Prasad Pai

Global simulation of atmospheric mercury concentrations and deposition fluxes

Results from a numerical model of the global emissions, transport, chemistry, and deposition of mercury (Hg) in the atmosphere are presented. Hg (in the form of Hg(0) and Hg(II)) is emitted into the atmosphere from natural and anthropogenic sources (estimated to be 4000 and 2100 Mg yr−1, respectively). It is distributed between gaseous, aqueous and particulate phases. Removal of Hg from the atmosphere occurs via dry deposition and wet deposition, which are calculated by the model to be 3300 and 2800 Mg yr−1, respectively. Deposition on land surfaces accounts for 47% of total global deposition. The simulated Hg ambient surface concentrations and deposition fluxes to the Earths surface are consistent with available observations. Observed spatial and seasonal trends are reproduced by the model, although larger spatial variations are observed in Hg(0) surface concentrations than are predicted by the model. The calculated atmospheric residence time of Hg is ∼1.7 years. Chemical transformations between Hg(0) and Hg(II) have a strong influence on Hg deposition patterns because Hg(II) is removed faster than Hg(0). Oxidation of Hg(0) to Hg(II) occurs primarily in the gas phase, whereas Hg(II) reduction to Hg(0) occurs solely in the aqueous phase. Our model results indicated that in the absence of the aqueous reactions the atmospheric residence time of Hg is reduced to 1.2 from 1.7 years and the Hg surface concentration is ∼25% lower because of the absence of the Hg(II) reduction pathway. This result suggests that aqueous chemistry is an essential component of the atmospheric cycling of Hg.

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.

Guidance for the Performance Evaluation of Three-Dimensional Air Quality Modeling Systems for Particulate Matter and Visibility

ABSTRACT Guidance for the performance evaluation of three-dimensional air quality modeling systems for particulate matter and visibility is presented. Four levels are considered: operational, diagnostic, mechanistic, and probabilistic evaluations. First, a comprehensive model evaluation should be conducted in at least two distinct geographical locations and for several meteorological episodes. Next, streamlined evaluations can be conducted for other similar applications if the comprehensive evaluation is deemed satisfactory. In all cases, the operational evaluation alone is insufficient, and some diagnostic evaluation must always be carried out. Recommendations are provided for designing field measurement programs that can provide the data needed for such model performance evaluations.

Particulate matter modeling in the Los Angeles basin using SAQM-AERO.

ABSTRACT The SARMAP air quality model, enhanced with aerosol modeling capability, and its associated components were developed to understand causes of ozone (O3) and particulate matter exceedances in the San Joaquin Valley of California. In order for this modeling system to gain increasing acceptance and use in guiding air quality management, it is important to assess how transportable this modeling system is across geographic domains. We describe the first application of the modeling system outside the “home” domain for which it was developed and evaluated. We have chosen the August 27-28, 1987, intensive monitoring period of the Southern California Air Quality Study to evaluate the performance of the modeling system and to assess its sensitivity to emission control options. The predicted surface concentrations of O3 and other gas-phase species were spatially and temporally correlated with measured data. The fractional normalized absolute error was 0.32 to 0.36 for O3, and somewhat larger for other species. The fractional normalized bias for O3 on August 27 and 28, 1987, was 0.02 to 0.04. The simulated PM2 5 mass and constituent species concentrations reproduced the magnitude and variability of the observed daytime concentrations at most locations; however, nighttime PM25 concentrations were over predicted by the model. The models response to emission control options was consistent with other models of the same genre.

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.

The Development of a Model to Examine Source-Receptor Relationships for Visibility on the Colorado Plateau

This paper describes the development and application of the Visibility and Haze in the Western Atmosphere (VISHWA) model to understand the source-receptor relationships that govern chemical species relevant to visibility degradation in the western United States. The model was developed as part of a project referred to as Visibility Assessment for Regional Emission Distributions (VARED), the objective of which is to estimate the contributions of various geographical regions, compounds, and emission sources to light scattering and absorption by particles on the Colorado Plateau. The VISHWA model is a modified version of a comprehensive Eulerian model, known as the Acid Deposition and Oxidant Model.1 The modifications were designed to obtain the computational efficiency required to simulate a one-year period at about 1/25th of real time, and at the same time incorporate mechanistic features relevant to realistic modeling of the fate and transport of visibility degrading species. The modifications included use of a condensed chemical mechanism; incorporation of reactions to simulate the formation of secondary organic particles; and use of a semi-Lagrangian advection scheme to preserve concentration peaks during advection. The model was evaluated with 1992 air quality data from Project MOHAVE (Measurements of Haze and Visual Effects) intensive experiments. An important conclusion of this evaluation is that aqueous-phase oxidation of SO2 to sulfate in nonprecipitating clouds makes a significant contribution to observed sulfate levels during winter as well as summer. Model estimates of ambient sulfate for the winter intensive were within a factor of 2 of the observations for 75% of the values. The corresponding statistic for the summer intensive was 90%. Model estimates of carbon were within a factor of 2 of the limited set of observations.

