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Meridional, distributions of NOx, NOy, and other species in the lower stratosphere and upper troposphere during AASE H

. The meridional distribution of NOx in the lower stratosphere and upper troposphere is inferred from 10 flights of the NASA DC-8 in the northern winter of 1992, along with like distributions of NOy, NOx/NOy, CO, and C2C14. In the lowest few km of the stratosphere there is little vertical gradient in NOx over the range of latitudes measured (40 ø- 90øN). There is a substantial latitudinal gradient, with 50 pptv above the pole and 120 pptv near 40øN. In the uppermost few km of the troposphere, background values range from 30 pptv over the pole to 90 pptv near 40øN. On two occasions higher values, up to 140 pptv in the mean, were seen 2-3 km below the tropopause in association with frontal systems. The meridional distributions of CO and C2C14 show the same feature, suggesting that the source of the elevated NOx is near the earth's surface.


Introduction
In the upper troposphere and lower stratosphere, NO and NO2 (NOx) play important roles in the production and destruction of 03. The region near the tropopause is one where 03 has its greatest effect as a greenhouse gas and where the lifetime of NOx is relatively long, so a given emission can have a larger impact. The global distribution of NOx in this region is only poorly known, however. Ehhalt et al. [1992] show   Weinheimer et al. [1993] for NO x and NOy, in  and references therein for the reactive carbon species). At low mixing ratios of NO and NO2 the overall uncertainty in 1-min NOx values is 20 pptv (including bias and precision errors). The percentage error decreases with increasing NOx, and it is -35% for the highest values shown.
The measurement of NOy (NO, NO 2, HNO 3, C1ONO 2, 2N20 5, HO2NO2, etc.) shows a significant lag in air with large gradients in HNO3 (an inlet problem), as when crossing the tropopause. The resulting errors can be rather large (-35%), but underestimates on ascent tend to cancel overestimates on descent, so the effect is reduced for the averages shown here. Altitudes are computed relative to the altitude of the tropopause, the height of which is determined at 14-s intervals along the flight track using remotely sensed temperature profiles, as described by Gary [1989]. Data from this instrument are not available for the January flights, so those flights are excluded from the present analysis. As the numbers of samples on the individual columns indicate, the sampling is extensive above the tropopause (except for the lowest latitude bin), but limited below. Nonetheless there is consistency among the altitude profiles for the different latitude bins. In descending from the stratosphere to the troposphere, there is a decrease in NOx in all four latitude bins immediately below the tropopause. With further descent there is a marked increase in the middle two latitude bins, spanning 45ø-75øN, with a slight increase for the 30ø-45øN range, and little change with altitude for the 75 ø-90ON range. For the two latitude ranges with data in the lowest altitude range, there is a marked decrease in NOx at the lowest level. This suggests the presence of a layer of NOx peaking at 2-3 km below the tropopause (see Figure 1 for the longitudes sampled). However, an examination of the individual flight profiles reveals that this is not a consistent feature and is primarily due to two events (discussed later) during the 10 flights. Thus the appearance of a layer at 2-3 km below the tropopause should be viewed as reflecting the occasional occurrence of relatively high values there, but no significance is attached to the exact shape, position, and latitudinal dimension of the apparent layer. Although the elevated NOx may be intermittent, it occurs frequently enough and with high enough values (at least in this sample) to have an impact on the means.   pptv) than for total NOx in AASE II (40-140 pptv). These differences are very striking, but it is not clear why they arise. One factor may be the difficulty in obtaining representative measurements for a species such as NOx which is relatively short-lived and has variable, localized sources. Also there is the possibility of systematic differences due to sampling different longitudes.

The Meridional Distribution of NO•
In contrast with this, the AASE II measurements are very similar in overall magnitude with those from AASE I [Carroll et al., 1990], where the high latitude (>59øN) median NOx was 40 pptv in the stratosphere, and 59 pptv in the troposphere.
The midlatitude (near 40øN) median NOx was 102 pptv in the stratosphere, and 56 pptv in the troposphere. Unlike the maximum at 6-7 km, the transition in NOx at the tropopause (Figure 2) is "smeared out" and is not readily apparent when altitude relative to the ground is used (Figure 3). There is enough variation in the height of the tropopause that the low values just below the tropopause are distributed over several bins of altitude above the ground, causing the transition to be lost in that representation. Figure 4 shows the meridional distribution of NOy relative to the height of the tropopause. There is little variation with latitude, and from 1 km below the tropopause to 3 km above, there is a uniform altitude gradient of-600 pptv/km. In the troposphere, there is a layer 2-3 km below the tropopause, similar to the apparent layer in NOx. Interestingly, the NOy layer does not completely disappear when the two high-NOx periods are excluded (not shown). The greater persistence of this feature for NOy may reflect its longer lifetime. The region of high NOy extends to lower altitudes over the pole which is due to the polluted air seen in a profile measured at the pole on 920314. Since there is little latitudinal gradient in NOy, the increase in NOx is due to a change in the NOy partitioning, one factor being the increased photolysis of HNO3 at lower latitudes. Over the pole at 2 km above the tropopause, NOx is 50 pptv and NOx/NOy is 3%. Toward 40øN, values approach 120 pptv and 6%.

Meridional Distributions of NOy and NOx/NOy
In the stratosphere at higher latitudes (50ø-90øN), the NOx/NOy ratio increases from -3% to -8% as the tropopause is approached from above. NOx is relatively constant at -70 pptv in the middle of this range of latitudes. One cause for the increase in the NOx/NOy ratio may be a slowing of the conversion of NOx to NOy as the tropopause is approached.
As the tropopause is approached, 03 and temperature decrease, and both of these trends are likely to slow the conversion of NO x to NOy, whether via gas phase conversion of NO 2 to

Summary
Meridional distributions of NOx in the lower stratosphere and upper troposphere are presented within the context provided by distributions of other species. In the lower stratosphere, there is no significant altitude gradient but a substantial latitude gradient (Figure 2b). In the upper troposphere, variability complicates the picture. For background conditions, that is, in the absence of recent pollution-laden convection (though not necessarily without such impact in the longer term), the pattern is similar to that in the lower stratosphere with little systematic variation with height, but a latitude gradient from 30øN to 90øN (Figure 2c). One difference is that there is a modest gradient at the tropopause, leading to smaller mixing ratios in the troposphere at a given latitude. In frontal regions (two cases), convection leads to higher values within a few km of the tropopause. Although sporadic, these values occurred with sufficient frequency and magnitude to have a significant impact on the mean values in AASE II (Figure 2b vs. Figure   2c).