As already noted, NOx has a typical lifetime of hours in the boundary layer and days in the troposphere. Because of this fact, little of this NOx can reach the stratosphere.
However, the stratosphere does contain appreciable amounts of NOx, which is formed from N2O transported up from the troposphere.
N2O is produced from dissolved nitrate by microbes in the soil. As already noted in relation to the production of NOx, this process is called denitrification. Although some microbes can further reduce N2O to N2, gaseous N2O can quickly diffuse out of the soil. Because N2O is an unreactive, stable gas, which is not water soluble and which does not absorb visible radiation it has a lifetime in the atmosphere that is estimated to be greater than 20 years.
In fact, the only sink for N2O is chemical reaction in the stratosphere, either by direct photolytic destruction or reaction with energetic oxygen atoms the photodissociation of stratospheric O3:
| N2O + hν | → | NO + O(1D) | (1) |
Equation 1 represents the major mechanism for the destruction of N2O in the stratosphere. The reaction itself does not deplete O3.
| N2O + O(1D) | → | 2 NO | (2) |
Equation 2 represents a mechanism that is responsible for only about 5% of the N2O destroyed in the stratosphere, but which produces NOx, and therefore a species that contributes to the destruction of O3.
| NO + O3 | → | NO2 + O2 | (3) |
Equation 3 is the mechanism for the conversion of NO into NO2 with the simultaneous destruction of O3.
Convective uplift from thunderstorms is an important transport mechanism for N2O (as well as NOx) from the boundary layer into the middle and upper troposphere.
Fig 1.1.4.1: Multiple thunderstorm cells over the Pacific Ocean as seen from the Space Shuttle.
Image: NASA STS064-83-099
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