Which compound can be found in the air at levels that can damage crops?

The compound known to be present in the air at damaging levels for crops is PAN, or Peroxyacyl Nitrate. PAN is a secondary air pollutant formed from the reaction of volatile organic compounds (VOCs) and nitrogen oxides (NOx) in the presence of sunlight. It can adversely affect plant health by causing leaf injury, impacting photosynthesis, and leading to reduced crop yields.

Ozone, while harmful to crops, is typically considered in the context of direct exposure as a primary pollutant rather than a secondary one like PAN. Carbon dioxide, although crucial for plant growth, is not considered damaging at current ambient levels, and high levels of sulfur dioxide can have harmful effects; however, it is less ubiquitous compared to PAN in certain environments when analyzing crop damage specifically linked to secondary air pollutants.

In summary, PAN is particularly notable for its crop-damaging potential as a secondary pollutant and is indicative of urban air pollution scenarios, thus making it the correct choice in the context of the question.