Philip M. Roth

On the treatment of point source emissions in urban air quality modeling

Abstract This paper presents the development and evaluation of an urban air quality model, the Plume-Airshed Reactive-Interacting System (PARIS), that is capable of providing a detailed treatment of large point source emissions. The PARIS model treats these large point sources by embedding one or more reactive plume models into the Systems Applications urban airshed model, which is a three-dimensional gridded model governed by the atmospheric diffusion equation. These embedded reactive plume models are used to decribe the chemistry and dynamics of large point source plumes and their interaction with the ambient urban environment. When the plume size becomes comparable to the airshed model grid cell size, a subgrid scale description is no longer necessary and the plume material is mixed into the airshed model grid cells. For purposes of evaluation and comparison, the Systems Applications urban airshed model and the PARIS model were applied to the St. Louis urban area for a one-day simulation and to the South Coast Air Basin (Los Angeles area) for a two-day simulation. Overall absolute errors between predictions and observations for the urban airshed and PARIS model simulations are on the order of 40–50% for O 3 and NO 2 concentrations. The overall differences in absolute errors between airshed and PARIS model predictions are between 1 and 5 %; the difference in overall model performance resulting from the PARIS model subgrid scale treatment of point sources is well within measurement uncertainties for both O 3 and NO 2 concentrations. These results indicate that for these two applications, detailed treatment of large point sources with the PARIS model has little effect on overall model performance. However, the model can provide information necessary for the study of the impact of individual point sources located in an urban environment.

Mathematical modeling of photochemical air pollution. II. A model and inventory of pollutant emissions.

Abstract In Part I a model for predicting the dynamic behavior of photochemical air pollution was formulated. To exercise the model, pollutant emissions must be specified as a function of time and location over the region of interest. In this paper (Part II) we present a general methodology for the compilation of a contaminant emissions inventory for an urban area. Particular attention is given to the description of motor vehicle emissions, which constitute the most important single source of pollutants in the region to which the model is applied, the Los Angeles airshed. The model is used to estimate the spatial and temporal distribution of carbon monoxide, hydrocarbon, and nitrogen oxide emissions in the Los Angeles airshed in Autumn 1969.

Sensitivity of a complex urban air quality model to input data

Abstract In recent years, urban-scale photochemical simulation models have been developed that are of practical value for predicting air quality and analyzing the impacts of alternative emission control strategies. Although the performance of some urban-scale models appears to be acceptable, the demanding data requirements of such models have prompted concern about the costs of data acquisition, which might be high enough to preclude use of photochemical models for many urban areas. To explore this issue, sensitivity studies with the Systems Applications, Inc. (SAI) Airshed Model, a grid-based time-dependent photochemical dispersion model, have been carried out for the Los Angeles basin. Reductions in the amount and quality of meteorological, air quality and emission data, as well as modifications of the model gridded structure, have been analyzed. This paper presents and interprets the results of 22 sensitivity studies. A sensitivity-uncertainty index is defined to rank input data needs for an urban phot...

Alternative base cases in photochemical modeling: their construction, role, and value

Abstract This study assesses the potential influence compensating errors in photochemical model inputs may have on estimates of the effects of emission control scenarios. Motivation stems from the apparent ability to achieve satisfactory model performance despite evidence suggesting the existence of significant biases in emissions estimates. Urban Airshed Model (UAM) sensitivity studies were carried out using simulations of two summer 1987 O3 episodes in the South Coast Air Basin of California. Since existing simulations exhibited inadequate performance, efforts were made to rectify these problems. Plausible conditions that might define acceptable base cases were established, and model runs were made to determine which alternative base cases provided a level of UAM performance comparable to the best achieved for the two summer episodes. The alternative base cases produced different estimates of the air quality benefits associated with hypothetical emission control scenarios. For example, one set of base cases indicated that NOx, controls would be counterproductive in helping to reduce the estimated peak O3 concentration in the eastern portion of the modeling domain. Another base case suggested that such controls would yield almost no change in the peak value. The results from alternative base case simulations provide a lower bound estimate of the uncertainty that attends future year modeling results. Such analyses should be incorporated into current photochemical modeling practice.

An appraisal of emissions control requirements in the California south coast air basin

Abstract The purpose of this study is to evaluate the effect of reductions of reactive organic gases (ROG) and NO x emissions on short-term O 3 and NO 2 concentrations and annual average NO 2 concentrations in the California South Coast Air Basin. Short-term air quality predictions were obtained by applying the Systems Applications Airshed Model to summer O 3 and autumn NO 2 episodes. Effects of emission controls on annual NO 2 concentrations were estimated using CDM and a new parcel tracking model NOXTRAK. Results for the summer O 3 episode indicate that ROG emission reduction in an effective means for reducing peak O 3 concentrations. NO x emission reduction imposed in addition to ROG emission reductions are counterproductive in reducing peak O 3 concentrations. The modeling results also suggest that attainment of the 1-h federal O 3 standard requires ROG emission reductions on the order of 80% from 1987 levels. Results for the autumn NO 2 episode indicate that NO x emission reductions approximating those recommended in a proposed Air Quality Management Plan (about 22%) will result in only small (about 5%) reductions in the peak NO 2 concentrations. ROG emission reduction may be more effective than NO x emission reduction in reducing the peak NO 2 concentration. For the episode studied, a reduction of 36% in ROG emissions is estimated to result in a reduction in peak NO 2 concentrations commensurate with that required to attain the 1-h state NO 2 standard. Model calculations also indicate that the federal NO 2 standard may not be meet by 1987 at one or two stations, but may blosely approached.

