Photooxidants from brown carbon and other chromophores in illuminated particle extracts
Authors: Kaur Richie , Labins Jacqueline R. , Helbock Scarlett S. , Jiang Wenqing , Bein Keith J. , Zhang Qi , Anastasio Cort 
Published: May 2019

Abstract. 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.