Environ. Sci. Technol. 1997, 31, 2829-2839
Updated Photochemical Modeling for California’s South Coast Air Basin: Comparison of Chemical Mechanisms and Motor Vehicle Emission Inventories R O B E R T A . H A R L E Y * ,† A N D ROBERT F. SAWYER‡ Department of Civil and Environmental Engineering and Department of Mechanical Engineering, University of California, Berkeley, California 94720-1710 JANA B. MILFORD Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309-0427
Large uncertainties remain in photochemical models used to relate emissions of VOC and NOx to ambient O3 concentrations. Bias in motor vehicle emission estimates for VOC has been a long-standing concern. An improved Eulerian photochemical model is described and applied to the August 27-28, 1987, period in southern California. The chemical mechanism used in the model is SAPRC93, which predicts peak ozone concentrations 22% higher on average than the LCC mechanism used previously. A revised motor vehicle emission inventory was developed using gasoline sales and infrared remote sensing data for CO and measured ambient NMOC/CO and NOx/CO ratios. On-road vehicle emissions for the South Coast Air Basin in summer 1987 were estimated to be (1800 ( 400) × 103 and (710 ( 160) × 103 kg day-1 for NMOC and NOx, respectively. These values are 2.4 and 1.0 times, respectively, the corresponding current official inventory estimates (MVEI 7G). Ozone concentrations predicted using the CIT airshed model matched observations more closely when the revised inventory was used in place of official emission estimates. If the vehicle fleet in 1987 were operating with no emission controls, NMOC and NOx emissions would have been 5900 × 103 and 1200 × 103 kg day-1 respectively. On average, peak predicted ozone concentrations for the controlled vehicle fleet operating in 1987 were 43% lower than values predicted for the uncontrolled vehicle fleet. The peak predicted ozone concentration with the uncontrolled vehicle fleet was 500 ppb.
1. Introduction Motor vehicles remain as an important source of air pollution problems despite control efforts that have been underway for more than 30 years. In the Los Angeles area in 1990, on-road motor vehicles have been estimated to contribute 76% of total carbon monoxide (CO) emissions, 47% of nonmethane organic compound (NMOC) emissions, and 51% of oxides of nitrogen (NOx) emissions (1). Vehicle emissions * Corresponding author. † Department of Civil and Environmental Engineering. ‡ Department of Mechanical Engineering.
S0013-936X(97)00056-4 CCC: $14.00
1997 American Chemical Society
have been estimated in the past using emission factor models such as MOBILE (2) and EMFAC (3), together with predictions of the spatial and temporal distribution of vehicle activity. However, the true emissions of CO and NMOC from motor vehicles are likely higher than past model estimates have indicated (4-6). This situation exists despite the fact that exhaust emissions from new cars today are only a few percent of the emission levels that were observed from precontrol cars in the early 1960s (7, 8). A major problem related to emissions of NOx and NMOC is the in situ photochemical formation of ozone and other oxidants in the atmosphere. Photochemical airshed models have been developed to study the complex couplings between atmospheric chemistry, meteorology, emissions of NOx and NMOC, and formation of ozone and other air pollutants (9). Recent experience has shown that model predictions for ozone are very sensitive to bias in emission estimates (10). Some studies (11) have attempted to compensate for bias in emissions by considering alternate cases with increased NMOC emissions (e.g., two times baseline). Objective procedures are needed to provide reliable estimates of present-day emissions as a necessary first step in the development of technically sound emission control policies. Progress in reducing air pollution has been slower than expected (12). Although ambient ozone concentrations and vehicular emissions of CO, NMOC, and NOx have not declined as rapidly as expected, it is still a significant accomplishment that air pollutant concentrations have been reduced. Azevedo and Lee (13) report that, between 1960 and 1987, total vehicle travel in California more than doubled; much of this growth in vehicle travel occurred in the Los Angeles area. In the absence of controls, vehicle emissions would have increased dramatically during this period. Therefore, a scenario with an uncontrolled vehicle fleet and 1987 traffic volumes is an insightful point of comparison to actual conditions observed in the Los Angeles area in summer 1987. The objectives of this study are to (1) Develop accurate estimates of motor vehicle emissions that are independent of emission factor and vehicle activity models. (2) Develop an emissions scenario in which 1987 traffic volumes are combined with precontrol vehicle emission factors. (3) Assess the impact of these emissions estimates on predicted ozone formation in California’s South Coast Air Basin (SoCAB). For this study, motor vehicle emissions of NMOC and NOx were estimated by using ambient ratios of NMOC/CO and NOx/CO and by using CO emissions derived previously from gasoline sales and infrared remote sensing data. Impacts on ozone air quality are estimated using the CIT airshed model applied to the SoCAB. For this study, the CIT airshed model (10) was updated to include the SAPRC93 chemical mechanism (14). The effects of updating the mechanism were explored through box model simulations that included a detailed sensitivity analysis. The chemical mechanism revisions, CIT airshed model, and airshed model simulation conditions are described in the next section of the paper. A third section presents a comparison of motor vehicle emissions estimates. Model results are presented and discussed in the fourth and fifth sections, respectively.
2. Model Description 2.1. SAPRC93 Chemical Mechanism. The chemical mechanism previously used in the CIT airshed model, the condensed version of the LCC mechanism (15), was replaced for this study with the SAPRC93 mechanism (14, 16). As
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TABLE 1. Species Used To Represent Organic Compound Emissions methane ethane propane n-butane
grouping alkane 1b alkane 2 alkene 1 alkene 2 alkene 3
ethene propene isoprene R-pinene
Explicit Species benzene m-xylene formaldehyde toluene p-xylene acetaldehyde ethylbenzene acetylene acetone o-xylene MTBE
Lumped Speciesa kOH (ppm-1 min-1) grouping
