ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-9-8469-2009 Observations of heterogeneous reactions between Asian pollution and mineral dust over the Eastern North Pacific during INTEX-B McNaughton C. S. 1 Clarke A. D. 1 Kapustin V. 1 Shinozuka Y. 1 9 Howell S. G. 1 Anderson B. E. 2 Winstead E. 2 Dibb J. 3 Scheuer E. 3 Cohen R. C. 4 Wooldridge P. 4 Perring A. 4 Huey L. G. 5 Kim S. 5 Jimenez J. L. 6 Dunlea E. J. 6 DeCarlo P. F. 6 10 Wennberg P. O. 7 Crounse J. D. 7 Weinheimer A. J. 8 Flocke F. 8 School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI, 96822, USA NASA Langley Research Center, Hampton, VA, 23665, USA University of New Hampshire, Durham, NH, 03824, USA University of California Berkeley, Berkeley, CA, 94720, USA Georgia Institute of Technology, Atlanta, GA, 30332, USA Cooperative Institute for Research in Environmental Sciences (CIRES) and University of Colorado, Boulder, CO, 80309, USA California Institute of Technology, Pasadena, CA, 91125, USA National Center for Atmospheric Research, Boulder CO, 80307, USA now at: NASA Ames Research Center, Moffett Field, CA, 94035, USA now at: Paul Scherrer Institute, Switzerland 31 03 2009 9 2 8469 8539 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/9/8469/2009/acpd-9-8469-2009.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/9/8469/2009/acpd-9-8469-2009.pdf

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 during long-range transport lead to irreversible sequestration of sulfur and nitrogen compounds 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, 10–30% of the CaCO<sub>3</sub> is converted to CaSO<sub>4</sub> or Ca(NO<sub>3</sub>)<sub>2</sub> through reactions with trace gases. The 11-year record from the Mauna Loa Observatory confirm these findings, indicating that, on average, 16% of the CaCO<sub>3</sub> has reacted to form CaSO<sub>4</sub> and 14% has reacted to form Ca(NO<sub>3</sub>)<sub>2</sub>. Heterogeneous reactions resulting in ~3% increase in dust solubility is shown to have 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 optics than changes to the hygroscopic:hydrophobic mass fractions of the aerosol. <br><br> In the presence of dust, nitric acid concentrations are reduced to &lt;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 &gt;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.

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