Modeling the long-term deposition trends in US over 1990—2010 and their impacts on the ecosystem assessment

Yuqiang Zhang1, Rohit Mathur2, Jesse Bash3 and Shawn Roselle4

Reactive nitrogen (Nr) is very important pollutant which at the same time plays a very important role on air and water quality, human health and biological diversity. The atmospheric nitrogen deposition can cause acidification and excess eutrophication, which brings damages to the ecosystems. Quantifying the total deposition is US is still a challenge due to the lack of the long-term observation data for the dry deposition.

For this study, we use a comprehensive coupled meteorology-air quality model (WRF-CMAQ) to simulate deposition changes in US over 1990—2010. The WRF-CMAQ model was run for the continental US using a 36km by 36km horizontal grid spacing, by using a consistent emission inventory recently developed by Jia et al., (2013). We found significant decreasing trend for the total inorganic nitrogen over the East and West coast of California, and increasing trend in the East North Central. The decreased total deposition was controlled by the oxidized nitrogen, as a result of the recent consistent NOx emission reductions due to air regulations such as the Clean Air Act and the NOx State Implementation Plan, consistent with other studies (Li et al., 2016; Schwede and Lear, 2014). The increased inorganic nitrogen deposition was dominated by the reduced nitrogen, which was attributed to the unregulated increasing ammonia (NH3) emissions. The dry and wet inorganic nitrogen deposition trends also have a different spatial patterns: wet deposition was decreasing all over the U.S., while the dry deposition decreased over the East and increased over the Central areas, which was controlled by the increased dry NH3 deposition. The contribution of the reduced nitrogen to the total nitrogen varies both spatially and temporally: it is usually above 50% in the North Central areas, and below 50% in the East and West of U.S. However we see a significant increasing trend for these contributions all over the U.S. as a result of the NOx emission reductions and ammonia emission increases.

Future effort will be investigated to compare the WRF-CMAQ model results with the estimated total deposition by using the hybrid approach developed by Schwede and Lear (2014). Then we will also quantify the ecosystem effects from the nitrogen deposition changes for the past 20 years on the critical loads and biodiversity.