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Fall 2001

The globalization of air pollution

Concentrations of carbon monoxide (CO) at an altitude of about 15,000 feet on 12 March 2000. Darkest areas in this image indicate highest concentrations of CO (450 parts per billion). Lightest areas indicate lowest concentrations (50 ppb). Notice the plume of high concentrations of CO extending eastward from eastern Asia across the Pacific. (Illustration courtesy MOPITT.)

For several days in mid-April of this year I noticed a whitening of the normally blue Colorado sky. It reminded me of the long period of "white skies" following the eruption of Mt. Pinatubo in 1991. Because of the April event's duration (several days) and its scale (the entire sky affected uniformly), I sensed this was a pollution event of global impact and importance.

The controversy and publicity over the human role in long-term global warming has overshadowed other important and immediate environmental impacts of the six billion humans on the planet. These include air and water pollution on a scale affecting regions far beyond those where the pollution is generated. In fact, recent evidence conclusively shows that air pollution has become global in nature, with emissions from Asia and China affecting the air over North America, emissions from North America affecting Europe, and emissions from Europe in turn affecting Asia. Old remedies, such as the one my close friend and colleague, the late Vern Suomi, once prescribed, with tongue in cheek—"The solution to pollution is dilution!"—are no longer applicable. Pollutants are now being created and carried half a hemisphere away faster than they can be diluted and removed from the atmosphere.

The global nature of air pollution has been documented by recent measurement campaigns, including the MOPITT (Measurements of Pollution in the Troposphere) satellite, INDOEX (Indian Ocean Experiment) and ACE-Asia (Aerosol Characterization Experiment–Asia). Below I touch on major findings from each of these. (Also, see the Web links below.)


Launched in December 1999, MOPITT measures carbon monoxide (CO) from NASA's Terra spacecraft. Both a toxin and a tracer for other types of pollution, CO also reacts with other gases to form ozone, a greenhouse gas and a human health hazard. The image below shows "hot spots" of CO extending thousands of miles downwind of sources. A dramatic time-lapse movie showing the movement of these pollution plumes is on the MOPITT Web site.

Two aerosol campaigns

The globalization of air pollution is not limited to gases such as carbon monoxide. Aerosols (particles approximately one micrometer in diameter, including dust, carbon, sulfate, salt, and nitrate) also are carried around the world before they are removed through processes involving chemical reactions and precipitation. Aerosols show their presence to the naked eye through visibility and sky color, the effect I saw in Boulder. Aerosol optical depth is a measure of the total extinction of light in the atmosphere caused by reflection and scattering by aerosols. In small concentrations (aerosol optical depth ~0.1), they create a light haze in the atmosphere and a whitish sky; in larger concentrations (aerosol optical depth ~0.5 or greater) they limit visibility to a mile or less and can obscure the stars, the Moon, and even the Sun.

Aerosols affect radiation directly when they reflect it back to space. Aerosols also influence cloud brightness and precipitation, each of which has an indirect effect on radiation. Both influences are important to Earth's overall radiative balance and thus to climate. Measurements from INDOEX show that particles originating from the Indian subcontinent persist for many days as they travel equatorward for thousands of kilometers over the Indian Ocean. The region's many aerosol sources include industrial and auto emissions, biomass burning, and soil dust. A thick layer of very polluted air, extending more than 1,000 kilometers offshore of the Indian subcontinent, covered the ocean almost constantly during the six-week INDOEX observing period. The NSF/NCAR C-130 aircraft recorded aerosol optical depths as high as 0.7. Aerosols comprised of soot, sulfates, nitrates, organic particles, fly ash, and mineral dust made up the haze, accompanied by CO and sulfur dioxide. Carried to the equator, these pollutants were swept upward in the deep cumulus convection of the intertropical convergence zone (ITCZ). On the other side of the ITCZ, in the Southern Hemisphere, the C-130 sampled almost completely clean air, with an aerosol optical depth of less than 0.1.

ACE-Asia, conducted during April and May 2001 from Japan, documented some severe dust outbreaks from Asia. The dust from the Mongolian and Chinese deserts interacts with human-made pollutants such as sulfur compounds. It was one of these dust storms that caused the white skies I observed in April.

Going global

Dust (indicated by the arrows) is visible blowing off the coast of China in this NOAA-16 satellite image. (Illustration courtesy NOAA.)

That Asian dust storm, which affected much of the Northern Hemisphere, is history, and the Colorado skies have temporarily returned to blue. But this is far from the end of the story. While the media and policy makers remain fixated on global warming, which has uncertain consequences many years into the future, the direct and immediate effects of human-produced pollution on the atmosphere and climate are largely ignored. The total impact of the globalization of air pollution on human health, agriculture, and climate is large now and will certainly increase over the next decades as the total population grows and as energy consumption per person also grows. As the three examples I've given indicate, scientists are already making important measurements that are helping us understand the global nature of the problem, and scientific attention is increasing. For example, a primary challenge of one of NCAR's new scientific initiatives, on chemistry-climate connections, is to understand the impacts of human activities on the chemistry of the atmosphere over urban to global scales.

The 17 August issue of Science summarizes the growing impact of air pollution on human health and morbidity and assesses some of the immediate health benefits that would result from reducing pollution associated with the emission of greenhouse gases. The magnitude of the problem is illustrated by a World Health Organization estimate that air pollution will cause more than eight million deaths worldwide by 2020. Thus, as the Science article points out, reducing greenhouse gas emissions worldwide would have an immediate positive effect on human health, along with the long-term effect of reducing global warming.

Eventually these and other impacts of global pollution will become large enough that the media and policy makers will take note. The results are quite predictable—there will be a lot of finger pointing and arguments against taking actions because of various political and economic pressures. Ironically, an important factor in the arguments against local action will be the severity of the global pollution problem itself. Any local action will have a relatively small local impact because much of the local pollution will have come from other cities, other states, and even other countries. For example, if 90% of a city's pollution comes from outside its borders, it will be difficult to convince that city to take potentially burdensome actions to reduce its 10% contribution. Arguments against enforcing U.S. Environmental Protection Agency standards for ozone in Georgia are already being made on the grounds that as much as half of the ozone precursors—volatile organic compounds and nitrogen oxides—are transported into Georgia from neighboring regions. If everyone in the world takes this position, we will have a classic "tragedy of the commons." Thus leaving pollution controls to local governments is not likely to work; solutions at the national and international level, perhaps analogous to the Montreal Protocol, will be needed. While these kinds of arguments rage, the planet's air will become dirtier and dirtier as the climate gets hotter and hotter.

On the Web:


http://www.atmosp.physics.utoronto.ca/MOPITT/home.html http://www.gsfc.nasa.gov/gsfc/earth/terra/co.htm


http://www.ucar.edu/communications/quarterly/summer99/INDOEX.htm l



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Edited by Bob Henson, bhenson@ucar.edu
Prepared for the Web by Jacque Marshall
Last revised: Tue Oct 23 11:26:05 MDT 2001