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2001 Progress Report: A Modeling Investigation of NHx Cycling in the Troposphere and Its Impact on Particulate Matter and Acidic Substances Budgets

EPA Grant Number: R826773
Title: 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, 2000 through September 30, 2001
Project Amount: $488,744
RFA: Air Pollution Chemistry and Physics (1998)
Research Category: Engineering and Environmental Chemistry

Description:

Objective:

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. The fate of nitrogen-containing species in the atmosphere is of considerable interest, given their role in the formation of acidic substances, particulate matter, tropospheric ozone, and potential eutrophication and nutrient loading effects resulting from their deposition. Although significant attention has been devoted towards studying oxidized nitrogen in the atmosphere, little effort has been devoted towards quantifying the budgets of reduced nitrogen species.

Progress Summary:

To investigate the processes governing the fate of NHx in a consistent modeling framework, and to develop a preliminary assessment of the adequacy of existing NH3 emissions, we conducted modeling analyses with two regional models: (1) the Extended-Regional Acid Deposition Model (RADM); and (2) the comprehensive gas-aerosol version of the Multiscale Air Quality Simulation Platform (MAQSIP).

The Extended-RADM is an enhanced version of the RADM, which includes detailed treatments of the physical and chemical processes regulating the fate of ammonia emissions. It also models the interaction and chemical and thermodynamic coupling between atmospheric NOx-SOx-NHx species. We used the Extended-RADM with the 1985 National Acid Precipitation Assessment Program (NAPAP) NH3 emission inventory as a starting point to develop seasonal and annual estimate emissions, ambient levels, and deposition amounts of reduced and oxidized nitrogen species through the simulation and aggregation of 30 cases following the approach previously used during the NAPAP study (Brook, et al., 1995). To account for uncertainties in magnitude and seasonal variation of ammonia emissions, we developed regional seasonal correction factors for ammonia emissions in the eastern United States through successive model applications and comparison with measurements from regional networks of ambient concentrations (Clean Air Status and Trends Network [CASTNet]) and deposition amounts (National Acid Deposition Program [NADP]) of various species. The resulting ammonia emissions show a distinct seasonal cycle with a maximum in summer followed by spring, fall, and winter. Our calculations suggest that the range between NH3 emissions during the cool and the warm season is a factor of three to four. Both model calculations and measurements indicate that during winter, large portions of the eastern United States are characterized by aerosols that are fully neutralized. Our model calculations for emission scenarios representative of the late 1980s to early 1990s period also indicate that reduced nitrogen species contribute 47(±8) percent of the total nitrogen wet deposition in the eastern United States; this is consistent with 43(±9) percent inferred from deposition measurements.

MAQSIP is a comprehensive urban to intercontinental scale atmospheric transport-transformation-deposition model that also has served as a prototype for EPA's Congestion Mitigation and Air Quality (CMAQ)/Models-3 system. We used MAQSIP to simulate the regional and local distributions of reduced and oxidized nitrogen and related species. We performed model simulations at three grid resolutions using grid nesting in: (1) the eastern United States (using a grid resolution of 36 km) to examine the regional characteristics; (2) North Carolina and surrounding states to examine local characteristics in vicinity of the high NH3 emissions in eastern North Carolina (using a finer resolution 12-km grid); and (3) eastern North Carolina's high NH3 emissions source region (using a 4-km resolution grid). We created meteorological inputs using the Mesoscale Meteorological Model, Version 5 (MM5). Emission inputs for these simulations were based on the National Emissions Trends (NET) 1996 inventory (now known as the National Emissions Inventory [NEI]), which provides the most current estimates for emissions of ammonia, sulfur, PM2.5, and PM10 in the eastern United States.

We rigorously compared model-predicted concentrations of various aerosol constituents against observations from the CASTNet and the Interagency Monitoring of Protected Visual Environments (IMPROVE) networks. Detailed analyses of model predictions also were performed to assess the model's representation of gas/particle partitioning of both oxidized and reduced nitrogen. As an example, Figure 1 illustrates the spatial distributions in the NH3/NHx ratio over eastern North Carolina, using a simulation of the three grid resolutions. Table 1 presents a comparison of modeled NH3/NHx ratio against limited available observations. The Extended RADM simulation results presented in this table are based on updated NH3 emissions from the 1985 NAPAP inventory. Finer resolution enables better simulation of the spatial variability in the near source (eastern farm site) and downwind (Clinton and Kinston) distributions and partitioning of NH3.

