Dairy cows may be a bigger contributor to smog in the Los Angeles area than many people thought, according to a new study.

The study, conducted by scientists from the University of Colorado and the National Oceanographic and Atmospheric Administration and published in Geophysical Research Letters, found that dairy farms and motor vehicles are about equally responsible for a sizeable fraction of the smog over LA, says this article in Ars Technica.

A large portion of the smog is ammonium nitrate, consisting of small particles from the interaction of ammonia and nitrogen oxide gas. Both cows and automobiles are responsible for ammonia, but it’s the automobiles that produce nitrogen oxide.     

Data gathered in and around the Los Angeles basin in May 2010 suggest that the region’s 9.9 million autos generate about 62 metric tons of ammonia each day, according to this article from Reuters. Ammonia emissions from dairy farms in the eastern portion of the basin ― home to about 298,000 cattle ― range between 33 and 176 metric tons per day, the article says.  

Yet, many of the dairy farms east of Los Angeles ― particularly those around Chino ― are no longer in business.

“There are not many dairies left in Southern Cal, and I am surprised to hear that these few dairies would have a major effect. The ammonia that they have measured does not come from the San Joaquin Valley with all its dairies,” says Frank Mitloehner, air-quality specialist in the Department of Animal Science at the University of California-Davis.

“I disagree with the notion that cows produce more smog than cars,” Mitloehner says. “Both contribute pollutants that need to mix to form PM2.5 (fine particulate matter). It is as if you were to say ‘flour is more important than the egg when baking a cake.’ You need both!”

The study appears to overstate the number of cows in the area.

There are about 100,000 mature cows -- milking and dry -- on about 110 facilities in Chino, Ontario and San Jacinto, according to Rob Vandenheuvel, general manager of the Milk Producers Council in Ontario, Calif. 

Ying Wang, director of Life Cycle Assessment research for the Innovation Center for U.S. Dairy, issued the following response to the study:

“We believe the ammonia emissions estimate of California dairy farms from the study published in Geophysical Research Letters is more than 50 percent higher than other research that we have reviewed,” Wang said.

For instance, the National Air Emissions Monitoring Study (NAEMS) study, which the National Milk Producers Federation commissioned with the U.S. Environmental Protection Agency, came up with significantly different findings, Wang said.  

“NAEMS was conducted from 2007 to 2009 at seven different dairy production facilities located in New York, Indiana, Texas, Wisconsin, California and Washington. The study is a credible representation of ammonia emissions from U.S. dairy farms,” Wang said.

According to the NAEMS study, the average ammonia emissions per cow per day are approximately 60.9 grams. To convert this to metric tons and compare it to the Geophysical Research Letters study:

  •   60.9 grams per cow per day multiplied by the 298,000 cows referenced in Geophysical Research Letters article equals 18,148,200 grams per day, which is about 18 metric tons per day. This is significantly less than the 33 to 176 metric tons per day range cited in that study.  
  • And, if there are only 100,000 cows, the final number would be one-third of that, or 6 metric tons.