Spatial/Temporal Variations and Trends of Aerosol Optical Depth at 380 nm Wavelength in China During 1980—2001

Study is undertaken of the spatial and temporal distributions and long-term trends of 380 nm Aerosol optical depth (AOD) over China of 1980—2001 TOMS/NASA monthly data which are then treated with techniques such as EOF (empirical orthogonal function), Morlet wavelet transform, Fourier power spectrum, linear trend, Mann-Kendall test (M-K test) and moving t-test. The conclusions as follows are drawn. AOD differs greatly in its meridional distribution all over China. On an annual-mean basis there are two AOD high-value regions covering wide areas and active throughout the whole year, which are the basins in South Xinjiang and Sichuan Basin, together with a short-term high-valued area in South China as well as low AOD covering Northeast China and the Tibetan Plateau. In spring, the northern values are higher in comparison with the southern ones except the high value zone in South China. In summer the southern values are slightly bigger than the northern ones and the reversal happens in winter. AOD in autumn is higher in the north than in the south to the west of 110°N and v.v. to the east. During the research period, the AOD at wavelength of 380 nm varies remarkably with time, thickest during spring in most part of China except the Tibetan Plateau and thinnest in different seasons for regions all over China. Furthermore, variations in seasonal AOD are dominated by significant geographic characteristics, which are of two patterns. The one for summer is a type with the south/west AOD opposite to north/east AOD and the other is a type with no difference in the general tendency throughout the whole country for the other three seasons. Eleven areas (Zone A—K) are selected for regional research of the spatial and temporal variations based on the annual distribution and seasonal EOF analysis of the AOD characteristics. There are five kinds of annual cycles for the AOD in the selected eleven regions. The differences among seasons are decreasing with the latitude and significant in lower latitudes. AOD in target areas and the national means show pronounced intra-seasonal/annual oscillations, as well as significant 1-, 2- and 4-yearly periods. The AOD anomaly of China and regional representatives have been weakened since late 1980s. During 1980—2001 annual-mean AODs experience the linear increasing trends on the national and regional scales, whose increase rates have been slowed down since the period from the late 1980s to the early 1990s, occurring from east to west and from north to south in China, with remarkable inter-annual abrupt changes in North China, i.e., Zone A-F, H and K in this study. Except the decreasing trend in January, most of the regional trends in different study months also have similar linear increasing trends and decreasing shifts. The turning points of seasonal AODs in eleven selected areas also occur during the period from the late 1980s to the early 1990s, especially significant in time series of spring and autumn AODs for the eleven research zones.