We continued model enhancement activities related to the use of an alternate thermodynamic aerosol equilibrium module (ISORROPIA by Nenes, et al., 1998) in MAQSIP. We included the ISORROPIA thermodynamic module into MAQSIP to help improve the representation of aerosol composition in coastal environments due to the presence of sea-salt emissions. We performed both box-model calculations and 3-dimensional (3-D) simulations to compare the results from ISORROPIA with RPMARES (the existing thermodynamics module in MAQSIP). Figure 2 presents comparisons of results from the 3-D simulations with these two approaches and show that for the SO42-- NO3--NH4+-H2O system, the predictions from the two models are very similar. Figure 3 presents box-model comparisons for a high-ammonia case in the presence of NaCl emissions due to sea-salt. In such environments, Na competes with NH3 for sulfate and nitrate aerosol formation.

Figure 1.

Figure 1. Effect of Grid Resolution on Resolving the Spatial Variations in Simulated NH3/NHx Ratio

Figure 2.

Figure 2. Comparison of Surface-Level Predictions With the ISORROPIA and RPMARES Aerosol Thermodynamic Equilibrium Models

Figure 3.

Figure 3. Comparison of Box-Model Predictions With the ISORROPIA and RPMARES Aerosol Thermodynamic Equilibrium Models for Varying Levels of NaCl. Initial values: SO42- = 10 µg/m3, NH3 = 15 µg/m3, HNO3 = 30 µg/m3.

Table 1. Comparison of NH3/NHx Ratio at Different Sites*

    Model
Site Observed Extended RADM MAQSIP (36 km) MAQSIP (12km) MAQSIP(4km)
Bondville, IL 0.29 0.31 0.39 - -
RTP, NC 0.24 0.17 0.50 0.38 0.35
Clinton, NC 0.71(0.17) 0.30 0.76 0.86 0.88
Kinston, NC 0.79 0.30 0.73 0.77 0.79
Eastern Farm, NC 0.91 0.30 0.87 0.92 0.94

*RTP data for 1989; Bondville data for 1989-90; Kinston, Clinton, Eastern Farm for 1997 Extended-RADM simulations based on "corrected" 1985 NAPAP NH3 emission inventory MAQSIP simulations based on 1996 National Emission Inventory (NEI).

References:
Brook JR, Samson PJ, Sillman S. Aggregation of selected three-day periods to estimate annual and seasonal wet deposition totals for sulfate, nitrate, and acidity. Part I: a synoptic and chemical climatology for eastern North America. Journal of Applied Meteorology 1995;34(2):297-325.

Nenes A, Plinis C, Pandis S. User's guide for the "ISORROPIA" thermodynamic equilibrium aerosol model. Presented at the Division of Marine and Atmospheric Chemistry of the Rosenstiel School of Marine and Atmospheric Science, University of Miami, Coral Gables, FL, 1998.

Future Activities:

Future activities include a detailed analysis of budgets of various modeled processes. In particular, we will assess modeled deposition fluxes and the impact of model grid resolution. Recently, we have made improvements to the NEI inventory related to the speciation of PM2.5 and PM10. These, along with Continuous Emissions Monitoring (CEM), will be used to further improve emission estimates to the model. We also will investigate improvements in spatial allocation of NH3 sources through the use of a data set containing location of individual livestock farms. We will investigate methods to introduce seasonal variation in NH3 emissions.

The anticipated model enhancement activities include further testing and assessment of alternate aerosol equilibrium modules and the investigation of potential improvements in dry-deposition velocity estimates for NH3.

We will conduct model simulations with the updated emissions to: (1) further study the sensitivity to grid resolution; (2) determine range of influence of reduced and oxidized nitrogen; and (3) study the impact of potential reductions in NOx and increases in NH3 emissions on PM composition.

Journal Articles:

No journal articles submitted with this report: View all 11 publications for this project

Supplemental Keywords:

air, atmospheric chemistry, chemical transport, regional models, model evaluation, deposition. , 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
Original Abstract
2002 Progress Report
Final Report

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The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.

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