Variability of nocturnal aerosol optical properties in China and correlations with meteorological variables during 2003-2022

Aerosol optical properties in daytime have been widely explored based on the ground -based and satellite -based passive remote sensing. However, few studies have investigated long-term variability of nocturnal aerosol optical properties and its impact factors mainly due to scarce observations. Nocturnal aerosol optical depth (NAOD) and & Aring;ngstro m exponent (NAE) derived from Copernicus Atmosphere Monitoring Service Reanalysis (CAMSRA) from 2003 to 2022 are analyzed to investigate the spatiotemporal variability of nocturnal aerosol optical properties and their meteorological impact factors in China. The high NAOD ranging from 0.8 to 1.4 was mainly found over inland of southeastern China, especially in the Sichuan basin and North and Central China plain, corresponding with heavily populated, economically developed and low-lying regions, but observably decreased since 2018. The NAE was less than 0.5 in arid and semiarid regions of the northwest China, while more than 0.7 in monsooninfluenced eastern China. The NAOD differs seasonally, and is generally higher in spring and summer. Reversely, the seasonal mean NAE increased from spring to winter. The monthly mean NAOD of China, which was high in February to October of 2006 - 2016, ranged from 0.9 to 1.1 related to the rapid urbanization and industrialization. The low NAE ranging from 0.7 to 0.8 mainly occurred in April, May and June due to the dust transportation from northwestern China in spring. The correlation between NAOD and potential evapotranspiration (PE), surface pressure (SP) and relative humidity (RH) was negative in northern China, while positive in southern China. The NAOD in most China was correlated positively to total precipitation (TP), but negatively to 10m ucomponent of wind (U10). The correlation between NAOD and 10m v -component of wind (V10) was positive in northeastern China, while negative in northwestern China. NAE in most China was correlated positively to PE and SP, while negatively to TP and T. The correlation between NAE and RH/V10 was positive in northwestern China, while negative in eastern China. The correlation between NAE and U10 was positive in northeastern and northwestern China, while negative in North and Central China. The findings in the study enable more comprehensive understanding of spatiotemporal variability of nocturnal aerosols optical properties and its impact factors in China.