Info

100 200 300

N20 Mixing Ratio, ppb

Latitude

FIGURE 5.6 (a) Vertical profiles of N20 over the tropics at equinox circa 1980. Circles denote balloonborne 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 diurnally averaged loss rate for N20 (in units of 1012 molecules cm"3 s 1) as a function of altitude and latitude, at equinox. The loss rate includes both photolysis and reaction with O('D) (Minschwaner et al. 1993).

altitude, largely because [M] is decreasing. Because the 03 profile eventually decreases with altitude, the concentration of 0(' D) reaches a maximum at a certain altitude. Also, since the N20 concentration is continually decreasing with altitude, its rate of destruction, 7n2o [N20], reaches a maximum at a certain altitude even though the light intensity is stronger at higher altitudes. Figure 5.6a shows vertical profiles of the N20 mixing ratio in the tropics. The circles denote balloonborne measurements at 9°N and 5°S; the squares represent aircraft measurements between 1.6°S and 9.9°N. The dashed curve is the average of satellite measurements at 5°N, equinox, between 1979 and 1981. Figure 5.6b shows the diurnally averaged 1980 loss rate for N20 (molecules cm"3 s"1) as a function of altitude and latitude for equinox calculated with a photochemical model (Minschwaner et al. 1993). The loss rate includes N20 photolysis and reaction with 0(1D). As seen from Figure 5.6, the global loss of N20 occurs mainly at latitudes between the equator and 30°. Also, N20 loss rates are largest in the 25-35 km altitude range.

The fractional yield of NO from N20 at any altitude is the ratio of the number of molecules of NO formed to the total molecules of N20 reacted:

The stratospheric O('D) concentration at any altitude is controlled by the source from 03 photolysis and the sink by quenching to ground-state atomic oxygen

so that the O('D) steady-state concentration is given by

For example, at 30km and 60 = 45°,./'n2o = 5 x 10~8 s"1 andj0,~>o('D) = 15 x 10~5 s-1. With £4 = 3.2 x 10~n cm3 molecule-1 s-1, the instantaneous steady-state concentration of 0(1D) from (5.19) at 30km and 0O = 45° is ~ 45 molecules cm"3. The NO yield under these conditions from (5.18) is 0.11 molecules of NO formed per molecule of N20 removed. The NO yield at any altitude would require using 24-h averages of the two j values. The fractional yield of NO from N20 loss varies with altitude and latitude; yet, the ratio of NOy (NO + N02 + all products of NOt oxidation) to N20 is nearly linear with altitudes (until NOy reaches its peak), suggesting a nearly constant yield with altitude and latitude (Figure 5.7). Transport smoothes out the variations of the fractional yield with

N20, ppb

FIGURE 5.7 Stratospheric NOv mixing ratio versus N20 mixing ratio observed by balloonborne in situ measurements at 44°N during October and November 1994 (WMO 1998). Solid line is linear fit to the data. Points labeled "AER" are results of a stratospheric two-dimensional (2D) model. [Adapted from Kondo et al. (1996).]

N20, ppb

FIGURE 5.7 Stratospheric NOv mixing ratio versus N20 mixing ratio observed by balloonborne in situ measurements at 44°N during October and November 1994 (WMO 1998). Solid line is linear fit to the data. Points labeled "AER" are results of a stratospheric two-dimensional (2D) model. [Adapted from Kondo et al. (1996).]

FIGURE 5.8 The NO, chemical family in relation to stratospheric NOA cycle 1.

altitudes and latitude. In the uppermost stratosphere, the reaction N + NO —> N2 + O converts NO back to N2; this is the explanation for the lack of adherence of the data to the straight lines in Figure 5.7.

Consider the following cycle involving NO, that converts odd oxygen (O3 + O) into even oxygen (02):

Was this article helpful?

0 0
How to Improve Your Memory

How to Improve Your Memory

Stop Forgetting and Start Remembering...Improve Your Memory In No Time! Don't waste your time and money on fancy tactics and overpriced

Get My Free Ebook


Post a comment