Computer Simulations of the Atmospheric Chemistry of Sulfate and Nitrate Formation

Simulations of the atmospheric chemistry of sulfate and inorganic nitrate formation have been carried out by means of a detailed gas phase—liquid phase chemical kinetic mechanism. Consideration has been given to the effect of changes in sulfur dioxide, nitrogen oxides, and reactive hydrocarbon concentrations on sulfate and nitrate formation for conditions typical of the midwestern and northeastern United States. The results indicate significant nonlinearities in the chemistry of acid formation, particularly between sulfur dioxide and sulfate concentrations.

Air Quality Modeling and Decisions for Ozone Reduction Strategies

Abstract Despite the widespread application of photochemical air quality models (AQMs) in U.S. state implementation planning (SIP) for attainment of the ambient ozone standard, documentation for the reliability of projections has remained highly subjective. An “idealized” evaluation framework is proposed that provides a means for assessing reliability. Applied to 18 cases of regulatory modeling in the early 1990s in North America, a comparative review of these applications is reported. The intercomparisons suggest that more than two thirds of these AQM applications suffered from having inadequate air quality and meteorological databases. Emissions representations often were unreliable; uncertainties were too high. More than two thirds of the performance evaluation efforts were judged to be substandard compared with idealized goals. Meteorological conditions chosen according regulatory guidelines were limited to one or two cases and tended to be similar, thus limiting the extent to which public policy makers could be confident that the emission controls adopted would yield attainment for a broad range of adverse atmospheric conditions. More than half of the studies reviewed did not give sufficient attention to addressing the potential for compensating errors. Corroborative analyses were conducted in only one of the 18 studies reviewed. Insufficient attention was given to the estimation of model and/or input database errors, uncertainties, or variability in all of the cases examined. However, recent SIP and policy‐related regional modeling provides evidence of substantial improvements in the underlying science and available modeling systems used for regulatory decision making. Nevertheless, the availability of suitable databases to support increasingly sophisticated modeling continues to be a concern for many locations. Thus, AQM results may still be subject to significant uncertainties. The evaluative process used here provides a framework for modelers and public policy makers to assess the adequacy of contemporary and future modeling work.

Chemical Transformation in Plumes

It is now recognized that a variety of chemical transformations occurs in plumes. The types of transformations, their rates of occurrence, and the magnitudes of the resultant pollutant concentrations depend on the chemical composition of a plume and of the ambient air entrained in a plume, the elevation and temperature of a plume, and the meteorological conditions (including solar intensity) experienced by a plume. It is postulated that these transformations contribute to atmospheric haziness and discoloration, atmospheric acidity, and extended sulfate and ozone concentrations in rural areas. In addition, it is postulated that transformations in plumes contribute to brief, elevated ground-level concentrations of nitrogen dioxide.

A methodology for the cost/health-benefit analysis of photochemical smog control

Abstract A methodology for the analysis of the costs and health-related benefits of photochemical smog control is presented. This methodology combines information on the cost of primary pollutant emission control, the effects of emission control on ambient concentrations of air pollutants, and the health effects of these air pollutants. The application of the methodology is illustrated for the control of ozone (O 3 ) and nitrogen dioxide (NOO 2 ) short-term concentrations in the Los Angeles metropolitan area.

Chemistry of Sulfate and Nitrate Formation

The atmospheric deposition of acidic species, i. e., sulfates and nitrates, has been known to occur for many decades. In recent years, concern about the ecological damage that acidic deposition may cause to streams and lake ecology, soils, forests, and materials has considerably increased (e. g., NRC, 1983; EPA, 1983; OTA, 1982). At the first meeting of the Convention on Long-Range Transboundary Air Pollution of the U.N. Economic Commission for Europe, held in Geneva in June 1983, Nordic countries proposed a 30 percent reduction in sulfur emissions to be implemented by 1993. In North America, the U.S./Canada Memorandum of Intent on Transboundary Air Pollution was set to define a common policy for the United States and Canada. Canada calls for a 50 percent reduction in SO2emission in both countries. In the United States, several reports acknowledge the contribution of sulfur dioxide (SO2) and nitrogen oxide (N0x) anthropogenic emissions to acidic deposition in northern America and the need for emission control (NRC, 1983; OSTP, 1983; OTA, 1982; EPA, 1983