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Major air pollution episode underway

By: Dr. Jeff Masters, 2:49 PM GMT on August 08, 2007

The Tropical Atlantic is exceptionally quiet, with very little thunderstorm activity anywhere. There are no threat areas to discuss, and none of the reliable models are forecasting development of a tropical storm over the next seven days.

What may be the most deadly weather event to affect the U.S. this year is underway across large portions of the central and eastern U.S. A large area of high pressure with light winds has settled over the region, bringing unhealthful levels of ozone and fine particulate matter to many major cities. High pressure systems are regions where the air gradually sinks, warming as it approaches the surface. This warming, sinking air creates a layer of air aloft (typically near 3000 feet in altitude) that is warmer than the air beneath it. This "upper air inversion" acts as a lid on the atmosphere, keeping pollutants trapped near the surface. Updrafts carrying surface air into the inversion suddenly encounter air that is warmer and less dense, so the updraft dies and the pollutants that they were trying to carry aloft settle back down towards the surface. If the high pressure region is large, an extensive area of light winds at the surface will exist, keeping the pollutants trapped under the inversion from being blown away horizontally. If the high pressure system stays in place for several days, pollutants will accumulate day by day, reaching levels harmful to human health and triggering a sharp rise in the death rate. "Particulate matter," also known as particle pollution or PM, is the pollutant that causes the largest rise in the death rate. Particulate matter less than 2.5 microns in diameter (PM 2.5) is particularly harmful to the lungs. A double-whammy dose of ozone pollution is also occurring this week, since temperatures are warm enough to drive the chemical reactions that form ozone.

Figure 1. Current wind speeds over the Southeast U.S.

Figure 2. Current visibility over the Southeast U.S.

The air pollution episode began August 1 over Indiana and Kentucky, then spread eastward to Ohio and Pennsylvania. By late last week, the stagnant conditions migrated southwards to the Southeast U.S. Wind speeds near the surface (Figure 1) have been less than 5 mph over much of the Southeastern U.S. the past few days, leading to visibilities less than seven miles due to fog and pollution (Figure 2). A small portion of the pollution is due to smoke from forest fires burning in the Northwestern U.S. (Figure 3).

Figure 3. Satellite image from NASA's Aqua satellite on Saturday, August 4, shows a large area of pollution over the Central and Southeast U.S. Forest fires burning in Montana and Idaho (bottom image) are visible, but smoke from these fires is being wafted northeastward into Canada.

On Tuesday, levels of fine particulate matter pollution exceeded the federal air quality standard of 35 ug/M3 over PA, NJ, MD, CT, DE, OH, SC, TN, GA and AL. The pollution episode is expected to be at its worst Wednesday in Georgia and North Carolinas; air pollution action days have been declared in major cites in those states, as well as St. Louis, Cincinnati, Philadelphia, Knoxville, and Wilmington. The air pollution episode should continue into Thursday, then ease on Friday when rains, higher winds, and more mixing of pollutants is expected.

Health Tip: Cut back on strenuous outdoor exercise when air quality is expected to be unhealthy. Exercise during the early morning or late evening hours when ozone levels are at the lowest levels of the day. This is especially important for children and other sensitive groups. This air pollution episode has likely contributed to the premature deaths of several hundred people already. As I discussed in detail in a blog in May, air pollution is thought to be responsible for tens of thousands of premature deaths across the U.S. each year. A major air pollution episode like this week's will contribute significantly to that toll.

Check out the EPA Airnow web site for detailed information on this ongoing pollution episode. Also, thanks go to Alex J. Sagady for providing information for this blog.

Jeff Masters

Air and Water Pollution

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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