Abstract
Volatile organic compounds (VOCs) regulate atmospheric oxidation capacity, and the reactions of VOCs are key in understanding ozone formation and its mitigation strategies. When evaluating its impact, most previous studies did not fully consider the role of oxygenated VOCs due to limitations of measurement technology. By using a proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) combined with gas chromatography mass spectrometer (GC-MS) technology, we are able to quantify a large number of oxygenated VOCs in a representative urban environment in southern China. Based on the new dataset, we find that non – formaldehyde (HCHO) OVOCs can contribute large fractions (22–44 %) of total ROX radical production, comparable or larger than the contributions from nitrous acid and formaldehyde. We demonstrate that constraints using OVOCs observations are essential in modeling radical and ozone production, as modelled OVOCs can be substantially lower than measurements, potentially due to primary emissions and/or missing secondary sources. Our results show that models without OVOC constraints using ambient measurements will underestimate P(ROX) and ozone production rate, and may also affect the determination of sensitivity regime in ozone formation. Therefore, a thorough quantification of photodegradable OVOCs species is in urgent need to understand accurately the ozone chemistry and to develop effective control strategies.
Highlights
Ground-level ozone is generated by photochemical oxidation of volatile organic compounds (VOCs) under the catalysis of nitrogen oxides (NOx) and hydroxide radicals (HOX=OH+HO2) (Atkinson, 2000;Monks et al, 2015)
~1.2×10-4,1.2×10-6~3.0×10-4 and 1.2×10-6~1.8×10-4 s-1, respectively
HOX radicals derived from photolysis of formaldehyde (HCHO)/kOH_NMHC and pyruvic acid/kOH_NMHC ratios displayed significant positive correlation with j(NO2). These results suggest that the enhancement of the photolysis rates converted more non-methane hydrocarbons (NMHCs) into secondary OVOCs, suggesting the crucial role of photolysis reactions in the airmass aging and the occurrence of secondary pollution
Summary
Ground-level ozone is generated by photochemical oxidation of volatile organic compounds (VOCs) under the catalysis of nitrogen oxides (NOx) and hydroxide radicals (HOX=OH+HO2) (Atkinson, 2000;Monks et al, 2015). In this process, photolysis reactions are a crucial driving force. The strong dependence of OH concentration on j(O1D) was found in a number of field measurements (Ehhalt and Rohrer, 2000;Rohrer et al, 2014b;Stone et al, 2012), implying the dominant role of ultraviolet radiation and photolysis reactions in the production of HOX radicals. Edwards et al (2014) found that the high ozone pollution in an oil and gas producing basin in the U.S in winter was caused by the photolysis of high concentrations of OVOCs to generate sufficient oxidants.
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