© 2001 by Leo Poppoff

May 10, 2001

Basin Watch

by

Leo Poppoff

WE'RE SURROUNDED BY GLOBAL AIR POLLUTANTS

An April 19 news article in the Tahoe World reported that "the thick
haze that hovered over Tahoe last week came from thousands of miles
away." According to the report, the haze started as a massive dust storm
in the Gobi Desert. It was another confirmation that we're not an
isolated basin. We're influenced by local pollution, of course, but also
by regional and global pollution.

We've covered a lot of this ground. We've discussed the general global
increase in nitrogen that's stimulating plant growth and worrying
ecologists. We looked at work by Alan Jassby of the Tahoe Research Group
on nutrients that fall on the lake and watershed. Thomas Cahill and
Steven Cliff of UC-Davis have determined that the basin's ozone comes
primarily from over the Sierra Crest, as does the particulate matter
that causes summer haze (sources of winter haze are local).

Alan Gertler of the Desert Research Institute finds that nitric acid is
the principal form of nitrogen falling into the Tahoe Basin. Alan
Heyvaert found mercury in the lake's sediments that could only have
blown in from the west.

It's been known for some time that chlorofluoromethane (Freon), has
diffused all over the globe. When it gets into the stratosphere, the
molecule is torn apart by ultraviolet radiation and the products cause
the depletion of ozone. Worldwide voyages have shown that global
carbon-monoxide levels have risen during the past several decades. Soot,
causing Arctic haze, has been identified as coming from Russia. Sahara
dust is found in Florida.

Carbon-dioxide, methane and nitrous-oxide, some natural and some
man-made, also spread far and wide. They're part of the mix of
"greenhouse gases" suspected of causing global warming. So, global-scale
pollution is really old news.

What's new is that the National Academy of Sciences (NAS) is now
concerned. It convened a panel of experts to study the problem and make
recommendations. Their report is imminent, but you can preview it on the
NAS website. Here's what they found.

Surface air from Asia can reach the upper troposphere over California in
two to four days. Carbon-monoxide over the west coast was traced back to
Asia in a 1997 study. A 1999 study found carbon-monoxide and ozone from
Asia over the State of Washington. Global chemical transport models
predict that "future developments in Asia could lead to emissions that
will significantly perturb free tropospheric ozone chemistry over the
Pacific."

According to the NAS panel, computer models calculate "that Asian
emissions have already raised mean background springtime levels of CO
(carbon-monoxide) and O3 (ozone) by an average of 34 and 4 ppb (parts
per billion), respectively, over the eastern North Pacific." Other
modeling studies predict that future Asian contributions to U.S. West
Coast ozone episodes could be as high as 40 ppb. To put these numbers in
perspective, note that California's ozone standard is 90 ppb and Tahoe's
carbon-monoxide standard is 6000 ppb.

In addition to CO and O3, the panel notes that the International Panel
on Climate Change has made worrisome projections of other pollutants. By
the year 2040, global levels of sulfur compounds could increase by 60
percent. Nitrogen compounds could double. Levels of volatile organic
compounds and methane are also expected to rise, though no numbers were
presented.

A very reactive compound called the hydroxyl radical is produced from
water vapor when it's radiated by ultraviolet light in the presence of
ozone. This transient chemical scavenges pollutants. It reacts with
carbon-monoxide, sulfur-dioxide and methane, removing them from air.
It's estimated that more than half of the sulfur-dioxide released into
the atmosphere is removed by reactions with hydroxyl radicals.
Increasing levels of pollutants are apparently overwhelming the ability
of hydroxyl radicals to cleanse the air. And levels of hydroxyl radicals
in the atmosphere are thought to be decreasing, according to studies by
Ronald Prinn. He is the Chairman of the Department of Earth, Atmospheric
and Planetary Sciences at MIT, and his paper was published in the May 4
issue of the journal "Science." Some scientists suggest that increased
haze has reduced the amount of ultraviolet light and that, in turn,
reduces the production of hydroxyl radicals.

Hydroxyl radicals are created and disappear so fast, it's very hard to
measure their concentrations, so values are inferred from other
measurements. In fact, it's hard to measure global levels of most
pollutants. So most results are calculated from global models, and they
have their limitations.

The NAS panel believes the potential global problem is serious. And
they're concerned that there are no adequate global monitoring programs
to keep track of the situation.

Closer to home, we also have a problem. We're still unsure about how
much of Tahoe's air and water pollution comes from sources out of the
basin. And we ought to be concerned that we don't have an adequate
monitoring program to tell us.