Model - photochemical

Because of the expanded scale and need to describe additional physical and chemical processes, the development of acid deposition and regional oxidant models has lagged behind that of urban-scale photochemical models. An additional step up in scale and complexity, the development of analytical models of pollutant dynamics in the stratosphere is also behind that of ground-level oxidant models, in part because of the central role of heterogeneous chemistry in the stratospheric ozone depletion problem. In general, atmospheric Hquid-phase chemistry and especially heterogeneous chemistry are less well understood than gas-phase reactions such as those that dorninate the formation of ozone in urban areas. Development of three-dimensional models that treat both the dynamics and chemistry of the stratosphere in detail is an ongoing research problem. 

The emission inventory and the initial and boundary conditions of pollutant concentrations have a large impact on the ozone concentrations calculated by photochemical models. 

The second model evaluated was the Lagrangian Photochemical Model LPM (54), a trajectory model. Backward trajectories were first determined so that starting positions could be used which would allow trajectories to reach station locations at the times of measurement. Measured concentrations ranged from 0.20 to 0.26 ppm and estimated concentrations from 0.05 to 0.53 ppm. 

Seinfeld (33) indicates that photochemical models estimating peak ozone concentrations in urban areas are generally within 30% of measured peaks. 

Mathews, H.B. and Rawlings, J.B., 1998. Batch crystallization of a photochemical Modelling, control and filtration. American Institution of Chemical Engineers Journal, 44(5), 1119-1127. 

One goal of tropospheric [HO ] or [H02 ] measurements is the generation of data for comparison with model calculations-to test or validate the models. Due to its high reactivity, HO comes into rapid photochemical equilibrium with its surroundings. Thus a test of a photochemical model, which compares measured and calculated HO concentrations, is mainly a test of the chemical mechanism that the model contains, and is relatively independent of... 

Uncertainties in Photochemical Models. The ability of photochemical models to accurately predict HO concentrations is undoubtedly more reliable in clean vs. polluted air, since the number of processes that affect [HO ] and [H02 ] is much greater in the presence of NMHC. Logan et al (58) have obtained simplified equations for [HO ] and [HO2 ] for conditions where NMHC chemistry can be ignored. The equation for HO concentration is given in Equation E6. The first term in the numerator refers to the fraction of excited oxygen atoms formed in R1 that react to form HO J refers to the photodissociation of hydrogen peroxide to form 2 HO molecules other rate constants refer to numbered reactions above. 

Dodge, M.C. Effect of Selected Parameters on Predictions of a Photochemical Model - EPA-mi3-77-048 U.S. EPA Research Triangle Park, N.C., 1977. 

Cmtzen, P. J. and Gidel, L. T. (1983). A 2-dimensional photochemical model of the atmosphere 2. the tropospheric budgets of the anthropogenic chloro-carbons CO, CH4, CH3CI and the effects of various NOj sources on tropospheric O3. /. Geophys. Res. 88, 6641-6661. 

Dodecylpyridinium iodide-reversed micelles trapping chlorophyll a have been suggested as interesting photochemical model systems [22] and water/AOT/chloroethylene systems as peculiar dry-cleaning solvents [64]. 

Located several kilometres above the Earth s surface is the stratosphere. Here the ozone layer acts as a filter, protecting life on Earth from harmful low-wavelength ultraviolet radiation known as UV-C, which damages biological macromolecules such as proteins and DNA. In order to understand the effects of anthropogenic input into the stratosphere, the production and destruction of the ozone layer has been studied by a variety of photochemical models and experimental methods. 

It is not difficult to see that ozone initially forms from the oxygen present in the air. Chapman [115] introduced the photochemical model of stratospheric ozone and suggested that the ozone mechanism depended on two photochemical and two chemical reactions ... 

Another Lagrangian photochemical model was developed by Wayne et al. This photochemical model uses a moving ceir or air mass that follows an air trajectory either from a specified source or to a specified receptor. The emission input from the ground-based sources is ex-... 

Whitney used actual measurements to evaluate studies of predicted values derived by photochemical models. To assess the validation tests sponsored by the epa for three different approaches, Whitney... 

The literature contains reviews of air quality modeling that stress special purposes. Some concentrate on meteorologic aspects, and others combine this with air chemistry. Proceedings of several conferences are another information resource. Recent surveys have been addressed specifically to photochemical modeling problems. It may be concluded that, although they are relatively complex, the photochemical-diffusion models perform as well as, if not better than, available inert-species models. 

Although there has been some controversy over whether there is indeed a true ozone deficit problem (e.g., Crutzen et al., 1995), a combination of measured concentrations of OH, HOz, and CIO with photochemical modeling seems to indicate that it may, indeed, exist (Osterman et al., 1997 Crtuzen, 1997), although the source of the discrepancy remains unclear. Measurements of CIO in the upper stratosphere have found concentrations that are much smaller (by a factor of 2) than predicted by the models (e.g., Dessler et al., 1996 Michelsen et al., 1996). Because of the chlorine chemistry discussed later, model overestimates of CIO will also result in larger predicted losses of 03 and hence smaller concentrations. 

Related to the use of indicator species is the use of species age in photochemical modeling studies (Venkatram et al., 1998). In this approach, the VOC and NOx are calculated at a particular point of interest in an air basin assuming no chemical reactions, i.e., only transport occurs. The age of the VOC and NOx since the time of emission is also calculated. The amount of 03 formed is then estimated using the VOC-NOx chemistry for that time period. This approach separates transport and chemistry in an explicit manner and allows the calculation of the effectiveness of various VOC and NOx reductions at a particular location. 

Dunker, A. M R. E. Morris, A. K. Pollack, C. H. Schleyer, and G. Yarwood, Photochemical Modeling of the Impact of Fuels and Vehicles on Urban Ozone Using Auto/Oil Program Data, Environ. Sci. Technol., 30, 787-801 (1996). 

Olson, J., M. Prather, T. Berntsen, G. Carmichael, R. Chatfield, P. Connell, R. Derwent, L. Horowitz, S. Jin, M. Kanakidou, P. Kasibhatla, R. Kotamarthi, M. Kuhn, K. Law, J. Penner, L. Perliski, S. Sillman, F. Stordal, A. Thompson, and O. Wild, Results from the Intergovernmental Panel on Climatic Change Photochemical Model Intercomparison (PhotoComp), J. Geophys. Res., 102, 5979-5991 (1997). 

Russell, A. G., K. F. McCue, and G. R. Cass, Mathematical Modeling of the Formation of Nitrogen-Containing Air Pollutants. 1. Evaluation of an Eulerian Photochemical Model, Environ. Sci. Technol., 22, 263-271 (1988). 

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