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

• Published in: Atmospheric Chemistry and Physics
• Main Authors: D. M. Westervelt, A. M. Fiore, C. B. Baublitz, G. Correa
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2021
• Subjects: Physics; QC1-999; Chemistry; QD1-999; Arctic; Human health
• Online Access: https://doi.org/10.5194/acp-21-6799-2021; https://doaj.org/article/00487b38770d44b298d87a0f3fdb4066

The unintended consequences of reductions in regional anthropogenic sulfur dioxide (SO 2 ) emissions implemented to protect human health are poorly understood. SO 2 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 SO 2 emissions from the US, Europe, and China by differencing a control simulation with an otherwise identical simulation in which 2015 anthropogenic SO 2 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 (HO 2 ) radicals are directly removed via heterogeneous chemistry on aerosol surfaces, including sulfate, in the model, and we find that sulfate aerosol produced by SO 2 emissions from the three individual northern mid-latitude regions strongly reduces both HO 2 and hydroxyl (OH) by up to 10 % year-round throughout most of the troposphere north of 30 ∘ N latitude. Regional SO 2 emissions significantly increase nitrogen oxides (NO x ) by about 5 %–8 % throughout most of the free troposphere in the Northern Hemisphere by increasing the NO x lifetime as the heterogeneous sink of HO 2 on sulfate aerosol declines. Despite the NO x 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, ...