Characteristics and formation of typical winter haze in Handan, one of the most polluted cities in China

Highlights: • The annual average PM_2.5 concentration was 122.4 μg m^-3, approximately 3.5 times higher than the guideline of NAAQS. • Nitrate and sulfate accounted for the largest proportion of PM_2.5. • High sulfate concentration in wintertime was primarily ascribed to high sulfur oxidation ratio, driven by high relative humidity (> 60%). Handan, a city within the North China Plain (NCP) region, is a typical city influenced by regional particulate matter (PM) pollution. One-year hourly semi-continuous observation was carried out in 2015 in Handan with the aim of identifying the chemical composition and variations in PM_2.5. Moreover, the concentration of aerosol precursors, meteorological factors, and secondary transformations are considered. The results demonstrate that the annual average PM_2.5 concentration in Handan is 122.35 μg m^{− 3}, approximately 3.5 times higher than the Chinese National Ambient Air Quality Standard (NAAQS) (35 μg m^{− 3}), and only 12 days were below the guideline. As expected, PM concentrations are highest in winter, especially in December. In addition, we measure the concentrations of five species commonly found in PM, nitrate, sulfate, ammonium, inorganic carbon, and organic carbon. Of these, nitrate and sulfate account for the largest proportion of PM_2.5; during periods when the PM_2.5 concentration was below 400 μg m^{− 3}, nitrate dominates, while above this concentration, sulfate dominate. This is likely related to the nitrogen and sulfur oxidation ratios, which are in turn, especially the sulfur oxidation ratio, driven by high relative humidity (> 60%). In addition, haze events are driven by other meteorological conditions, wind speed and direction, where low wind speeds from the south and southwest enable pollutant accumulation, which are infrequently interspersed with brief periods with high wind speeds that promote pollutant dispersal. Even though Handan is among the ten most polluted cities in China with regard to air pollution, few studies beyond model simulations have analyzed air pollutant concentrations in this city. Therefore, this study makes a significant contribution to understanding air pollution in Handan, which can further be used to improve our understanding of regional pollution in the highly populated North China Plain. These results have implications for the creation of policies and legislation, as well as other pollution control measures.