Impact of regional Northern Hemisphere mid-latitude anthropogenic sulfur dioxide emissions on local and remote tropospheric oxidants
Authors: Westervelt Daniel M. , Fiore Arlene M. , Baublitz Colleen B. , Correa Gustavo 
Published: May 2021

Abstract. The unintended consequences of reductions in regional
anthropogenic sulfur dioxide (SO2) emissions implemented to protect
human health are poorly understood. SO2 decreases began in the 1970s in
the US and Europe and are expected to continue into the future, while recent
emissions decreases in China are also projected to continue. In addition to
the well-documented climate effects (warming) from reducing aerosols,
tropospheric oxidation is impacted via aerosol modification of photolysis
rates and radical sinks. Impacts on the hydroxyl radical and other trace
constituents directly affect climate and air quality metrics such as surface
ozone levels. We use the Geophysical Fluid Dynamics Laboratory Atmospheric
Model version 3 nudged towards National Centers for Environmental Prediction
(NCEP) reanalysis wind velocities to estimate the impact of SO2
emissions from the US, Europe, and China by differencing a
control simulation with an otherwise identical simulation in which 2015
anthropogenic SO2 emissions are set to zero over one of the regions.
Springtime sulfate aerosol changes occur both locally to the emission region
and also throughout the Northern Hemispheric troposphere, including remote
oceanic regions and the Arctic. Hydroperoxy (HO2) radicals are directly
removed via heterogeneous chemistry on aerosol surfaces, including sulfate,
in the model, and we find that sulfate aerosol produced by SO2
emissions from the three individual northern mid-latitude regions strongly
reduces both HO2 and hydroxyl (OH) by up to 10 % year-round
throughout most of the troposphere north of 30∘ N latitude.
Regional SO2 emissions significantly increase nitrogen oxides
(NOx) by about 5 %–8 % throughout most of the free troposphere in the
Northern Hemisphere by increasing the NOx lifetime as the heterogeneous
sink of HO2 on sulfate aerosol declines. Despite the NOx
increases, tropospheric ozone decreases at northern mid-latitudes by 1 %–4 %
zonally averaged and by up to 5 ppbv in summertime surface air over China,
where the decreases in HO2 and OH suppress O3 production. Since
2015 anthropogenic SO2 emissions in China exceed those in the US or
Europe, the oxidative response is greatest for the China perturbation
simulation. Chemical effects of aerosols on oxidation (reactive uptake)
dominate over radiative effects (photolysis rates), the latter of which are
only statistically significant locally for the large perturbation over
China. We find that the SO2 emissions decrease in China, which has yet
to be fully realized, will have the largest impact on oxidants and related
species in the Northern Hemisphere free troposphere compared to future
decreases in Europe or the US. Our results bolster previous calls for a
multipollutant strategy for air pollution mitigation to avoid the
unintended consequence of aerosol removal leading to surface ozone increases
that offset or mask surface ozone gains achieved by regulation of other
pollutants, especially in countries where current usage of high-sulfur
emitting fuels may be phased out in the future.