Optimizing a twin-chamber system for direct ozone production rate measurement.

High Ozone Production Rate (OPR) leads to O3 pollution episodes and adverse human health outcomes. OPR observation (Obs-OPR) and OPR modelling (Mod-OPR) have been obtained from observed and modelled peroxy radicals and nitrogen oxides. However, discrepancies between them remind of an imperfect understanding of O3 photochemistry. Direct measurement of OPR (Mea-OPR) by a twin-chamber system emerges. Herein, we optimized Mea-OPR design, i.e., minimizing the chamber surface area to volume ratio (S/V) to 9.8 m-1 from 18 m-1 and the dark uptake coefficient of O3 to 9.9 × 10-9 from 7.1 × 10-8 in the literature. In addition, control experiments further revealed and quantified a photo-enhanced O3 uptake, and therefore recommended an essential correction of Mea-OPR. We finally characterized a measurement uncertainty of ±38% and a detection limit of 3.2 ppbv h-1 (3SD), which suggested that Mea-OPR would be sensitive enough to measure OPR in urban or suburban environments. Further application of this system in urban Beijing during the Beijing 2022 Olympic Winter Games recorded a noontime OPR of 7.3 (±3.3, 1SD) ppbv h-1. These observational results added up to our confidence in future field application of Mea-OPR, to facilitate pollution control policy evaluation and to shed light on O3 photochemistry puzzle.

Heterogeneous reactions significantly contribute to the atmospheric formation of nitrated aromatic compounds during the haze episode in urban Beijing.

Nitrated aromatic compounds (NACs) are key components of air pollution; however, due to the presence of complex mixtures of primary and secondary species, especially in urban environments, their atmospheric formation is poorly understood. Here we conducted a field campaign during a winter haze episode in urban Beijing, China to monitor gaseous and particulate NACs at 2-h time resolution. Through a standard-independent non-targeted approach, a total of 238 NACs were screened, of which 127 species were assigned chemical formula and 25 structures were confirmed. Four main classes were identified: nitrated aromatic hydrocarbons, nitrophenols, oxygenated nitrated aromatic compounds, and nitrated heterocyclic aromatic compounds. Hierarchical clustering analysis revealed disparate temporal variances of diurnal or nocturnal elevation, among which different nitration formations were captured, i.e., daytime photochemical oxidation and nighttime heterogeneous reactions. Isomeric information, particularly the substitution position of the nitro group on biphenyl, further demonstrated a potential heterogeneous mechanism of electrophilic nitration by NO2+. Assisted by source apportionment, we found that nighttime heterogeneous reactions significantly contributed to NAC formation, e.g., 31.3 % and 60.8 %, respectively, to 2-nitrofluoranthene and 2-nitropyrene, which were previously considered as classical daytime gas-phase products. This study provides comprehensive information on urban NAC species and highlights the importance of unheeded heterogeneous reactions in the atmosphere.