Number range CHAPTER© 2005 by Leo Poppoff

September 8, 2005

Basin Watch

by

Leo Poppoff

BREEZES THAT BLOW FROM AFAR BRING POLLUTANTS

If you were to look at South Shore from a hillside on the North Shore,
during one of those atmospherically stable days, you'd probably see a
layer of smog obscuring your view of the casinos. This is air pollution,
mainly caused by local sources, with some contributions from the
Sacramento Valley. If you'd looked toward Freel Peak, above the smog
layer, visibility would be much better. Yet, it might appear to be a bit
hazy. And, believe it or not, that haze might well be caused by
pollutants from across the Pacific Ocean.

How do we know this? A team of atmospheric scientists recently published
a paper in the Journal of Geophysical Research that details the
transport of aerosols (tiny particles suspended in air) from the Asian
continent to the Western United States. The team was composed of Richard
A. VanCuren from the California Air Resources Board, Steven S. Cliff
from UC-Davis, Kevin D. Perry from the University of Utah and Michael
Jimenez-Cruz from the

Lawrence Berkeley National Laboratory.

VanCuren and his associates instrumented five sites in Northern
California and collected aerosol samples and meteorological data from
April 15 to May 25, 2002. Aerosols were collected with a special
rotating drum particle sampler developed by UC-Davis scientists.
Chemical analysis of the aerosols was made with a very sensitive X-ray
fluorescence technique using the Lawrence Berkeley laboratory
synchrotron. The sites were at Trinidad Head, the Trinity Alps, Mount
Lassen, Crater Lake, and White Mountain. Monitoring these sites was part
of a larger Intercontinental Transport and Chemical Transformation
experiment.

Sampling at the Trinidad Head station yielded mostly local sea-salt
because it's usually socked in with a rather persistent coastal marine
layer. But, occasionally, when vertical mixing occurred, Asian aerosols
were detected. Aerosol collections at the other, higher altitude, sites,
above and away from the coastal marine layer, were dominated by Asian
dust.

Ratios of local and Asian aerosols varied from day to night at the
higher altitude sites because of the influence of vertical air currents
that are typical of mountains. During the day, as mountains heat up,
local winds blow up the slopes carrying local aerosols with them. At
night, as the landscape cools, winds blow down the slopes, bringing
Asian aerosols from the higher levels of the atmosphere. Or as Steve
Cliff puts it, "the Asian aerosol fumigates the site with downslope
wind."

How did the researchers know that the aerosols were from Asia? It seems
that Asian soil, lofted into the atmosphere, has a unique character. It
has a much higher ratio of calcium to iron than local soils. \"It\'s
interesting,\" Cliff said, \"even though we have high calcium from
potential

regional sources in the southwest, like in Texas, we don\'t see evidence
of these (in the western U.S. aerosol samples). I\'m not sure why, but
it appears that those soils aren\'t liberated as easily or otherwise
readily transported to the mountain west.\" The high calcium to iron
ratio of Asian aerosols has also been confirmed by measurements made in
Korea and by sampling Pacific airflow from Asia at a station in Hawaii.

Cliff explained that there is a "rapid transport regime . . . a warm
conveyor belt. The idea is that you have a very rapid transport from
Asia and the aerosols are long lived enough to make it across the
Pacific Basin." Analysis of the trajectory of winds that cross the
Pacific show that they sweep across China on their way to the United
States.

It's been known for many years that prevailing winds can carry soils and
pollutants a long ways. Saharan sand often blows over Florida and Puerto
Rico. And a persistent Arctic haze has been traced to combustion
products from Russia.

Would Asian aerosol affect regional visibility in the Tahoe Basin? "I
think it definitely would," Cliff said. He noted that another UC-Davis
atmospheric scientist, Tom Cahill, had reported some observations, which
indicated that maybe Asian aerosol was important for visibility
reduction in the Tahoe Basin. Cliff added that "given that you're always
going to have some Asian soil, then it's going to be one of the causes
of visibility reduction. When there's lots of Asian soil, in some cases
you might have huge impacts. But even on a very clean day, when you have
very few local sources, or when local sources are suppressed, that long
range transported stuff is going to be the primary contributor to
reduction in visibility."

These studies of intercontinental travel of aerosols pose another
question. Since these aerosols are persistent over the north Pacific
Basin, do they affect climate by altering the radiation budget of this
part of the earth? According to Cliff, Asian aerosols may well be

important in global climate models. He and his associates are working
with climate modelers at the Lawrence Livermore Laboratory to answer
this very interesting question.

Questions or comments? Send them to basinwatch@sbcglobal.net