The global distribution of dust column burden derived from MODIS Deep Blueaerosol products is compared to NH column burden retrieved from IASIinfrared spectra. We found similarities in their spatial distributions, inparticular their hot spots are often collocated over croplands and to alesser extent pastures. Globally, we found 22% of dust burdencollocated with NH, with only 1% difference between land-usedatabases. This confirms the importance of anthropogenic dust fromagriculture. Regionally, the Indian subcontinent has the highest amount ofdust mixed with NH (26%), mostly over cropland and during thepre-monsoon season. North Africa represents 50% of total dust burden butaccounts for only 4% of mixed dust, which is found over croplands andpastures in Sahel and the coastal region of the Mediterranean. In order toevaluate the radiative effect of this mixing on dust optical properties, wederive the mass extinction efficiency for various mixtures of dust andNH, using AERONET sunphotometers data. We found that for dusty daysthe coarse mode mass extinction efficiency decreases from 0.62 to0.48 m g as NH burden increases from 0 to 40 mg m.The fine mode extinction efficiency, ranging from 4 to 16 m g,does not appear to depend on NH concentration or relative humidity butrather on mineralogical composition and mixing with other aerosols. Ourresults imply that a significant amount of dust is already mixed withammonium salt before its long range transport. This in turn will affect dustlifetime, and its interactions with radiation and cloud properties.
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