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2002 Progress Report: A Modeling Investigation of NHx Cycling in the Troposphere and Its Impact on Particulate Matter and Acidic Substances Budgets
EPA Grant Number: R826773Title: A Modeling Investigation of NHx Cycling in the Troposphere and Its Impact on Particulate Matter and Acidic Substances Budgets
Investigators: Mathur, Rohit , Coats, Carlie J. , Houyoux, Marc , McHenry, John , Shankar, Uma
Current Investigators: Mathur, Rohit , Adelman, Zac , Alapaty, Kiran , Houyoux, Marc , Shankar, Uma
Institution: MCNC / North Carolina Supercomputing Center
EPA Project Officer: Shapiro, Paul
Project Period: October 1, 1998 through September 30, 2001 (Extended to March 31, 2003)
Project Period Covered by this Report: October 1, 2001 through September 30, 2002
Project Amount: $488,744
RFA: Air Pollution Chemistry and Physics (1998)
Research Category: Engineering and Environmental Chemistry
Description:
Objective:The fate of nitrogen-containing species in the atmosphere is of considerable interest, given their role in the formation of acidic substances, particulate matter (PM), tropospheric ozone, and potential eutrophication and nutrient loading effects resulting from their deposition. Although we have devoted significant attention towards studying oxidized nitrogen in the atmosphere, we have devoted little effort towards quantifying the budgets of reduced-nitrogen species. The overall objective of this research project is to improve the current understanding of the cycling of reduced-nitrogen compounds in the atmosphere and to investigate the coupling of such compounds with atmospheric aerosols and other criteria pollutants responsible for the acidifying atmospheric load, through the development, enhancement, and continuous evaluation of comprehensive multipollutant regional models.
Progress Summary:During this project period, we continued with our investigations on improving the model representation of: (1) sea-salt emissions and chemistry and its impact on modeled aerosol composition; and (2) dry deposition velocity for NH3. Sea-salt particles provide a significant source of Na+, Cl-, and sea-salt sulfate, and can influence aerosol composition in coastal environments. We used the approach of Monahan, et al. (1986) to calculate the production rate of sea-salt particles in the open ocean. We included this module in the SMOKE (Sparse Matrix Operator Kernel for Emissions) emissions processing system. We also investigated the development of different approaches for partitioning the bulk composition predicted by ISORROPIA between fine and coarse aerosol modes. These involved partitioning based on surface areas and also through the explicit thermodynamic calculations in the fine and coarse modes. Detailed analysis and evaluation of the results from these experiments are ongoing.
Compared to several atmospheric species (e.g., O3, SO2, NO2), the dry deposition of NH3 to most surfaces is a relatively efficient removal process. In conditions of large ambient concentrations, NH3 is taken up rapidly by moist surfaces that do not have a high pH and through leaf stomata. Current estimates of the dry deposition velocity of NH3 (Vd) are quite uncertain. In recent years, however, we have devoted significant effort to quantifying and parameterizing the dry deposition velocity of NH3 for a variety of surface conditions in Europe (Sutton, et al., 1993; Duyzer, 1994; Erisman, et al., 1994; Fletchard, et al., 1999; and references therein). Sutton, et al. (1994) summarize several observation studies and report typical Vd values in the range of 0.5-5 cm s-1 for a variety of natural and forested ecosystems.
Our current estimation of Vd in the model is based on the resistance analog method of Wesley (1989) and Walmsley and Wesley (1996). The value of Vd is dependent on three terms representing the bulk properties of the lower atmosphere and the underlying surface: the aerodynamic resistance (ra), the laminar sublayer resistance (rb), and the canopy resistance (rc). For the various cases considered in this study, over most of the continental eastern United States, we found the mean daytime values of Vd computed using this approach to be about 0.5-0.7 cm s-1. These estimates are considerably lower than those suggested by studies in Europe, but they cannot be verified, given the lack of comprehensive observational data on NH3 deposition velocity over the eastern United States. These lower estimates of Vd could be related to the computed canopy resistance (rc) in the model. The ra and rb are dependent on prevalent meteorological conditions and standard methods for their estimation are widely used. The canopy or surface resistance describes the uptake process at the surface, and in turn, is dependent on the vegetative characteristics of the underlying surface, with smaller rc for surfaces with stronger uptake.
