The stratospheric sink of N2O has been well characterized and can be used to put constraints on the magnitude of the source fluxes. N2O is primarily removed by photolysis in the stratosphere with secondary contributions by its reaction with O(1D). Its absorption spectrum peaks near 185 nm, though the available photolysing radiation in the stratosphere shifts the region of peak photolysis to 195-205 nm. Maximum loss of N2O occurs in the tropics at an altitude of ~30 km and drops rapidly at higher latitudes. The global loss of N2O is about 13 Tg N/year. According to this loss and an observed increase of 4 Tg N/year in the total atmospheric burden, the global source of N2O must be ~17 Tg N/year.

Experimental and theoretical studies indicate that photolysis isotopically fractionates N2O and enriches the stratosphere in heavy N2O species. Some of these species are returned to the troposphere via stratospheric-tropospheric exchange processes, balancing the flux of light N2O from surface sources. They combine to produce global mean enrichments of S15N ~7%0 and S18O ~20%o in the troposphere.

The destruction of N2O and the subsequent enrichment of the heavy isotopologues can be described as a Rayleigh fractionation process, where each isotopologue is characterized by its own unique fractionation constant £. The fractionation follows the order I e4561 > I e4481 > I e5461 > I e4471. As the fractionation constants are dependent on wavelength and temperature, we expect to see e vary throughout the stratosphere. While measurements clearly show that e is non-constant, increasing from the lower to the upper stratosphere, its variability can be explained primarily through the mixing of air masses with different transport histories.

The understanding of the stratospheric sinks of N2O is essentially complete. There remain some outstanding questions regarding the variability in the ratio of fraction-ation constants throughout the stratosphere and the possibility of non-standard chemical sinks. These are likely to be quite small and have little influence on the vertical structure of N2O. Finally, it appears likely that the oxygen anomaly is primarily due to an isotopic transfer of heavy oxygen from ozone to N2O. These unknown components of the stratospheric sink are small compared with the much larger uncertainties in the individual source fluxes of N2O. Therefore, more measurements of the enrichment of N2O in the lower stratosphere will be useful to better constrain the isotopic flux to the troposphere, and in turn to better constrain individual source fluxes.

Was this article helpful?

0 0

Post a comment