© 2001 by Leo Poppoff

March 22, 2001

Basin Watch

by

Leo Poppoff

TAHOE BASIN AIR IS GOOD, BUT NOT GOOD ENOUGH

The cold lake covering the bottom of the Tahoe Basin, encircled by
relatively steep mountains, produces our most common meteorological
condition. Radiation cooling at night generates gentle downslope winds
that carry air from ridgetops over urban areas and out over the lake.
They carry pollutants, trapped by an inversion, which is caused by the
layer of cold air over the lake covered by warmer air aloft. And some of
those trapped pollutants deposit onto the lake.

In Chapter 3 of Volume I of the Lake Tahoe Watershed Assessment, Steven
Cliff and Thomas Cahill describe this and other Tahoe meteorological
regimes. Cliff and Cahill are atmospheric researchers at UC-Davis who
have been studying Tahoe Basin pollution for many

years. In their chapter, they show how various meteorological conditions
affect air quality in the basin.

The second most common regime is caused by heating later in the day,
Cliff and Cahill explain. It produces a chimney effect and upslope winds
that bring air (and pollutants) from the Sacramento Valley over the
Tahoe Basin. The condition starts in April, lasts through October, and
is strongest in summer. It's also strengthened, according to
researchers, by prevailing westerly winds.

Winter storms overcome the terrain-influenced regime. They cause strong
vertical mixing, dilute local and upwind pollutants, and bring in clean
North Pacific air. So, fall rains and winter snows have very low
concentrations of nitrates and sulfates.

In summer, low pressure cells over the basin-and-range region (east of
Tahoe) circulate moisture, often forming thunderstorms along the Sierra
crest. Also, the authors write, strong high pressure patterns north and
northwest of Tahoe can bring strong dry winds at almost any time of the
year.

What all this boils down to is this. During most of the year, local
pollution from autos, fireplaces, etc. is trapped for at least part of
the day in the basin. In the summer, ozone, nitrates, sulfates, and
smoke from urban and agricultural activities move into the basin from
the Sacramento Valley. But, when we have winter storms, summer
thunderstorms, or dry northern winds, the basin's air is cleansed.

Cliff and Cahill write that as early as 1963, a team of experts
suggested that deposition of atmospheric pollutants -- nitrogen and
phosphorus -- should be considered an important part of the lake's
nutrient budget. In the early 1970s, spot checks along the South Shore
revealed high carbon monoxide and airborne particulate levels. Routine
monitoring was started and the UC-Davis Air Quality Group began their
studies. They found that automobile emissions and wood smoke are the
main sources of local pollution today.

By most urban standards, current air quality in the Tahoe Basin is very
good to excellent, according to the UC-Davis scientists. Violations of
state and federal standards are few. However, Cliff and Cahill point
out, the Tahoe Basin is not a big city. It's a unique scenic area with a
nutrient-sensitive lake that covers much of the basin. And folks are
more sensitive to carbon monoxide at high altitudes. So, new standards
were adopted for the basin with lower carbon monoxide and higher
visibility values.

Tahoe Basin air pollutants include carbon monoxide; nitric oxides and
hydrocarbons from auto exhaust; methane, which is partly from natural
sources; sulfur dioxide, which is a combustion product; and ozone formed
by sunlight acting on auto exhaust. According to Cliff and Cahill, most
of those pollutants are produced locally, except for ozone. All are well
below state, federal and basin standards. Because of improved auto
engines and fuels, all are decreasing, except ozone.

The thing that we notice, however, isn't ozone, carbon monoxide or other
gaseous pollutants. They're mostly invisible. What bothers most folks is
visibility \-- or lack of it. Visibility is affected by tiny particles
of nitrates, sulfates, and smoke in the air that scatter and absorb
light. Cliff and Cahill write that summer visibility in the basin is
dominated by particles blown in, over the Sierra crest, from the west.
In winter, visibility is dominated by local wood smoke.

Ozone values are highest in summer when solar radiation is highest. But
several things puzzled the scientists about ozone values in the basin.
The values peaked in late afternoons, evenings and nights, rather than
midday when solar radiation is highest. Also, auto exhaust components --
nitric oxides and hydrocarbons -- that cause ozone to form in sunlight
are actually decreasing in the basin. So, air pollution experts believe
that ozone is coming from upwind sources -- like Sacramento.

So far, ozone is below standards, but pines are already experiencing
slight damage from ozone. Increasing ozone concentrations are a
potential problem for the basin's forests and other Sierra forests. Yet,
if you've driven through Sacramento lately, you've probably noticed that
traffic continues to increase -- adding to our ozone levels, diminishing
our visibility, and adding nutrients to the lake.