Inter-comparisons of VOC oxidation mechanisms based on box model: A focus on OH reactivity.

Volatile organic compounds (VOCs) oxidation processes play a very important role in atmospheric chemistry, and the chemical reactions are expressed in various manners in chemical mechanisms. To gain an improved understanding of VOCs evolution during oxidation processes and evaluate the discrepancies of VOCs oxidation schemes among different mechanisms, we used the total VOC reactivity as a diagnostic and evaluated tool to explore the differences for six widely used chemical mechanisms. We compared the total VOC reactivity evolution under high-NOx conditions for several sets of precursors, including n-pentane, toluene, ethene, isoprene and a mixture of 57 Photochemical Assessment Monitoring Stations (PAMS) species in a 0-D photochemical box model. Inter-comparison of total VOC reactivity of individual precursor simulations showed discrepancies to different extent of the oxidation schemes among the studied mechanisms, which are mainly attributed to the different lumping approaches for organic species. The PAMS simulation showed smaller discrepancy than individual precursor cases in terms of total VOC reactivity. SAPRC07 and RACM2 performances are found to better match the MCM for simulation of total VOC reactivity. Evidences suggest that the performance in simulating secondary organic products, OH concentrations and NOx concentrations are related to the OH reactivity discrepancies among various chemical mechanisms. Information in this study can be used in selection of chemical mechanisms to better model OH reactivity in different environments. The results in this study also provide directions to further improve the ability in modelling total VOC reactivity with the chemical mechanisms.

Elucidating contributions of volatile organic compounds to ozone formation using random forest during COVID-19 pandemic: A case study in China.

Ozone has been reported to increase despite nitrogen oxides reductions during the COVID-19 pandemic, and ozone formation needs to be revisited using volatile organic compounds (VOCs), which are rarely measured during the pandemic. Here, a total of 98 VOCs species were monitored in an economy-active city in China from January 2021 to August 2022 to assess contributions to ozone formation during the pandemic. Total VOCs concentrations were 35.55 ± 21.47 ppb during the entire period, among which alkanes account for the largest fraction (13.78 ppb, 38.0%), followed by aromatics (6.16 ppb, 16.8%) and oxygenated VOCs (OVOCs, 5.69 ppb, 15.7%). Most VOCs groups (e.g., alkenes, OVOCs) and individual species (e.g., isoprene, methyl vinyl ketone) display obvious seasonal and diurnal variations, which are related to their sources and reactivities. No weekend effects of VOCs suggest limited influences from traffic emissions during pandemic. Aromatics and alkenes are the major contributors (39% and 33%) to ozone formation potential, largely driven by o/m/p-xylene (21%), ethylene (15%), toluene (9%). Secondary organic aerosol formation potential is dominated by toluene (>50%) despite its low proportion (5%). Further inclusion of VOCs and meteorology in the Random Forest model shows good ozone prediction performance (R2 = 0.77-0.86, RMSE = 11.95-19.91 µg/m3, MAE = 8.89-14.58 µg/m3). VOCs and NO2 contribute >50% of total importance with the largest difference in importance ratio of VOCs/NO2 in the summer and winter, implying ozone formation regime may vary. No seasonal variations in importance of meteorology are observed, while importance of other variables (e.g., PM2.5) is highest in the summer. This work identifies critical VOCs groups and species for ozone formation during the pandemic, and demonstrates the feasibility of machine learning algorithms in elucidation of ozone formation mechanisms.