Atmospheric Chemistry and Physics (Nov 2009)
Observations of heterogeneous reactions between Asian pollution and mineral dust over the Eastern North Pacific during INTEX-B
Abstract
In-situ airborne measurements of trace gases, aerosol size distributions, chemistry and optical properties were conducted over Mexico and the Eastern North Pacific during MILAGRO and INTEX-B. Heterogeneous reactions between secondary aerosol precursor gases and mineral dust lead to sequestration of sulfur, nitrogen and chlorine in the supermicrometer particulate size range. <br><br> Simultaneous measurements of aerosol size distributions and weak-acid soluble calcium result in an estimate of 11 wt% of CaCO<sub>3</sub> for Asian dust. During transport across the North Pacific, ~5–30% of the CaCO<sub>3</sub> is converted to CaSO<sub>4</sub> or Ca(NO<sub>3</sub>)<sub>2</sub> with an additional ~4% consumed through reactions with HCl. The 1996 to 2008 record from the Mauna Loa Observatory confirm these findings, indicating that, on average, 19% of the CaCO<sub>3</sub> has reacted to form CaSO<sub>4</sub> and 7% has reacted to form Ca(NO<sub>3</sub>)<sub>2</sub> and ~2% has reacted with HCl. In the nitrogen-oxide rich boundary layer near Mexico City up to 30% of the CaCO<sub>3</sub> has reacted to form Ca(NO<sub>3</sub>)<sub>2</sub> while an additional 8% has reacted with HCl. <br><br> These heterogeneous reactions can result in a ~3% increase in dust solubility which has an insignificant effect on their optical properties compared to their variability in-situ. However, competition between supermicrometer dust and submicrometer primary aerosol for condensing secondary aerosol species led to a 25% smaller number median diameter for the accumulation mode aerosol. A 10–25% reduction of accumulation mode number median diameter results in a 30–70% reduction in submicrometer light scattering at relative humidities in the 80–95% range. At 80% RH submicrometer light scattering is only reduced ~3% due to a higher mass fraction of hydrophobic refractory components in the dust-affected accumulation mode aerosol. Thus reducing the geometric mean diameter of the submicrometer aerosol has a much larger effect on aerosol optical properties than changes to the hygroscopic:hydrophobic mass fractions of the accumulation mode aerosol. <br><br> In the presence of dust, nitric acid concentrations are reduced to <50% of total nitrate (nitric acid plus particulate nitrate). NO<sub>y</sub> as a fraction of total nitrogen (NO<sub>y</sub> plus particulate nitrate), is reduced from >85% to 60–80% in the presence of dust. These observations support previous model studies which predict irreversible sequestration of reactive nitrogen species through heterogeneous reactions with mineral dust during long-range transport.
