Reaction of Organic Matter with Nitronium Ion as a Source of Organic Nitrate Esters in Proxies for Organic Aerosols

Organicnitrate ester compounds (RONO2) are an importantsink for nitrogen oxides (NO x & EQUIV;NO + NO2) that impact ozone and organic aerosol formationin the lower atmosphere. While RONO2 formation from gasphase oxidation chemistry and heterogeneous reactions involving reactiveuptake of NO3 into organic aerosols is well documented, in situ bulk-aqueous-phase production of RONO2 within organic aerosols has not been explored as extensively. Here,we provide evidence that RONO2 species are produced duringaqueous phase reactions of nitronium (NO2 (+))with humic and fulvic acids, which are used as surrogates for aerosolorganic matter. X-ray photoelectron spectroscopy (XPS) and high-resolutionmass spectrometry were used to characterize nitrogen functional groupsand changes in bulk chemical composition during nitration reactions.Correlations between the organic N abundance in reacted humic andfulvic acids and functional group abundance measured by solid-state C-13 NMR indicate that NO2 (+) targets thelipid fraction of the organic matter. Fourier transform infrared spectroscopy(FTIR) analysis of model lipid compounds shows that the reaction ofNO(2) (+) with terminal alcohols (ROH) and alkenesis a source of stable RONO2 compounds. Further, the low-pHenvironment of most nitrate aerosols is expected to enhance the totalterminal ROH pool through acid-catalyzed ester hydrolysis within thelipid-like fraction of natural organic matter. Our findings demonstratethat multiphase oxidation of organic matter by NO2 (+), along with air-particle partitioning of products from gas-phasechemistry, may be an important source of particle-phase organic nitrates.