Photochemical Formation of C-C Alkyl Nitrates in Suburban Hong Kong and over the South China Sea.

Alkyl nitrates (RONO) are important reservoirs of atmospheric nitrogen, regulating nitrogen cycling and ozone (O) formation. In this study, we found that propane and n-butane were significantly lower at the offshore site (WSI) in Hong Kong ( p < 0.05), whereas C-C RONO were comparable to the suburban site (TC) ( p > 0.05). Stronger oxidative capacity at WSI led to more efficient RONO formation. Relative incremental reactivity (RIR) was for the first time used to evaluate RONO-precursor relationships. In contrast to a consistently volatile organic compounds (VOC)-limited regime at TC, RONO formation at WSI switched from VOC-limited regime during O episodes to VOC and nitrogen oxides (NO ) colimited regime during nonepisodes. Furthermore, unlike the predominant contributions of parent hydrocarbons to C-C RONO, the production of C-C RONO was more sensitive to other VOCs like aromatics and carbonyls, which accounted for ∼40-90% of the productions of C-C alkylperoxy (RO) and alkoxy radicals (RO) at both sites. This resulted from the decomposition of larger RO/RO and the change of OH abundance under the photochemistry of other VOCs. This study advanced our understanding of the photochemical formation of RONO, particularly the relationships between RONO and their precursors, which were not confined to the parent hydrocarbons.