Articles | Volume 14, issue 18
Atmos. Chem. Phys., 14, 9555–9566, 2014
Atmos. Chem. Phys., 14, 9555–9566, 2014

Research article 16 Sep 2014

Research article | 16 Sep 2014

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

L. Kritten1,*, A. Butz2,*, M. P. Chipperfield3, M. Dorf4,*, S. Dhomse3, R. Hossaini3, H. Oelhaf2, C. Prados-Roman5,*, G. Wetzel2, and K. Pfeilsticker6 L. Kritten et al.
  • 1Institute for Space Sciences (WEW), Free University Berlin, Berlin, Germany
  • 2Karlsruhe Institute of Technology, IMK-ASF, Karlsruhe, Germany
  • 3Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds, UK
  • 4Max-Planck-Institute for Chemistry, Mainz, Germany
  • 5Atmospheric Chemistry and Climate Group, Institute of Physical Chemistry Rocasolano (CSIC), Madrid, Spain
  • 6Institute of Environmental Physics (IUP), University of Heidelberg, Heidelberg, Germany
  • *formerly at: Institute of Environmental Physics (IUP), University of Heidelberg, Heidelberg, Germany

Abstract. 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 (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 NO2 by limb scanning DOAS (differential optical absorption spectroscopy) measurements (Weidner et al., 2005; Kritten et al., 2010), supported by detailed photochemical modelling of NOy (NOx(= NO + NO2) + NO3 + 2N2O5 + ClONO2 + HO2NO2 + BrONO2 + HNO3) photochemistry and a non-linear least square fitting of the model result to the NO2 observations. Simulations are initialised with O3 measured by direct sun observations, the NOy partitioning from MIPAS-B (Michelson Interferometer for Passive Atmospheric Sounding – 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 NO2 is found if the σN2O5(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. As a 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.

Final-revised paper