Abstract. Volatile organic compounds (VOCs) play important roles in the tropospheric atmosphere. In this study, VOCs were measured at an urban site in Guangzhou, one of the megacities in the Pearl River Delta (PRD), using a gas chromatograph–mass spectrometer/flame ionization detection (GC–MS/FID) and a proton transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS). Diurnal profile analyses show that stronger chemical removal by OH radicals for more reactive hydrocarbons occurs during the daytime, which is used to estimate the daytime average OH radical concentration. In comparison, diurnal profiles of oxygenated volatile organic compounds (OVOCs) indicate evidence of contributions from secondary formation. Detailed source analyses of OVOCs, using a photochemical age-based parameterization method, suggest important contributions from both primary emissions and secondary formation for measured OVOCs. During the campaign, around 1700 ions were detected in PTR-ToF-MS mass spectra, among which there were 462 ions with noticeable concentrations. VOC signals from these ions are quantified based on the sensitivities of available VOC species. OVOC-related ions dominated PTR-ToF-MS mass spectra, with an average contribution of 73 % ± 9 %. Combining measurements from PTR-ToF-MS and GC–MS/FID, OVOCs contribute 57 % ± 10 % to the total concentration of VOCs. Using concurrent measurements of OH reactivity, OVOCs measured by PTR-ToF-MS contribute greatly to the OH reactivity (19 % ± 10 %). In comparison, hydrocarbons account for 21 % ± 11 % of OH reactivity. Adding up the contributions from inorganic gases (48 % ± 15 %), ∼ 11 % (range of 0 %–19 %) of the OH reactivity remains `missing”, which is well within the combined uncertainties between the measured and calculated OH reactivity. Our results demonstrate the important roles of OVOCs in the emission and evolution budget of VOCs in the urban atmosphere.

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Isocyanic acid (HNCO) is a potentially toxic atmospheric pollutant, whose atmospheric concentrations are hypothesized to be linked to adverse health effects. An earlier model study estimated that concentrations of isocyanic acid in China are highest around the world. However, measurements of isocyanic acid in ambient air have not been available in China. Two field campaigns were conducted to measure isocyanic acid in ambient air using a high-resolution time-of-flight chemical ionization mass spectrometer (ToF-CIMS) in two different environments in China. The ranges of mixing ratios of isocyanic acid are from below the detection limit (18 pptv) to 2.8 ppbv (5 min average) with the average value of 0.46 ppbv at an urban site of Guangzhou in the Pearl River Delta (PRD) region in fall and from 0.02 to 2.2 ppbv with the average value of 0.37 ppbv at a rural site in the North China Plain (NCP) during wintertime, respectively. These concentrations are significantly higher than previous measurements in North America. The diurnal variations of isocyanic acid are very similar to secondary pollutants (e.g., ozone, formic acid, and nitric acid) in PRD, indicating that isocyanic acid is mainly produced by secondary formation. Both primary emissions and secondary formation account for isocyanic acid in the NCP. The lifetime of isocyanic acid in a lower atmosphere was estimated to be less than 1 day due to the high apparent loss rate caused by deposition at night in PRD. Based on the steady state analysis of isocyanic acid during the daytime, we show that amides are unlikely enough to explain the formation of isocyanic acid in Guangzhou, calling for additional precursors for isocyanic acid. Our measurements of isocyanic acid in two environments of China provide important constraints on the concentrations, sources, and sinks of this pollutant in the atmosphere.