A bum rap for agricultural dust?

Agricultural Research, May, 2004 by Don Comis

Look at those dust clouds kicked up by a plow in West Texas! Is that dust affecting public health as much as public perception and dust samplers would indicate?

"No," says Michael Buser, an agricultural engineer in the Cotton Production and Processing Research Unit at ARS's Cropping Systems Research Laboratory in Lubbock, Texas. Why not? Because current sampling methods tend to overestimate the amount of very small particles in agricultural dust, his research shows.

Buser's lab is located in the heart of Dust Bowl country. Scientists there and at the Texas A&M Center for Agricultural Air Quality Engineering and Science have been working on characterizing and defining dust emitted from agricultural sources. They also want to develop economically feasible ways to reduce dust emissions to comply with federal and state air quality regulations.

"In agriculture," says Buser, "we're typically dealing with 'boulders,' in comparison to the particle sizes that the EPA (U.S. Environmental Protection Agency) is concerned about from a health-risk standpoint." Agricultural activities will generate some dust particles in the size range that EPA is concerned with, but the amounts are greatly overestimated in most situations because of inherent sampler errors, he says.

EPA is concerned with particulate matter 10 micrometers ([micro]m) or less in diameter (PM10), with a special concern for particles 2.5 [micro]m or less in diameter (PM2.5).

"To put these particle sizes in perspective, the average diameter of a human hair is 75 [micro]m," Buser says.

Complying With the Law

Through the authority of the Federal Clean Air Act, EPA has established National Ambient Air Quality Standards, or concentration limits, for PM10 and PM2.5. Multiple violations of these standards can lead to a "nonattainment" designation for an area, with a corresponding reduction in allowable emission rates for all sources of particulate matter in that area.

State air pollution regulatory agencies have authority to set or limit emissions from individual businesses through use of operating permits. If an agency determines that a business is exceeding limits set in the permit--or if it becomes necessary to reduce emissions in a region--the agency could require the business to cut its emissions by adopting alternative management techniques or incorporating additional control devices.

Buser and his colleagues are evaluating samplers and sampling methods regulatory agencies use to determine particulate matter levels emitted by agricultural sources, such as cotton gins, cattle feedlots, dairy operations, grain elevators, and tillage and harvesting operations. They have found that sampler measurements can indicate that agricultural sources are emitting much more PM10 or PM2.5 than they actually are. This is because the preseparator in the samplers blocks some of the smaller particles from passing through it and being deposited on the filter, but lets some of the bigger particles pass through.

Basically, the sampler has two errors: one causing oversampling and another causing undersampling. A common assumption made in the regulatory community to circumvent this problem is that one error offsets the other. In other words, the mass of smaller particles prevented from reaching the filter is equal to the mass of larger particles that mistakenly make it to the filter. But this assumption is not valid outside of a laboratory setting where there is a uniform distribution of particle sizes. Outdoors, the distribution of particle sizes in dust from most agricultural operations is far more uneven, with larger particles tending to predominate.

A More Precise Sampling Tool

EPA and state air quality regulatory agencies now use two basic types of samplers: Ambient air samplers sample air over a wide area, and "stack" samplers measure particulate matter emitted from an individual business or farm operation. EPA has a network of several thousand ambient air samplers permanently in place across the United States. A region's air quality agency may temporarily install a stack sampler at a particular business before issuing an operating permit or to determine whether the business has to reduce emissions.

Buser and his colleagues have found a third, more accurate method to determine PM10 and PM2.5 concentrations from agricultural sources. It uses total suspended particulate (TSP) samplers to obtain a total concentration of dust, followed by laboratory analysis of the filter to determine particle-size distribution and percentage of smaller dust particles.

Buser's research shows that this method would do a better job than stack samplers in measuring dust output from agriculture.

It Saves Money--In the Long Run

This proposed method of determining concentrations emitted from agricultural sources will most likely increase the cost of sampling. But, says Buser, "By improving the samplers and sampling protocol used to regulate agricultural businesses, we could save that sector of the economy a lot of money in the long run, without any harm to air quality."