# The formation of nitro-aromatic compounds under high NO x and anthropogenic VOC conditions in urban Beijing, China

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 both high-inline-formulaNOx and anthropogenic VOC dominated conditions. We investigated the factors that influence NAC formation (e.g., inline-formulaNO2, 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 minline-formula−3, higher than those reported in other summertime studies (0.14–6.44 ng minline-formula−3). 4-Nitrophenol (4NP, 32.4 %) and 4-nitrocatechol (4NC, 28.5 %) were the top two most abundant NACs, followed by methyl-nitrocatechol (MNC), methyl-nitrophenol (MNP), and dimethyl-nitrophenol (DMNP). The oxidation of toluene and benzene in the presence of inline-formulaNOx was found to be a more dominant source of NACs than primary biomass burning emissions. The inline-formulaNO2 concentration level was found to be an important factor influencing the secondary formation of NACs. A transition from low- to high-inline-formulaNOx regimes coincided with a shift from organic- to inorganic-dominated oxidation products. The transition thresholds were inline-formulaNO2 ∼ 20 ppb for daytime and inline-formulaNO2∼25 ppb for nighttime conditions. Under low-inline-formulaNOx conditions, NACs increased with inline-formulaNO2, while the inline-formula $M14inlinescrollmathmlchem{\mathrm{normal NO}}_{normal 3}^{-}$ 25pt16ptsvg-formulamathimgc5e3e0772eea57309f236de17ca43cb8 acp-19-7649-2019-ie00001.svg25pt16ptacp-19-7649-2019-ie00001.png concentrations and inline-formula $M15inlinescrollmathmlchem\left({\mathrm{normal NO}}_{normal 3}^{-}\right)/\mathrm{normal NACs}$ 66pt16ptsvg-formulamathimgb11e1e5398608eb8df751ed83b541a68 acp-19-7649-2019-ie00002.svg66pt16ptacp-19-7649-2019-ie00002.png ratios were lower, implying organic-dominated products. Under high-inline-formulaNOx conditions, NAC concentrations did not further increase with inline-formulaNO2, while the inline-formula $M18inlinescrollmathmlchem{\mathrm{normal NO}}_{normal 3}^{-}$ 25pt16ptsvg-formulamathimg7dd3c683c0655cd2a5c1ed2d08ea01e9 acp-19-7649-2019-ie00003.svg25pt16ptacp-19-7649-2019-ie00003.png concentrations and inline-formula $M19inlinescrollmathmlchem\left({\mathrm{normal NO}}_{normal 3}^{-}\right)/\mathrm{normal NACs}$ 66pt16ptsvg-formulamathimgc4832a949e671b238f8731d693d75742 acp-19-7649-2019-ie00004.svg66pt16ptacp-19-7649-2019-ie00004.png ratios showed increasing trends, signaling a shift from organic- to inorganic-dominated products. Nighttime enhancements were observed for 3M4NC and 4M5NC, while daytime enhancements were noted for 4NP, 2M4NP, and DMNP, indicating different formation pathways for these two groups of NACs. Our analysis suggested that the aqueous-phase oxidation was likely the major formation pathway of 4M5NC and 3M5NC, while photo-oxidation of toluene and benzene in the presence of inline-formulaNO2 could be more important for the formation of nitrophenol and its derivatives. Using the (3M4NCinline-formula+4M5NC) inline-formula∕ 4NP ratios as an indicator of the relative contribution of aqueous-phase and gas-phase oxidation pathways to NAC formation, we observed that the relative contribution of aqueous-phase pathways increased at elevated ambient RH and remained constant at RH > 30 %. We also found that the concentrations of VOC precursors (e.g., toluene and benzene) and aerosol surface area acted aspage7650 important factors in promoting NAC formation, and photolysis as an important loss pathway for nitrophenols.

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Wang, Yujue / Hu, Min / Wang, Yuchen / et al: The formation of nitro-aromatic compounds under high NOx and anthropogenic VOC conditions in urban Beijing, China. 2019. Copernicus Publications.

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