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In-situ airborne measurements of trace gases, aerosol size distributions,chemistry and optical properties were conducted over Mexico and the EasternNorth Pacific during MILAGRO and INTEX-B. Heterogeneous reactions betweensecondary aerosol precursor gases and mineral dust lead to sequestration ofsulfur, nitrogen and chlorine in the supermicrometer particulate size range. Simultaneous measurements of aerosol size distributions and weak-acidsoluble calcium result in an estimate of 11 wt% of CaCO3 for Asiandust. During transport across the North Pacific, ~5–30% of theCaCO3 is converted to CaSO4 or Ca(NO3)2 with anadditional ~4% consumed through reactions with HCl. The 1996 to2008 record from the Mauna Loa Observatory confirm these findings,indicating that, on average, 19% of the CaCO3 has reacted to formCaSO4 and 7% has reacted to form Ca(NO3)2 and ~2%has reacted with HCl. In the nitrogen-oxide rich boundary layer near MexicoCity up to 30% of the CaCO3 has reacted to form Ca(NO3)2while an additional 8% has reacted with HCl. These heterogeneous reactions can result in a ~3% increase in dustsolubility which has an insignificant effect on their optical propertiescompared to their variability in-situ. However, competition betweensupermicrometer dust and submicrometer primary aerosol for condensingsecondary aerosol species led to a 25% smaller number median diameter forthe accumulation mode aerosol. A 10–25% reduction of accumulation modenumber median diameter results in a 30–70% reduction in submicrometerlight scattering at relative humidities in the 80–95% range. At 80% RHsubmicrometer light scattering is only reduced ~3% due to a highermass fraction of hydrophobic refractory components in the dust-affectedaccumulation mode aerosol. Thus reducing the geometric mean diameter of thesubmicrometer aerosol has a much larger effect on aerosol optical propertiesthan changes to the hygroscopic:hydrophobic mass fractions of theaccumulation mode aerosol. In the presence of dust, nitric acid concentrations are reduced to <50%of total nitrate (nitric acid plus particulate nitrate). NOy as afraction of total nitrogen (NOy plus particulate nitrate), is reducedfrom >85% to 60–80% in the presence of dust. These observationssupport previous model studies which predict irreversible sequestration ofreactive nitrogen species through heterogeneous reactions with mineral dustduring long-range transport

Observations of heterogeneous reactions between Asian pollution and mineral dust over the Eastern North Pacific during INTEX-B