London’s “Great Smog” of 1952 killed thousands and led the U.K. to enact clean air laws in subsequent years. Severe haze events still present a serious air-quality problem, particularly in China, but even after all these years the chemistry behind how airborne chemicals and particulates combine to produce such haze remains hazy. A key component is aqueous oxidation of sulfur dioxide to sulfate by nitrogen dioxide, says an international team of researchers led by Gehui Wang of the Chinese Academy of Sciences, Renyi Zhang of Texas A&M University, and Mario J. Molina of the University of California, San Diego (Proc. Natl. Acad. Sci. USA 2016, DOI: 10.1073/pnas.1616540113). Sulfur dioxide and nitrogen dioxide are coproduced during combustion of coal and other fuels. Through lab experiments and field studies, the researchers found significant SO2 oxidation under two conditions: when cloud droplets are available, as in London, or on fine aerosol particles when there is high humidity and enough ammonia around to neutralize the pH, as is the case in haze events in China. The results suggest that NO2 and NH3 emissions must be controlled along with SO2 to reduce severe haze.