Question

How is agriculture a source of air pollution?

Answer 1

If someone burns agricultural residues on the field, air pollution occurs

Explanation:

Heavy machinery, if it is used frequently, cause air pollution in agricultural sector.

Another issue of burning agricultural residues. This is an air pollution issue.

In some areas, such as rice fields, anaerobic processes cause methane formation. When harvesting is completed, such gases (hydrogen sulfide, methane, etc.) cause air pollution.

Answer 2

Nitrogen-rich fertilizers, animal waste, and, combustion emissions are forms of agriculture air pollution.

Explanation:

Nitrogen-fertilizers and animal waste release ammonia and nitrous oxide. Ammonia can combine with combustion emissions to form tiny solid particles or aerosols.
http://www.earthinstitute.columbia.edu/articles/view/3281
https://www.epa.gov/ghgemissions/overview-greenhouse-gases#nitrous-oxide
Also cows produce large amounts of methane a very potent greenhouse gas. I think is also safe to assume farm vehicles release carbon dioxide.