Photochemical Formation of C1-C5 Alkyl Nitrates in Suburban Hong Kong and over the South China Sea.

Alkyl nitrates (RONO2) are important reservoirs of atmospheric nitrogen, regulating nitrogen cycling and ozone (O3) 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 C3-C4 RONO2 were comparable to the suburban site (TC) ( p > 0.05). Stronger oxidative capacity at WSI led to more efficient RONO2 formation. Relative incremental reactivity (RIR) was for the first time used to evaluate RONO2-precursor relationships. In contrast to a consistently volatile organic compounds (VOC)-limited regime at TC, RONO2 formation at WSI switched from VOC-limited regime during O3 episodes to VOC and nitrogen oxides (NO x) colimited regime during nonepisodes. Furthermore, unlike the predominant contributions of parent hydrocarbons to C4-C5 RONO2, the production of C1-C3 RONO2 was more sensitive to other VOCs like aromatics and carbonyls, which accounted for ∼40-90% of the productions of C1-C3 alkylperoxy (RO2) and alkoxy radicals (RO) at both sites. This resulted from the decomposition of larger RO2/RO and the change of OH abundance under the photochemistry of other VOCs. This study advanced our understanding of the photochemical formation of RONO2, particularly the relationships between RONO2 and their precursors, which were not confined to the parent hydrocarbons.