Hourly variation characteristics of PM2.5 and main components in Beijing based on wind direction

Abstract

The annual average concentration of PM2.5 in Beijing consistently met the standard between 2021 and 2023. However, the pollution situation persists, particularly during autumn and winter. With the diminishing capacity for local emission reduction in Beijing, the significance of regional transport has increased. To examine the impact of various air masses on PM2.5 and its chemical constituents, a detailed analysis was conducted using wind direction data, hourly PM2.5 values, and concentrations of major components from 2015 to 2020 in Beijing. The findings reveal that the southwest transport corridor remained a critical pathway for regional pollutant influx into Beijing, although the influence of pollutants transported from the east has intensified. During October to February, when the hourly increase in NO3− surpassed 10 μg/m³ and increases in SO42−, NH4+, and OM exceeded 6 μg/m³, the likelihood of southwest and southern transport ranged from 56% to 60%, peaking at 56%–72% from December to February. From March to September, when the hourly increase in NO3− exceeded 10 μg/m³, eastward transport was probable at 63%, reaching 69% from May to July. Sharp increases in SO42−, NH4+, and OM, related to combustion, primarily occurred during autumn and winter evenings, whereas a sharp increase in NO3− was observed mainly during spring and summer mornings. This study elucidates the correlation between hourly concentration growth of PM2.5 chemical components and transport direction, offering a scientific foundation for mitigating regional air pollution in the Beijing-Tianjin-Hebei area.