[N2OJa 1I1d1

where a(l) is the absorption cross section at wavelength l, I(l) is the spectrum of the photolysing radiation or actinic flux and [N2O] is the local concentration of N2O. The actinic flux is a measure of the direct, scattered and reflected radiation, and varies significantly with time and location. This produces considerable variation in the photolysis rate. It is not surprising then that the N2O photolysis rate peaks near the equator where the solar radiation is greatest. In the remainder of the section we briefly comment on the factors that determine the photolysis rate in Eq. 14.4.

Nitrous oxide absorption spectrum

The absorption spectrum of N2O over relevant wavelengths in the stratosphere is shown in Fig. 14.1. The absorption between 174 and 320 nm peaks near 180 nm and falls to near zero at 260 nm (Johnston and Selwyn, 1975; Selwyn et al., 1977). The spectrum is characterized by a featureless absorption continuum that underlies a series of structured vibrational bands whose peak-to-trough intensities increase with both energy and temperature. The continuum is produced by transitions from the ground electronic state X(1Z+) to the repulsive excited electronic state B(1A). Banded structure arises from transitions from a vibrationally excited ground state to the X(1Z-) state. Although this transition would be normally

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1.15

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1.15

180 182 184 Wavelength (nm)

Fig. 14.1. Experimental absorption spectrum of nitrous oxide (N2O) showing structure at high energies. (From Selwyn et al., 1977. Reprinted with permission.)

180 182 184 Wavelength (nm)

Fig. 14.1. Experimental absorption spectrum of nitrous oxide (N2O) showing structure at high energies. (From Selwyn et al., 1977. Reprinted with permission.)

weak, in both states the N2O molecule is bent, and this enhances the transition due to quantum mechanical effects. As the temperature increases the vibrationally bent modes become more populated and the intensity of the absorption increases, especially towards the low energy side of the spectrum. Between 151 and 485 K, absorption cross sections increase by a factor of 2 at 188 nm and by a factor of 1.3 at 172 nm (Selwyn and Johnston, 1981). The temperature-dependent UV absorption spectrum can be expressed at low spectral resolution by:

where l is wavelength (nm), T temperature (K), and the coefficients are:

A3 = 4.301146 x 10-2 B3 = 1.111572 x 10-2 A4 = -1.777846 x 10-4 B4 = -1.881058 x 10-5 (Selwyn et al., 1977).

The function is plotted in Fig. 14.2 over a range of temperatures spanning the stratosphere. The greatest relative changes in absorption intensity occur at the red end of the spectrum where photolysis is strongest. At 200 nm near the photolysis peak, the increase in the cross section is about 20%. Since temperatures increase with altitude in the stratosphere, absorption intensities will be larger in the upper stratosphere.

Opacity of the stratosphere

Although N2O absorbs UV radiation most efficiently at wavelengths near 180 nm, the greatest rate of N2O loss is near 200 nm. This is due to the opacity of the stratosphere. Molecular oxygen is an efficient absorber of radiation at wavelengths where N2O absorption is greatest. Due to the large concentration of oxygen, the opacity of the stratosphere is high between 130 and 200 nm (Schumann-Runge system) and low between 200 and 300 nm (Herzberg continuum). Between 180 and 195 nm, the Schumann-Runge system consists of absorption bands. Between these bands the opacity is modest and radiation is transmitted. Ozone in turn is a strong absorber of radiation between 200 and 300 nm (Hartley bands) and prevents harmful radi

175 185 195 205 215 225

Wavelength (nm)

Fig. 14.2. Absorption spectra of nitrous oxide (N2O) using polynomial fits of Selwyn and Johnston (1981) at stratospherically relevant temperatures. The intensity of the absorption increases at higher temperature representing the lower stratosphere.

175 185 195 205 215 225

Wavelength (nm)

Fig. 14.2. Absorption spectra of nitrous oxide (N2O) using polynomial fits of Selwyn and Johnston (1981) at stratospherically relevant temperatures. The intensity of the absorption increases at higher temperature representing the lower stratosphere.

ation from reaching the troposphere. This prevents the photolysation of N2O in the troposphere.

Thus over the wavelengths that N2O absorbs most effectively, radiation can penetrate only through a small window centred about 200 nm and through some smaller windows within the Schumann-Runge band system. Hence, when the absorption spectrum of N2O is convolved with the actinic flux of the stratosphere, the resulting spectrum peaks near 200 nm (Fig. 14.3).

Photolytic loss rates

The final factor contributing to the photolysis rate is the distribution of N2O throughout the stratosphere. The concentration of N2O decreases with altitude above the troposphere, falling to near zero at ~50 km (Toyoda et al., 2004). Throughout the stratosphere, concentrations are highest in the tropics where strong tropical convection sweeps gas-rich air from the troposphere to higher altitudes (Johnston et al., 1979; Minschwaner et al., 1993).

Photolysis rates peak at altitudes between 30 and 35 km at noon when actinic flux is highest (Fig. 14.4). Global annual loss rates are estimated by integrating local rates over the whole stratosphere and for all seasons. Estimates of the annual loss rate range from 12.2 to 13.1 Tg N/year, which require an instantaneous global atmospheric lifetime of ~120 years for current N2O burdens (Table 14.1). This is close to its steady-state lifetime since the ratio between its stratospheric and tropospheric abundances is relatively constant.

Uncertainties in the loss rate come from a variety of sources. The Schumann-Runge bands of oxygen are complex and finely structured, and thousands of overlapping lines make it difficult to accurately specify the actinic flux in the atmospheric layers where N2O is photolyzed. High spectral resolution modelling by Minschwaner et al. (1992, 1993) has reduced the errors of the Schumann-Runge cross sections between 175 and 210 nm to 15%. Other uncertainties include the global distribution of N2O concentrations, absolute solar irradiance, the

180 190 200 210 220 230 240 Wavelength (nm)

Fig. 14.3. Calculated photolysis rates of nitrous oxide (N2O) in units of molecules/cm3/s. The rates are from a model run at 5° latitude, at noon, during equinox. (From Minschwaner et al., 1 993. Reproduced by permission of American Geophysical Union.)

180 190 200 210 220 230 240 Wavelength (nm)

Fig. 14.3. Calculated photolysis rates of nitrous oxide (N2O) in units of molecules/cm3/s. The rates are from a model run at 5° latitude, at noon, during equinox. (From Minschwaner et al., 1 993. Reproduced by permission of American Geophysical Union.)

0 T3

Fig. 14.4. Total loss rates for nitrous oxide (N2O) including contributions from both photolysis and photo-oxidation. The rates are diurnal averages with units of 102 molecules/cm3/s. (From Minschwaner et al., 1 993. Reproduced by permission of American Geophysical Union.)

Fig. 14.4. Total loss rates for nitrous oxide (N2O) including contributions from both photolysis and photo-oxidation. The rates are diurnal averages with units of 102 molecules/cm3/s. (From Minschwaner et al., 1 993. Reproduced by permission of American Geophysical Union.)

cross sections in the Herzberg continuum and the absorption cross sections of N2O. These limit the certainty of the global N2O loss rate to about 20-30% (Minschwaner et al., 1993; McLinden et al., 2003).

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