Investigation of Ambient Reactive Nitrogen Emissions Sources and Deposition in the Columbia River Gorge National Scenic Area

Anthropogenic reactive nitrogen is emitted into the atmosphere from fossil fuel combustion (nitrogen oxides) and agricultural activities (nitrogen oxides and ammonia). Nitrogen oxide emissions have long been controlled for their role in ambient air pollution and human health effects. However, reactive nitrogen deposition is less understood even though it can play a significant role in altering biodiversity, impairing ecosystem and biogeochemical function and degrading cultural artifacts. Although nitrogen deposition is a natural part of biogeochemical cycling, many ecosystems across the United States are at risk of exceeding the critical nitrogen deposition load. While nitrogen oxides are routinely measured in urban areas, far less is known in non-urban landscapes where ecosystems may be especially sensitive. Regional chemical transport models have been used to predict the impacts of ambient reactive nitrogen deposition in non-urban areas, but models have difficulty simulating reactive nitrogen due to poorly quantified emissions, especially from the agricultural sector. My research explores the speciated deposition of reactive nitrogen through monitoring and modeling in the unique field setting of the 150 mile Columbia River Gorge (CRG) located along the border of Oregon and Washington. This site is ideally suited for this investigation due to the large sources of reactive nitrogen at either end of the CRG and unique seasonally driven channel wind flow. Seasonally driven wind allowed us to look at the reactive nitrogen emissions flowing through the CRG to assess ambient the reactive nitrogen partitioning and deposition gradient. Using data collected by the United States Forest Service to control ambient haze in the CRG and our co-located nitrogen oxides (NOx) gas analyzer, we first characterized the influence of seasonal, bimodal wind distributions on the spatial distribution of reactive nitrogen. We found that during winter months with predominantly easterly winds, particulate nitrate and ammonium and gas-phase nitrogen dioxide levels create a gradient from the eastern end to the western end. Particulate nitrate and sulfate mass concentrations influence the CRG gradient during summer months with predominantly western winds. We also found that the magnitude of the impact from east is greater than the magnitude of impact from the west. When we compared our observations to regional chemistry transport models, we found that models are significantly under-predicting levels of reactive nitrogen in the CRG. This bias is not isolated to a single station within the Gorge, but throughout the whole Columbia Basin. Our results indicate that there are under-represented emissions in the