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Aqueous Atmospheric Organic Processing: Effects of Fog and Cloud Composition

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June 8, 2016
Ali Boris
Hosted by Jeff Collett (advisor), Jeff Pierce, Sonia Kreidenweis, Delphine Farmer (Chemistry)

Abstract

Cloud and fog droplets are well-suited venues for organic reactions leading to the formation of suspended particulate matter in the atmosphere. Suspended particular matter formed through aqueous reactions is called "aqueous secondary organic aerosol" or aqSOA, and can interact with solar radiation and adversely impact human ecosystem health. Although atmospheric observations and lab simulations have verified the formation of aqSOA, little is known about where and when it occurs in the atmosphere. The organic (carbonaceous) reactions leading to aqSOA formation also degrade chemicals in the atmosphere, impacting the potential health effects of fog water deposited to ecosystems and crops. In the present work, studies are described that approach these aqueous oxidation reactions from field and lab perspectives, capturing both complex and simple experiments. Some results will be presented that capture the dynamics of aqSOA formation from studies of in-situ fog chemistry, but the lack of control over environmental variables in these observations will be highlighted. Lab-based reactions of fog and cloud water will also be presented, which oppositely underscore the missing variables in such simplified lab experiments. Despite the need for more advanced experimental design to quantify aqSOA formation and identify its sensitivities to real atmospheric variables, these field and lab approaches have garnered new insight into some key aspects of aqueous oxidation.

Fog at Baengnyeong Island (BYI) in the Yellow Sea of Korea was collected in July 2014. Fog chemistry was exemplary of aged atmospheric components: sulfur was almost entirely oxidized (98.9 to 99.8% was present as S(VI) versus S(IV)), and peroxides, which can serve as oxidants, were depleted. Organic acids at times accounted for >50% of the total organic carbon (TOC) by carbon mass, indicating that organic matter was highly oxidized. Although formic and acetic acids were the most abundant, concentrations of ten out of the 18 organic acids quantified were above 1 μM. Some organosulfate and nitroxyorganosulfate species were additionally observed, which may have formed during aqueous reactions in the fog or in humid conditions as air traveled to BYI. Back trajectories demonstrated that the relative humidities of the air masses arriving at BYI were typically >80%, suggesting that oxidation could have taken place in the aqueous phase.

The Southern California coast is frequently foggy during the summer months, but in contrast to BYI, is closer to many atmospheric chemical emissions sources. Fog water was collected at Casitas Pass (CP) near Ventura, California in June 2015. Regional oil drilling and/or refinery emissions influenced the composition of foggy air, as did biogenic and marine emissions. Only 20% of TOC on average was contributed by organic acids, suggesting influence of fresher organic emissions than observed at BYI. After 3-5 hours of foggy conditions, however, organosulfates and nitroxyorganosulfates were observed, suggesting possible in-fog oxidation. A contrast between the 2015 study and a 1985/6 study demonstrated improved air quality compared to 1985/6, with lower concentrations of anthropogenically derived species (NH4+, NO3-, SO42-, acetate, formate, and formaldehyde), but similar concentrations of naturally derived species (Na+, Cl-, Ca2+, and Mg2+).

Lab work involving aqueous oxidation within real cloud water revealed that organic constituents of cloud water caused oxidation reactions to slow due to competition for photons and/or oxidant. Inorganic species (NH4+, SO42- and NO3-) at concentrations relevant to polluted cloud water did not have a statistically significant effect on oxidation. Mechanisms of oxidation were also surprisingly unaffected by cloud water components: similar low molecular mass organic acids were observed as products of oxidation in pure and cloud water.

Oxidation of real cloud water sample constituents in the lab revealed that organosulfate species were produced when sufficient SO42- and organic species concentrations were present. Four fog and cloud water samples were oxidized, demonstrating different oxidation regimes: a BYI fog was clearly more aged such that organosulfate esters were formed; cloud water from Mount Tai, China contained biomass burning and anthropogenic aromatic emissions and produced organic acids similar to those observed from nitrophenol chemical standard oxidations; and fog water from CP containing fresher emissions produced mainly low molecular mass organic acids.

The aqueous oxidation of biomass burning emissions collected using a mist chamber resulted in the formation of a variety of low molecular mass organic acids. No apparent structure-activity relationship was observed: aliphatic and aromatic species were oxidized at similar rates when exposed toŸOH radicals. The degradation of potentially toxic organonitrate species as well as net production of semi-volatile organic acid products were observed, demonstrating that in-cloud oxidation of biomass burning emissions likely contributes to the chemical evolution and organic aerosol mass within smoke plumes.

Overall, there is still a need for advanced experiment development in the field of aqueous organic atmospheric chemistry. The finding that physical processes obscured effects of aqueous reactions during fog field studies should, likewise, guide future field work toward the concurrent measurement of microphysical parameters and possible development of higher efficiency techniques for droplet collection and/or real-time chemical analyses. However, the combination of bulk reactions and fog studies employed within this thesis has allowed the effects of real fog and cloud water chemistry on aqSOA formation to be demonstrated. The common oxidation products identified under most aqueous atmospheric regimes, including low molecular mass organic acid species, but specific environmental requirements for other products such as organosulfates, should guide future research in identifying molecular tracers of aqSOA and sensitivity studies of aqSOA formation to environmental factors.