Linking Improvements in Sulfur Dioxide Emissions to Decreasing Sulfate Deposition by Combining Satellite and Surface Observations with Trajectory Analysis

Sulfur dioxide (SO₂), a criteria pollutant, and sulfate (SO₄^2-) deposition are major environmental concerns in the eastern U.S. and both have been on the decline for two decades. Satellite column SO₂ data from the Ozone Monitoring Instrument (OMI), and SO₄^2- wet deposition data from the NADP (National Atmospheric Deposition Program) were used to investigate the temporal and spatial relationship between the downward trends in SO₂ emissions and sulfate deposition over the eastern U.S. from 2005 to 2015. To establish the relationship between SO₂ emission sources and receptor sites, we conducted a Potential Source Contribution Function (PSCF) analysis using HYSPLIT back trajectories for five selected Air Quality System (AQS) sites - (Hackney, OH, Akron, OH, South Fayette, PA, Wilmington, DE, and Beltsville, MD) - in close proximity to NADP sites with large downward SO₄^2- trends since 2005. Back trajectories were run for three summers (JJA) and three winters (DJF) and used to generate seasonal climatology PSCFs for each site. The OMI SO₂ and interpolated NADP sulfate deposition trends were normalized and overlapped with the PSCF, to identify the areas that had the highest contribution to the observed drop. The results suggest that emission reductions along the Ohio River Valley have led to decreases in sulfate deposition at Hackney, Akron and South Fayette. Farther to the east, emission reductions in southeast PA have resulted in improvements in sulfate deposition at Wilmington, DE, while for Beltsville, reductions from both the Ohio River Valley and nearby have had an impact on sulfate deposition. For Beltsville, sources closer than 300 km from the site contribute roughly 56% observed deposition trends in winter, and 82% in summer, reflecting seasonal changes in transport pattern as well as faster oxidation of sulfur in summer. This suggests that emissions and deposition are not only linked through location of sources relative to the observing sites, but also the weather patterns characteristic to the region, as evidenced by a west to east shift in the contribution between winter and summer. The method developed here is applicable to other regions with significant trends such as China and India, and can be used to estimate the potential benefits of emission reduction in those areas.