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

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 NO_x 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.