The source of NO^ in the stratosphere is primarily N20 oxidation by 0(1D) in the upper stratosphere. Most of the mass that is transported into the troposphere, however, comes from the lower stratosphere. Peak levels of activity for this source occur primarily in the spring. This activity is found to be most vigorous near the subtropical and polar jet streams as well as in mid and high-latitude regions affected by atmospheric overturning associated with largc-scalc low-pressure disturbances and frontal systems. Even though the NOx flux estimates are small, transported stratospheric odd nitrogen may still be a significant source of free tropospheric NOx for some remote regions. For example, depending on how much of the NO t production from lightning is transported to the stratosphere, estimates of the average global flux of NO^ from the stratosphere range from approximately 0.3 to 1 TgN/yr (Ko et al., 1986; Murphy and Fahey, 1994). These values are close to balancing the stratospheric production of NOx from N20 (Kasibhatla et al., 1991).
Although the stratospheric NO,, source seems small compared to boundary layer NO,, sources, it is comparable in magnitude to other free tropospheric sources such as emissions from subsonic aircraft. Unlike subsonic aircraft emissions and lightning, which predominantly release NO directly into the free troposphere, input of NO,, from the middle stratosphere should primarily consist of HN03 with only a small, approximately 25% or less (i.e., 0.25TgN/yr), contribution from NO, (Russell et al., 1988; Notholt et al., 1995). However, because removal of NO^ in the upper troposphere is extremely inefficient compared to that in the lower atmosphere, the NO,, from the stratosphere may have a long enough lifetime to impact the distribution of NO, through recycling reactions, thereby influencing ozone production in the free troposphere.
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