Source contributions to the size and composition distribution of urban particulate air pollution

Abstract

A mechanistic air quality model has been constructed which is capable of predicting the contribution of individual emissions source types to the size- and chemical-composition distribution of airborne particles. This model incorporates all of the major aerosol processes relevant to regional air pollution studies including emissions, transport, deposition, gas-to-particle conversion and fog chemistry. In addition, the aerosol is represented as a source-oriented external mixture which is allowed to age in a more realistic fashion than can be accomplished when fresh particle-phase emissions are averaged into the pre-existing atmospheric aerosol size and composition distribution. A source-oriented external mixture is created by differentiating the primary particles emitted from the following source types: catalyst-equipped gasoline engines, non-catalyst-equipped gasoline engines, diesel engines, meat cooking, paved road dust, crustal material from sources other than paved road dust, and sulfur-bearing particles from fuel burning and industrial processes. Discrete primary seed particles from each of these source types are emitted into a simulation of atmospheric transport and chemical reaction. The individual particles evolve over time in the presence of gas-to-particle conversion processes while retaining information on the initial source from which they were emitted. The source- and age-resolved particle mechanics model is applied to the 1987 August SCAQS episode and comparisons are made between model predictions and observations at Claremont, CA. The model explains the origin of the bimodal character of the sub-micron aerosol size distribution. The mode located between 0.2 and 0.3 μm particle diameter is shaped by transformed emissions from diesel engines and meat cooking operations with lesser contributions from gasoline-powered vehicles and other fuel burning. The larger mode located at 0.7–0.8 μm particle diameter is due to fine particle background aerosol that has been further transformed by fog and by other gas-to-particle conversion processes. Smaller contributions to the mode located at 0.7–0.8 μm particle diameter are made by food cooking and by the fine particle fraction of paved and unpaved road dust. Separation of the contributions of particles from different sources helps to visualize the effect that different emissions control programs would have in advance of their adoption.