A study by UC Davis’ department of land, air and water resources, found that agricultural soils contribute 25 to 41 percent of nitrogen oxide emissions in California. The primary source is fertilized cropland. Two links that are worth your time. A video from the Sacramento Bee and an article from the Fresno Bee can be found here: Fertilizer on farm fields is a major source of California smog, researchers say
While carbon dioxide is the major greenhouse gas, nitrogen oxide plays a central role in global warming and is a major factor in smog formation. “Emissions of NOx from agricultural areas are much higher than we used to believe and could be a major source of atmospheric NOx statewide,” the researchers wrote in the article.