A study of the atmospheric photochemical loss of N₂O based on trace gas measurements

Abstract

Nitrous oxide (N₂O) plays an important role in ozone chemistry as well as in greenhouse warming. It is the source of NO_x radicals in the stratosphere which are the dominant catalysts for ozone depletion. Recently, doubts have been raised on the global N₂O budget. One approach to solve this problem has been the consideration of new mechanisms for atmospheric production and destruction of N₂O. In parallel, N₂O sinks have been constrained from observed tracer correlations in the lower stratosphere based on aircraft measurements, which are limited up to an altitude of only 20 km. We use vertical distributions of N₂O and other trace gases measured simultaneously from Hyderabad, India (17.5°N, 78.6°E) in 1987, 1990, 1994 and 1998 using balloon-borne cryogenic air samplers covering the altitude range of about 8-37 km to study these issues together with 2-D model simulations. The slopes of N₂O correlations with CH₄, CFC-12 and CFC-11 compare well with the model derived slopes, with exceptions in cases where dynamical perturbations are strong. Average N₂O lifetimes of 85±43 and 111±38 years have been estimated using the observed slopes and two sets of reference lifetimes for the correlated tracers. This average lifetime compares well within the spread, with the lifetime estimated from the sink of N₂O in the model, suggesting that the present estimate of the N₂O photochemical sink incorporated in the model is adequate.