American Chemical Society
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Photodegradation of Secondary Organic Aerosols by Long-Term Exposure to Solar Actinic Radiation

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journal contribution
posted on 2020-06-11, 17:04 authored by Vahe J. Baboomian, Yiran Gu, Sergey A. Nizkorodov
Sunlight-driven chemical transformations of secondary organic aerosol (SOA) are important for understanding the climate- and health-relevant properties of atmospheric particulate matter, but these photochemical processes are not well understood. We measured the photodegradation rates of SOA by observing condensed-phase photochemical processes over many days of UV exposure. The experiments relied on a quartz crystal microbalance to quantify the mass loss rate from SOA materials prepared by ozonolysis of d-limonene and α-pinene and photo-oxidation of toluene under either high or low NOx conditions. We observed that 254 nm irradiation degraded SOA almost entirely after 24 h. The mass loss rates were higher for toluene-derived SOA, which absorbs strongly at 254 nm. Irradiation at 305 nm, which is more relevant for the troposphere, resulted in larger mass loss rates from SOA generated from α-pinene and d-limonene, even though toluene-derived SOA had a higher absorption coefficient. In all 305 nm irradiation experiments, the initial mass loss rate was high (corresponding to 1–5% fractional mass loss per hour), but it slowed down after 24 h of irradiation, with a photorecalcitrant fraction of SOA degrading much slower (<1% fractional mass loss per hour). The mass loss rates were observed to increase at a higher relative humidity because volatile photoproducts could diffuse out of SOA faster. Long-term changes in the chemical composition of limonene ozonolysis SOA were examined using high-resolution electrospray ionization mass spectrometry and revealed a more complex mixture of species after photodegradation compared to the initial SOA. The compounds in the photodegraded sample had on average lower molecular weights, lower H/C ratios, and higher O/C ratios compared to the compounds in the un-photolyzed sample. These experiments confirm that condensed-phase photochemistry is an important aging mechanism for SOA during long-range transport.