Ozone Formation in California's San Joaquin Valley: A Critical Assessment of Modeling and Data Needs

ABSTRACT Data from the 1990 San Joaquin Valley Air Quality Study/ Atmospheric Utility Signatures, Predictions, and Experiments (SJVAQS/AUSPEX) field program in Californias San Joaquin Valley (SJV) suggest that both urban and rural areas would have difficulty meeting an 8-hr average O3 standard of 80 ppb. A conceptual model of O3 formation and accumulation in the SJV is formulated based on the chemical, meteorological, and tracer data from SJVAQS/ AUSPEX. Two major phenomena appear to lead to high O3 concentrations in the SJV: (1) transport of O3 and precursors from upwind areas (primarily the San Francisco Bay Area, but also the Sacramento Valley) into the SJV, affecting the northern part of the valley, and (2) emissions of precursors, mixing, transport (including long-range transport), and atmospheric reactions within the SJV responsible for regional and urban-scale (e.g., downwind of Fresno and Bakersfield) distributions of O3. Using this conceptual model, we then conduct a critical evaluation of the meteorological model and air quality model. Areas of model improvements and data needed to understand and properly simulate O3 formation in the SJV are highlighted.

Environmental policy analysis: modeling atmospheric particulate matter.

The new National Ambient Air Quality Standards (NAAQS) for particulate matter (PM), promulgated by the U.S. Environmental Protection Agency, include 24-hour and annual average standards for fine particles (PM25), in addition to the previous PM10 standards. Numerical models are needed to develop the emission control strategies that will bring polluted areas into attainment of the standards. Because the fraction of PM material that is formed in the atmosphere (secondary PM) is more significant in PM25 than in PM10, the numerical models required to develop reliable source receptor relationships must take this secondary PM into account. We review numerical modeling techniques in terms of their ability to address the PM standards. We recommend that various techniques be used (sometimes in combination) to address the different PM standards. Further model development and evaluation, additional field data collection, and training of agency staff in the use of more advanced modeling techniques are recommended. In July 1997, EPA promulgated NAAQS for atmospheric PM that include annual and 24-hour average standards of 50 ug/m and 150 ug/m, respectively, for PM10, as well as new annual and 24-hour average standards of 15 ug/m and 65 pg/m, respectively, for PM2 5 (fine particles). For areas that are designated by EPA to be in nonattainment of these standards, state and local air quality agencies will need to prepare State Implementation Plans (SIPs) that present the emission controls proposed to bring those areas into attainment of the standards. The new fine particle standards will prompt emission controlstrategies that may significantly differ from those used to meet the existing PM standards because PM contains a large fraction of particulate material that was formed in the atmosphere (secondary PM) whereas PM tends to be dominated by particulate material that was directly emitted into the atmosphere (primary PM) Models that were develODed for PM will not aDDly to PM if they cannot properly address secondary PM

On artificial dilution of point source mercury emissions in a regional atmospheric model

Previously, we developed and applied a regional atmospheric mercury model to a domain covering most of North America at a horizontal grid resolution of 100 km. The implication of using this coarse resolution is that point sources of mercury emissions are instantaneously spread over a grid volume of horizontal dimensions 100 x 100 km2 and a vertical dimension equal to the depth of the grid cell where the point source emissions are released. Since point sources comprise a significant majority of a regional mercury emissions inventory, it is important to understand what effect this artificial dilution may have on calculated mercury concentrations and deposition fluxes. To understand this effect, we conducted model simulations using a finer grid, embedded within the original coarse grid, over a sub-domain that includes over 50% of the largest mercury point sources in the north-eastern United States. The horizontal resolution of the fine grid is 20 km, i.e. it is five times smaller than that of the coarse grid. We compared short-term (daily) and long-term (annual) averaged mercury concentrations, and deposition (wet and dry) fluxes on the coarse and fine grids. As expected, the effect of grid resolution is more clearly seen in close proximity to point sources than at remote locations. For short-term averages near major point sources, the peak concentrations and dry deposition fluxes of mercury from the fine grid are almost a factor of two greater than the corresponding estimates from the coarse grid. At remote locations, however, the concentrations and dry deposition peaks estimated by the two model grid resolutions are more comparable. For total wet deposition of mercury, the distinction between the fine and the coarse grid model results is less significant, regardless of the location. This could be due to the redistribution of precipitation fields or the effect of mercury aqueous chemistry. The effect of grid resolution is more important when model estimates are averaged over short time periods, e.g. daily, as opposed to over long periods, e.g. seasonally and annually.

Sensitivity analysis of mercury human exposure

We present a comprehensive analysis of the sensitivity of mercury (Hg) human exposure to environmental variables using a multimedia model of the fate and transport of Hg in the environment. The results of the analysis show that the Hg dose is most sensitive to the lake pH, the burial rate of Hg adsorbed to sediments, and the chemical speciation of Hg emissions to the atmosphere. The lake pH has a strong non-linear effect on the methylation rate and bioaccumulation of Hg in fish. The burial of sediments is a major pathway for removing Hg from the lake cycling. The speciation of Hg emissions is important because Hg(II) is deposited much more rapidly than Hg(0). These results highlight the importance of key variables that should be investigated through well-designed field programs, so that we can minimize the overall uncertainties associated with the modeling of mercury fate and transport.