Reducing one pollutant increases another

Because of the Clean Air Act of 1970, emissions of fine particles from power plants and automobiles have been decreasing for decades. Premature deaths from respiratory problems have been prevented, and air quality has improved. Most of the reductions in particulate matter have happened in summertime; the concentrations of sulfate and nitrate particles have not decreased as much in the winter. Limited observations have made it difficult to identify a reason for the asymmetry. Now Viral Shah , at the University of Washington, and colleagues have investigated the seasonal discrepancy by using atmospheric chemistry observations over the eastern US from the 2015 Wintertime Investigation of Transport, Emissions, and Reactivity campaign and the GEOS-Chem chemical transport model. Hydrogen peroxide is the key player. Its effectiveness at forming ${\mathrm{SO}}_4^{2–}$ from SO₂ via photochemical oxidation is, unlike other atmospheric gases, unaffected by winter’s reduced sunlight. Whereas modeled wintertime SO₂ concentration dropped 58% from 2007 to 2015, ${\mathrm{SO}}_4^{2–}$ concentration only decreased 40% due to H₂O₂‘s oxidation efforts. Shah and colleagues also found that the sulfur reactions affect the fate of nitrogen pollutants. With less atmospheric ${\mathrm{SO}}_4^{2–}$, particulates have an increased pH. Simulations revealed that higher pH favors the formation of ${\mathrm{NO}}_3^{–}$ in particle phase over shorter-lived aqueous nitric acid. As a result, ${\mathrm{NO}}_3^{–}$ concentrations remained higher than expected despite slashes in nitrogen oxides emissions. To effectively improve wintertime air quality, future reductions in SO₂ and nitrogen oxides emissions will need to be larger to compensate for the chemical feedbacks. (V. Shah et al., Proc. Natl. Acad. Sci. USA, 2018, doi:10.1073/pnas.1803295115 .)