Projections of Atmospheric Nitrogen Deposition to the Chesapeake Bay Watershed
Campbell, P., J. Bash, Chris Nolte, T. Spero, E. Cooter, K. Hinson, AND L. Linker. Projections of Atmospheric Nitrogen Deposition to the Chesapeake Bay Watershed. Journal of Geophysical Research - Biogeosciences. American Geophysical Union, Washington, DC, 12(11):3307-3326, (2019). https://doi.org/10.1029/2019JG005203
To probe the impacts of changing climate and emissions on atmospheric N deposition from a global-to-regional scale, we use data from the Community Earth System Model (CESM) driven by a Representative Concentration Pathway (RCP) scenario that assumes stabilized anthropogenic radiative forcing at 4.5 W m-2 (RCP4.5) by the year 2100. Output from CESM is dynamically downscaled using the Weather Research and Forecasting (WRF) model create downscaled meteorology for a historical decade 1995-2004 and a future scenario 2045-2055. Historical and future agriculture was modeled using the USDA’s Environmental Policy Integrated Climate (EPIC) model driven by the historical and future meteorological simulations. The EPIC output was then used to simulate ammonia emissions from cropping systems in the Community Multiscale Air Quality (CMAQ). The CMAQ model then simulated these scenarios using emissions from 2011 for historical and 2040 for future scenarios. The WRF historical modeled meteorology was then evaluated against historical meteorological observations and the CMAQ historical modeled nitrogen deposition was evaluated against observations to assess the skill of the modeling system. Future simulations were run with both 2011 and 2040 emissions to quantify the individual impacts of emission reductions and changes in meteorology. The mean meteorological and atmospheric nitrogen deposition model estimates for the historical period matched the observations well and were comparable to typical retrospective simulations that use observational data assimilation. The future meteorological simulations estimated that the Chesapeake Bay watershed to be warmer, more humid and receive more precipitation by 2050. The future projections also had more variability in the precipitation. EPIC simulations estimated earlier planting dates and fertilizer application dates due to warmer spring time temperatures. Future CMAQ simulations estimated a 21% reduction in atmospheric nitrogen deposition to the watershed the future simulation with historical emission rates indicates a 4% increase in nitrogen deposition due to meteorological changes particularly the increase in precipitation. The projection of additional reductions in nitrogen oxide emissions, NOx, results in higher proportion of the nitrogen deposition in the form of ammonium. This study should be useful in understanding the relative contribution of climate change and emissions to the atmospheric nitrogen deposition to the Chesapeake Bay Watershed. This should be useful information for air and water quality managers to better understand how current regulations are forecast to decrease nitrogen deposition and if additional controls may be necessary to maintain the Chesapeake Bay Total Maximum Daily Load goals beyond the 2025 restoration target. We estimate that the climate impact on nitrogen deposition is relatively small, by about a factor of 5 lower, when compared to emissions reductions. The data generated here are may also be useful in forecasting how eutrophication events may change in this future scenario. Additionally, the shift in the composition of nitrogen deposition from nitrogen oxides to ammonium may alter biogeochemical cycling and merit further investigation into its ecological implications.
Atmospheric deposition is among the largest pathways of nitrogen loading to the Chesapeake Bay Watershed (CBW). The interplay between future climate and emission changes in and around the CBW will shift the future nutrient deposition abundance and regime (e.g., oxidized vs. reduced nitrogen). In this work, a Representative Concentration Pathway (RCP) from the Community Earth System Model is dynamically downscaled using the Weather Research and Forecasting (WRF) and Community Multiscale Air Quality (CMAQ) model coupled to the agro-economic Environmental Policy Integrated Climate (EPIC) model. The relative impacts of emission and climate changes on atmospheric nutrient deposition are explored for a recent historical period and a period centered on 2050. The projected regional emissions in CMAQ reflect current federal and state regulations, which use baseline and projected emission years 2011 and 2040, respectively. The historical simulations of 2-m temperature and precipitation have cool and dry biases, and temperature and precipitation are projected to both increase. Ammonium wet deposition agrees well with observations, but nitrate wet deposition is underpredicted. Climate and deposition changes increase future ammonium fertilizer application. In the CBW at 2050, these changes (along with widespread decreases in anthropogenic nitrogen oxide and sulfur oxide emissions, and relatively constant NH3 emissions) decrease total nitrogen deposition by 21%, decrease annual average oxidized nitrogen deposition by 44%, and increase reduced nitrogen deposition by 10%. These results emphasize the importance of air quality regulations on the control of future nitrogen loading to the Chesapeake Bay in a changing climate.