Aerosol-PBL relationship under diverse meteorological conditions: Insights from satellite/radiosonde measurements in North China

Abstract

The planetary boundary layer height (PBLH) plays crucial roles in regulating air pollution levels; however, its relationships with PM_2.5 (fine particulate matter with diameter ≤ 2.5 μm) under diverse meteorological conditions, as well as potential causes, are not yet well understood. This study leverages approximately three years of satellite/radiosonde derived PBLH to investigate PBLH-PM_2.5 relationships under six typical circulation patterns in North China, and some novel physical explanations for the varying PBLH-PM_2.5 relationship are proposed. The six circulation patterns are dominated by three high- and three low-pressure systems, with significantly negative PBLH-PM_2.5 correlations exhibited in high-pressure patterns, while weak or even insignificant relationships in low-pressure patterns. Meteorological factors, particularly humidity and vertical winds, can largely explain the varying PBLH-PM_2.5 relationships across different synoptic patterns. Under high-pressure patterns, elevated PM_2.5 aligns well with high humidity within boundary layer, which restricts the magnitude of the PBLH and thus the negative PBLH-PM_2.5 correlation. However, under low-pressure patterns, humidity in boundary layer and in free atmosphere exert conflicting effects on PBLH-PM_2.5 relationship. Higher PM_2.5 is observed when only the boundary layer is moist, whereas when thewhole column is moist or supersaturated, nucleation, transitions, and wet scavenging lead to reduced PM_2.5 and lower PBLH, thereby the positive PBLH-PM_2.5 correlation. Additionally, the column feature of vertical wind within boundary layer can also help explain the positive PBLH-PM_2.5 relationship. These findings provide deeper insights into understanding boundary layer processes and pollution dynamics.