Low-NO atmospheric oxidation pathways in a polluted megacity
2021
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.
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