Impact of regional Northern Hemisphere mid-latitude anthropogenic sulfur dioxide emissions on local and remote tropospheric oxidants

Westervelt, Daniel M.; Fiore, Arlene M.; Baublitz, Colleen B.; Correa, Gustavo

The unintended consequences of reductions in regional anthropogenic sulfur dioxide (SOinline-formula2) emissions implemented to protect human health are poorly understood. SOinline-formula2 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 SOinline-formula2 emissions from the US, Europe, and China by differencing a control simulation with an otherwise identical simulation in which 2015 anthropogenic SOinline-formula2 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 (HOinline-formula2) radicals are directly removed via heterogeneous chemistry on aerosol surfaces, including sulfate, in the model, and we find that sulfate aerosol produced by SOinline-formula2 emissions from the three individual northern mid-latitude regions strongly reduces both HOinline-formula2 and hydroxyl (OH) by up to 10 % year-round throughout most of the troposphere north of 30inline-formula N latitude. Regional SOinline-formula2 emissions significantly increase nitrogen oxides (NOinline-formulax) by about 5 %–8 % throughout most of the free troposphere in the Northern Hemisphere by increasing the NOinline-formulax lifetime as the heterogeneous sink of HOinline-formula2 on sulfate aerosol declines. Despite the NOinline-formulax 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 HOinline-formula2 and OH suppress Oinline-formula3 production. Since 2015 anthropogenic SOinline-formula2 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 SOinline-formula2 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.

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Westervelt, Daniel M. / Fiore, Arlene M. / Baublitz, Colleen B. / et al: Impact of regional Northern Hemisphere mid-latitude anthropogenic sulfur dioxide emissions on local and remote tropospheric oxidants. 2021. Copernicus Publications.

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