Constraining the Nsub2/subOsub5/sub UV absorption cross section from spectroscopic trace gas measurements in the tropical mid-stratosphere

摘要

pstrongAbstract./strong The absorption cross section of Nsub2/subOsub5/sub, σsubNsub2/subOsub5/sub/sub(λ, iT/i), which is known from laboratory measurements with the uncertainty of a factor of 2 (Table 4-2 in (Jet Propulsion Laboratory) JPL-2011; the spread in laboratory data, however, points to an uncertainty in the range of 25 to 30%, 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 NOsub2/sub by limb scanning DOAS (differential optical absorption spectroscopy) measurements (Weidner et al., 2005; Kritten et al., 2010), supported by detailed photochemical modelling of NOsuby/sub (NOsubx/sub(= NO + NOsub2/sub) + NOsub3/sub + 2Nsub2/subOsub5/sub + ClONOsub2/sub + HOsub2/subNOsub2/sub + BrONOsub2/sub + HNOsub3/sub) photochemistry and a non-linear least square fitting of the model result to the NOsub2/sub observations. Simulations are initialised with Osub3/sub measured by direct sun observations, the NOsuby/sub partitioning from MIPAS-B (Michelson Interferometer for Passive Atmospheric Sounding a?? Balloon-borne version) observations in similar air masses at night-time, and all other relevant species from simulations of the SLIMCAT (Single Layer Isentropic Model of Chemistry And Transport) chemical transport model (CTM). Best agreement between the simulated and observed diurnal increase of NOsub2/sub is found if the σsubNsub2/subOsub5/sub/sub(λ, iT/i) is scaled by a factor of 1.6 ?± 0.8 in the UV-C (200a??260 nm) and by a factor of 0.9 ?± 0.26 in the UV-B/A (260a??350 nm), compared to current recommendations. As a consequence, at 30 km altitude, the Nsub2/subOsub5/sub lifetime against photolysis becomes a factor of 0.77 shorter at solar zenith angle (SZA) of 30° than using the recommended σsubNsub2/subOsub5/sub/sub(λ, iT/i), and stays more or less constant at SZAs of 60°. Our scaled Nsub2/subOsub5/sub photolysis frequency slightly reduces the lifetime (0.2a??0.6%) of ozone in the tropical mid- and upper stratosphere, but not to an extent to be important for global ozone./p.