Stratospheric Source of NO, from

The principal natural source of NO (NO + N02) in the stratosphere is N20. Approximately 90% of N20 in the stratosphere is destroyed by photolysis  

Reaction 2a is the main source of NO in the stratosphere. About 58% of the N20 + O( D) reaction proceeds via channel 2a, the remaining 42% by channel 2b. 

The main influx of tropospheric species into the stratosphere occurs in the tropics, and N20 enters via this route. Upon crossing the tropopause, N20 advects slowly upward. During its ascent, N20 is diluted by N20-poor air that mixes in from outside the tropical stratosphere and, at the same time, disappears by reactions 1 and 2. The higher a molecule of N20 rises in the stratosphere, the more energetic the photons that are encountered, and the more rapid its photodissociation by reaction 1. 

Even though the photodissociation of N20 produces 0(ID), the main source of 0( D) in the stratosphere is photodissociation of 03 (reaction 3 in Section 5.2), and the concentration of O( D) at any altitude is determined by its source from 03 photolysis and its sink from quenching by 0(1D) + M. The concentration of 0( D) increases with 

FIGURE 5.6 (a) Vertical profiles of N20 over the tropics at equinox circa 1980. Circles denote balloonbome measurements at 9°N and 5°S squares represent aircraft measurements between 1.6°S and 9.9°N. Dashed curve refers to the average of satellite measurements at 5°N, equinox, between 1979 and 1981. This compilation of data was presented by Minschwaner et al. (1993), where the original sources of data can be found. The dotted curve indicates the vertical profile used by Minschwaner et al. to estimate the lifetime of N20. (b) Calculated diumally averaged loss rate for N20 (in units of 1012 molecules cm 3 s ) as a function of altitude and latitude, at equinox. The loss rate includes both photolysis and reaction with O( D) (Minschwaner et al. 1993). 

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