The Oxides of Nitrogen: Their Relation to Tropospheric Ozone

Abstract

Reactive (or “odd”) nitrogen is emitted into the atmosphere in a variety of forms, with the most important being NOx (NO and NO2), ammonia (NH3), and nitrous oxide (N2O). Emissions of these species into the atmosphere have been summarized, for example, by the IPCC Fourth Assessment Report (the AR4; IPCC, 2007). Some discussion of NOx emissions and trends has also been presented in Chapter I. Emissions of NOx are mainly the result of anthropogenic activity associated with fossil fuel combustion and industrial activity. For the 1990s, the AR4 estimates total anthropogenic NOx emissions of 33.4 TgN yr−1, with natural emissions (mostly from soil and lightning) accounting for an additional 8.4–13.7 TgN yr−1. Ammonia emissions are comparable in magnitude to those for NOx, with anthropogenic emissions (45.5 TgN yr−1) again exceeding natural emissions (10.6 TgN yr−1). Although the majority of the ammonia produces aerosols or is scavenged by aerosol and is subsequently lost from the atmosphere, some gas phase oxidation does occur, which can in part lead to NOx production. The N2O source strength is about 17.7 TgN yr−1, with natural sources outweighing anthropogenic ones (IPCC, 2007). However, N2O is essentially inert in the troposphere, and thus the vast majority of its photooxidation and concomitant NOx release occurs in the stratosphere. The major NOx − related reactions occurring in the Earth’s troposphere are summarized in Figure III-A-1. As just alluded to, the species NO and NO2 are jointly referred to as NOx and are often treated collectively. This is because, under daytime conditions, these two species are rapidly interconverted, with the interconversion occurring on a much shorter timescale than the loss of either species.