Ozone Formation

NOx refers to both nitrogen oxide molecules (NO) and nitrogen dioxide molecules (NO₂). The other category is a class of hydrocarbons called volatile organic compounds (VOCs). Without the combination of lots of ultraviolet sunlight, NOx, and VOCs, ground-level ozone does not form.

About half of all of the NOx and VOCs in the United States, as well as in Texas, enters the atmosphere from the combustion of fuels in vehicles. The exact percentage of the VOC and NOx emissions that comes from vehicles varies by city. For example, in cities with lots of petrochemical plants, such as the Beaumont/Port Arthur area, around 7 percent of the VOCs and 13 percent of the NOx comes from vehicles. On the other hand, in areas such as Dallas/Fort Worth, 32 percent of the VOCs and 60 percent of the NOx comes from vehicles.