Mehmet T. Odman

Multiscale air quality modeling: Application to southern California

A multiscale air quality model has been developed to follow accurately and efficiently the long-range transport of pollutants emitted from urban areas. The model employs a two-dimensional finite element scheme to follow the horizontal transport. The multiscale capability is obtained by using local finite element refinements. The model is applied to a 3-day intensive measuring period (August 27–29, 1987) over southern California. Uniform and nonuniform grid systems are employed in the simulations. Uniform grid systems used resolutions of 5×5 km2, 10×10 km2, and 20×20 km2, horizontally. Two nonuniform grid systems are used. The first combines the 5×5 km2 grid over urban Los Angeles with the 10×10 km2 grid over the rest of the domain, while the second uses the 5×5 km2 grid followed by the 10×10 km2 grid and 20×20 km2 grid over the ocean and sparsely populated areas. Uniform coarsening of the grid impacted the urban areas by diluting NOx emissions, leading to higher levels of urban ozone predictions. The impact on regional areas was complex. The ozone being transported downwind was not followed as accurately as on the fine grid and the regional levels usually decreased. On the other hand, the artificial dilution of NOx emissions had the adverse effect of increasing regional ozone levels. Therefore grid coarsening sometimes leads to what appears to be satisfactory levels of regional ozone for the wrong reason. By using fine grids over the source areas, more satisfactory predictions of both urban and regional ozone levels were obtained. This is also computationally more efficient than using uniform fine grids over the entire domain.

A comparison of fast chemical kinetic solvers for air quality modeling

Abstract Two methods for solving stiff systems of ordinary differential equations describing nonlinear chemical kinetics in air quality models, namely the hybrid and the quasi-steady state approximation (QSSA) schemes, are compared with respect to their accuracy and computational speed. Their implementation for parallel and vector processing computers is discussed. Tests are conducted using two different photochemical mechanisms. Also, a new test problem is developed to represent various degrees of stiffness encountered in urban and regional air quality simulations. It is concluded that the hybrid scheme is more accurate than the QSSA scheme. Different techniques are considered to eliminate conservation errors and make the solutions more accurate. As for the computational efficiency, the QSSA scheme is approximately two to four times faster than the hybrid scheme.

A multiscale finite element pollutant transport scheme for urban and regional modeling

Abstract Accurately describing the transport from urban to regional areas, and vice versa, is necessary for effective emission control strategies. However, this task is complicated by the need to use a fine spatial scale in cities to resolve the steep concentration gradients, and the desire to use a coarser scale in rural areas for computational efficiency. A multiscale transport scheme is developed for use in multiscale air quality models. This Petrov-Galerkin finite element scheme accommodates a fine-grid mesh within a coarse-grid mesh. Spurious waves are eliminated locally, without loss of high-order accuracy, by applying a mass-conservative smoothing filter. Some aliasing errors are observed due to the changes in the mesh size. Nevertheless, the scheme performs bettern than others used for following the transport of pollutants in regional models and has the added advantage of accurately describing the transition from urban to rural areas, and vice versa.

Airshed calculation of the sensitivity of pollutant formation to organic compound classes and oxygenates associated with alternative fuels

This study uses a 3-D Eulerian photochemical model and an advanced chemical reaction mechanism to evaluate the sensitivity of pollutant levels to changes in emissions. In particular, the ozone forming potentials of classes of organic compounds are calculated, with particular emphasis on oxygenated organics associated with alternative fuels. Methanol, ethanol, MTBE, alkane and toluene emissions were found to add about one-fifth the ozone (on a carbon mass basis) as alkenes, aldehydes, non-toluene aromatics and ethene. On a per-carbon basis, formaldehyde added about ten times as much ozone as the least reactive organics tested. The results of the trajectory model-based study usually compare well with those found here. The pollution formation potentials can now be used in assessing the relative impact of various exhaust gas compositions.

