Multiday haze in the East Asia: Transport and chemical aging of hygroscopic particles

Abstract. East Asian countries (Korea and China) have been experiencing exceedingly high concentrations of particulate matter (PM) that threatens health and potentially alters climate. However, the formation of East Asian PM is poorly understood. Furthermore, during the winter high PM concentrations at Seoul, Korea often occurs via the combination of the transport of Chinese haze through the prevailing Westerlies and the local formation at Seoul, and this complicates the PM formation. A severe multiday haze event occurred at Seoul during February 24–28, 2014, and the synoptic meteorological conditions suggest the combination the transport and the local formation (Seo et al., 2017). PM at Seoul and Deokjeok Island during this haze period was sampled and analyzed. Deokjeok Island provides background for Seoul PM since it is upwind and emission free. We hypothesize that transported PM from China (PM at Deokjeok Island) is hygroscopic (thus contains water due to high RH) and undergoes multiphase photochemical aging at Seoul. To validate our hypothesis, we conducted smog chamber experiments. In a humid smog chamber, photochemistry of NO_x initiates hygroscopic growth of particles and aerosol liquid water in turn facilitates aqueous chemistry forming organonitrates and oligomers. This multiphase chemistry provides chemical insights of chemical aging during a haze event in East Asia. NO_x effects on photochemical aging of particles are substantial. Even a few ppb of NO_x form nitrates in aerosol liquid water, and acidify neutral particles. NO_x reduction is not likely to be an effective strategy for nitrate- and acidity-related health.