Constraining the N2O5 UV absorption cross-section from spectroscopic trace gas measurements in the tropical mid-stratosphere.

The absorption cross-section of N₂O₅, σN₂O₅(λ, T), which is known from laboratory measurements with the uncertainty of a factor of 2 (Table 4-2 in JPL-2011, Sander et al., 2011), was investigated by balloon-borne observations of the relevant trace gases in the tropical mid-stratosphere. The method relies on the observation of the diurnal variation of NO₂ supported by detailed photochemical modelling of NO_y (NO_x(= NO + NO₂) + NO3 + 2N₂O₅ + ClONO₂ + HO₂NO₂ +BrONO₂ + HNO3) photochemistry. Simulations are initialised with O₃ measured by direct sun observations, the NOy partitioning from MIPAS-B (Michelson Interferometer for Passive Atmospheric Sounding-Balloon) observations in similar air masses at nighttime, and all other relevant species from simulations of the SLIMCAT chemical transport model (CTM). Best agreement between the simulated and observed diurnal increase of NO₂ is found if the sN₂O₅(λ, T) is scaled by a factor of 1.6 ± 0.8 in the UV-C (200-260 nm) and by a factor of 0.9 ± 0.26 in the UV-B/A (260-350 nm), compared to current recommendations. In consequence, at 30 km altitude, the N₂O₅ lifetime against photolysis becomes a factor of 0.77 shorter at solar zenith angle (SZA) of 30° than using the recommended σN₂O₅ (λ, T), and stays more or less constant at SZAs of 60°. Our scaled N₂O₅ photolysis frequency slightly reduces the lifetime (0.2-0.6%) of ozone in the tropical mid- and upper stratosphere, but not to an extent to be important for global ozone. [ABSTRACT FROM AUTHOR]