Photochemical Formation of C 1 -C 5 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.