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We use a 3-D global chemical transport model (GEOS-Chem) to interpretaircraft observations of nitrate and sulfate partitioning in transpacificdust plumes during the INTEX-B campaign of April–May 2006. The modelincludes explicit transport of size-resolved mineral dust and itsalkalinity, nitrate, and sulfate content. The observations show thatparticulate nitrate is primarily associated with dust, sulfate is primarilyassociated with ammonium, and Asian dust remains alkaline across thePacific. This can be reproduced in the model by using a reactive uptakecoefficient for HNO3 on dust (γ(HNO3) ~10−3)much lower than commonly assumed in models and possibly reflectinglimitation of uptake by dust dissolution. The model overestimates gas-phaseHNO3 by a factor of 2–3, typical of previous model studies; we showthat this cannot be corrected by uptake on dust. We find that the fractionof aerosol nitrate on dust in the model increases from ~30% infresh Asian outflow to 80–90% over the Northeast Pacific, reflecting inpart the volatilization of ammonium nitrate and the resulting transfer ofnitrate to the dust. Consumption of dust alkalinity by uptake of acid gasesin the model is slow relative to the lifetime of dust against deposition, sothat dust does not acidify (at least not in the bulk). This limits thepotential for dust iron released by acidification to become bio-availableupon dust deposition. Observations in INTEX-B show no detectable ozonedepletion in Asian dust plumes, consistent with the model. Uptake ofHNO3 by dust, suppressing its recycling to NOx, reduces Asianpollution influence on US surface ozone in the model by 10–15% or up to1 ppb

Impact of mineral dust on nitrate, sulfate, and ozone in transpacific Asian pollution plumes