Abstract. Little is known about the formation processes of nitrooxy organosulfates (OSs) by nighttime chemistry. Here we characterize nitrooxy OSs at a molecular level in firework-related aerosols in urban Beijing during Chinese New Year. High-molecular-weight nitrooxy OSs with relatively low H / C and O / C ratios and high unsaturation are potentially aromatic-like nitrooxy OSs. They considerably increased during New Year's Eve, affected by the firework emissions. We find that large quantities of carboxylic-rich alicyclic molecules possibly formed by nighttime reactions. The sufficient abundance of aliphatic-like and aromatic-like nitrooxy OSs in firework-related aerosols demonstrates that anthropogenic volatile organic compounds are important precursors of urban secondary organic aerosols (SOAs). In addition, more than 98 % of those nitrooxy OSs are extremely low-volatility organic compounds that can easily partition into and consist in the particle phase and affect the volatility, hygroscopicity, and even toxicity of urban aerosols. Our study provides new insights into the formation of nitrooxy organosulfates from anthropogenic emissions through nighttime chemistry in the urban atmosphere.

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Organosulfates (OSs) are some of the important components of secondary organic aerosols in ambient air. However, little is known about the stable carbon and sulfur isotopic compositions of OSs. Here, we identified the isotopic pair 12 C n / 13 C 12 C n−1 and 32 S n / 34 S 32 S n−1 formulas in urban aerosols using ultra-highresolution Fourier transform ion cyclotron resonance mass spectrometry. Family series of 694−1649 13 C 32 S OSs and 40−144 12 C 34 S OSs with completely coherent carbon chains were detected, accounting for 37−52% and 2.2−4.5%, respectively, of the observed 12 C 32 S OSs. OSs with detected isotopic peaks have relatively high levels of saturation and high volatilities, especially those containing 34 S isotopes with H/C > 1.5. Moreover, the δ 13 C and δ 34 S values of OSs were estimated with mean values ranging from −22‰ to −0.87‰ and from −1‰ to 26‰, respectively. Notably, unsaturated OSs might be more enriched with 34 S than aliphatic ones. Furthermore, all detected 12 C 34 S OSs are in the low-O/C corridor with a low carbon oxidation state, which are mostly volatile and low-volatile organic compounds with high volatility, implying that they are generated by gasphase oxidation. These findings represent a first step toward identifying the individual molecules of 13 C and 34 S isotopic OSs in atmospheric aerosols.

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