Abstract: Based on CALIPSO satellite data and ERA5 reanalysis data, the correlation between the optical properties of tropospheric aerosols and relative humidity in the marine areas within the longitude range of 108° E–124° E and the latitude range of 18° N–41° N (i.e. China's offshore waters) from 2007 to 2016 is analyzed. The results show that during the study period, the 532 nm total backscattering coefficient and the 532 nm extinction coefficient of aerosols in the sea area increases with the increase of relative humidity, while the depolarization ratio of aerosols decreases with the increase of relative humidity. This may be due to the change of the hygroscopicity and overall morphology of aerosol particles caused by the increase of relative humidity. At the same time, the values of various aerosol optical properties are the lowest in summer as a whole, while the values in spring, autumn and winter will change alternately with the increase of relative humidity, which may be related to the different proportions of various types of aerosols in different seasons. In the other hand, the values of aerosol optical properties in 2014–2016 were significantly lower than those in the previous seven years, which may be the result of the strong implementation of China's environmental protection policies. After further dividing the offshore waters of China, it is found that from 2007 to 2016, the proportion of marine derived aerosols increases in order of the Bohai and Yellow Seas, the East China Sea and the South China Sea, and the overall value of each optical property of aerosols decreases in order of the Bohai and Yellow Seas, the East China Sea and the South China Sea. Further analysis on the division of altitude layers in the sea area shows that, within the range of 0–7 km, the higher the altitude, the smaller the proportion of marine source aerosols and the larger the proportion of land source aerosols, and at the same time, the overall value of aerosol depolarization ratio continues to increase.

Key words: aerosol, optical properties, cloud-aerosol lidar and infrared pathfinder satellite observation; ERA5, China offshore