Using satellite data to estimate particulate air quality in a subtropical city: an evaluation of accuracy and sampling issues

Abstract

Although satellite data are increasingly being used for particulate air quality studies, the applicability of satellite-derived aerosol optical depth (AOD or τ) products for use over tropical or subtropical cities with frequent cloud cover should be carefully examined. Using eight years of ground-based and satellite-based observations, we assess the accuracy and sampling issues of using satellite data to study particulate air quality over a typical subtropical city, Hong Kong, at monthly to yearly timescales. The validation of Moderate Resolution Imaging Spectroradiometer (MODIS) AOD products shows that 64.6% of the retrievals fall within an expected error envelope of ± (0.05 + 0.15τ) and that they have a low bias during the eight-year study period, thus suggesting that the accuracy of current satellite-derived AOD data still needs to be improved. In addition, the availability of satellite observations is typically less than 30% during the months in spring and summer and less than 35% over seasonal and yearly timescales due to the cloudy and rainy weather. Inadequate sampling issues result in large biases over monthly and seasonal timescales; however, satellite data do not have major sampling issues on the yearly timescale despite the positive bias due to the washout effects of rain.