* Critical Findings
ASSESSMENT
An Air-Quality Strategy
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The Degradation of Lake Tahoe
The Lake Tahoe air basin, especially the built-up areas at the south and north ends
of the lake, experience seriously degraded air quality each winter. In addition to
health and visibility effects, the ecological effect of air pollution on Lake Tahoe
appears significant. Figure 9.5 shows transport of materials into the Lake Tahoe Basin
as a function of time of year.
 
Figure 9.5 (Actual View 64K)
Concentrations of aerosols at two sites at Lake Tahoe (South Lake Tahoe and D.L. Bliss State Park) over the seasons, 1989-94. (From volume II, chapter 48.)
The site at D. L. Bliss State Park, near Emerald Bay
on the west shore, in effect, samples air equivalent to that in the Desolation Wilderness
Area and the associated transports from upwind sources. The difference between the
Bliss data and the South Lake Tahoe data thus represents the local contribution to
fine particles. For sulfates, local sources are minor at all times. For other pollutants,
such as organic matter and nitrates, there are massive winter enhancements at South
Lake Tahoe, at which time transport from upwind sources is sharply decreased because
of the trapping of pollutants in the Central Valley.
The low concentration of transported nitrates, and their small particulate size, must
be contrasted with the high levels of local nitrates from the highways that ring
the lake. The coarser winter nitrate particles are more likely to settle into Lake
Tahoe than the fine particles transported from the Central Valley each summer, but the latter
extend across the entire area of the lake. Direct nitrate deposition measurements
are difficult to do, and the issue is still controversial. Fine particles are thus
triply important at Lake Tahoe as they affect visibility, degradation of the lake, and
human health concerns.
The Future of Air Quality
Air quality in the Sierra Nevada is at a critical point, with moderate to severe degradation
becoming all too often accepted as the status quo. Ozone is in a holding pattern
despite massive efforts to control primary transportation sources. Clearly, the Central Valley is not like Californias urban areas, where ozone is in decline; this
fact increases concerns for future Sierra Nevada air quality as valley and foothill
populations grow. Except at Lake Tahoe, little effort is being made to address reduced
visibility, which, to the average visitor, is the most evident sign of degraded air quality.
Yet most of the haze seen today comes from the Central Valley, a change from the
past. Very little is known about the effects of other substances, including herbicides and pesticides, that may be transported into the Sierra Nevada from sources such
as the Central Valley. We also now know that there is an air-quality component, local
and transported, in the decline in the clarity of Lake Tahoes waters. The future
will almost certainly bring more forest smoke, from either wildfires or prescribed burns,
in response to the fuel buildup of the past ninety years.
There are areas within the Sierra Nevada for which air quality is improving. It was
no accident that areas that were in rapid and demonstrable decline, first Lake Tahoe
and then Mono Lake, also engendered the most effective scientific, legal, political,
and regulatory responses. The dust problems at Mono Lake will soon decline as the lakes
water level rises in response to recent legal and regulatory actions. Now urbanized
enclaves in the mountains (Lake Tahoe, Mammoth Lakes) are also showing some progress,
partly through improvement in vehicles, partly through controls on woodstoves and
other sources.
Finally, there are factors of potential importance to air quality that are still quite
uncertain; many of these are tied to changes in the global climate. Scientists are
confident that there has been a 25% increase in carbon dioxide and a doubling of
methane in the past century, and an order of magnitude increase in chlorofluorocarbons
in the past twenty years. Also, an overall small temperature rise and a small (less
than 3%) increase in ultraviolet radiation are becoming apparent and may have influences
yet to be determined. Less certain are predictions of increased climatic variability,
an increase in summer rain, decrease in summer snow, and more frequent El Niño events,
which can generate either drought or intense rainfall at different phases of the
cycle. Model predictions for these and other potential air-quality influences are uncertain.
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