HOCl chemistry in the Antarctic Stratospheric Vortex 2002, as observed with the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS)

Abstract

Abstract. In the 2002 Antarctic polar vortex enhanced HOCl mixing ratios were detected by the Michelson Interferometer for Passive Atmospheric Sounding both at altitudes of around 35 km (1000 K potential temperature), where HOCl abundances are ruled by gas phase chemistry and at around 18-24 km (475-625 K), which belongs to the altitude domain where heterogeneous chlorine chemistry is relevant. At altitudes of 33 to 40 km polar vortex HOCl mixing ratios were found to be around 0.14 ppbv as long as the polar vortex was intact, centered at the pole, and thus received relatively little sunlight. This is the altitude region where in midlatitudinal and tropic atmospheres peak HOCl mixing ratios significantly above 0.2 ppbv (in terms of daily mean values) are observed. After deformation and displacement of the polar vortex in the course of a major warming, ClO-rich vortex air was more exposed to sunlight, where enhanced HO x abundances led to largely increased HOCl mixing ratios (up to 0.3 ppbv), exceeding typical midlatitudinal and tropical amounts significantly. The HOCl increase was preceded by an increase of ClO. Model runs could reproduce these measurements only when the Stimpfle et al. (1979) rate constant for the reaction ClO+HO 2 →HOCl+O 2 was used but not with the current JPL recommendation. At an altitude of 24 km, HOCl mixing ratios of up to 0.15 ppbv were detected. This HOCl enhancement, which is already visible in 18 September data, is attributed to heterogeneous chemistry, which is in agreement with observations of polar stratospheric clouds. The measurements were compared to a model run where no polar stratospheric clouds appeared during the observation period. The fact that HOCl still was produced in the model run sugCorrespondence to: T. von Clarmann (thomas.clarmann@imk.fzk.de) gests that a significant part of HOCl was generated from ClO rather than directly via heterogeneous reaction. Excess ClO, lower ClONO 2 and earlier loss of HOCl in the measurements are attributed to ongoing heterogeneous chemistry which is not reproduced by the model. On 11 October, polar vortex mean daytime mixing ratios were only 0.03 ppbv.

Diurnal variation of stratospheric and lower mesospheric HOCl, ClO and HO<sub>2</sub> at the equator: comparison of 1-D model calculations with measurements by satellite instruments

“…Results from Stimpfle et al (1979) and Nickolaisen et al (2000) give the worst agreement with the observations. This result contradicts the study by Kovalenko et al (2007), which was done for lower altitudes (below 36 km) and is also not fully consistent with the results of von Clarmann et al (2012), whose global data covered all seasons but whose analysis was focused on altitudes below 45 km.…”

“…Our result contradicts the study of k 1 by Kovalenko et al (2007) which has been done for lower altitudes (below 36 km) and is also not fully consistent with the results of von Clarmann et al (2012), whose global data covered all seasons and a wide range of temperatures, but whose analysis was focused on altitudes below 45 km. The reaction rate k 1 is temperature dependent, so one may obtain different results at different altitudes.…”

“…The reaction rate k 1 is temperature dependent, so one may obtain different results at different altitudes. Note that the HOCl formation rate constants from JPL 2009 (Sander et al, 2009), assessed by von Clarmann et al (2012), are identical to those in JPL 2011 (Sander et al, 2011). Our results are consistent with the quantitative analysis of k 1 by Kuribayashi et al (2013).…”

“…Kovalenko et al (2007) reported that the value of k 1 published by Stimpfle et al (1979) is a factor of two higher than JPL 2006 recommendations (Sander et al, 2006) and is more consistent with their measurements. The conclusion of Kovalenko et al (2007), based on two balloon-borne instruments (FIRS-2, Far-InfraRed emission Spectrometer and MkIV, mid-infrared solar absorption spectrometer), has been confirmed by von Clarmann et al (2009). The rate constant, k 1 , has later been revised in JPL 2009 (Sander et al, 2009) and JPL 2011 (Sander et al, 2011) (JPL 2009 andJPL 2011 are identical).…”

“…Hypochlorous acid, HOCl, is considered to be a reservoir for active chlorine, ClO x , and odd hydrogen, HO x , in the stratosphere and the lower mesosphere (von Clarmann et al, 2012). It is produced primarily by the reaction of ClO and HO 2 (peroxy radical) and is destroyed mainly by photodissociation at wavelengths shorter than 420 nm which returns OH and Cl radicals (Hickson et al, 2007).…”