The China Daily described China Project research on carbon pricing and air pollution; The Economist cited results on the effectiveness of China's SO2 controls from our book, Clearer Skies Over China, also a subject of a New York Times op-ed by Project researchers Chris NIELSEN and Mun HO. That SO2 controls might be effective even as wintertime haze episodes have grown more severe may be counterintuitive but is not inconsistent. PM2.5 is not a single pollutant, but a class of them, defined only by physical size (particles less than 2.5 microns in diameter). As it can have any chemical composition, there are many chemical pathways for the formation of PM2.5, both as a primary pollutant emitted directly from sources and as a secondary one formed chemically in the air from reactions of precursor gases. This means there are a variety of chemical sources of PM2.5 and it does not follow from high observed PM2.5 that a given policy to reduce one form, such as sulfate particles from SO2, has therefore failed. Separately, such episodes cannot occur without unusually stagnant meteorological conditions, and whether the wintertime meteorology in north China has been unusual during recent episodes and/or may be changing over time is the subject of intensive ongoing research. The chemical and meteorological factors behind high PM2.5 episodes in the particular environments of northern Chinese air basins are not as well understood as many assume.
March 14, 2014