Urban airshed models

Urban Airshed Model (UAM) is a du-ee-dimensional grid based photochemical sinuudtion model for urban scale domains. 

Developing control strategies for ozone is very different than for relatively unreactive species such as CO. In the latter case, the concentrations in air are a direct result of the emissions, and all things being equal, a reduction in emissions is expected to bring about an approximately proportional reduction in concentrations in ambient air. However, because O, is formed by chemical reactions in air, it does not necessarily respond in a proportional manner to reductions in the precursor emissions. Indeed, as we shall see, one can predict, using urban airshed or simple box models, that under some conditions, ozone levels at a particular... 

CIT = California Institute of Technology/Carnegie Institute of Technology UAM = Urban Airshed Model RADM 2 = Regional Acid Deposition Model. G-P = gas-particle. 

Al-Wali, K. I., and P. J. Samson, Preliminary Sensitivity Analysis of Urban Airshed Model Simulations to Temporal and Spatial Availability of Boundary Layer Wind Measurements, Atmos. Environ., 30, 2027-2042 (1996). 

Chock, D. P G. Yarwood, A. M. Dunker, R. E. Morris, A. K. Pollack, and C. H. Schleyer, Sensitivity of Urban Airshed Model Results for Test Fuels to Uncertainties in Light-Duty Vehicle and Biogenic Emissions and Alternative Chemical Mechanisms. Auto/Oil Air Quality Improvement Research Program, Atmos. Environ., 29, 3067-3084 (1995). 

Sistla, G N. Zhou, W. Hao, J.-Y. Ku, S. T. Rao, R. Bornstein, F. Freedman, and P. Thunis, Effects of Uncertainties in Meteorological Inputs on Urban Airshed Model Predictions and Ozone Control Strategies, Atmos. Environ., 30, 2011-2025 (1996). 

Several types of models are commonly used to describe the dispersion of atmospheric contaminants. Among these are the box, plume, and puff models. None are suitable, however, for describing the coupled transport and reaction phenomena that characterize atmospheres in which chemical reaction processes are important. Simulation models that have been proposed for the prediction of concentrations of photochemically formed pollutants in an urban airshed are reviewed here. The development of a generalized kinetic mechanism for photochemical smog suitable for inclusion in an urban airshed model, the treatment of emissions from automobiles, aircraft, power plants, and distributed sources, and the treatment of temporal and spatial variations of primary meteorological parameters are also discussed. 

Portions of the material described here are derived from a comprehensive airshed modeling program in which the authors are participating (17). This chapter focuses on urban airshed models however novel models have been proposed for urban air pollution problems of a more restricted scale— particularly, the prediction of concentrations in the vicinity of major local sources, notably freeways, airports, power plants, and refineries. In discussing plume and puff models earlier we pointed out one such class of models. Other work is the model proposed by Eschenroeder (18) to predict concentrations of inert species in the vicinity of roadways and the modeling of chemically reacting plumes, based on the Lagrangian similarity hypothesis, as presented by Friedlander and Seinfeld (19). 

Several approaches to airshed modeling based on the numerical solution of the semi-empirical equations of continuity (7) are now discussed. We stress that the solution of these equations yields the mean concentration of species i and not the actual concentration, which is a random variable. We emphasize the models capable ot describing concentration changes in an urban airshed over time intervals of the order of a day although the basic approaches also apply to long time simulations on a regional or continental scale. 

An example of a generalized mechanism suitable for inclusion in an urban airshed model is presented below. In particular, we wish to illustrate the scope of such a mechanism and the level of detail that must be included to ensure accuracy while avoiding undue complexity. We have selected the mechanism proposed by Hecht and Seinfeld (45) for this purpose. This mechanism fulfills the requirements for suitability summarized earlier, has predicted accurately the concentration-time behavior of pollutant species in a smog chamber for a variety of hydrocarbon-NO mixtures, and can be included in any of the airshed models described without difficulty. We note, however, that the mechanism is not a unique description of atmospheric chemistry modified and improved versions may well be developed during the next few years. 

While all pertinent factors that affect the distribution of vehicular emissions cannot be taken into account, emissions rates may still be represented with suflBcient accuracy to merit inclusion in an urban airshed model. For simplicity a vehicle emissions inventory can be divided into two parts ... 

Aircraft Emissions. Aircraft emissions may be represented in various ways. For example, a model that describes each aircraft departure and arrival might be considered. Such a model would be appropriate in a study limited to the estimation of concentration levels in the immediate vicinity of an airport. However, an aircraft emissions model suitable for inclusion in a comprehensive urban airshed model need not be so detailed. The model we shall describe here was formulated on the idea that it would be a part of an airshed model having a spatial resolution of the order of two miles and a temporal resolution of the order of several minutes to one hour. 

Area sources of either a selected chemical or a precursor present a common problem for modeling. In particular, the rich and complex patterns of hydrocarbon emissions from general urban and industrial sources either include or might produce through atmospheric photochemical reactions some of the species on the analysis list. The treatment of such species in photochemical airshed modeling is difficult (8, 9). The effort required for any one such exercise is substantial, and the effort required for a comprehensive analysis of all urban regions relevant to this program would be prohibitive. 

Urban aerosols are complicated systems composed of material from many different sources. Achieving cost-effective air particle reductions in airsheds not meeting national ambient air quality standards requires identification of major aerosol sources and quantitative determination of their contribution to particle concentrations. Quantitative source Impact assesment, however, requires either calculation of a source s impact from fundamental meteorological principles using source oriented dispersion models, or resolving source contributions with receptor models based on the measurement of characteristic chemical and physical aerosol features. Q)... 

T Trban airshed models are mathematical representations of atmospheric transport, dispersion, and chemical reaction processes which when combined with a source emissions model and inventory and pertinent meteorological data may be used to predict pollutant concentrations at any point in the airshed. Models capable of accurate prediction will be important aids in urban and regional planning. These models will be used for ... 

---