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

Abstract

Organic nitrate ester compounds (RONO 2 ) are an important sink for nitrogen oxides (NO x ≡ NO + NO 2 ) that impact ozone and organic aerosol formation in the lower atmosphere. While RONO 2 formation from gas phase oxidation chemistry and heterogeneous reactions involving reactive uptake of NO 3 into organic aerosols is well documented, in situ bulk-aqueous-phase production of RONO 2 within organic aerosols has not been explored as extensively. Here, we provide evidence that RONO 2 species are produced during aqueous phase reactions of nitronium (NO 2 + ) 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 13 C NMR indicate that NO 2 + targets the lipid fraction of the organic matter. Fourier transform infrared spectroscopy (FTIR) analysis of model lipid compounds shows that the reaction of NO 2 + with terminal alcohols (ROH) and alkenes is a source of stable RONO 2 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 NO 2 +, along with air-particle partitioning of products from gas-phase chemistry, may be an important source of particle-phase organic nitrates.