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

Kritten, L. and Butz, A. and Chipperfield, M. P. and Dorf, M. and Dhomse, S. and Hossaini, R. and Oelhaf, H. and Prados-Roman, C. and Wetzel, G. and Pfeilsticker, K. (2014) Constraining the N2O5 UV absorption cross section from spectroscopic trace gas measurements in the tropical mid-stratosphere. Atmospheric Chemistry and Physics, 14 (18). pp. 9555-9566. ISSN 1680-7316

Abstract

The absorption cross section of N2O5, sigma N2O5(lambda, 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 + 2N(2)O(5) + ClONO2 + HO2NO2 + BrONO2 + HNO3) photochemistry and a non-linear least square fitting of the model result to the NO2 observations. Simulations are initialised with O-3 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 sigma N2O5(lambda, 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 degrees than using the recommended sigma N2O5(lambda, T), and stays more or less constant at SZAs of 60 degrees. 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.