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

Abstract 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 SO 4 2− (balanced by NH 4 + ) in the accumulation mode range; SO 4 2− often exhibited a diurnal variation that was primarily the result of fog deposition overnight and photochemical formation during the day. SO 4 2− in the 3.1–6.2 μm particles was mostly of marine origin. Most of the smaller particle SO 4 2− was from anthropogenic sources and it is estimated that about 75% of the SO 4 2− in the smaller particles were due to secondary processes. Oxalate, C 2 O 4 2− , although at low concentrations (generally 4 2− ), displayed a bi-modal size distribution, the accumulation mode being similar to SO 4 2− and the coarse mode peaking at 1–1.8 μm. Frequently at night, there was also a significant increase in NO 3 − in the accumulation mode that is attributed to the co-condensation of gas phase NH 3 +HNO 3 to form aerosol NO 3 − and the reactive uptake of N 2 O 5 onto aerosols. In the coarse mode, the sea salt Cl − was frequently deficient relative to Na + yet accompanied by a corresponding increase in NO 3 − ; this was attributed to reaction of NO 3 − precursors (e.g., HNO 3 and N 2 O 5 ) with sea salt aerosol Cl − .