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

Abstract

Organic nitrate ester compounds (RONO2) are an important sink for nitrogen oxides (NO x ≡ NO + NO2) that impact ozone and organic aerosol formation in the lower atmosphere. While RONO2 formation from gas phase oxidation chemistry and heterogeneous reactions involving reactive uptake 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 during aqueous phase reactions of nitronium (NO2 +) with humic and fulvic acids, which are used as surrogates for aerosol organic matter. X-ray photoelectron spectroscopy (XPS) and high-resolution mass spectrometry were used to characterize nitrogen functional groups and changes in bulk chemical composition during nitration reactions. Correlations between the organic N abundance in reacted humic and fulvic acids and functional group abundance measured by solid-state 13C NMR indicate that NO2 + targets the lipid fraction of the organic matter. Fourier transform infrared spectroscopy (FTIR) analysis of model lipid compounds shows that the reaction of NO2 + with terminal alcohols (ROH) and alkenes is a source of stable RONO2 compounds. Further, the low-pH environment of most nitrate aerosols is expected to enhance the total terminal ROH pool through acid-catalyzed ester hydrolysis within the lipid-like fraction of natural organic matter. Our findings demonstrate that multiphase oxidation of organic matter by NO2 +, along with air-particle partitioning of products from gas-phase chemistry, may be an important source of particle-phase organic nitrates.