Ionic composition and size characteristics of particles in the Lower Fraser Valley: Pacific 2001 field study

The size distribution of aerosol chemical compositions was studied over a 2-week period in August 2001 at three locations (one urban and two semi-rural) in the Lower Fraser Valley (LFV) of British Columbia, Canada. The size distributions varied diurnally and were bimodal with a pronounced peak at about 0.3–0.55 μm (accumulation mode) and 2–7 μm (coarse mode). Generally, on an equivalent basis, the aerosol was dominated by SO42− (balanced by NH4+) in the accumulation mode range; SO42− often exhibited a diurnal variation that was primarily the result of fog deposition overnight and photochemical formation during the day. SO42− in the 3.1–6.2 μm particles was mostly of marine origin. Most of the smaller particle SO42− was from anthropogenic sources and it is estimated that about 75% of the SO42− in the smaller particles were due to secondary processes. Oxalate, C2O42−, although at low concentrations (generally <0.1 SO42−), displayed a bi-modal size distribution, the accumulation mode being similar to SO42− and the coarse mode peaking at 1–1.8 μm. Frequently at night, there was also a significant increase in NO3− in the accumulation mode that is attributed to the co-condensation of gas phase NH3+HNO3 to form aerosol NO3− and the reactive uptake of N2O5 onto aerosols. In the coarse mode, the sea salt Cl− was frequently deficient relative to Na+ yet accompanied by a corresponding increase in NO3−; this was attributed to reaction of NO3− precursors (e.g., HNO3 and N2O5) with sea salt aerosol Cl−.