Project at a Glance

Title: The use of multi-isotope ratio measurements as a new and unique technique to resolve NOx transformation, transport and nitrate deposition in the Lake Tahoe basin.

Principal Investigator / Author(s): Thiemens, Mark; Michalski, Greg

Contractor: UC San Diego

Contract Number: 03-317

Research Program Area: Atmospheric Processes, Ecosystem & Multimedia Effects

Topic Areas: Ecosystem Impacts, Impacts, Lake Tahoe Study, Monitoring


The enrichment of oxygen 17 (17O) observed in photochemically produced nitrate is a powerful tracer of the deposition and transformation of atmospherically derived nitrogen (N) to watersheds. This novel approach was applied, on a small scale, to address the question of the importance of atmospheric N deposition to Lake Tahoe eutrophication. Measurements of 17 oxygen isotopes in nitrate aerosols collected in the Tahoe Basin showed similar enrichments as those analyzed from southern California with an average 17Oof 24. This atmospheric isotopic signal was assumed to be the same at Lake Tahoe based on comparative measurements, which was deposited to the lake surface generating a water column nitrate 17O average of 4. This means that ~ 20% of water column nitrate has retained its photochemical isotopic character and has not been recycled by biota. Using N deposition estimates from other Tahoe researchers, we derive a lake average nitrification rate of 1 mg/m2/yr. A significant number of atmospheric and lake nitrate samples were lost during the analytical process due to unexpected loading of organic material in the solutions. Consequently, a portion of the samples collected have been archived and will be analyzed and reported to the ARB once a more effective analytical method has been developed. Initial test results on this new analytical method suggest that it will be applicable by the end of 2007.

For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753

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