Trends in aerosol optical properties over the Bohai Rim in Northeast China from 2004 to 2010

Strong seasonal variations in regional aerosol optical depth at 550 nm (AOD) and aerosol type were investigated over China’s Bohai Rim based upon six years’ worth of data from four sites. Aerosol loading was higher and particle sizes were larger in the spring and summer than in the autumn or winter in the region. The regional mean background AOD and Angstrom exponent (α) were 0.20 ± 0.03 and 0.74 ± 0.17, respectively, and these values decreased by roughly 0.01 and 0.40 over the past six years. The annual mean AOD and α were 0.50 ± 0.06 and 1.09 ± 0.23 at three typical observation stations in the Bohai Rim economic zone. AOD increased by 0.07 and α decreased by 0.18 over the past six years, presumably due to rapid economic growth in the special zone. The accuracy of the MODIS C051 AOD product was relatively low over this region, and 52% of MODIS AOD product was within the expected error. The averaged standard deviation between annual mean ground-based and MODIS retrievals of AOD was ±0.11, but the monthly mean and seasonal mean differences ranged from −0.34 to +0.45 and from −0.17 to +0.24, respectively, at the four sites studied. The calibrated MODIS AOD was adjusted by the seasonal liner regression functions between daily ground-observed AOD and raw MODIS AOD product. The averaged standard deviations of the calibrated MODIS AOD values were ±0.02 for the annual means and ±0.08 for the monthly means. The accuracy of the calibrated MODIS AOD was found to be reliable.

► There was a “dimming” trend in AOD in the Bohai Rim, the third largest economic zone in China.
► There was a “slight brightening” trend in the regional background AOD over the past six years.
► Retrieval differences of MODIS AOD can be further propagated when deriving the monthly mean.
► Calibrated by the ground observation, the accuracy of MODIS AOD was reliable.