Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 4 2 2004 10.5194/acpd-4-1623-2004 http://www.atmos-chem-phys-discuss.net/4/1623/2004/ http://www.atmos-chem-phys-discuss.net/4/1623/2004/acpd-4-1623-2004.html http://www.atmos-chem-phys-discuss.net/4/1623/2004/acpd-4-1623-2004.pdf 1623 1663 2004-03-16 Aerosol particle size distributions in the lower Fraser Valley: evidence for particle nucleation and growth M. Mozurkewich mozurkew@yorku.ca T.-W. Chan Y.-A. Aklilu B. Verheggen Department of Chemistry and Centre for Atmospheric Chemistry, York University, 4700 Keele Street, Toronto, ON M3J 1P3, Canada Particle size distributions from 9 to 640 nm diameter were measured at Eagle Ridge in the lower Fraser Valley from 13 August to 1 September 2001 as part of the Pacific 2001 Air Quality Study. The site was on top of a ridge, about 300 m above the valley floor, in a predominantly agricultural area about 70 km ESE of Vancouver. To further characterize the particles, their hygroscopic properties (affinity for water) were measured. The maximum of the number distributions was generally between 40 and 100 nm diameter, but the number distribution was sometimes dominated by ultrafine particles with diameters below 20 nm. These ultrafine particles, which appeared to some extent on all days, were frequently associated with elevated levels of CO and NO_x, as expected for fresh vehicular emissions. The appearance of these fresh emissions was most pronounced when the growing mixed layer reached the altitude of the site. In contrast, pronounced nucleation events occurred on the five cleanest days; these resulted in particle number concentrations as high as 5×10⁴ particles cm^−3 and growth rates of 5 to 10 nm hr^−1. These events appear to have been driven by the photochemical oxidation of biogenic organic compounds and to have been triggered when the UV flux reached about 25 W m^−2. Dramatic growth events were also observed on the afternoons of the more polluted days; these produced an extremely narrow mode (σ<0.3) at a diameter of about 40 nm. Rainy days showed low number concentrations with the size distributions shifted to small sizes. On one of these days there was evidence of nucleation not far from the site; this may have been occurring in the vicinity of the clouds.