"NOx" is a general term used to describe oxides of nitrogen, which include two pollutants from cars:
- NO (nitric oxide)
- NO2 (nitrogen dioxide).
The high-temperature environment of a combustion engine mainly creates NO, which then converts to NO2 later in the presence of oxygen.
NOx does not have any impact on climate change. The main issue with NOx is that it is a key ingredient in smog. NOx mixes with volatile organic compounds (VOCs) and, in the presence of sunlight, creates ground-level ozone (O3) which is otherwise known as urban smog. Ground-level ozone is recognized by the the U.S. Environmental Protection Agency as a criteria pollutant, as is NOx. More NOx does not always cause more ozone. It depends on the proportion of NOx to VOCs, and how much sunlight you have. In certain cases, more NOx can actually lower ozone levels, at least theoretically, which makes the whole NOx debate more than a little confusing. In general, most regulatory agencies assume that less NOx is a good thing, and they make their rules accordingly. A secondary issue with NOx is that it causes acid rain, but it’s not the main culprit for this problem. When it rains, it pours.
Nitrous oxide (N2O) is not a major pollutant from cars. Most of it comes from many natural sources, including bacteria in soil and the oceans. Cultivation of crops like soybeans in nitrogen-rich soil can release a lot of N2O, at least according to studies at the University of California at Davis.
N2O is sometimes used in cars to increase performance (by the Fast and Furious crowd) since it breaks down in the engine and yields oxygen to the combustion process. It is also used as an anesthetic (a.k.a. "laughing gas"). The problem with N2O is that it is a really strong greenhouse gas, roughly 25 times more powerful climate change agent than carbon dioxide. So growing more soybeans, for example, to make biodiesel could inadvertently release more N2O into the atmosphere, thereby causing more climate change even though biodiesel replaces more carbon-intensive fuels.
A conversation about oxides of nitrogen is only slightly less painful than root canal surgery—unless you use nitrous oxide, of course.