Size distribution and dry deposition of particulate mass, sulfate and nitrate in an urban area

Atmospheric size distributions for mass, sulfate and nitrate were measured in Chicago, Illinois during the Spring–Summer, and Fall of 1994 and 1995. Nineteen samples were collected over two or three daytime periods using a Wide Range Aerosol Classifier system (Noll Rotary Impactor and Cascade Impactor). The Atmospheric Aerosol was divided into 10 size range between 0.1 and 100 μm. The total particulate mass contains three peaks, a fine peak (mode) with an MMD of 0.43 μm, and two coarse particulate peaks at 6.75 (mode 2) and 28.58 μm (mode 3). The total concentration of particles and the location of the concentration peaks varied significantly depending on wind velocity. Data shows that sulfate and nitrate existed in both the fine (<2.5 μm) and coarse (>2.5 μm) particles. The atmospheric concentration of sulfate in the fine and coarse particles varied between 1.0–7.0 and 0.3–1.0 μg/m3, respectively. The nitrate concentrations were 0.3–5.3 μg/m3 in the fine and 0.3–2.9 μg/m3 in the coarse fraction. Dry deposition fluxes were simultaneously measured with a knife edged surrogate surface containing greased mylar films. The measured dry deposition fluxes of sulfate and nitrate were between 1.0–4.0 and 0.5–3.9 mg/(m2 day), respectively. A multi-step method was used to calculate total and cumulative deposition flux with the Semhel–Hodgson deposition velocity model and atmospheric particulate concentration. The results indicated that calculated flux compares well with measured flux, and greater than 99% of the total flux was due to particles larger than 5 μm diameter for both sulfate and nitrate. The average ratios of calculated/measured fluxes were 0.93 for sulfate and 0.91 for nitrate.