Modeling C₁–C₄ Alkyl Nitrate Photochemistry and Their Impacts on O₃ Production in Urban and Suburban Environments of Hong Kong

As intermediate products of photochemical reactions, alkyl nitrates (RONO₂) regulate ozone (O₃) formation. In this study, a photochemical box model incorporating master chemical mechanism well reproduced the observed RONO₂ at an urban and a mountainous site, with index of agreement in the range of 0.66–0.73. The value 0.0003 was identified to be the most appropriate branching ratio for C₁ RONO₂, with the error less than 50%. Although levels of the parent hydrocarbons and nitric oxide (NO) were significantly higher at the urban site than the mountainous site, the production of C₂–C₃ RONO₂ was comparable to or even lower than at the mountainous site, due to the lower concentrations of oxidative radicals in the urban environment. Based on the profiles of air pollutants at the mountainous site, the formation of C₂–C₄ RONO₂ was limited by NO_x (volatile organic compounds (VOCs)) when total volatile organic compounds (TVOCs)/NO_x was higher (lower) than 10.0 ± 0.4 parts per billion by volume (ppbv)/ppbv. This dividing ratio decreased (p < 0.05) to 8.7 ± 0.4 ppbv/ppbv at the urban site, mainly due to the different air pollutant profiles at the two sites. For the formation of C₁ RONO₂, the NO_x-limited regime extended the ratio of TVOCs/NO_x to as low as 2.4 ± 0.2 and 3.1 ± 0.1 ppbv/ppbv at the mountainous and urban site, respectively. RONO₂ formation led to a decrease of simulated O₃, with reduction efficiencies (O₃ reduction/RONO₂ production) of 4–5 parts per trillion by volume (pptv)/pptv at the mountainous site and 3–4 pptv/pptv at the urban site. On the other hand, the variations of simulated O₃ induced by RONO₂ degradation depended upon the regimes controlling O₃ formation and the relative abundances of TVOCs and NO_x.