Abstract
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 were nominally anti-correlated with the increases in the number concentrations of coarse particles. The ion 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. Suspension of dust with deposited organic material and scavenging of organic materials by dust near anthropogenic sources are suggested, and if any secondary organic aerosol (SOA) was formed during transport from Asian source regions across the Pacific it 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.
Highlights
The transport of trace gases and particles at the intercontinental scale is a concern for atmospheric chemistry, climate forcing and the national regulation of air pollutants
The INTEX-B period at Whistler Peak was marked by intervals with the temperature below 0◦C and the relative humidity (RH) above 90% alternating with intervals when the temperatures reached above 0◦C and the RH went below 60% (Fig. 5)
The chemical composition of most of the plumes was dominated by sulphate that ranged from 1–5 μg m−3 for particles measured with the AMS; the AMS samples particles 0.1–0.5 μm geometric diameter (GD) with approximately 100% efficiency
Summary
The transport of trace gases and particles at the intercontinental scale is a concern for atmospheric chemistry, climate forcing and the national regulation of air pollutants. Al., 2001; Huebert et al, 2003; Prospero et al, 2003; Brock et al, 2004) These studies indicate that the main transport of Asian aerosol to North America occurs during the spring and, despite different regional sources, dust and sulphate are often but not always coincident in the plumes. Modelling studies have identified pathways, transport times and potential impacts on North American air quality (e.g. Walker et al, 2008; van Donkelaar et al, 2008; Zhao et al, 2006; Gong et al, 2006; Heald et al, 2006; Holzer et al 2003, 2005; Jacob et al 1999; Song and Carmichael, 1999). Implications for the impact of this aerosol on climate forcing are suggested
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