The formation of nitro-aromatic compounds under high NO_<i>x</i> and anthropogenic VOC conditions in urban Beijing, China

<p>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-<span class="inline-formula">NO_<i>x</i></span> and anthropogenic VOC dominated conditions. We investigated the factors that influence NAC formation (e.g., <span class="inline-formula">NO₂</span>, 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&thinsp;ng&thinsp;m<span class="inline-formula">⁻³</span>, higher than those reported in other summertime studies (0.14–6.44&thinsp;ng&thinsp;m<span class="inline-formula">⁻³</span>). 4-Nitrophenol (4NP, 32.4&thinsp;%) and 4-nitrocatechol (4NC, 28.5&thinsp;%) 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 <span class="inline-formula">NO_<i>x</i></span> was found to be a more dominant source of NACs than primary biomass burning emissions. The <span class="inline-formula">NO₂</span> concentration level was found to be an important factor influencing the secondary formation of NACs. A transition from low- to high-<span class="inline-formula">NO_<i>x</i></span> regimes coincided with a shift from organic- to inorganic-dominated oxidation products. The transition thresholds were <span class="inline-formula">NO₂ ∼ 20</span>&thinsp;ppb for daytime and <span class="inline-formula">NO₂∼25</span>&thinsp;ppb for nighttime conditions. Under low-<span class="inline-formula">NO_<i>x</i></span> conditions, NACs increased with <span class="inline-formula">NO₂</span>, while the <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M14" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="c5e3e0772eea57309f236de17ca43cb8"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-19-7649-2019-ie00001.svg" width="25pt" height="16pt" src="acp-19-7649-2019-ie00001.png"/></svg:svg></span></span> concentrations and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M15" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mo>(</mo><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup><mo>)</mo><mo>/</mo><mi mathvariant="normal">NACs</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="66pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="b11e1e5398608eb8df751ed83b541a68"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-19-7649-2019-ie00002.svg" width="66pt" height="16pt" src="acp-19-7649-2019-ie00002.png"/></svg:svg></span></span> ratios were lower, implying organic-dominated products. Under high-<span class="inline-formula">NO_<i>x</i></span> conditions, NAC concentrations did not further increase with <span class="inline-formula">NO₂</span>, while the <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M18" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="7dd3c683c0655cd2a5c1ed2d08ea01e9"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-19-7649-2019-ie00003.svg" width="25pt" height="16pt" src="acp-19-7649-2019-ie00003.png"/></svg:svg></span></span> concentrations and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M19" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mo>(</mo><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup><mo>)</mo><mo>/</mo><mi mathvariant="normal">NACs</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="66pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="c4832a949e671b238f8731d693d75742"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-19-7649-2019-ie00004.svg" width="66pt" height="16pt" src="acp-19-7649-2019-ie00004.png"/></svg:svg></span></span> 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 <span class="inline-formula">NO₂</span> could be more important for the formation of nitrophenol and its derivatives. Using the (3M4NC<span class="inline-formula">+</span>4M5NC)&thinsp;<span class="inline-formula">∕</span>&thinsp;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&thinsp;&gt;&thinsp;30&thinsp;%. We also found that the concentrations of VOC precursors (e.g., toluene and benzene) and aerosol surface area acted as<span id="page7650"/> important factors in promoting NAC formation, and photolysis as an important loss pathway for nitrophenols.</p>