Contributions of global and regional sources to mercury deposition in New York State

A modeling system that includes a global chemical transport model (CTM) and a nested continental CTM (TEAM) was used to simulate the atmospheric transport, transformations and deposition of mercury (Hg). Three scenarios were used: (1) a nominal scenario, (2) a scenario conducive to local deposition and (3) a scenario conducive to long-range transport. Deposition fluxes of Hg were analyzed at three receptor locations in New York State. For the nominal scenario, the anthropogenic emission sources (including re-emission of deposited Hg) in New York State, the rest of the contiguous United States, Asia, Europe, and Canada contributed 11-1, 25-9, 13-19, 5-7, and 2-5%, respectively to total Hg deposition at these three receptors. Natural sources contributed 16-4%. The results from the local deposition and long-range transport scenarios varied only slightly from these results. However, there are still uncertainties in our understanding of the atmospheric chemistry of Hg that are likely to affect these estimates of local, regional and global contributions. Comparison of model simulation results with data from the Mercury Deposition Network suggests that local and regional contributions may currently be overestimated.     

Modeling atmospheric mercury deposition in the vicinity of power plants

Two mathematical models of the atmospheric fate and transport of mercury (Hg), an Eulerian grid-based model and a Gaussian plume model, are used to calculate the atmospheric deposition of Hg in the vicinity (i.e., within 50 km) of five coal-fired power plants. The former is applied using two different horizontal resolutions: coarse (84 km) and fine (16.7 km). More than 96% of the power plant Hg emissions are calculated with the plume model to be transported beyond 50 km from the plants. The grid-based model predicts a lower fraction to be transported beyond 50 km: >91% with a coarse resolution and >95% with a fine resolution. The contribution of the power plant emissions to total Hg deposition within a radius of 50 km from the plants is calculated to be <8% with the plume model, <14% with the Eulerian model with a coarse resolution, and <10% with the Eulerian model with a fine resolution. The Eulerian grid-based model predicts greater local impacts than the plume model because of artificially enhanced vertical dispersion; the former predicts about twice as much Hg deposition as the latter when the area considered is commensurate with the resolution of the grid-based model. If one compares the local impacts for an area that is significantly less than the grid-based model resolution, then the grid-based model may predict lower local deposition than the plume model, because two compensating errors affect the results obtained with the grid-based model: initial dilution of the power plant emissions within one or more grid cells and enhanced vertical mixing to the ground.     

Using a sub-grid scale modeling approach to simulate the transport and fate of toxic air pollutants

Several air toxics are emitted from mobile sources on roadways and these emissions account for a significant fraction of the health risks to the population. In addition, health effect studies are now becoming more comprehensive and some account for the spatial heterogeneities of air pollutant concentration fields (as is the case near roadways). Standard models can simulate either the near-source concentration fields or the urban background, but no model can handle both spatial scales in the vicinity of roadways in a coherent and scientifically correct manner. Here, we present a model that provides such an integrated treatment by combining a grid-based air quality model of the urban background with a plume-in-grid representation of roadway emissions. The model is applied to simulate near-roadway concentrations due to emissions from a busy interstate highway in New York City. Qualitative comparisons with typical measured concentration profiles show that the model captures the observed features of toxic air pollutant concentrations near roadways.     

Trace metal analysis of solvents used as fuel in a cement kiln

The kilns used to manufacture Portland cement provide an excellent environment for the safe disposal of certain combustible, hazardous wastes. Moreover, the fuel value of these wastes may yield significant savings in fuel costs.In order to evaluate part of the air pollution potential associated with these fuels, spent solvents were analyzed for trace metals using flame atomic absorption spectroscopy. Lead, zinc, chromium, nickel, selenium, cadmium, beryllium and arsenic levels were examined in 55 samples of solvents that had been obtained by a single distillation of unblended spent solvents, 185 samples of undistilled solvents, and 10 samples of coal. All samples of distilled solvents showed metal concentrations near the limit of detection.The mixed, undistilled solvents showed metals concentrations that were relatively higher than the distilled product, and very scattered except for cadmium, beryllium and arsenic, which were always below the limit of detection. The average values (mg/l) for lead, zinc, chromium, nickel and selenium were 42, 58, 30, 14 and 4, respectively.In order to determine the potentialnet increase or decrease of trace metals in the stack gases, ten coal samples were tested. These coal samples were obtained from a nearby cement kiln. The average concentrations for coal were 20 mg/l lead, 38 mg/l zinc, 43 mg/l chromium (corrected), 60 mg/l nickel 2 mg/l beryllium, and <0.62 mg/l cadmium. No determinations were made for selenium and arsenic. Most of the metal content of the spent solvents was contained on or in the participate matter in the solvents. It follows that the supernatant liquid from a settling tank should be a relatively safe fuel for use at cement kilns.     

Responses of future air quality to emission controls over North Carolina,Part I: Model evaluation for current-year simulations

The prediction of future air quality and its responses to emission control strategies at national and state levels requires a reliable model that can replicate atmospheric observations. In this work, the Mesoscale Model (MM5) and the Community Multiscale Air Quality Modeling (CMAQ) system are applied at a 4-km horizontal grid resolution for four one-month periods, i.e., January, June, July, and August in 2002 to evaluate model performance and compare with that at 12-km. The evaluation shows skills of MM5/CMAQ that are overall consistent with current model performance. The large cold bias in temperature at 1.5 m is likely due to too cold soil initial temperatures and inappropriate snow treatments. The large overprediction in precipitation in July is due likely to too frequent afternoon convective rainfall and/or an overestimation in the rainfall intensity. The normalized mean biases and errors are ?1.6% to 9.1% and 15.3–18.5% in January and ?18.7% to ?5.7% and 13.9–20.6% in July for max 1-h and 8-h O₃ mixing ratios, respectively, and those for 24-h average PM_2.5 concentrations are 8.3–25.9% and 27.6–38.5% in January and ?57.8% to ?45.4% and 46.1–59.3% in July. The large underprediction in PM_2.5 in summer is attributed mainly to overpredicted precipitation, inaccurate emissions, incomplete treatments for secondary organic aerosols, and model difficulties in resolving complex meteorology and geography. While O₃ prediction shows relatively less sensitivity to horizontal grid resolutions, PM_2.5 and its secondary components, visibility indices, and dry and wet deposition show a moderate to high sensitivity. These results have important implications for the regulatory applications of MM5/CMAQ for future air quality attainment.