Control of Atmospheric Fine Primary Carbon Particle Concentrations

Abstract

The adverse health effects and urban visibility degradation associated with atmospheric carbon particle concentrations suggest that control of this class of air pollutants is desirable, especially in the event of an increase in the usage of diesel vehicles. In this study, procedures for the engineering design of fine carbonaceous particulate matter abatement strategies have been developed and tested in the Los Angeles area. Carbon particle abatement strategies are evaluated based on the results of an emissions to air quality model, the performance of which is verified by comparison to measurements of ambient aerosol concentrations taken in the South Coast Air Basin during 1982. As a result of this research, the long-term average behavior of fine aerosol carbon concentrations has been characterized in the Los Angeles area for the first time. The highest concentrations of fine particulate total carbon were observed in areas of heavy traffic density. The annual average fine total carbon concentration at downtown Los Angeles was 12.2 pg m^-3 during 1982, which constituted 37% of the fine aerosol (particle diameter < 2.1 [mu]m) mass collected at that location. Aerosol carbon concentrations were observed to decrease with distance inland from downtown Los Angeles. The 1982 annual average fine total carbon concentration at Rubidoux, which is about 80 km east of Los Angeles. was only 8.2 pg m^-3. There is a pronounced winter peak and summer minimum in carbonaceous aerosol concentrations in the western portion of the air basin. The monthly average fine total carbon concentration at downtown Los Angeles reached a high of 22.3 pg m^-3 during December 1982, and dropped to 7.4 pg m^-3 during June 1982. At eastern locations in the air basin, the seasonal trend becomes less significant, with monthly average fine total carbon concentrations at Rubidoux observed to be between 6.4 and 10.8 pg m^-3 during all months of 1982. Elemental carbon in the atmosphere is inert and is due solely to direct (primary) aerosol emissions from sources while organic carbon could be directly emitted as primary aerosol or could be formed in part from condensation of the low vapor pressure products of atmospheric chemical reactions (secondary formation). Examination of the spatial and temporal trends of the ratio of fine total carbon to fine elemental carbon concentration leads to the conclusion that secondary organic carbon aerosol formed in the atmosphere from hydrocarbon precursors was not the overwhelming contributor to overall long-term average total carbon levels in the Los Angeles area during the year 1982. At downwind locations, such as Azusa or Rubidoux, it was found that, at most, between 16% and 22% of the annual average total carbon concentration (or 27% to 38% of the organic carbon) may be due to secondary aerosol formation in excess of that found at Lennox (a nearcoastal site next to a heavily travelled freeway source of primary aerosol). Comparison of fine elemental and organic carbon particle concentrations against the ratio of those two aerosol species found in basin-wide source emissions farther indicates that, over long averaging times during 1982, primary aerosol carbon particle emissions are responsible for the majority of atmospheric carbon particle concentrations. The particulate air quality data collected during 1982 were used to verify the performance of a mathematical model for long-term average air quality. The Lagrangian particle-in-cell air quality model previously developed by Cass (1977, 1981) was improved to handle near-source dispersion from ground level sources. The model was tested against emissions, elemental carbon air quality, and meteorological data for 1982 in the Los Angeles area. It was found that the model adequately predicts the long-term average concentration of this primary pollutant. The predictions and observations of monthly average elemental carbon particle concentrations have a positive correlation coefficient of 0.78. The model also determines the source classes responsible for fine carbon particle air quality. Many source types: including highway vehicles, charcoal broilers; and fireplaces contribute to primary total carbon particle concentrations, while elemental carbon concentrations are due mostly to emissions from diesel engines. The source class contributions computed by the air quality model were used to determine the optimal emission control strategy for attaining any desired level of improved carbon particle air quality. Linear programming techniques were employed to solve for the least costly set of emission control measures which would enable an air quality goal to be met. Solutions indicate that application of a few control measures, aimed almost entirely at diesel engines, will reduce the basin-wide maximum annual average fine elemental carbon concentration approximately by half at an annual cost of about $69 million. The maximum annual average fine primary particulate total carbon concentration may be reduced by about 55% at a cost of $102 million per year. A control program for visibility improvement would preferentially require the reduction of atmospheric fine elemental carbon particle concentrations, whereas a program designed to control fine aerosol mass would benefit from total carbon particle concentration reductions. It was determined that a control strategy that is optimal for total carbon control may be near-optimal for elemental carbon control. However, an emissions control strategy designed to optimize for elemental carbon control may produce peak total carbon concentrations that exceed those which would result from a control strategy optimized for total carbon by as much as 8%. In summary, it has been demonstrated that the air quality model is useful both in predicting long-term average carbon particle air quality and in determining the sources responsible for that air quality outcome. It was found that emissions from diesel engines were responsible for a large portion of the atmospheric fine carbon particle concentrations in the Los Angeles area during 1982. Control of emissions from diesel engines is therefore important, and it was determined that the least costly set of emission control measures for reducing carbon particle concentrations includes diesel engine emission controls.

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