Rural ozone can be fed by feed (as in silage)

Livestock operations take a lot of flak for polluting. Manure lagoons not only irritate neighbors’ noses but also leak nitrogen — sometimes fostering dead zones up to 1,000 miles downstream. And ruminants can release copious amounts of methane, a greenhouse gas. Researchers are now linking ozone to livestock as well. But this time the pollution source is not what comes out the back end of an animal but what’s destined to go in the front.

State air-quality managers have been puzzling over why some rural areas suffer high ozone pollution. It’s been a real conundrum in California’s San Joaquin Valley, home to three of the nation’s six most ozone-ravaged counties.

In big cities, combustion products spewed out of tailpipes and smokestacks play a big role in cooking up ozone. But there’s a paucity of these in rural America.

What there is a lot of: farming.

Indeed, the San Joaquin Valley alone produces almost a tenth of the nation’s total agricultural output. Its tomatoes and almond trees aren’t big ozone sources, obviously. So when the California Air Resources Board pondered the region’s ozone problem a few years back, it suggested that perhaps volatile emissions from animals and their wastes might be responsible when they mixed with nitrogen dioxide and other reactive organic air pollutants.

Environmental engineer Cody Howard and his colleagues at the University of California at Davis looked into this idea because it certainly made sense. A ruminant’s digestive system ferments grains. A byproduct of that — alcohol — is a reactive organic gas that can drive the atmospheric chemistry responsible for making ozone.

But Howard’s group showed in a 2008 paper that while the idea was valid, in fact the animals’ emissions just weren’t big enough to explain the magnitude of the Valley’s ozone problem. So they began investigating animal feed, Howard says — especially silage, grain “which has been [deliberately] fermented” and as such comes laced with a lot of alcohol.

The UC-Davis team sequentially tested each of seven different types of feed in a one-meter-square tented chamber. Into this mobile cube they infused a mix of gases that matched the background composition of the Valley’s air. When they illuminated the chamber’s interior with lamps to simulate sunlight’s ultraviolet rays, sure enough, the silage cooked up additional ozone. Lots of it during each six minute test.

Corn silage generated about 125 parts per billion ozone, alfalfa silage a little less, and mixed oat-wheat silage a whopping 210 ppb.

Alcohols and aldehydes in silages emerged as leading contributors to regional ozone formation. “Ethanol and especially larger alcohol species account for more than 50 percent of the ozone formation for most types of feed,” Howard and his colleagues now report online, ahead of print, in Environmental Science & Technology.

The ozone-formation potency of silage pales in comparison to that of tailpipe emissions from a gasoline-powered car. However, Howard’s team calculates, the San Joaquin Valley has so much silage that animal feed would appear to be the single biggest driver of the region’s smog-ozone problem. Whereas cars and light-duty trucks can generate some 13 metric tons of ozone per day, the researchers find that feed for the Valley’s 10 million head of dairy cattle can produce another 24.5 million tons of ozone per day (with corn silage accounting for all but about 8 percent of the feed’s share).

And the animals themselves: Their burping out alcohol or eliminating such compounds in wastes should contribute only about 3 tons of the Valley’s ozone per day, Howard says.

I asked him how applicable his findings might be for other livestock centers around the country. Not very, he suspects. The San Joaquin Valley’s geography and agricultural intensity make it rather unique. “It’s completely surrounded by mountains,” he notes. “So anything emitted in the Valley typically stays there for a long time — which presents a problem when you’re talking about ozone formation.”