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 O-3 formation. Although concern about O-3 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 (O-x = O-3 + 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 O-2. Carbonyls are important precursors of Ox, but their photochemical reactions do not result in RONO2 production. Therefore, O-x/RONO2 could indicate the relative importance of carbonyls to Ox formation. On the basis of the measured ratios of O-x/2-BuONO2, we estimated the maximum contributions of secondary carbonyls to Ox at the NW site to be 25-32% during 12:00-16:00 in summer. (C) 2013 Elsevier Ltd. All rights reserved.