Rainforest-like Atmospheric Chemistry in a Polluted Megacity

Abstract

<p><strong>Abstract.</strong> The impact of volatile organic compound (VOC) emissions 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 <q>high-NO</q> environments, while remote regions such as rainforests, with minimal anthropogenic influences, are considered to be <q>low-NO</q>. Policy to reduce urban air pollution is typically developed assuming that the chemistry is controlled by the high-NO regime. 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 significant formation of gas and aerosol phase oxidation products associated with the low-NO <q>rainforest-like</q> regime during the afternoon. This is caused by a surprisingly low concentration of NO, coupled with high concentrations of VOCs and of the atmospheric oxidant hydroxyl (OH). 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 %). With increasing global emphasis on reducing air pollution, the modelling tools used to develop urban air quality policy need to adequately represent both high- and low-NO regimes if they are to have utility.</p>

Strong anthropogenic control of secondary organic aerosol formation from isoprene in Beijing

“…High levels of peroxy radicals were observed, with mean afternoon concentrations of HO 2 and RO 2 of around 3 × 10 8 and 1.5 × 10 9 molecule cm −3 respectively. Zero-dimensional box modelling indicates that up to 35 % of the isoprene-derived RO 2 radicals can react with HO 2 in the afternoon on some days (Newland et al, 2020). Thus, the diurnal profile in Fig.…”

“…S5 and Newland et al, 2020) signal increases during the afternoon as the NO levels drop to below 1 ppb. The low NO levels indicate that up to 30 % of the isoprene peroxy radical from OH oxidation can react with HO 2 rather than NO at this site, meaning IEPOX can be formed locally (Newland et al, 2020). There is also likely to be a regional source of IEPOX and 2-MT-OS, suggesting both local and regional anthropogenic influences.…”

“…The highest concentrations generally occurred in the morning between 04:00 and 07:00 LT and steadily decreased during the day. On some days, the mixing ratio of NO was very low in the afternoon as a result of reaction with O 3 and other unknown sinks(Newland et al, 2020). The mean mixing ratio of NO 2 was 22.3 ppbv, which was much higher than NO, with a range from 3.7 to 95 ppbv.…”

Strong anthropogenic control of secondary organic aerosol formation from isoprene in Beijing

“…High levels of peroxy radicals were observed, with mean afternoon concentrations of HO2 and RO2 of around 3 x 10 8 molecule cm -3 and 1.5 x 10 9 molecule cm -3 , respectively. Zero-dimensional box modelling indicates on some days up to 35 % of the isoprene-derived RO2 radicals can react with HO2 in the afternoon (Newland et al, 2019). Thus, the diurnal profile seen in Figure 2b, measured in samples during the measurement period suggests that IEPOX was formed at 405 this urban location by the reaction of OH with local isoprene emissions, with a fraction of the RO2 radicals formed reacting with HO2 rather than NO, and subsequent uptake to aerosol forming 2-MT-OS.…”

Using highly time-resolved online mass spectrometry to examine biogenic and anthropogenic contributions to organic aerosol in Beijing

“…15 These have been associated with high emissions of VOC from diverse sources 16,17 which lead to enhanced ozone production 18,19 and SOA formation 20 alongside regional transport which is an important source of semi-volatile organic compounds (SVOC), intermediate-volatility organic compounds (IVOC), POA and SOA in Beijing during the summertime. 21,22 Historically, source apportionment of OA has been limited to low timeresolution high molecular precision offline measurements of molecular tracers, focusing on the behaviour of specic VOC systems including isoprene, [23][24][25][26] apinene 27 and toluene. 28,29 More recently, highly time-resolved bulk composition measurements by aerosol mass spectrometry (AMS) and aerosol chemical speciation monitors (ACSM) have been used to apportion OA to broad source classi-cations including fossil fuels (FFOA), biomass burning (BBOA) and oxidised organic aerosol (OOA).…”