Abstract
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.
Highlights
Volatile organic compounds (VOCs) play important roles in the tropospheric atmosphere
These results indicate that Oxygenated volatile organic compounds (OVOCs) with more than two oxygen atoms may be underestimated by PTR-time-of-flight mass spectrometry (ToF-MS), possibly as a result of losses to the inlet of PTR-ToF-MS instrument used in this study (Riva et al, 2019)
It should be noted that a substantial fraction of OVOCs with three or more oxygen atoms is attributed to the background, which may be due to the tubing delay effects as the result of the gas wall partitioning of these lower volatility multifunctional species (Pagonis et al, 2017)
Summary
Volatile organic compounds (VOCs) play important roles in the tropospheric atmosphere. The unmeasured OVOC species have proved to contribute large fractions to the missing reactivity, based on previous observation studies (Kim et al, 2016) and model simulations (Lou et al, 2010; Whalley et al, 2016). We performed a systematic analysis on diurnal profiles to investigate the photochemical losses of various hydrocarbons and the secondary formation of OVOCs. After considering VOC signals from uncalibrated species in PTRToF-MS, we used the combined data set to analyze contributions of different VOC groups to total VOC concentrations. A photochemical age-based parameterization method was applied to quantify the contributions from different atmospheric processes to concentrations of OVOCs. In the end, we evaluated the contributions of measured VOCs by both conventional methods and newly quantified species from PTR-ToFMS to OH reactivity in this region
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