Science News - December 15, 2004

Quantifying herbicides latched onto soil dust

The fine dust kicked up when farmers till or disk their fields contains up to 250 times more of a popular herbicide than the soil does, according to ES&T research published on the web this week (es049210s). This first-of-its-kind study by researchers in Connecticut and California could be a clue to why fine particulate matter (PM_2.5) is linked to increased health risks.

Currently, the U.S. ambient air standards are mass-based and do not really pertain to what is attached to the fine dust particles, says one of the paper’s authors, Britt Holmén, an assistant professor at the University of Connecticut. However, she adds that there have been studies showing that “these smaller particles are more detrimental to human health, without the mechanistic understanding of why that is so; the fact that we find those herbicides on these particles could potentially contribute to understanding the health effects.”

Over the course of two years, Holmén and her team followed a tractor in Davis, Calif., as it worked the land and applied polar herbicides that prevent weed germination. The 2 herbicides studied, metolachlor and pendimethalin, are among the 10 most widely used in the United States. The group used filters to collect PM_2.5 and gas-phase samples downwind at 1, 3, and 5 meters above the ground. They discovered high levels of pendimethalin attached to fine particles, whereas metolachlor was more likely to partition to the air. Laboratory studies helped to rule out a filter artifact that could have skewed the results.

With so little known about particle–gas partitioning of polar herbicides, this kind of careful data interpretation to improve the analytical techniques is very important, says Tom Harner, an atmospheric scientist with the Meteorological Service of Environment Canada. “We need to understand that partitioning in order to better evaluate [the] long-range transport potential and environmental fate [of these herbicides]. This paper highlights some of the potential caveats or pitfalls associated with this kind of sampling.”

Decision makers who translate this type of data to standards might take cues from the diesel research community in the years to come, says Holmén, who also conducts diesel particulate research. She points out that the diesel community is considering a number-based standard, such as particles per cubic centimeter, to replace the mass-based standard. Combustion sources put out large numbers of particles that are highly toxic but low in mass, similar to those found in her group’s field tests, she explains. —RACHEL PETKEWICH