Photochemical Aging of α-pinene and β-pinene Secondary Organic Aerosol formed from Nitrate Radical Oxidation

The nitrate radical (NO3) is the dominant nighttime oxidant in most urban and rural environments and reacts rapidly with biogenic volatile organic compounds to form secondary organic aerosol (SOA) and organic nitrates (ON). Here, we study the formation of SOA and ON from the NO3 oxidation of two monoterpenes (alpha-pinene and beta-pinene) and investigate how they evolve during photochemical aging. High SOA mass loadings are produced in the NO3+beta-pinene reaction, during which we detected 41 highly oxygenated gas- and particle-phase ON possessing 4 to 9 oxygen atoms. The fraction of particle-phase ON in the beta-pinene SOA remains fairly constant during photochemical aging. In contrast to the NO3+beta-pinene reaction, low SOA mass loadings are produced during the NO3+alpha-pinene reaction, during which only S highly oxygenated gas- and particle-phase ON are detected. The majority of the particle-phase ON evaporates from the alpha-pinene SOA during photochemical aging, thus exhibiting a drastically different behavior from that of beta-pinene SOA. Our results indicate that nighttime ON formed by NO3+monoterpene chemistry can serve as either permanent or temporary NO sinks depending on the monoterpene precursor.