During the summer and fall of 2002, aircraft measurements of meteorological and air quality variables were obtained over the western Sierra Nevada and the Lake Tahoe Basin. During the winter of 2003, similar measurements were made close to the lake’s surface using a small research vessel on the lake. Aircraft air quality sampling included real time monitoring of ozone, NO, NOy, and particulate concentrations plus grab samples of gaseous and particulate nitrogen species using annular denuder-filter pack (DFP) assemblies. Boat sampling was the same except that no ozone monitor was aboard. The primary objective of these field efforts was to document the concentrations of nitrogen-containing species as well as other pollutants in the air over and upwind of the lake, as these species can deposit into the lake and act as nutrients that accelerate eutrophication. This report describes the techniques used to acquire the data, assure their quality and summarizes the general conditions encountered. Descriptions of instrument calibrations and of the formats used for the QA/QC-ed data sets transferred to the ARB are also included. Sampling was conducted on 20 days during the summer and fall with an aircraft and on 6 days during the winter with a boat. Two additional days were devoted to joint aircraft-boat sampling in the fall. Data recovery for the continuous real time measurements was nearly 100 percent. Analyses of the DFP samples from the aircraft also went well, although there were issues with blank levels for several chemical species. During our sampling days, the concentration of atmospheric N over Lake Tahoe ranged from 33 to 360 nmol-N m –3 -air, with an average value of 120 nmol-N m –3 -air. Gaseous ammonia was typically the dominant component, accounting for an average of 55% of total N, while particulate ammonium contributed an additional 10% of total N on average. Nitric acid/nitrate and organic nitrogen (gaseous and particulate) were also significant components that, on average, accounted for 20% and 14% of the total atmospheric N burden. In contrast, levels of nitrous acid and nitrite were generally insignificant. A variety of weather conditions were encountered which clearly affect pollutant levels measured both in the Tahoe Basin and over the mountains to the west. On most days, late afternoon air quality was slightly to significantly worse to the west of the basin than in the basin. In the mornings, the variations among locations were more random. A preliminary analysis of our DFP measurements, in conjunction with meteorological data, suggests that nitrogen levels in the air above Lake Tahoe can be affected by a number of sources and factors including the regional “background” pollution level, in-basin emissions, local and distant forest fires, and pollution from the Central Valley.
For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753
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