Aging of secondary organic aerosol from α-pinene ozonolysis: Roles of hydroxyl and nitrate radicals

摘要

This work investigates the oxidative aging of preformed secondary organic aerosol (SOA) derived from α-pinene ozonolysis (~100 ppb_v hydrocarbon HC_x with excess of O_3) within the University of California-Riverside Center for Environmental Research and Technology environmental chamber that occurs after introduction of additional hydroxyl (OH) and nitrate (NO_3) radicals. Simultaneous measurements of SOA volume concentration, hygroscopicity, particle density, and elemental chemical composition (C:O:H) reveal increased particle wall-loss-corrected SOA formation (1.5, 7.5, and 15.1), increase in oxygen-to-carbon ratio (O/C; 15.6, 8.7, and 8.7), and hydrophilicity (4.2, 7.4, and 1.4) after addition of NO (ultraviolet UV on), H_2O_2 (UV on), and N_2O_5 (dark), respectively. The processing observed as an increase in O/C and hydrophilicity is attributed to OH and NO_3 reactions with first-generation vapor products and UV photolysis. The rate of increase in O/C appears to be only sufficient to achieve semivolatile oxygenated organic aerosol (SV-OOA) on a day time scale even at the raised chamber radical concentrations. The additional processing with UV irradiation without addition of NO, H_2O_2, or N_2O_5 is observed, adding 5.5 wall-loss-corrected volume. The photolysis-only processing is attributed to additional OH generated from photolysis of the nitrous acid (HONO) offgasing from chamber walls. This finding indicates that OH and NO_3 radicals can further alter the chemical composition of SOA from α-pinene ozonolysis, which is proved to consist of first-generation products.Secondary organic aerosol (SOA) may undergo aging processes once formed in the atmosphere, thereby altering the physicochemical and toxic properties of aerosol. This study discusses SOA aging of a major biogenic volatile organic compound (VOC; α-pinene) after it initially forms SOA. Aging of the α-pinene ozonolysis system by OH (through NO or H_2O_2 injection), NO_3 (through N_2O_5 injection), and photolysis is observed. Although the reaction rate appears to be only sufficient to achieve semivolatile oxygenated organic aerosol (SV-OOA) level of oxygenation on a 1-day scale, it is important that SOA aging be considered in ambient air quality models. Aging in this study is attributed to further oxidation of gas-phase oxidation products of α-pinene ozonolysis.Supplemental Materials: Supplemental materials are available for this paper. Go to the publisher's online edition of the Journal of the Air Waste Management Association for information on the referenced α-pinene ozonolysis reaction and chamber reactor temperature.