Wind-Borne Pollutants May Travel Thousands Of Miles

A team of geoscientists -- June-Soo Park, Steve Sweet, and Terry Wade -- at Texas A&M's Geochemical and Environmental Research Group (GERG) came to these conclusions while studying how pollutants such as polynuclear aromatic hydrocarbons (PAHs), pesticides, and polychlorinated biphenyls (PCBs) are transported in the atmosphere to Galveston and Corpus Christi bays and removed by rain and dust. The scientists also studied how gaseous pollutants are exchanged between the air and water.

They found that air pollutants could be transported over long distances instead of being trapped in the ocean or the soil, and that gaseous water pollutants could evaporate into the atmosphere instead of staying in the ocean. In both cases, the airborne pollutants could lead to deposition of pollutants long distances from where they were produced or used.

"Most scientists used to think that organic pollutants were not present as gas in the air," Wade says. "To our surprise, we have learned over the last 30 years that organic pollutants can be in the vapor phase, which means that they can be transported over long distances."

For example, high levels of pesticides such as DDT, chlordane and toxaphene are present in beluga whales from the Arctic, where they were not used.

"What we do here in Texas can affect what is happening in the Arctic," Wade says. "We have added these contaminants to the environment, and now we can see that they are showing up in areas where we would not have expected them to be."

To assess the whereabouts of the pollutants, Wade and his collaborators used rain and air samplers that they installed close to Galveston and Corpus Christi bays.

A rain sampler that opens only when it is raining was used. The sampling site was also equipped with sensors that look at wind speed and direction, relative humidity, and barometric pressure.

"The general principle of the experiments," Wade says, "is that you take a sample of air or rain and you measure how much contaminant is present in the rain or in the air. Then using models, you try to estimate the amount of contaminants that are coming down into the bay."

In the rain sampler, the rain goes first through a filter that holds back the particles present in the rain and then through an absorbent that soaks up the dissolved pollutants. The volume of rain going through the sampler is measured

The air sampler contains a filter to collect particles in the air (dust) and polyurethane foam plugs that trap gaseous air pollutants.

Wade and his collaborators recently collected pollutants in about 3,000 cubic meters of air and up to 120 liters of rain. Knowing the amount of pollutant deposited in the samplers, the scientists estimated the corresponding amounts that would enter the entire bay area.

"To determine how much of some pollutants deposited in Corpus Christi Bay during rain events," Wade says, "we analyzed the rainfall events over more than a year's period and determined the yearly amount of deposition in our sampler, then we estimated the corresponding amount to the entire surface area of Corpus Christi Bay."

The estimation of pollutant inputs from the atmosphere is good for rain, but more uncertain for particle (dust) deposition and gas exchange. It is more difficult to measure the ground deposition of air pollutants than rain pollutants.

One of the most interesting results of the scientists' work is related to the gas exchange between air and water. "Our most surprising result is that there is a flux of contaminants currently coming out of Corpus Christi Bay to the atmosphere," Wade says," while we previously assumed that all the flux of contaminants would be into the water." Wade suggests that PAH might come from the evaporation from small petroleum spills.

"In an area where we produce petroleum, there is natural seepage and accidental releases," he says, "so if you spill PAH in the ocean, a lot of it evaporates and then can be transported long distances."

Their research also indicates that DDT and PCB evaporate into the atmosphere and are transported over long distances.

"Scientists assumed that when you spray DDT on crops, the insecticides stay in the soil. They might be washed into rivers when it rains and be transported down rivers to coastal areas," Wade says. "To our surprise, we discovered that the pesticide can volatilize into the gaseous state and be transported in the air over long distances fairly rapidly."

Wade adds that scientists are now trying to design pesticides that break down faster and can be biodegraded more rapidly to prevent them from being transported to remote locations by the atmosphere.

"We may alter our environment as long as we keep it sustainable," Wade says. "If we can put something in the environment that degrades at a rate that prevents it from building up, then there should be no harm caused to the environment."