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. Satellite aerosol products such as the Dark Target (DT)produced from the MODerate resolution Imaging Spectroradiometer (MODIS) areuseful for monitoring the progress of air pollution. Unfortunately, the DToften fails to retrieve during the heaviest aerosol events as well as themore moderate events in winter. Some of the literature attributes this lack ofretrieval to the cloud mask. However, we found this lack of retrieval is mainlytraced to thresholds used for masking of inland water and snow.Modifications to these two masks greatly increase 50 % of the retrievalsof aerosol optical depth at 0.55 µm (AOD) greater than 1.0. The“extra”-high-AOD retrievals tend to be biased when compared witha ground-based sun photometer (AErosol RObotic NETwork, AERONET). Reducing bias in new retrievalsrequires two additional steps. One is an update to the assumed aerosoloptical properties (aerosol model); the haze in this region is both lessabsorbing and lower in altitude than what is assumed in the globalalgorithm. The second is accounting for the scale height of the aerosol,specifically that the heavy-aerosol events in the region are much closer tothe surface than what is assumed by the global DT algorithm. The resultingcombination of modified masking thresholds, new aerosol model, and loweraerosol layer scale height was applied to 3 months of MODIS observations(January–March 2013) over eastern China. After these two additional steps areimplemented, the significant increase in new retrievals introduces nooverall bias at a high-AOD regime but does degrade other overall validationstatistics. We also find that the research algorithm is able to identifyadditional pollution events that AERONET instruments may not due todifferent spatial sampling. Mean AOD retrieved from the research algorithmincreases from 0.11 to 0.18 compared to values calculated from theoperational DT algorithm during January to March of 2013 over the studyarea. But near Beijing, where the severe pollution occurs, the new algorithmincreases AOD by as much as 3.0 for each 0.5∘ grid box over theprevious operational-algorithm values.

atmospheric measurement techniques

A Dark Target research aerosol algorithm for MODIS observations over eastern China: increasing coverage while maintaining accuracy at high aerosol loading