Other Oxides of Nitrogen

Other oxides of nitrogen also are being added to the atmosphere in increased amounts. Anthropogenic emissions of nitrogen compounds in the US, as reported by EPA, are shown in Figure 4.

NO and $NO$₂ are gases are collectively taken together with $NO$₃ aerosol to comprise what we call $NO$_X. Annual $NO$_X emissions in the US over the last 80 years have increased dramatically, from less than 1 to about 22 Tg (teragrams, or millions of metric tons). On a global basis natural sources of $NO$_X include soils (5-20 Tg annually) and lightning (2-20 Tg annually) and give an annual total amount of 8 to 41 Tg. Anthropogenic sources contribute annually about 27 to 38 Tg from fossil fuel combustion, biomass burning, and tropospheric aircraft. These numbers suggest that humans are emitting oxides of nitrogen to the atmosphere in total annual amounts that are comparable in magnitude to natural sources (Figure 5).

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