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. Nitro-aromatic compounds (NACs), as important contributors to the light absorption by brown carbon, have been widely observed in various ambient atmospheres; however, their formation in the urban atmosphere was little studied. In this work, we report an intensive field study of NACs in summer 2016 at an urban Beijing site, characterized by bothhigh- NOx and anthropogenic VOC dominated conditions. We investigated the factors that influence NAC formation (e.g., NO2 , VOC precursors, RH and photolysis) through quantification of eight NACs, along with major components in fine particulate matter, selected volatile organic compounds, and gases. The average total concentration of the quantified NACs was 6.63 ng m −3 , higher than those reported in other summertime studies (0.14–6.44 ng m −3 ). 4-Nitrophenol (4NP, 32.4 %) and 4-nitrocatechol (4NC,28.5 %) were the top two most abundant NACs, followed bymethyl-nitrocatechol (MNC), methyl-nitrophenol (MNP), anddimethyl-nitrophenol (DMNP). The oxidation of toluene and benzene in thepresence of NOx was found to be a more dominant source of NACs than primary biomass burning emissions. The NO2 concentration level was found to be an important factor influencing the secondary formation of NACs. A transition from low- to high- NOx regimes coincided with a shift fromorganic- to inorganic-dominated oxidation products. The transitionthresholds were NO2 ∼ 20  ppb for daytime and NO2∼25  ppb for nighttime conditions. Under low- NOx conditions, NACs increased with NO2 , while the NO 3 - concentrations and ( NO 3 - ) / NACs ratios were lower, implying organic-dominated products. Under high- NOx conditions, NAC concentrations did not further increase with NO2 , while the NO 3 - concentrations and ( NO 3 - ) / NACs ratios showedincreasing trends, signaling a shift from organic- to inorganic-dominatedproducts. Nighttime enhancements were observed for 3M4NC and 4M5NC, whiledaytime enhancements were noted for 4NP, 2M4NP, and DMNP, indicatingdifferent formation pathways for these two groups of NACs. Our analysissuggested that the aqueous-phase oxidation was likely the major formationpathway of 4M5NC and 3M5NC, while photo-oxidation of toluene and benzene inthe presence of NO2 could be more important for the formation ofnitrophenol and its derivatives. Using the (3M4NC + 4M5NC)  ∕  4NP ratios as an indicator of the relative contribution of aqueous-phase and gas-phaseoxidation pathways to NAC formation, we observed that the relativecontribution of aqueous-phase pathways increased at elevated ambient RH andremained constant at RH > 30 %. We also found that theconcentrations of VOC precursors (e.g., toluene and benzene) and aerosolsurface area acted as important factors in promoting NAC formation, andphotolysis as an important loss pathway for nitrophenols.

atmospheric chemistry and physics

The formation of nitro-aromatic compounds under high NO&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; and anthropogenic VOC conditions in urban Beijing, China