Air Pollution in the Tahoe Basin


 John Carroll and Cort Anastasio
Atmospheric Science Section
Department of LAWR
U.C. Davis


CONCERNS:

  1. Photochemical smog produces phytotoxic secondary pollutants in sufficiently high concentrations, which can increase mortality and morbidity in forest flora.
  2. Deposition of nitrogen containing species onto forest systems can act as fertilizers to alter the nutrient status of plants and change species distributions.
  3. Deposition of nitrogen containing species onto surface waters can increase primary productivity, promote algae growth and change species distributions.
  4. Acid deposition into soils and surface waters changes nutrient availability and the flux of various compounds to and from sediment beds.
  5. Transport of toxic substances from industrial sources and the use of agricultural chemicals to alpine ecosystems and can poison organisms there.


CLIMATOLOGY:

Warm Season:

  1. Inflow of marine air through various gaps in the coastal mountains and up the mountain slopes on 72% of the warm season days.
  2. Strong diurnal wind cycle of daytime up slope and nighttime downslope flows.
  3. Near ground wind flow in the Tahoe Basin is predominantly from the southwest at speeds greater than 7 mph (> 50% of the daytime hours), light and variable or locally down slope nights.
Cool Season:
  1. Absent storms light winds with the Central Valley filled with cool air that drains slowly through the gaps in the coastal mountains.
  2. Valley pollutants (mostly particles, CO, NOx and other N-species) are trapped below the inversion present at or below 1500 feet altitude.
  3. Storms provide strong winds, clear away polluted air and improve air quality.
  4. Tahoe basin similar to the Central Valley: cold drainage fills lower basin except when stormy. Local emissions trapped near the surface.


RECENT RESEARCH FINDINGS:

  1. The pattern of occurrence of the annual average of the 30 hours of peak ozone concentrations at Tahoe correlates better with rural NE California than with Sacramento or the foothills directly to the SW. Peak concentrations are relatively low: for 14 of the last 17 years the average of the 30 highest hourly ozone concentrations per year has been less than 80 ppbv.
  2. Strong daytime ventilation and predominance of SW winds suggests that local emissions do not significantly increase ozone concentrations.
  3. Multiple aircraft measurements show only occasional, small increases in ozone concentrations (+10 ppbv) between the south and north shores.
  4. Nitrogen deposition has transformed the lake from N limited to P limited. N input to the lake appears to be largely atmospheric. The deposition data are highly variable from week to week and preliminary analysis of one yearŐs sequence shows no apparent seasonal trends.
  5. Aircraft samples of gas N species at 1200 feet above the lake (4 samples) show 32 to 194 nmol N/ m3 of air, of which 69% is ammonia and the rest mostly nitric acid.
  6. Aircraft samples of Particulate N at 1200 feet above the lake (4 samples) show 11 to 34 nmol N/ m3 of air of which 56% is ammonium, 27% nitrate and the rest mostly organic N.
  7. Photochemical reactions of N containing species in water have been shown to quickly transform deposited benign species to bio-available species. Hence knowing the speciation of what is deposited to surface waters is not equivalent to accessing their impact.
  8. Historically peak CO concentrations were higher at Tahoe than in Sacramento: e.g. 18 ppmv for Tahoe versus 12 ppmv for Sacramento in 1985. These averages have become about 6 ppmv at these locations by 1999.