FastPhotochemistry in Wintertime Haze: Consequencesfor Pollution Mitigation Strategies

In contrast to summer smog, the contribution of photochemistryto the formation of winter hazein northern mid-to-high latitude is generally assumed to be minor due to reduced solar UV and water vapor concentrations. Our comprehensive observations of atmospheric radicals and relevant parameters during several hazeevents in winter 2016 Beijing, however, reveal surprisingly high hydroxyl radical oxidation rates up to 15 ppbv/h, which is comparable to the high values reported in summer photochemical smog and is two to three times larger than those determined in previous observations during winter in Birmingham (Heard et al. Geophys. Res. Lett. 2004, 31, (18)), Tokyo (Kanaya et al. J. Geophys. Res.: Atmos. 2007, 112, (D21)), and New York (Ren et al. Atmos. Environ. 2006, 40, 252– 263). The active photochemistryfacilitates the production of secondary pollutants. It is mainly initiated by the photolysis of nitrous acidand ozonolysisof olefins and maintained by an extremely efficiently radical cycling process...