Vertical structure of the aerosols in the troposphere over the North China Plain: An analysis based on observations and simulations from 2007 to 2022

Abstract

This study systematically investigates the vertical structure characteristics and driving mechanisms of tropospheric aerosols over the North China Plain (NCP) based on Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) aerosol profiles from 2007 to 2022, combining K-means clustering, regression analysis, and Weather Research and Forecasting with Chemistry model (WRF-Chem) simulations. Seasonal cluster analysis identified four typical vertical structures during daytime, with the Elevated Aerosol Layer (EAL) in summer being the most prominent. Significant aerosol accumulation occurs within the 500–1500 m height range during this season, contributing nearly 50 % of the tropospheric Aerosol Optical Depth (AOD). Analysis of long-term CALIOP data further revealed a significant decreasing trend in AOD over the study region between 2013 and 2020, a change closely linked to emission reduction policies implemented during the same period. To delve deeper into the causes of the aerosol accumulation phenomenon at 500–1500 m and the influencing factors and vertical distribution characteristics of the AOD decline, we designed WRF-Chem sensitivity experiments combined with regression analysis. The results indicate that both emission factors and meteorological factors influence AOD, but their dominance varies with height: emissions primarily affect near-surface aerosols below 500 m, while meteorology dominates the variations at heights of 500–1500 m. Specifically, hygroscopic growth, thermal convection, bounce and resuspension of coarse particles, and long-range transport collectively enhance aerosol accumulation at 500–1500 m. Consequently, policy-driven emission reductions significantly lowered near-surface aerosol concentrations, which is crucial for mitigating population exposure risks. However, the aerosol accumulation phenomenon at 500–1500 m persists regardless of emission changes, as it is primarily driven by meteorological factors.