Fresno California has air quality problems and the huge number of cows in the San Joaquin Valley has been taking the blame. Turns out that ozone may not be coming from the cows after all, but from their feed.
There are 15 million cows in the valley, the largest dairy producing area in the United States, which is under a federal mandate to reduce its ozone levels. Farmers had installed methane digesters to treat manure, to no avail. Ozone levels kept rising, even though there isn’t nearly as much auto traffic as in urban areas and efforts were being made to reduce it.