Increases in the concentration of NO x from aircraft generally will increase the rate of ozone production by speeding the oxidation of CO and CH 4 . Sources and Sinks of NO x Aircraft emissions are one of many sources of NO x in the troposphere and the stratosphere. As a result of this source, NO x concentrations in the LS are quite large, increasing from about 100 pptv at the tropopause to as much as 3000 pptv at an altitude of 20 km. Aircraft exhaust provides an additional source of NO x , but there is no evidence that this source has appreciably altered the concentration of stratospheric NO x . The primary sources of NO x in the troposphere are fossil fuel combustion, biomass burning, soil emissions, lightning, transport from the stratosphere, ammonia oxidation, and aircraft exhaust.
Biomass burning occurs primarily in the continental tropics.
The contribution of NO x produced by lightning is very uncertain because it is extremely difficult to measure directly. The relative impact of NO x produced from aviation is critically dependent on the strength of this source. Despite this uncertainty, it is likely that averaged over the globe, the lightning source of NO x in the UT is 2 to 8 times as large as the aircraft source. Although lightning, aircraft emissions, and transport of NO x from the boundary layer during deep convection clearly provide significant sources of NO x to the UT, the contribution of each in determining the total NO x concentration is not well understood. In situ observations have been made in the UT to help identify and quantify sources of NO x .