Much to the chagrin of California tourism promoters, smog is likely one of the things you picture when you think about the city of Los Angeles. The haze of pollutants that often hangs over the city is more than just an eyesore; it's a source of considerable respiratory stress. And where does that air pollution come from? Smokestacks, tailpipes, and cows. You read that right—cows. In fact, a new study estimates that cows contribute at least as much as automobiles.

There are two main factors that control the formation of smog. The first is the air pollution. This includes a range of volatile organic compounds, as well as oxides of nitrogen (referred to as NO x compounds. These pollutants react in sunlight to produce the ground-level ozone that triggers asthma advisories. The second necessary condition is stagnant air. A stiff breeze moving through the city will clear away pollution before it can collect. Areas that experience smog typically have the right topographical and meteorological conditions for air masses to hang around for a while.

A large portion of the small particles, or aerosols, that contribute to the haze are comprised of ammonium nitrate (NH 4 NO 3 ). The nitrate comes from NO x emissions, but the ammonium mostly comes from a couple specific sources.

One is automobiles. While the catalytic converter on your car’s exhaust system cuts way down on your NO x emissions, it generates some NH 3 (ammonia) in the process. Another purpose for the catalytic converter is to oxidize carbon monoxide to carbon dioxide, and it does so by stealing oxygen from water molecules. The abandoned hydrogen from the water is released as hydrogen gas, which happily gets together with some of the nitrogen in your exhaust to form ammonia.

Animal waste is another source of ammonia. It’s one of the delightful odors in that distinctive potpourri that graces livestock barns. Enzymes in feces break down the urea in the urine, resulting in the emission of ammonia.

To provide an accounting of these ammonia sources in Los Angeles, researchers from the University of Colorado-Boulder and the National Oceanographic and Atmospheric Administration organized eighteen flights of a plane carrying instruments that could make detailed atmospheric measurements.

Some flew transects across the area just downwind of urban Los Angeles, and others flew across the area downwind of the large dairy farms east of the city. Downwind of the city, the measured ratio of ammonia to carbon monoxide matched that of emissions from downtown traffic. Using the average concentration of ammonia and the area covered, they estimated that vehicles were contributing 62±24 metric tons of ammonia per day to the air above Los Angeles.

Downwind of the dairies, the story was different. There, where carbon monoxide was absent, the ammonia was more concentrated, but present in smaller areas. In total, estimates of the ammonia from dairies ranged (depending on the flight) from 33±16 to 176±88 metric tons per day. That’s considerably higher than previous estimates that relied on extrapolations from average emissions per animal.

That means that, contrary to expectations, traffic and dairy farms appear to be about equally responsible for a sizeable fraction of the haze over Los Angeles. Actually, it’s a little more complex than that—because the ammonia downwind of the dairies is much more concentrated, conditions favor greater NH 4 NO 3 particle formation there. So, even for equal amounts of total ammonia, dairies will have an outsized impact on air quality.

Obviously, that’s important information for efforts to clean up the air in Los Angeles. The biggest impact would come from helping dairy operations use ammonia reducing practices, like treatment of waste, proper storage, and dietary changes. That could make a difference for the over 4 million people who live downwind.

Geophysical Research Letters, 2012. DOI: DOI:10.1029/2012GL051197 (About DOIs).