Evidence for Significant Particulate Organic Nitrate Formation in the Los Angeles Basin at Night

Oxidn. of biogenic volatile org. compds. (BVOCs) by the nitrate radical (NO₃) represents a potentially efficient secondary org. aerosol (SOA) formation pathway. However, the contribution of nighttime NO₃-BVOC chem. to urban OA prodn. remains poorly constrained. Here, we present measurements of submicrometer aerosol compn. obtained at Caltech (Pasadena, CA) with an Aerodyne HR-ToF-AMS in May-June 2020. At night (19:00-6:00), a majority (>70%) of AMS-derived aerosol nitrate is org., and particulate org. nitrates (pONs) constitute up to 20-30% of total AMS-derived OA. Notably, clear increases in pON mass loadings are obsd. after sunset that correlate strongly with the prodn. rate of NO₃. The mass spectral signature of a pON-assocd. org. aerosol factor extd. using pos. matrix factorization (PMF) is compared to lab. SOA formed from NO₃ oxidn. of individual monoterpenes to aid source identification. Box model simulations reproduce obsd. overnight pON formation using reasonable SOA yields from NO₃ oxidn. of monoterpenes (~40%). Comparison to data collected in 2010 suggests that a major shift in the dominant nighttime nitrate aerosol formation pathway has occurred in the last decade. The observation of substantial NO₃-derived pON prodn. highlights the increasing importance of biogenic emissions to urban aerosol loadings and NO_x loss processes.