Effect of emissions control strategies on the size and composition distribution of urban particulate air pollution

The predicted behavior of the size- and chemical-composition distribution of airborne particles in the Los Angeles area is examined as it changes in response to specific emissions control strategies. Model calculations indicate that strategies currently envisioned to control the emissions of primary particles in the Los Angeles area effectively reduce the atmospheric concentrations of particles between 0.1-0.3 mu m particle diameter and above 2.5 mu m particle diameter but do little to reduce particulate concentrations between 0.6 and 0.8 mu m particle diameter. Analysis reveals that in Los Angeles, most atmospheric particles with diameters between 0.6-0.8 mu m begin as water-soluble nonsea salt background particles over the Pacific Ocean which then are transformed by significant accumulation of gas-to-particle conversion products as they are advected across the urban area. Control of primary particulate emissions alone does not reduce the amount of secondary aerosol which forms in the atmosphere and may even serve to redistribute this secondary material to particles with diameters that scatter light more efficiently Strategies originally designed to reduce ambient ozone concentrations through the control of emissions of reactive organic gases (ROG) and oxides of nitrogen (NOx) would reduce fine particle mass concentrations at Claremont CA on August 28, 1987 by 9.5% under the conditions studied here, primarily by reducing aerosol nitrate concentrations. Additional controls on ammonia emissions would suppress aerosol nitrate formation further. The simultaneous use of all gas-phase and particle-phase emissions control measures studied here would reduce atmospheric particle concentrations by 46% at Claremont, CA, relative to the base case 1987 summer conditions.