
suggests that hydroxyl radical concentrations in particles will be somewhat
lower than values in fog and cloud drops, a result of size-dependent changes
in mass transport from the gas phase. In contrast, oxidants formed from
illumination of brown carbon will be enhanced in particles: moving from very
dilute drops (3×10-5µg PM/µg H2O) to
concentrated particles (1 µg PM/µg H2O) we predict that the
concentration of 1O2* will increase by approximately a factor of
2400, while concentrations of oxidizing triplets will increase between a
factor of 30 and 2000. The higher 1O2* concentrations predicted in
particles lead to a large decrease in the lifetimes of highly water soluble
organic compounds compared to foggy conditions, even though the liquid water
content of the particles is roughly 104 times lower than the fog. It
appears that triplets are also more significant oxidants for individual
organic compounds in PM than in fog, but there is too much uncertainty in
our data to properly assess this increase. In contrast, ⚫OH is
important for the oxidation of organics that react only slowly with
1O2* and 3C* but is otherwise a minor oxidant for the
organics we considered since the particulate ⚫OH concentration
is quite low.
While our results suggest that oxidants derived from brown carbon are very
significant in water-containing particles, there are several large
uncertainties. Most significantly, because of experimental limitations on
the maximum PM concentration in our extracts, we need to extrapolate oxidant
measurements over a very large range (approximately a factor of 1000) to
predict oxidant levels in ambient water-containing particles. This results
in very large uncertainties. As part of this uncertainty, it is difficult to
assess how reactions in the particles might suppress concentrations of
1O2* and 3C*. Secondly, while calculations suggest that
unaccounted oxidants are minor sinks for our triplet probes, if these
species were important our triplet concentrations would be biased high.
Finally, it is unclear how widely our results, which are for one season and
one location, can be applied to other particles containing brown carbon.
However, PME3, our one sample collected during both daytime (with little
biomass burning) and night (with significant biomass burning), had similar
reactivity to the other samples, which were collected only at night.
Regardless, since these are the first measurements of 1O2* and
3C* in particles, strengthening and improving our findings
requires more measurements, especially for other seasons and locations.
Measurements under much higher particle mass/water mass ratios, ideally
under ambient conditions, are also needed.
Despite the uncertainties, our results indicate that BrC-derived
photooxidants such as singlet molecular oxygen and organic triplet excited
states can be important oxidants in atmospheric particles. Currently these
oxidants are not included in atmospheric models, although our calculations
suggest that 1O2* and 3C* can dominate the
processing of highly soluble organic molecules in aerosol particles.
Data availability
Light absorption data have been submitted to the data
repository Pangaea, cited in the text, and are available at
10.1594/PANGAEA.896422 (Kaur et al., 2018). Other data are available
upon request.
The supplement related to this article is available online at: https://doi.org/10.5194/acp-19-6579-2019-supplement.
Author contributions
CA and RK developed the research goals and designed the experiments. KB lent
and set up the sampler, while RK, CA, and WJ collected samples. RK, JRL, and
SH performed the photochemistry experiments while WJ analyzed ions and OC. RK
analyzed the data and prepared the manuscript with contributions from all
co-authors. CA reviewed, wrote portions of, and edited the manuscript. CA and
QZ provided supervision and oversight during the experiments and writing.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
We thank Ann Dillner, Alexandra Boris, and April Chaney (UC
Davis, Air Quality Research Center) for use of a microbalance and an
anonymous reviewer for extensive and helpful comments.
Financial support
This research has been supported by the National
Science Foundation (grant no. AGS-1649212); the California Agricultural
Experiment Station (project CA-D-LAW-6403-RR); the University of California,
Santa Cruz (Guru Gobind Singh Fellowship); and the University of California,
Davis (Donald G. Crosby Graduate Fellowship in Environmental Chemistry as
well as James and Rita Seiber International Student Support Award).
Review statement
This paper was edited by Manabu Shiraiwa and reviewed by three anonymous referees.
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