Which pollutant is most commonly associated with acid rain?

Acid rain is primarily caused by the presence of sulfur dioxide (SO2) and nitrogen oxides (NOx) in the atmosphere. These pollutants originate from the combustion of fossil fuels, which release sulfur and nitrogen compounds into the air. Once in the atmosphere, these compounds undergo chemical reactions that produce sulfuric and nitric acids. When these acids mix with rainwater, they lower the pH of the precipitation, resulting in acid rain.

Among the options provided, nitrogen oxides are indeed a significant contributor to acid rain formation. They react with water in the atmosphere to produce nitric acid, one of the two main acids responsible for acid rain, highlighting the critical role of nitrogen oxides in this environmental issue.

In contrast, while carbon monoxide is a harmful air pollutant, it is not associated with acid rain formation. Lead, while toxic and a pollutant, does not contribute to the acidity of rain. Chlorofluorocarbons (CFCs) are primarily known for their role in ozone depletion rather than acid rain, further separating them from this environmental concern. Thus, nitrogen oxides are accurately identified as the pollutant most commonly linked to acid rain.