Volatile organic compounds (VOCs) combine with nitrogen oxides in the presence of sunlight to form the criteria pollutant ozone. The term "volatile organic compound" referrs to any carbon compound that can react photochemicaly in the atmosphere.
Source: U.S. Environmental Protection Agency. 1999. Handbook for criteria pollutant inventory development: a beginner's guide for point and area sources. EPA-OAQPS-454-037. Research Triangle Park, NC. U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards.