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

Abstract:

Oxidation of biogenic volatile organic compounds (BVOCs) by the nitrate radical (NO₃) represents a potentially efficient secondary organic aerosol (SOA) formation pathway. However, the contribution of nighttime NO₃-BVOC chemistry to urban OA production remains poorly constrained. Here, we present measurements of submicrometer aerosol composition obtained at Caltech (Pasadena, CA) with an Aerodyne HR-ToF-AMS, SMPS, and 12-h filter samples analyzed offline with LC/ESI-MS. Preliminary analyses suggest that at night (19:00-6:00), a majority (>70%) of AMS-derived aerosol nitrate is organic, and particulate organic nitrates (pONs) constitute up to 20-30% of total AMS-derived OA. Notably, clear increases in pON mass loadings are observed after sunset that correlate strongly with the production rate of NO₃. Correlations between pON and NO₃ production rates together with speciated BVOC data are used to estimate “effective” ambient pON yields from NO₃-initiated oxidation of BVOCs, which provide a useful point of comparison to those derived from laboratory experiments for use in regional models. LC/ESI-MS analysis of 12-hr filter samples affords identification and quantification of individual pON species, as well as molecular insight into the dominant precursor classes (e.g., monoterpenes) and physicochemical properties (e.g., O:C and functionality) of the pONs. The observation of substantial NO₃-derived pON production highlights the persistent influence of NO_x on urban air quality in the LA Basin despite both significant decadal declines in NO_x emissions and notable recent reductions due to the COVID-19 pandemic.