Visibility-derived aerosol optical depth over global land from 1980 to 2021

Abstract

Abstract. Long-term and high spatial resolution aerosol optical depth (AOD) data are necessary for climate change detection and attribution. Global ground-based AOD observation stations are sparse, and satellite AOD observations have a low time frequency, as well low accuracy before 2000. In this study, AOD was derived from hourly visibility observations collected at more than 5000 stations of the Automated Surface Observing System (ASOS) over global land from 1980 to 2021. The AOD retrievals of the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Aqua Earth observation satellite were used to train the machine learning method, and the ERA5 reanalysis boundary layer height was used as input. The predicted result has correlation coefficients of 0.54 and 0.51 with Terra MODIS satellite retrievals and AERONET ground observations. The correlation coefficients are higher at monthly and annual scales, which are 0.81 and 0.61 for the monthly and 0.91 and 0.62 for the annual, when compared with Terra MODIS and AERONET AOD, respectively. The visibility-derived AOD at ASOS stations was gridded into a 0.5-degree resolution by area-weighted ordinary kriging interpolation. Analysis of visibility-derived AOD indicates that the global mean AOD over land is 0.16, which is 0.24, 0.22, 0.11, 0.11, 0.130, and 0.12 for Africa, Asia, Europe, North America, Oceania, and South America, respectively. The mean AOD over global land, the Northern Hemisphere, and the Southern Hemisphere demonstrated decreasing trends of -0.0026/10a, -0.0018/10a, and -0.0059/10a, respectively, from 1980 to 2021. The visibility-derived AOD at station and grid scales over global land from 1980 to 2021 are available at National Tibetan Plateau / Third Pole Environment Data Center (https://doi.org/10.11888/Atmos.tpdc.300822) (Hao et al., 2023).