Prakash Karamchandani

Multiscale modeling of the atmospheric fate and transport of mercury

A numerical simulation of the atmospheric fate and transport of mercury (Hg) was conducted using a multiscale approach. Two different spatial scales were used to simulate (1) the global cycling of atmospheric Hg and (2) the atmospheric deposition of Hg in potentially sensitive areas. The global simulation was conducted using an updated version of our global Hg chemical transport model (CTM). The imbedded continental simulation was conducted using an updated version of the regional/continental CTM, TEAM. Simulations were conducted using 1998 meteorology and 1998/1999 emission inventories. Model simulation results show improved performance compared to earlier simulations. For example, the global simulation shows background concentrations of Hg species, interhemispheric gradients, and vertical gradients that are consistent with available measurements. The comparison of simulated Hg wet deposition fluxes with data from the Mercury Deposition Network in the United States shows a coefficient of determination (r2) of 0.75, little bias (−3%), and an average gross error of 21%. The major remaining sources of uncertainties, which include speciation of Hg emissions, Hg atmospheric chemistry, and dry and wet deposition processes for Hg species, are discussed.

Simulation of the regional atmospheric transport and fate of mercury using a comprehensive eulerian model

Abstract This paper describes the development, testing and evaluation of a comprehensive mercury simulation model. The model was developed by starting with an existing model that simulates the sulfur/NOx/VOC system and then substituting modules specific to mercury (e.g. chemistry and scavenging). The transport and fate of mercury emissions in the contiguous United States was simulated with the mercury simulation model. We first tested the model by simulating the sulfur system using emissions from the 1990 U.S. EPA criteria pollutants inventory. This testing provided a measure of the uncertainties in the meteorological input data (e.g. gridded wind, cloud and precipitation fields) and model parameterizations (e.g. transport:) that are common to both the sulfur and mercury systems. The mercury simulation model was then evaluated by comparing model estimates of annual average concentrations and wet deposition amounts of mercury against published measurement data. The mercury evaluation showed that the model captured the range of observed values in most regions where observations were available. Moreover, observed spatial gradients in mercury wet deposition amounts and ambient concentrations were also seen in the model results. For the scenario considered here, the simulation results lead to an annual mercury wet deposition amount that is roughly twice the estimated annual dry deposition amount, for most regions of the modeling domain.

The development and application of a simplified ozone modeling system (SOMS)

Abstract This paper describes the development and evaluation of a computationally efficient semi-empirical photochemical model that can be used as a screening tool to obtain quick estimates of the effect of a large number of VOC and NO x emission control strategies on ozone concentrations. Selected control strategies can subsequently be examined with a more complex model. The model is one component of an ozone management system, the regional ozone decision model (RODM), designed to examine the costs and environmental consequences of alternate ozone abatement strategies. The model was developed by systematic simplification of a detailed photochemical model. At each step of the simplification, the simplified model was tested against observations and against results from the detailed model. The first major simplification was the introduction of a highly parameterized chemistry mechanism, originally developed by Azzi et al. (1992 Proc. 11th Int. Clean Air Conf., 4th Regional IUAPPA Conf. ). This modification resulted in a factor of 5 improvement in the computational efficiency of the model. The model with the simplified chemistry was then tested by applying it to a photochemical oxidant episode in the San Joaquin Valley of California. Further improvements in computational speed and efficiency were obtained by uncoupling the chemistry from the transport of VOC and NO x .

The role of non-precipitating clouds in producing ambient sulfate during summer: Results from simulations with the Acid Deposition and Oxidant Model (ADOM)

Abstract A comprehensive acid deposition model was used to investigate the importance of non-precipitating stratus clouds for the production of ambient sulfate. A comparison of model estimates of ambient sulfate and SO2 concentrations with corresponding observations for an episode in the summer of 1988 showed that the model underestimated ambient sulfate concentrations and overestimated ambient SO2 concentrations when non-precipitating stratus clouds were ignored in the model formulation. When the model was modified to include non-precipitating stratus clouds, a distinct improvement in model performance was obtained.

Sensitivity of simulated atmospheric mercury concentrations and deposition to model input parameters

Previously, we have simulated the atmospheric transport and fate of mercury emissions in North America and derived estimates of ambient concentrations and dry and wet deposition of mercury. In this study we quantify sensitivity of the derived estimates to model input parameters that we believe have the largest potential to influence model estimates. We vary five input parameters: emission speciation, Hg(II) dry deposition velocity, precipitation amount, concentration of redox species, and Hg(II) boundary conditions, within their plausible range of values. Our results show that emission speciation has the largest influence and Hg(II) boundary conditions have the smallest influence on the derived estimates. The sensitivity of simulated wet deposition to emission speciation and redox species concentration is non-linear and varies by region. In regions with low wet deposition (5–15 μg m−2 yr−1), emission speciation and chemistry show comparable influence, whereas in regions with high wet deposition (15–30 μg m−2 yr−1), emission speciation shows greater influence than chemistry. The interregional differences in sensitivity suggest that different pathways control total wet deposition for different regions. While in our previous study we evaluated the modeling system against observations, the sensitivity studies described in this paper enabled us to obtain new insights on atmospheric mercury by focusing on the dynamics of the system, i.e., response of the system to variation in its inputs. This analysis is essential before model-simulated results are used to investigate source-receptor relationships. Our findings also indicate that there is a critical need to get additional data on mercury speciation of major emission sources.