To assess the uncertainty associated with estimation of rc and to improve the model's representation of dry deposition processes, we compared the Vd values estimated using three different formulations: (1) a radiation-based scheme driven by shortwave radiation following Wesley (1989); (2) using the detailed Pleim-Xiu (PX) land-surface model in MM5, which also accounts for variations in air temperature, relative humidity, and soil moisture; and (3) a photosynthesis-based formulation based on CO2 assimilation by plants. We included/used these three formulations in MM5 to estimate Rs for each model grid. Figure 1 presents comparisons of the computed canopy conductance (proportional to Vd) using these three schemes for different land-use types. In general, both the PX and the photosynthesis-based schemes estimate canopy conductance values, which are higher than those estimated by the Wesley (1989) approach and suggest that the dry deposition sink for NH3 in current models may be underestimated. This is further illustrated in Figure 2, which presents the spatial distributions of average NH3 deposition rates over the eastern United States for the June 1996 case.
We also analyzed modeled budgets of various processes on both local and regional scales. Preliminary results from these analyses suggest that for NH3, source regions in North Carolina (Sampson County), during typical dry conditions during summer, a large fraction (87 percent) of the ammonia emitted at the surface is transported from the model surface layer. Only about 6 percent of the emitted NH3 is deposited locally within the county scale. If the budgets are computed through the depth of the daytime boundary layer, our calculations suggest that 27 percent of the NH3 emitted from Sampson County is converted to NH4+ aerosol, 6 percent is dry-deposited locally, and the rest is transported out of the county. We are also performing similar analyses of model budgets for other counties and the entire eastern United States.
Figure 1. Comparisons of Computed Canopy Resistances for NH3 for Different Land Use Types Using the Wesley (1989) Scheme, the PX Land-Surface Model, and the Photosynthesis-Based Formulation
Figure 2. Comparisons of NH3 Dry Deposition Rates (kg-N/hr) for June 1996 Estimated, Using the Wesley Scheme, the PX Land-Surface Model, and the Photosynthesis-Based Formulation
References:
Duyzer J. Dry deposition of ammonia and ammonium aerosols over heathland. Journal
of Geophysical Research 1994;99:18757-18763.
Erisman JW, van Pul A, Wyers P. Parameterization of surface resistance for quantification of atmospheric deposition of acidifying pollutants and ozone. Atmospheric Environment 1994;28:2595-2607.
Fletchard C, Fowler D, Sutton MA, Cape JN. Modeling of ammonia and sulfur dioxide exchange over moorland vegetation. Quarterly Journal of the Royal Meteorological Society 1999;125:2611-2641.
Sutton MA, Fowler D, Montcrieff JB. The exchange of atmospheric ammonia with vegetated surfaces (I). unfertilized vegetation. Quarterly Journal of the Royal Meteorological Society 1993;119:1047-1070.
Sutton MA, Asman WAH, Schjørring JK. Dry deposition of reduced nitrogen. Tellus 1994;46B:255-273.
Wamsley JL, Wesley ML. Modification of coded parameterizations of surface resistances to gaseous dry deposition. Atmospheric Environment 1996;30:1181-1188.
Wesley ML. Parameterizations of surface resistances to gaseous dry deposition in regional scale numerical models. Atmospheric Environment 1989;23:1293-1304.
Future Activities:Future activities include a detailed analysis of budgets of various modeled processes. We also will perform a similar analysis to assess the impact of different dry deposition formulations on regulating atmospheric NHx budgets. Results from our modeling investigations are being summarized in manuscripts, which are currently in preparation.
Journal Articles on this Report: 2 Displayed | Download in RIS Format
| Other project views: | All 11 publications | 4 publications in selected types | All 4 journal articles |
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Dennis RL, Mathur R. Airshed domains for modeling atmospheric deposition of oxidized and reduced nitrogen to the neuse/pamlico system. Hydrological Science & Technology 2001;17(1-4):107-117. |
R826773 (2002) R826773 (Final) |
not available |
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Mathur R, Dennis RL. Seasonal and annual modeling of reduced nitrogen compounds over the eastern United States: Emissions, ambient levels, and deposition amounts. Journal of Geophysical Research - Atmospheres 2003;108(D15):2201-2219. |
R826773 (2002) R826773 (Final) |
not available |
air, atmospheric chemistry, chemical transport, regional models, model evaluation, deposition, particulate matter, PM. , Air, Scientific Discipline, RFA, Engineering, Chemistry, & Physics, Ecology, Environmental Chemistry, tropospheric ozone, fate and transport, regional scale, sulfur, ambient air, fine particles, ozone, eutrophication, aerosol particles, nitrogen removal, oxides, ambient aerosol particles, troposphere, particulates, PM2.5, particulate matter, air modeling, air pollution models, ambient aerosol, human exposure
Progress and Final Reports:
1999 Progress Report
2000 Progress Report
2001 Progress Report
Original Abstract
Final Report