Photocatalyzed reactions between NOx and hydrocarbons create ozone and particulates. Back in the day, US EPA focused on reducing hydrocarbon emissions from vehicles, paint and so on. Because it was easier than reducing NOx emissions from vehicles. The problem is that some areas, such as the Blue Ridge Mountains, have high natural hydrocarbon levels from trees. Plus personal care products. Without the NOx pollution, hydrocarbon emissions wouldn't be so problematic.