PM2.5 pollution in China’s Guanzhong Basin and the USA’s San Joaquin Valley mega-regions

The Guanzhong Basin (GZB) of northwest China is examined as a mega-region containing the mega-city of Xi'an. The concept of a "mega-region" is more accurate than that of a "mega-city" for air quality management as there is an interaction between urban and non-urban emissions. Parallels are drawn between the GZB and the San Joaquin Valley (SJV) mega-region of central California for excessive wintertime PM2.5 concentrations. Long-term chemical composition measurements show the usual high levels of organic and elemental carbon, minerals, sulfate, nitrate, and ammonium. Wintertime provides prolonged high pressure systems in both areas punctuated by unstable precipitation events. Sluggish nighttime winds allow pollutants such as ammonia from distant agricultural operations, to mix with urban pollutants, sometimes serving as a reactor to create secondary sulfates and nitrates, and possibly some secondary organic compounds. A shallow surface layer forms at night in the SJV and GZB that couples to an upper level inversion after sunrise, allowing pollutants accumulated aloft to mix to the surface. Although current air quality management strategies have focused on urban emissions, and PM2.5 levels are on a downward trend, future management efforts must consider reducing emissions from a variety of sources in the larger region of these basins. Agricultural emissions are important in the SJV, but are just being addressed in the GZB. Tactics developed for the SJV would probably be effective in other areas of the world with similar emissions, topography, and meteorology. Experiments related to agricultural NOx emissions, emission inventory enhancements, source apportionment, and estimates of precursor limitations for ammonium nitrate formation, have been conducted in the SJV that can be tested in the GZB.