Regional trends in aerosol optical depth and particle size parameters since 2002 and 2012 for Terra, Aqua, and SNPP

The NASA Dark Target (DT) algorithm retrieves aerosol optical depth (AOD) from passive imagery, originally Terra and Aqua MODIS (beginning 2000 and 2002, respectively) and continuing on to Suomi-NPP VIIRS (beginning 2012). It is currently in testing for the ABI and AHI geostationary imagers, and we plan to adapt the same retrieval algorithm to NOAA-20 VIIRS and to future VIIRS launches. We require at least a 30-year period of continuous measurements to form a climate data record, which is beyond the expected end of the MODIS mission. However, coherent regional trends are already emerging in the AOD retrievals made over the past 18 years: some apparently resulting from policy changes, others with sources yet to be determined, but with good agreement among the three sensors for which DT is operational. One way to shed light on the causes of these trends is to examine other parameters in the DT product. Regional trends in Ångström exponent and fine mode fraction would indicate a change in particle size distribution, which could come from differences in the source and composition of particles. However, the disagreement among the three sensors is greater for the particle size trends than for trends in AOD, and the reasons need to be understood. Figure. Eight-year trend (March 2012-February 2021) in aerosol optical depth as measured by Dark Target for VIIRS SNPP (top) and for MODIS Aqua (bottom). Each plotted 11 grid cell meets a significance threshold of p 0.05.