Abstract
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.
Highlights
The NOx ozone loss cycle dominates catalytic ozone loss in the mid-stratosphere (25–45 km) and the ozone production via coupling with the HOx cycle below about 20 km (e.g. Brasseur and Solomon, 2005)
In the following we describe how the parameters s1 and s2 are retrieved from the measured and modelled field of NO2
After 10 iterations the global minimum is found at s1 = 1.6 and s2 = 0.9, i.e. our retrieval indicates a σN2O5 (λ, T ) a factor 1.6 larger in the UV-C spectral range and a factor 0.9 smaller in the UV-B/A spectral range as compared to the recommended value in JPL-2011, with retrieval uncertainties of ±0.80 and ±0.26 for s1 and s2, respectively
Summary
The NOx ozone loss cycle dominates catalytic ozone loss in the mid-stratosphere (25–45 km) and the ozone production via coupling with the HOx cycle below about 20 km (e.g. Brasseur and Solomon, 2005). Reaction (R13) is relevant for the daytime increase of NOx in the tropical mid-stratosphere, since the product of photolysis rate JN2O5 and concentration of N2O5 is the largest among the reservoir species under the given conditions (see Fig. 1). Time-dependent profiles of stratospheric NO2 are used, which were measured over north-eastern Brazil observations (O3, and a suite of NOy species) taken within the so-called tropical pipe over north-eastern Brazil during (5.1◦ S, 43.6◦ W) on 30 June 2005 (see Fig. 8 in Kritten et al, 2010). From modelled concentration field of NO2 as a function of time and height For this purpose σN2O5 (λ, T ) is scaled by two free parameters (denoted by s1 and s2) in the extreme. Further on in the study, the re-normalised NO2 field (shown in the lower panel of Fig. 4) is used, rather than the field inferred in Kritten et al (2010) (see Fig. 8 therein)
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