The 450,000 dairy cows in Southern California are contributing to some of the dirtiest air in the country — and regulators want to clamp down. The cows kick up tons of dust and their manure emits ammonia that joins with nitrogen oxide from vehicle tailpipes to create particulate pollution. Dairies in the area account for only 1 percent of the emissions that lead to air pollution, a trifling amount compared to the 57 percent contributed by vehicles. But every bit counts, and the South Coast Air Quality Maintenance District is considering rules to cut ammonia emissions by altering the composition of the cow feed and changing manure cleanup and storage methods. The AQMD also hopes that dairy farmers simply move away from the region.