Development of an extended chemical mechanism for global- through-urban applications

Abstract The interactions between climate and air quality are receiving increasing attention due to their high relevancy to climate change. Coupled climate and air quality models are being developed to study these interactions. These models need to address the transport and chemistry of atmospheric species over a large range of scales and atmospheric conditions. In particular, the chemistry mechanism is a key component of such models because it needs to include the relevant reactions to simulate the chemistry of the lower troposphere, the upper troposphere, and the lower stratosphere, as well as the chemistry of polluted, rural, clean, and marine environments. This paper describes the extension of an existing chemistry mechanism for urban/regional applications, the 2005 version of the Carbon Bond Mechanism (CB05), to include the relevant atmospheric chemistry for global and global–through–urban applications. Updates to the mechanism include the most important gas–phase reactions needed for the lower stratosphere as well as reactions involving mercury species, and a number of heterogeneous reactions on aerosol particles, cloud droplets, and Polar Stratospheric Clouds (PSCs). The extended mechanism, referred to as CB05 for Global Extension (CB05_GE), is tested for a range of atmospheric conditions using a zero–dimensional box–model. A comparison of results from the extended mechanism with those from the original starting mechanism for both clean and polluted conditions in the lower troposphere shows that the extended mechanism preserves the fidelity of the original mechanism under those conditions. Simulations of marine Arctic conditions, upper tropospheric conditions, and lower stratospheric conditions with the box model illustrate the importance of halogen chemistry and heterogeneous reactions (on aerosol surfaces as well as PSCs for stratospheric conditions) for predicting ozone and elemental mercury depletion events that are often observed during these conditions. Depletions that are comparable to observed depletions are predicted by the box model for very clean conditions (extremely low or zero concentrations of aldehydes and other VOCs) because, in the absence of continuous sources of active halogens, these conditions result in less conversion of active chlorine and bromine to more stable products, such as HCl and HBr.

On the effect of spatial resolution on atmospheric mercury modeling.

Mathematical modeling of the atmospheric fate and transport of mercury (Hg) was conducted using three nested domains covering global, continental and regional scales with horizontal resolutions of approximately 1000, 100 and 20 km, respectively. Comparisons of modeling results with wet deposition fluxes show a coefficient of determination (r(2)) of 0.45 for the continental simulation and 0.14 for the continental/regional simulation. The poor correlation obtained in the regional simulation results to a large extent from the fact that the model predicts an increasing gradient in Hg wet deposition from Minnesota to Pennsylvania, which is not observed in the monitoring network. The use of a finer spatial resolution (20 km) improves model performance in Minnesota and Wisconsin (upwind of major Hg emission sources) but degrades model performance in Pennsylvania (downwind of major Hg emission sources). We suggest the hypothesis that some key Hg chemical transformations are likely missing in current models of atmospheric Hg.

Simulation of Sulfate and Nitrate Chemistry in Power Plant Plumes

The rate of formation of secondary particulate matter (PM) in power plant plumes varies as the plume material mixes with the background air. Consequently, the rate of oxidation of sulfur dioxide (SO2) and nitrogen dioxide (NO2) to sulfate and nitric acid, respectively, can be very different in plumes and in the background air (i.e., air outside the plume). In addition, the formation of sulfate and nitric acid in a power plant plume is a strong function of the chemical composition of the background air and the prevailing meteorological conditions. We describe the use of a reactive plume model, the Reactive and Optics Model of Emissions, to simulate sulfate and nitrate formation in a power plant plume for a variety of background conditions. We show that SO2 and NO2 oxidation rates are maximum in the background air for volatile organic compound (VOC)-limited airsheds but are maximum at some downwind distance in the plume when the background air is nitrogen oxide (NOx)-limited. Our analysis also shows that it is essential to obtain measurements of background concentrations of ozone, aldehydes, peroxyacetyl nitrate, and other VOCs to properly describe plume chemistry.

Formulation of a Second-Generation Reactive Plume and Visibility Model

Abstract The formulation of a second-generation reactive plume and visibility model, the Reactive and Optics Model of Emissions (ROME), is presented. This model presents the following improvements over existing plume visibility models. Chemical transformations in the gas phase, aqueous phase, and particles are simulated by means of a comprehensive chemical kinetic mechanism. Aerosol dynamics is simulated using a sectional representation of the aerosol size distribution. This approach allows an efficient treatment of radiative transfer calculations. Plume diffusion is treated according to several options, including a second-order closure algorithm for instantaneous plume concentrations. ROME is applied to a case study to illustrate the relative effects of NOx and primary particulate emissions on plume visual appearance.

Development and Application of a Multipollutant Model for Atmospheric Mercury Deposition

Abstract A multipollutant model, the Community Multiscale Air Quality model paired with the Model of Aerosol Dynamics, Reaction, Ionization, and Dissolution (CMAQ-MADRID), is extended to include a comprehensive treatment of mercury processes and is applied to the simulation of the atmospheric deposition of sulfate and mercury over the United States during 1996. Model performance is evaluated first by comparison with annual sulfate wet deposition data from the National Atmospheric Deposition Program’s National Trends Network; the coefficient of determination r 2 is 0.77, and the model normalized error and bias are 53% and −8%, respectively. When actual precipitation data are used to scale the deposition fluxes, r 2 improves to 0.91 and the error and bias change to 42% and −41%, respectively. The scaled results underscore a tendency of the model to underestimate sulfate wet deposition. Model performance for mercury wet deposition is then evaluated by comparison with data from the Mercury Deposition Network....