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, however, both parameters can be substantially enhanced under humid atmospheric conditions via hygroscopic growth. Here we use 532-nm polarization lidar observations and ERA5 humidity data 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 30.7% higher under ambient atmospheric conditions. We found values of AOD_amb=0.404 and AOD_dry=0.309. During China’s rapid air-cleaning period of 2010–2017,  AOD_amb declined significantly by −0.068 yr^-1; in contrast, the rate of decrease of AOD_dry was −0.049 yr^-1 which is 28% slower, but the decrease of the dry aerosols more accurately captures aerosol emission reductions. Hygroscopic-growth-induced net AOD ( ΔAOD_RH) 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, ΔAOD_RH has remained high attributing to rising humidity conditions but weakening hygroscopicity. While AOD_amb suggests peak pollution in summer, AOD_dry 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.