The Central American Smoke Event of May 1998.

A Summary Based on Reports and Data on the Web, May 17, 18, RBH

During a ten-day period, May 7-17, 1998, smoke from fires in Central America drifted northward into the USA and Canada and caused exceedances of the PM standard, health alerts, and impairment of air traffic, as well as major reductions of visual range, discoloration of the sky, and red sunsets. It was a major air pollution event covered by the national and local media.

Background

Throughout the spring of 1998, thousands of fires in Central America have been burning as it happens every spring but the 1998 fires are said to be about twice as intense as the normal year. As a result, smoke has been lingering over southern Mexico, Guatemala and Honduras since late February 1998 throughout the spring and smaller incursions of smoke palls have reached the US Gulf Coast, particularly Florida. Unlike earlier years, the research community has followed with keen interest the 1998 Central American fires by a variety of UV, visible and infrared remote sensors from satellites.

The Two Smoke Episodes

Two major smoke palls drifted northward throughout the ten-day period. The first covered the Gulf states on May 8 and 9, 1998, while the second swept through the entire eastern USA and Southeast Canada between May13 and 17, 1998. Both episodes occurred under meteorologically similar conditions. There was a stagnation and poor ventilation over Central America for several days resulting in an accumulation of thick smoke over the source area. The smoke pall was about 1000 miles in diameter. Subsequently, the accumulated smoke was swiftly transported northward and stretched into an elongated ribbon-like shape. During the first episode, May 8-9, the smoke pall entered the USA through the Gulf Coast at Texas and turned eastward blanketing Louisiana, Arkansas, Alabama, Tennessee, Georgia and northern Florida with smoke. However, the entire first episode was confined to about a day.

The second episode began with stagnation over Central America on May 10, 11, and 12, 1998. By May 12, a remarkably thick pall of smoke covered virtually the entire Gulf of Mexico. On May13, the smoke pall began a swift journey to the north from Texas along the Mississippi Valley. By May 14, the smoke pall had passed through Missouri and reached Wisconsin with the leading edge entering western Ontario. As a result by May 15, a ribbon of smoke was created stretching from Central America to the Hudson Bay making the source of the smoke clearly visible. Over the next two days the north-south smoke pall was literally shoved eastward by an approaching cold front, resulting in a remarkable contrast of haziness (smokiness) in the front and behind the front. By May 16, the smoke reached Ohio, Pennsylvania, and West Virginia. On Sunday, May 17, virtually the entire Eastern Seaboard was blanketed by a pall of smoke and, as of this writing, it is still there.

Early Reports on the Effects of Smoke

At this early stage an assessment of the multiple effects of smoke is not possible. However, it is known that the Texas Natural Resources Office has carefully monitored smoke and before the main smoke pall arrived, issued a health alert for the Texas coastal region on May 12 1998, and subsequently extended it to May 18. The fine particle concentrations in Texas have been said to have reached 200 m g/m3. The health complaints prompted public health researchers to quantitatively measure the respiratory effects of the persisting smoke.

As the smoke pall stretched northward reaching St. Louis on May 15, the hourly PM10 concentrations rose sharply from 20-60 m g/m3 to 156 m g/m3 . The 24-hour PM2.5 levels also increase from 27 m g/m3 on May 13th to 68 m g/m3 on the 15th - evidently the largest recorded concentration over the past ten years. The St. Louis media as well as media elsewhere, have reported numerous citizen complaints of poor air quality. At the St. Louis Lambert Airport one of the runways was closed resulting in a 30 % reduction in traffic and significant flight delays.

The transport of smoke toward Wisconsin was monitored by GOES satellite and once it arrived in Madison it appeared from the ground as thick haze. The smoke filled sky has blocked out much of the sun both in St. Louis and Madison.

Was This Smoke Event Unusually Intense?

Yes, from the US/Canada perspective, the level of haze aerosol concentration caused by the Central American Fires is most unusual - maybe a once a century event. However, there are many parts of the world where smoke of comparable magnitude occurs yearly including West/Central Africa, Indonesia, China, Brazil/Bolivia.

Are the Distance and Direction of Aerosol Transport From Central America to Canada Unusual?

The transport distance is not unusual. For example, smoke from West African Savannah fires and windblown dust from Sahara regularly reaches South America. In fact, just recently in April 1998, a windblown dust pall from Western China was shown to cause over 100 m g/m3 PM10 concentrations 10,000 miles away in Washington state - half way around the globe. However, on the average, the aerosol transport distance is < 1000 miles, particularly in moist air with precipitation. The smoke transport direction and path from Central America to Canada is probably rather unusual for this part of the year. In the summer, moist air from the Gulf of Mexico regularly reaches Southeastern Canada.