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\begin{document}

\title{Global Changes in Secondary Atmospheric Pollutants during the 2020
COVID-19 Pandemic}



\author[1]{Benjamin Gaubert}%
\author[2]{Idir Bouarar}%
\author[3]{Thierno Doumbia}%
\author[4]{Yiming Liu}%
\author[5]{Trissevgeni Stavrakou}%
\author[2]{Adrien Michel Deroubaix}%
\author[6]{Sabine Darras}%
\author[7]{Nellie Elguindi}%
\author[8]{Claire Granier}%
\author[9]{Forrest Gerhart Lacey}%
\author[10]{Jean-François Müller}%
\author[2]{Xiaoqin Shi}%
\author[1]{Simone Tilmes}%
\author[11]{Tao Wang}%
\author[2]{Guy P. Brasseur}%
\affil[1]{National Center for Atmospheric Research (UCAR)}%
\affil[2]{Max Planck Institute for Meteorology}%
\affil[3]{Laboratoire d'Aerologie, University de Toulouse, CNRS, UPS}%
\affil[4]{Hong Kong Polytechnic University}%
\affil[5]{Royal Belgian Institute for Space Aeronomy}%
\affil[6]{Observatoire Midi-Pyrenees}%
\affil[7]{Centre national de la recherche scientifique}%
\affil[8]{University of Toulouse}%
\affil[9]{National Center for Atmospheric Research}%
\affil[10]{Royal Belgian Institute for Space Aeronomy (BIRA-IASB)}%
\affil[11]{The Hong Kong Polytechnic University}%


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  \date{June 16, 2024}


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\begin{abstract}
We use the global Community Earth System Model to investigate the
response of secondary pollutants (ozone O\textsubscript{3}, secondary
organic aerosols SOA) in different parts of the world in response to
modified emissions of primary pollutants during the COVID-19 pandemic.
We quantify the respective effects of the reductions in NOx and in VOC
emissions, which, in most cases, affect oxidants in opposite ways. Using
model simulations, we show that the level of NOx has been reduced by
typically 40 \% in China during February 2020 and by similar amounts in
many areas of Europe and North America in mid-March to mid-April 2020,
in good agreement with space and surface observations. We show that,
relative to a situation in which the emission reductions are ignored and
despite the calculated increase in hydroxyl and peroxy radicals, the
ozone concentration increased only in a few NOx-saturated regions
(northern China, northern Europe and the US) during the winter months of
the pandemic when the titration of this molecule by NO\textsubscript{x}
was reduced. In other regions, where ozone is NOx-controlled, the
concentration of ozone decreased. SOA concentrations decrease in
response to the concurrent reduction in the NO\textsubscript{x} and VOC
emissions. The model also shows that atmospheric meteorological
anomalies produced substantial variations in the concentrations of
chemical species during the pandemic. In Europe, for example, a large
fraction of the ozone increase in February 2020 was associated with
meteorological anomalies, while in the North China Plain, enhanced ozone
concentrations resulted primarily from reduced emissions of primary
pollutants.%
\end{abstract}%



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