1Science and Technology Branch, Environ. Canada, Toronto, Ontario, M3H 5T4, Canada
2Scripps Institution of Oceanography, University of California, San Diego, 92093, USA
3Dept. of Physics and Atmospheric Science, Dalhousie Univ., Halifax, NS M3J 1P3, Canada
4State University of New York at Albany, Albany, New York, USA
5University of British Columbia, Vancouver, BC, Canada V6T 1Z2, Canada
6Department of Chemistry, University of Toronto, Ontario, Canada
7University of Colorado, Boulder, Colorado, USA
Abstract. Several cases of aerosol plumes resulting from trans-Pacific transport were observed between 2 km and 5.3 km at Whistler, BC from 22 April 2006 to 15 May 2006. The fine particle (<1 μm) chemical composition of most of the plumes was dominated by sulphate that ranged from 1–5 μg m−3 as measured with a Quadrapole Aerosol Mass Spectrometer (Q-AMS). Coarse particles (>1 μm) were enhanced in all sulphate plumes. Fine particle organic mass concentrations were relatively low in most plumes and anti-correlated with the increases in the coarse particles. The chemistry of coarse particles sampled at Whistler Peak was dominated by calcium, sodium, nitrate, sulphate and formate. Scanning transmission X-ray microscopy of coarse particles sampled from the NCAR C-130 aircraft relatively close to Whistler indicated carbonate, potassium and organic functional groups, in particular the carboxyl group. Asian plumes reaching Whistler, BC during the INTEX-B study were not only significantly reduced of fine particle organic material, but organic compounds were attached to coarse particles in significant quantities. Scavenging of organic aerosol precursors by dust near source regions is suggested, and any formation of secondary organic aerosol (SOA) during transport from Asian source regions across the Pacific was principally associated with the coarse particles. An average of profiles indicates that trans-Pacific transport between 2 and 5 km during this period increased ozone by about 10 ppbv and fine particle sulphate by 0.2–0.5 μg m−3. The mean sizes of the fine particles in the sulphate plumes were larger when dust particles were present and smaller when the fine particle organic mass concentration was larger and dust was absent. The coarse particles of dust act to accumulate sulphate, nitrate and organic material in larger particles, diminishing the role of these compounds in indirect radiative forcing, but potentially enhancing their roles in direct radiative forcing.