Hygroscopic growth obscures actual variation in anthropogenic aerosol optical depth over central China during 2010–2024

Abstract

Particle bulk optical property parameters play an essential role in evaluating air quality, which, however, can be substantially enhanced under humid atmospheric conditions via hygroscopic growth. Here we use 532 nm polarization lidar observations, ERA5 humidity data, and Hänel hygroscopic-growth parameters from our previous study (Jing et al., 2026) during 2010–2024 to retrieve vertical profiles of ambient and dry aerosol backscatter and extinction coefficients of anthropogenic pollution over central China. Particle hygroscopic growth led to enhanced particle backscatter coefficient by 11 %–46 % below 2 km on an annual basis for the considered time frame. Anthropogenic Aerosol Optical Depth (AOD) was ∼30 % higher under ambient atmospheric conditions. We found values of AODamb=0.404 and AODdry=0.315. In Jing et al. (2025), we reported a significant decline of -0.068 yr-1 for AODamb during China's rapid air-cleaning period of 2010–2017. Here, we find a 28 % lower decline rate for AODdry (-0.049 yr-1), which more accurately reflects the reduction in anthropogenic aerosol emission. Hygroscopic-growth-induced net AOD (ΔAODRH) dropped sharply in 2011–2014, most likely as a result of emission mitigation and drier atmospheric conditions, then rebounded in 2014–2019 as rising humidity conditions and the presence of hydrophilic aerosols. Since 2020, ΔAODRH has remained high attributed to rising humidity conditions but weakening hygroscopicity.AODamb suggests peak pollution in summer, while AODdry identifies winter as the true air-pollution maximum. These results highlight the significant impact of aerosol water uptake on its optical properties; therefore, it must be accounted for to ensure accurate air quality assessments.