Formation Mechanism of Heavy PM2.5 Pollution in Harbin in January 2020

Abstract

In the context of China's commitment to improve air quality enhancement, cities within severe cold climate zones, exemplified by Harbin continue to confront the exigent issue of fine particulate matter (PM2.5) pollution, notably during winter. Despite nationwide initiatives aimed at mitigating air pollution, Harbin's unique geographical and climatic conditions, combined with its predominant economic activities, have led to consistently high levels of PM2.5 during winter. An episode of severe PM2.5 pollution is observed in January 2020, with the monthly average mass concentration of PM2.5 peaking at 155.0 μg·m-3, significantly surpassing national standards. To explore the mechanism of this severe PM2.5 pollution episode, an integrated analysis of chemical compositions and influencing factors is conducted during this period. In the meantime, methods of backward trajectory clustering and the weighted potential source contribution function (WPSCF) are employed to investigate source areas and transport pathways of air pollutants. Results indicate that the severe PM2.5 pollution in Harbin mainly originates from primary emissions, with biomass burning contributing significantly. During the observation period, the concentration of levoglucosan in PM2.5, a common tracer of biomass burning, reaches as high as 1.1 μg·m-3, which is 3.7 to 5.5 times higher than that in other regions experiencing severe biomass burning pollution during winter. Furthermore, research findings indicate that meteorological conditions play a significant role in exacerbating PM2.5 pollution in Harbin. High relative humidity (averaging at 80.0%) combined with extremely low temperatures (averaging at -18.0 ℃) provided favorable conditions for secondary aerosol formation. Under such low-temperature and high-humidity conditions, the average sulfur oxidation rate reaches as high as 25.6%, and the nitrogen oxidation rate reaches 10.8%. It significantly increases the contribution of secondary aerosols to PM2.5 in Harbin. Additionally, this study also reveals the impact of regional transportation on the air quality of Harbin. It indicates that the air quality of Harbin is influenced not only by local emissions but also by the transportation of pollutants from neighboring cities such as Suihua, Daqing, Changchun, and Songyuan. The inter-city transfer of pollution highlights the close connection of air pollution issues within the region. Through the comprehensive analysis of causes of a severe PM2.5 pollution event in Harbin during winter from multiple perspectives, a scientific basis is provided for understanding causes for air pollution in similar cold climate.

Keywords

• cold climate
• biomass burning
• humidity
• regional transmission