Lake Tahoe is a high altitude (1898 m) lake located in the northern Sierra Nevada mountains at the California-Nevada border. Starting in the second half of the 20th century, decline in Lake Tahoe's water clarity and degradation in the basin's air quality have become major concerns due to its unique scenic features. Nitrogen (N) and phosphorus (P) loading via direct atmospheric deposition and sediment transport to the lake have been implicated as responsible for its eutrophication. Previous estimates suggest that atmospheric N deposition contributes half of the total N loading to the lake, but sources of the atmospheric nitrogen remain unclear. In order to develop an understanding of the origin of atmospheric pollutants leading to the decline in Tahoe's water clarity, we undertook a series of studies designed to quantify ambient levels of key N species in the basin and applied advanced modeling techniques to apportion in-basin vs. out-of-basin sources of the observed pollutants. This paper presents the results of these studies.