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The accumulation of secondary acids and ammonium on individual mineral dustparticles during ACE-Asia has been measured with an online single-particlemass spectrometer, the ATOFMS. Changes in the amounts of sulphate, nitrate,and chloride mixed with dust particles correlate with air masses fromdifferent source regions. The uptake of secondary acids depended on theindividual dust particle mineralogy; high amounts of nitrate accumulated  on calcium-richdust while high amounts of sulphate accumulated on aluminosilicate-rich dust. Oxidation ofS(IV) to S(VI) by iron in the aluminosilicate dust is a possible explanationfor this enrichment of sulphate, which has important consequences for thefertilization of remote oceans by soluble iron. This study shows thesegregation of sulphate from nitrate and chloride in individual aged dust particlesfor the first time. A transport and aging timeline provides anexplanation for the observed segregation. Our data suggests that sulphatebecame mixed with the dust first. This implies that the transport pathwayis more important than the reaction kinetics in determining which speciesaccumulate on mineral dust. Early in the study, dust particles involcanically influenced air masses were mixed predominately with sulphate.Dust mixed with chloride then dominated over sulphate and nitrate when amajor dust front reached the R. V. Ronald Brown. We hypothesize that therapid increase in chloride on dust was due to mixing with HCl(g) releasedfrom acidified sea salt particles induced by heterogeneous reaction withvolcanic SO<sub>2</sub>(g), prior to the arrival of the dust front. The amount ofammonium mixed with dust correlated strongly with the total amount ofsecondary acid reaction products in the dust. Submicron dust and ammoniumsulphate were internally mixed, contrary to frequent reports that they existas external mixtures. The size distribution of the mixing state of dust withthese secondary species validates previous mechanisms of the atmosphericprocessing of dust and generally agrees with simulated aerosol chemistryfrom the STEM-2K3 model. This series of novel results has importantimplications for improving the treatment of dust in global chemistry modelsand highlights a number of key processes that merit further investigationthrough laboratory and field studies

Direct observations of the atmospheric processing of Asian mineral dust