Spatial and temporal variations of AOD over land at the global scale

Abstract

Spatial and temporal variations of aerosol optical depth (AOD) over land at the global scale were investigated using the MODIS C005 data set from July 2002 to June 2007. Global average AOD, AOD_c (coarse mode AOD) and AOD_f (fine mode AOD) were mapped and analysed. It was found that AOD in Asia and Africa is nearly 2 times higher than that in South America, Europe and North America. A high level of η (fine model fraction (FMF) of AOD) was recorded where AOD is relatively low. Meanwhile, low η (0.05–0.3) was observed where AOD is extremely high. It suggests that coarse particles contribute much more than fine particles to global AOD over land. High AOD level in Nigeria and north India is dominated by coarse particles due to dust events. The average AOD in China (∼0.7) is almost 50% higher than that in India (∼0.55) and nearly 2–3 times higher than that in North America and Western Europe (0.2–0.3), due to spring dust in the west and intensive anthropogenic activities in the eastern part of China. Overall, the average AOD in the USA was limited in a relatively low level. At the same time, AOD in the western part of the USA (AOD_c = 0.28 and AOD_f = 0.06) is much higher than that in the east (AOD_c = 0.06 and AOD_f = 0.17). The annual cycle of global mean AOD showed a sinusoidal shape, where peaks occur during June–August and the lowest AODs appear in November–January. The peak month, peak value and possible sources for six major continents are Asia (April, AOD_peak = 0.31, dust; June–July, AOD_peak = 0.27, secondary aerosol), Africa (March, AOD_peak = 0.28, dust; August, AOD_peak = 0.28, wildfires), South America (September, AOD_peak = 0.31, wildfires), North America (May–July, AOD_peak = 0.18, dust and wildfires), Europe (March–April, AOD_peak = 0.18, secondary aerosols), Oceania (December–January, AOD_peak = 0.09, secondary aerosols). The inter-annual variation of AOD was insignificant at the global scale. However, the inter-annual variations at the regional scale were substantial. The abnormal peak of AOD during summer 2003 was probably due to dry and hot weather and intensive wildfires.

Acknowledgements

Project (41071318) was supported by the National Natural Science Foundation of China (NSFC). We thank the MODIS science data support team for processing C005 aerosol data sets.