The formation of nitro-aromatic compounds under high NO<sub>x</sub>-anthropogenic VOCs dominated atmosphere in summer in Beijing, China

Abstract. Nitro-aromatic compounds (NACs), as important contributors to ultraviolet light absorption by brown carbon, have been widely observed in various ambient atmospheres, however, few field studies has been focused on their formation in urban atmospheres. In this work, NACs in Beijing were comprehensively quantified and characterized in summer, along with major components in fine particulate matter and selected volatile organic compounds. Field observations in this high NOx-anthropogenic VOCs dominated urban atmosphere were analyzed to investigate the NAC formation and influence factors. The total concentration of quantified NACs was 6.63 ng/m3, higher than other summertime studies (0.14–6.44 ng/m3). 4-Nitrophenol (4NP, 32.4 %) and 4-nitrocatechol (4NC, 28.5 %) were the most abundant ones among all the quantified NAC species, followed by methyl-nitrocatechol (MNC), methyl-nitrophenol (MNP) and dimethyl-nitrophenol (DMNP). The oxidation of toluene and benzene in the presence of NOx were found to be more dominant sources of NACs than biomass burning emissions. The NO2 level was an important factor influencing the secondary formation of NACs. A transition from low- to high-NOx regimes coincided with a shift from organic- to inorganic-dominated oxidation products. The transition thresholds were NO2∼20 ppb for daytime and NO2∼25 ppb for nighttime conditions. Under low-NOx conditions, NACs increased with NO2, while the NO3− concentrations and (NO3−)/NACs ratios were lower, implying organic-dominated products. Above the NOx regime transition values, NO2 was excess for the oxidation of ambient VOCs. Under this condition, NAC concentrations did not further increase obviously with NO2, while the NO3− concentrations and (NO3−)/NACs ratios showed significant increasing trends, when shifting from organic- to inorganic-dominated products. Obvious nighttime enhancements of 3M4NC and 4M5NC, daytime enhancements of 4NP, 2M4NP and DMNP indicated their different formation pathways. The aqueous-phase oxidation was the major formation pathways of 4M5NC and 3M5NC, and photo-oxidation of toluene and benzene in the presence of NO2 could be more important for the formation of nitrophenol and its derivatives. Thus, the (3M4NC + 4M5NC)/4NP ratios was employed to indicate the relative contribution of aqueous-phase and gas-phase oxidation to NAC formation. The relative contribution of aqueous-phase pathways was observed to increase at elevated ambient RH and remain constant at RH > 30 %. In addition, the concentrations of VOC precursors (e.g. toluene and benzene) and aerosol surface area were also important factors promoting NAC formation, and photolysis was an important loss pathway of NACs.