1. Robin L. Dennis, Jesse O. Bash, Kristen M. Foley, Rob Gilliam, Robert W. Pinder U.S. Environmental Protection Agency, National Exposure Research Laboratory, Atmospheric Modeling and Analysis Division, RTP NC 27711 The interconnection of wet and dry deposition and the response of deposition to incorporation of new process understanding in regional models Background Deposition cleanses the atmosphere and delivers important environmental species to the biosphere, particularly reactive nitrogen which is a necessary nutrient, but in excess becomes a stressor. The nitrogen system is inter-connected and very dynamic with interactions over a range of scales. Regional models are tools (numerical laboratories) to represent and understand processes, scales, and effect on budgets and to provide answers to management questions. Regional models are subject to error. Through model evaluation studies and field-level process studies we identified (1) biases in convective precipitation, (2) lack of NO production from lightning, (3) improper diurnal pattern of animal ammonia emissions, and (4) the fact that air-surface exchange of ammonia is bi-directional. We examined the individual effects of these four model improvements on wet and dry deposition for a 2006 continental US (CONUS) simulation using 12km grids with the Community Multiscale Air Quality (CMAQ) model. kg/ha 2006 Wet Total-Nitrate without Lightning2006 Wet Total-Nitrate with Lightning New Trigger: NMB = 1% RMSE = 29 cm Observed Precipitation (cm) Modeled Precipitation (cm) Default: NMB = 12% RMSE = 30 cm 1.New Convective Trigger in WRF Meteorological Model 2. Lightning NO Production CONUS Precipitation bias significantly reduced from +12% to +1% Wet NH 4 bias increase from -13% to -20% (compensating error) 14.4% decrease in dry reduced N deposition 7.6% increase in wet reduced N deposition 6.6% decrease in total reduced N deposition 3. New CAFO NH 3 Diurnal Emission Profile Mean diurnal NH 3 emissions profile for the static profile (red) and the dynamic profile (blue) for the month of July over the continental US A.In general, with the rapid vertical transfer of the increased NH 3 emissions during the day and smaller emissions at night, the surface concentrations and, hence, dry deposition close to the CAFOs will decrease the most. Farther from the CAFOs air concentrations and dry deposition will reduce less. Where emissions of NH 3 are very small or nonexistent, as in the Adirondacks or the Western US, the long-range transport of lofted NH 3 and resultant NH 4 aerosols will enhance reduced-N dry deposition. B.The increased lofting of NH 3 emissions from the surface will mostly enhance wet deposition. C.For total reduced-N deposition, wet and dry changes cancel out in many areas. However, the larger dry deposition changes dominate the pattern of change of total reduced-N deposition. Note: We are not certain what is producing the decreased deposition in the Oklahoma-Texas panhandle. A BC 4. Bi-Directional NH 3 Air-Surface Exchange 1 Fraction 1 Fraction 1 Fraction 2006 New CAFO Profile/Static Profile Wet Red-N Dry Red-N Total Red-N Use CMAQ X 19% increase in wet Red-N deposition 30% reduction in dry Red-N deposition 10% reduction in total Reduced-N deposition USDA Environmental Policy Integrated Climate Model (EPIC) Fraction Wet Red-N Dry Red-N 2006 NH 3 BiDi/Unidirectional Base Total Red-N 2006 NH 3 BiDi/Unidirectional Base Fraction ABC A.With incorporation of bi-directional NH 3, dry deposition is reduced, allowing NH 3 emissions to mix away from the surface, participate in long rang transport, and be scavenged by clouds, increasing wet deposition. Where CAFOs emissions are dominant the soil compensation points are not as large and the wet deposition does not increase as much at those locations. B.Where the soil compensation point is high due to fertilizer and manure application, air-surface gradients of NH 3 will be reduced and the dry deposition of the gradient-driven flux will be significantly reduced compared to a unidirectional approach. Dry deposition for locations with little or no agriculture, such as the Adirondacks or western deserts, will not change much. C.For total reduced N deposition, wet and dry changes nearly cancel, but CAFO areas show a distinct reduction in deposition. Note: The West Coast states have a decrease in wet deposition because fertilizer estimates used in the bi-directional formulation are noticeably lower than in the standard emissions inventory, offsetting the expected increase in wet deposition. Precipitation, CAFO Profile, Bi-Directional NH 3 Flux Together Large improvement in Reduced-N wet deposition bias Normalized Mean Bias from -18% to -4% (addressing compensating error) kg/ha Prior SimulationNew Simulation Old to New CONUS Deposition Total Red-N 17% Decrease Total Ox-N 3.1% Increase Total-N 5.6% Decrease Old = CMAQ 5.0 with lightning NO New = CMAQ 5.0.2 with lightning NO, new convective precipitation, new CAFO, BiDi NH 3 Wet Deposition NMB OldNew Red-N -18% -4% Ox-N -4% -5% Sulfur +7% -5% NMB = Normalized Mean Bias NME = Normalized Mean Error RMSE= Root Mean Square Error Nitrate Concentrations at IMPROVE NMBNMERMSE Old Simulation 20% 113% 1.5 New Simulation 4% 73% 1.0 Aerosol Concentrations Improved Monthly model biases for Old Simulation (red) and New Simulation (blue) assessed at IMPROVE sites for NO 3 -. Box encloses 25 th and 75 th percentiles; whiskers extend to the 5 th and 95 th percentiles. Point is mean. Summary With the new convective trigger, the precipitation over- prediction bias was significantly reduced. The negative bias in ammonia wet deposition was then increased, illuminating a compensating error. The inclusion of NO production from lightning reduced the annual NO 3 wet deposition bias. The bi-directional NH 3 exchange and the new animal NH 3 (dynamic CAFO) diurnal emissions profile reduced dry deposition and increased wet deposition of ammonia. The increase in wet deposition removed the precipitation- related compensating error. The four changes led to improvements in CMAQ ambient aerosol as well as wet deposition estimates compared to measurements. Removal is a competition between dry and wet deposition that is influenced by surface processes and gas-aerosol partitioning, even for very soluble species. Improving processes can improve ambient air concentration as well as deposition simulations, especially aerosols. The gas-aerosol partition changes favor longer range transport of ammonia into areas of very low ammonia emissions for subsequent deposition. Evaluation is complicated by the dynamic system, nonlinearities in it, and compensating errors. Wet NO 3 DepositionNMBNMERMSE No Lightning NO -14% 24% 2.4 With Lightning NO -5% 22% 2.3