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Particle size distributions from 9 to 640nm diameter were measured at EagleRidge in the lower Fraser Valley from 13 August to 1 September 2001 as partof the Pacific 2001 Air Quality Study. The site was on top of a ridge, about300m above the valley floor, in a predominantly agricultural area about 70kmESE of Vancouver. To further characterize the particles, theirhygroscopic properties (affinity for water) were measured. The maximum ofthe number distributions was generally between 40 and 100nm diameter, butthe number distribution was sometimes dominated by ultrafine particles withdiameters below 40nm. These ultrafine particles, which appeared to someextent on all days, were frequently associated with elevated levels of COand NOx, as expected for fresh vehicular emissions. The appearance ofthese fresh emissions was most pronounced when the growing mixed layerreached the altitude of the site. In contrast, pronounced nucleation eventsoccurred on the five cleanest days; these resulted in particle numberconcentrations as high as 5x104 particles cm-3 and growthrates of 5 to 10nmhr-1. Nucleation appears to have been triggeredwhen the UV flux reached about 25Wm-2. The growth of thesenewly formed particles was probably driven bythe photochemical oxidation of biogenic organic compounds. Dramatic growthevents were also observed on the afternoons of the more polluted days; theseproduced an extremely narrow mode σ<0.3) at a diameter of about40nm. Rainy days showed low number concentrations with the sizedistributions shifted to small sizes. On one of these days there wasevidence of nucleation not far from the site; this may have been occurringin the vicinity of the clouds

Aerosol particle size distributions in the lower Fraser Valley: evidence for particle nucleation and growth