Abstract
Abstract. The diurnal variation of HOCl and the related species ClO, HO2 and HCl measured by satellites has been compared with the results of a one-dimensional photochemical model. The study compares the data from various limb-viewing instruments with model simulations from the middle stratosphere to the lower mesosphere. Data from three sub-millimetre instruments and two infrared spectrometers are used, namely from the Sub-Millimetre Radiometer (SMR) on board Odin, the Microwave Limb Sounder (MLS) on board Aura, the Superconducting Submillimeter-wave Limb-Emission Sounder (SMILES) on the International Space Station, the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board ENVISAT, and the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) on board SCISAT. Inter-comparison of the measurements from instruments on sun-synchronous satellites (SMR, MLS, MIPAS) and measurements from solar occultation instruments (ACE-FTS) is challenging since the measurements correspond to different solar zenith angles (or local times). However, using a model which covers all solar zenith angles and data from the SMILES instrument which measured at all local times over a period of several months provides the possibility to verify the model and to indirectly compare the diurnally variable species. The satellite data were averaged for latitudes of 20° S to 20° N for the SMILES observation period from November 2009 to April 2010 and were compared at three altitudes: 35, 45 and 55 km. Besides presenting the SMILES data, the study also shows a first comparison of the latest MLS data (version 3.3) of HOCl, ClO, and HO2 with other satellite observations, as well as a first evaluation of HO2 observations made by Odin/SMR. The MISU-1D model has been carefully initialised and run for conditions and locations of the observations. The diurnal cycle features for the species investigated here are generally well reproduced by the model. The satellite observations and the model agree well in terms of absolute mixing ratios. The differences between the day and night values of the model are in good agreement with the observations although the amplitude of the HO2 diurnal variation is 10–20% lower in the model than in the observations. In particular, the data offered the opportunity to study the reaction ClO+HO2 → HOCl+O2 in the lower mesosphere at 55 km. At this altitude the HOCl night-time variation depends only on this reaction. The result of this analysis points towards a value of the rate constant within the range of the JPL 2006 recommendation and the upper uncertainty limit of the JPL 2011 recommendation at 55 km.
Highlights
Hypochlorous acid, HOCl, is considered to be a reservoir for active chlorine, ClOx, and odd hydrogen, HOx, in the stratosphere and the lower mesosphere
HO2 data from Sub-Millimetre Radiometer (SMR) are averaged over the period October 2003 to October 2004 and HOCl data from Michelson Interferometer for Passive Atmosphere Sounding (MIPAS) are averaged from November 2003 to April 2004, as these data are not available for the period November 2009 to April 2010
In order to remove the offset between the different datasets and to investigate the amplitude of the diurnal variations, the mean of the night-time or morning volume mixing ratios at the respective levels is subtracted from the observations at other solar zenith angles
Summary
Hypochlorous acid, HOCl, is considered to be a reservoir for active chlorine, ClOx, and odd hydrogen, HOx, in the stratosphere and the lower mesosphere (von Clarmann et al, 2012). It is produced primarily by the reaction of ClO and HO2 (peroxy radical) and is destroyed mainly by photodissociation at wavelengths shorter than 420 nm which returns OH and Cl radicals (Hickson et al, 2007). The Stimpfle et al (1979) formula for calculation of the rate constant is based on non-Arrhenius behaviour of the rate coefficient with strong negative temperature dependency, while JPL 2006 (Sander et al, 2006) to JPL 2011 (Sander et al, 2011) use a standard Arrhenius expression
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