title: Aircraft measurements of nitrogen and phosphorus in and around the Lake Tahoe Basin: implications for possible sources of atmospheric pollutants to Lake Tahoe.
abstract: Atmospheric deposition of nitrogen (N) and phosphorus (P) into Lake Tahoe appears to have been a major factor responsible for the shifting of the lake's nutrient response from N-limited to P-limited. To characterize atmospheric N and P in and around the Lake Tahoe Basin during summer, samples were collected using an instrumented aircraft flown over three locations: the Sierra Nevada foothills east of Sacramento ("low-Sierra"), further east and higher in the Sierra ("mid-Sierra"), and in the Tahoe Basin. Measurements were also made within the smoke plume downwind of an intense forest fire just outside the Tahoe Basin. Samples were collected using a denuder-filter pack sampling system (DFP) and analyzed for gaseous and water-soluble particle components including HNO3/ NO3-, NH3 /NH4+, organic N (ON), total N, SRP (soluble reactive phosphate) and total P. The average total gaseous and particulate N concentrations (+/- 1sigma) measured over the low- and mid-Sierra were 660 (+/- 270) and 630 (+/- 350) nmol N/m3-air, respectively. Total airborne N concentrations in the Tahoe samples were one-half to one-fifth of these values. The forest fire plume had the highest concentration of atmospheric N (860 nmol N/m3-air) and a greater contribution of organic N (ON) to the total N compared to nonsmoky conditions. Airborne P was rarely observed over the low- and mid-Sierra but was present at low concentrations over Lake Tahoe, with average +/- 1sigma) concentrations of 2.3 +/- 2.9 and 2.8 +/- 0.8 nmol P/m3-air under typical clear air and slightly smoky air conditions, respectively. Phosphorus in the forestfire plume was present at concentrations approximately 10 times greater than over the Tahoe Basin. P in these samples included both fine and coarse particulate phosphate as well as unidentified, possibly organic, gaseous P species. Overall, our results suggest that out-of-basin emissions could be significant sources of nitrogen to Lake Tahoe during the summer and that forest fires could be important sources of both N and P.
authors.affiliation: Atmospheric Science Program, Department of Land, Air and Water Resources, University of California, One Shields Avenue, Davis, California 95616-8627, USA.
authors.forename: Qi
authors.initials: Q
authors.lastname: Zhang
authors.forename: John J
authors.initials: JJ
authors.lastname: Carroll
authors.forename: Alan J
authors.initials: AJ
authors.lastname: Dixon
authors.forename: Cort
authors.initials: C
authors.lastname: Anastasio
mesh_terms: D000392:Air Movements; D000393:Air Pollutants / Q000032:analysis*; D000401:Aircraft; D002140:California; D004784:Environmental Monitoring / Q000379:methods; D005390:Fires; D005740:Gases; D009584:Nitrogen / Q000032:analysis*; D010316:Particle Size; D010758:Phosphorus / Q000032:analysis*; D012621:Seasons; D014197:Trees
chemical_list: D000393:Air Pollutants; D005740:Gases; D010758:Phosphorus; D009584:Nitrogen