Low-NO atmospheric oxidation pathways in a polluted megacity

Abstract

Abstract. The impact of emissions of volatile organic compounds (VOCs) to the atmosphere on the production of secondary pollutants, such as ozone and secondary organic aerosol (SOA), is mediated by the concentration of nitric oxide (NO). Polluted urban atmospheres are typically considered to be “high-NO” environments, while remote regions such as rainforests, with minimal anthropogenic influences, are considered to be “low NO”. However, our observations from central Beijing show that this simplistic separation of regimes is flawed. Despite being in one of the largest megacities in the world, we observe formation of gas- and aerosol-phase oxidation products usually associated with low-NO “rainforest-like” atmospheric oxidation pathways during the afternoon, caused by extreme suppression of NO concentrations at this time. Box model calculations suggest that during the morning high-NO chemistry predominates (95 %) but in the afternoon low-NO chemistry plays a greater role (30 %). Current emissions inventories are applied in the GEOS-Chem model which shows that such models, when run at the regional scale, fail to accurately predict such an extreme diurnal cycle in the NO concentration. With increasing global emphasis on reducing air pollution, it is crucial for the modelling tools used to develop urban air quality policy to be able to accurately represent such extreme diurnal variations in NO to accurately predict the formation of pollutants such as SOA and ozone.

Formation of condensable organic vapors from anthropogenic and biogenic volatile organic compounds (VOCs) is strongly perturbed by NO<sub><i>x</i></sub> in eastern China

“…In addition to multi-nitrates (C 5 H 10 O 7−8 N 2 , C 5 H 8 O 6−9 N 2 , and C 5 H 9 O 10 N 3 summarized in Table S6 in the Supplement), several mononitrate series (C 4 H 7 O 5−7 N, C 5 H 9 O 4−9 N, C 5 H 7 O 5−8 N, and C 5 H 11 O 5−6 N) of this factor are also abundant in the isoprene nitrate type-II factor in Centreville (Massoli et al, 2018). Many of the isoprene nitrates here have been especially investigated in our previous observations in the YRD (Xu et al, 2021) and have been discovered in other field measurements (Lee et al, 2016;Massoli et al, 2018) and in many laboratories (Ng et al, 2008;Lambe et al, 2017). Generally, these compounds are second-and third-generation OH oxidation products of isoprene under high-NO x conditions (Wennberg et al, 2018).…”

“…7a), most probably generated from double OH attack proceeding with double RO 2 + NO termination (Jenkin et al, 2015). C 5 H 10 O 8 N 2 can be also produced in an NO 3 + isoprene system (Ng et al, 2008;Zhao et al, 2021), whereas in this study, the nocturnal C 5 H 10 O 8 N 2 is principally from the isoprene-derived OOM (Isop-OOM) factor (Fig. 8b) which will be discussed later.…”

“…This phenomenon suggests a probability of HO 2 -driven chemistry of this factor under low-NO conditions since NO can consume HO 2 and compete with HO 2 for RO 2 . Such low-NO atmospheric oxidation pathways have been suggested to be non-negligible in the afternoon in central Beijing (Newland et al, 2021).…”

Highly oxygenated organic molecules produced by the oxidation of benzene and toluene in a wide range of OH exposure and NO<sub><i>x</i></sub> conditions

“…Yet, quantitative understanding in the effects of NO x on the formation of oxygenated products is limited for aromatic precursors compared with those for biogenic VOCs (Nah et al, 2016;Lambe et al, 2017;Sarnela et al, 2018). The high-NO x experiments herein were conducted with [NO 2 ] : [NO] ratios of 20 to 120 (Table S1), which may represent urban afternoon conditions when fresh NO x emission is mostly converted to NO 2 , and intense photochemistry fuels the oxidation of aromatics accompanying the NO x emission (Newland et al, 2021 termination are probably minor because of the low concentrations of RO 2 (< 0.9 ppt). Given the low and narrow range of NO x levels in the OFR254-iN 2 O1.1 experiments, the product concentrations for all lamp voltages (i.e., a range of OH exposure) are averaged and shown as the first data point in each panel of Fig.…”

Comment on acp-2021-201

“…Yet, quantitative understanding in the effects of NOx on the formation of oxygenated products is limited for aromatic precursors compared with those for biogenic VOCs (Nah et al, 2016;Lambe et al, 2017;Sarnela et al, 2018). The high-NOx experiments herein were conducted with [NO2]:[NO] ratios of 20 to 120 (Table S1), which may represent urban afternoon conditions when fresh NOx emission is mostly converted to NO2 and intense photochemistry fuels the oxidation of aromatics accompanying the NOx emission (Newland et al, 2021). Figure 4 -1.5 ppb to 0.5 -0.7 ppb as a result of the switch of 1.1% of N2O injection to 4.4% (Table S1).…”