Geoscience Reference
In-Depth Information
the “planetary boundary layer,” where friction with the Earth's surface alters airfl ow.
And there's one last complication—the mixing within the planetary boundary layer
that determines the level of pollutants near the ground.
So far, scientists have begun to get the best handle on these variables for PM, nitro-
gen dioxide (NO
2
), carbon monoxide (CO), and formaldehyde. They are also making
some progress with ozone, but SO
2
and most volatile organic compounds (VOCs) still
pose a signifi cant challenge. Other important pollutants that are known to be widely
distributed—among them polycyclic aromatic hydrocarbons, ammonia, pesticides,
and metals such as lead and mercury—are hard to distinguish from the PM to which
they are attached or have spectral signatures that are diffi cult to detect amidst all the
complicating atmospheric conditions, says John Burrows, co-director of the Institute
of Remote Sensing and Environmental Physics at the University of Bremen. Tracking
such pollutants will require better instruments, Burrows says.
Indeed, we still have a long way to go before we can fully document and accurately
forecast pollution streams and their effects on the ground. But data from satellites, air-
craft, balloons, ground-level monitors, chemical transport models, and other tools are
already improving our understanding of the global transport of pollutants as individual
nations and international collaborations struggle to better address emissions and the
effects they have on downwind countries and continents.
A FINE VIEW OF PARTICULATE MATTER
The PM is created by combustion of fuels in vehicles, power plants, domestic cooking,
and industrial processes, and by natural processes such as wildfires, volcanic erup-
tions, wind-blown dust, and bursting of sea salt-laden bubbles at the ocean's surface.
The PM can cause premature death and a range of respiratory and cardiovascular
problems. Because of its various sources and residence time in the atmosphere, PM
is highly variable in space and time. In contrast to gaseous pollutants, PM character-
ization must also include composition or type, particle size, and shape—all important
dimensions for assessing detrimental effects of particulates on human health.
By carefully quantifying how much visible and near-infrared sunlight a PM plume
refl ects back up into space, scientists can accurately estimate the average size of the
individual particles within the plume. Particles of natural origin (such as windblown
dust) tend to be larger in size than human-produced particles, which mostly originate
from the process of combustion (from fi res or factories) and are therefore broken down
into smaller particles.
The PM was not considered a major long-range transport issue until about the mid-
1990s. Even now, the 51 countries that participate in the United Nations Convention
on Long-Range Transboundary Air Pollution are just beginning to address PM, dis-
cussing whether and how to add policies for this pollutant to the convention, says Andre
Zuber, co-chair with Keating and a policy offi cer with the European Commission.
Satellite images indicate how far PM can travel. Almost all continents and regions
can be a PM hotspot at one time or another. Some areas are a major source almost
year-round, including central and southern Africa, eastern Asia, Indonesia, Europe, the
eastern US, and northern and central South America.
