Measurements of C1-C4 alkyl nitrates and their relationships with carbonyl compounds and O3 in Chinese cities

Ambient alkyl nitrates (RONO2) are important byproducts of O3 formation. Although concern about O3 pollution has increased recently, few studies have investigated RONO2 chemistry and distributions in China. In this study, ambient levels of C1–C4 RONO2 were measured in Chinese cities, and their relationships with parent hydrocarbons (RH), carbonyls, and total oxidant (Ox = O3 + NO2) were investigated. Our measurements showed that 2-butyl nitrate (2-BuONO2) was the most abundant RONO2 species, with mixing ratios of 48–88 pptv, followed by 2-propyl nitrate (2-PrONO2), ethyl nitrate (EtONO2), methyl nitrate (MeONO2), and 1-propyl nitrate (1-PrONO2). The measured RONO2 species exhibited maximum levels in the early afternoon (13:00–14:00) of summer, suggesting the importance of RONO2 photochemical production. Relative to 2-BuONO2/n-butane, the measured 1-PrONO2/propane agreed well with the modeled ratio based on laboratory kinetic data, suggesting that propane was the dominant precursor of ambient 1-PrONO2. However, the measured ratios for MeONO2/methane, EtONO2/ethane, and summertime 2-PrONO2/propane showed significant positive deviations from the predicted values, indicating the existence of additional sources other than OH oxidation of the parent hydrocarbons. Initial mixing ratios of C1–C3 carbonyls during 08:00–12:00 in summer at the PKU site exhibited significant correlations with RONO2 levels, suggesting the importance of secondary sources for ambient carbonyls. The measured ratios of formaldehyde/MeONO2 were close to the theoretical ratio, whereas the derived ratios for acetaldehyde/EtONO2, propanal/1-PrONO2, and acetone/2-PrONO2 were higher than the kinetic ratios, indicating that these carbonyls might be produced from sources other than the reaction of alkoxy radicals with O2. Carbonyls are important precursors of Ox, but their photochemical reactions do not result in RONO2 production. Therefore, Ox/RONO2 could indicate the relative importance of carbonyls to Ox formation. On the basis of the measured ratios of Ox/2-BuONO2, we estimated the maximum contributions of secondary carbonyls to Ox at the PKU site to be 25–32% during 12:00–16:00 in summer.