A nonlinear filtering algorithm for multi-dimensional finite element pollutant advection schemes

Abstract A nonlinear filter is developed for use with finite element methods in solving the atmospheric diffusion equation. Usually, high-order accurate finite element methods lead to ripples near sharp gradients. This is an undesirable feature in air quality modeling. The filter eliminates these ripples by adding artificial diffusion along the direction of the streamlines. Since it is applied only in regions where the ripples are located, the accuracy of the solution is maintained. Here, the filter was used with the streamline upwind Petrov-Galerkin method. It displayed good performance characteristics in the standard rotating puff test, and in a new test where the angular velocity profile is parabolic. It did not cause excessive crosswind diffusion that was present in a previously developed two-dimensional filter, or Forester filters applied to one-dimensional, spatially split algorithms. The new test problem was designed specifically to show that the rotating puff test may not always be the appropriate test to evaluate the performance of transport schemes, especially those that split the horizontal transport into one-dimensional operators. As expected, a one-dimensional splitting scheme (Chapeau function) displayed worse performance than fully two-dimensional schemes under more severe conditions of the new test problem.

Airshed model evaluation of reactivity adjustment factors calculated with the maximum incremental reactivity scale for transitional-low emission vehicles

Abstract The California Air Resources Board recently adopted regulations for light- and medium-duty vehicles that require reductions in the ozone-forming potential or “reactivity,” rather than the mass, of nonmethane organic gas (NMOG) emissions. The regulations allow sale of all alternatively fueled vehicles (AFVs) that meet NMOG exhaust emission standards equivalent in reactivity to those set for vehicles fueled with conventional gasoline. Reactivity adjustment factors (RAFs), the ratio of the reactivity (per gram) of the AFV exhaust to that of the conventionally fueled vehicle (CFV), are used to correct the stringent exhaust emission standards. Complete chemical speciation of the exhaust and conversion of each NMOG species to an appropriate mass of ozone using the maximum incremental reactivity (MIR) scale of Carter determines the RAF. The MIR approach defines reactivity where NMOG control is the most effective strategy in reducing ozone concentrations, and assumes it is not important to define reactiv...

Future directions in photochemical air quality modeling

Photochemical air quality models provide the most defensible method for relating future air quality to changes in emission, and hence are the foundation for determining the effectiveness of proposed control strategies. However, strategies, based primarily on controlling reactive organic gas emissions, have not provided the expected benefits. This raises the question of what have been the deficiencies in previous studies utilizing these tools? Furthermore, what changes are necessary, and desired, to improve upon past efforts? The current generation of models have matured within their original frameworks to represent, relatively accurately, the important physical and chemical processes affecting pollutant dynamics in urban atmospheres. The ability to follow regional dynamics is less well demonstrated. Current regional models have a single horizontal resolution scale. Multiscale models will enable detailed treatment of urban chemistry, and also effectively follow long range transport and chemistry. Improved computational capabilities will allow more detailed chemistry and heterogeneous processes to be followed within the models. The practice of photochemical modeling will benefit greatly from recent and future intensive field studies. The advancements in both the model framework and practice will allow much more accurate evaluation of proposed control strategies, and lead to a much improved understanding of pollutant dynamics.

Multiscale Horizontal Transport for Urban and Regional Air Quality Modeling

Many processes involved in the formation of air pollution problems such as regional oxidant and acid deposition occur on a wide range of spatial scales. Recent studies point to the importance of resolving fine-scales in order to get a better understanding of regional and global air quality problems (e.g. Lamb, 1983; Lin et al., 1988; Cho et al., 1989; Sillman et al., 1990). The current generation of regional air quality models have fixed horizontal scales. Modeling large regions with the scale fixed at the smallest resolvable length is not desirable, because it is computationally very inefficient and expensive. Therefore, fine- scale (2–5 km) resolution should be incorporated within coarse-scale (20–50 km) models only where needed, i.e., in sources regions where observed pollutant concentration gradients are large. Such multiscale or nested grid models will replace the existing air quality models as the next generation (Russell and Odman, 1991).