Remote‐sensing measurements in the polar vortex: Comparison to in situ observations and implications for the simultaneous retrievals and analysis of the NO₂ and OClO species

Abstract

[1] Nighttime remote-sensing balloon observations conducted by the SALOMON instrument in the arctic polar vortex in January 2006 reveal high amounts of stratospheric NO 2 in the lower stratosphere similarly to previously published profiles. NO 2 concentration enhancements are also present in the vertical profiles observed by the GOMOS instrument on board the Envisat satellite and obtained coincidently to the balloon measurements. Such quantities are not present in in situ observations obtained by the SPIRALE instrument in similar geophysical conditions. While OClO amounts are acceptably reproduced by Chemistry Transport Model (CTM) calculations, NO 2 simulated values are well below the observed quantities. The examination of the slant column densities of NO 2 obtained at float altitude highlights unexpected strong enhancements with respect to the elevation angle and displacement of the balloon. It is shown that these fluctuations result from NO 2 spatial inhomogeneities located above the balloon float altitude. Potential vorticity maps reveal the presence of midlatitude NO 2 -rich air in the upper stratosphere or lower mesosphere as a result of the perturbed dynamical situation of the vortex. The presence of spatial inhomogeneities crossed by the lines of sight leads to artificial high concentration values of NO 2 in the vertical profile retrieved from the slant column densities assuming spatial homogeneity. A direct implication is that the differences previously observed between measurements of NO 2 and OClO and model results are probably mostly due to the improper inversion of NO 2 in the presence of perturbed dynamical conditions or when mesospheric NO x production events occur. The dynamical situation will have to be systematically analyzed in future studies involving remote-sensing observations.Citation: Berthet, G., et al. (2007), Remote-sensing measurements in the polar vortex: Comparison to in situ observations and implications for the simultaneous retrievals and analysis of the NO 2 and OClO species,

Stratospheric aerosols from the Sarychev volcano eruption in the 2009 Arctic summer

“…Another explanation for the differences between both datasets could be related to the issue of comparing in situ data (the aerosol counter) with remote-sensing observations obtained by integrating over tens of kilometres (Micro-RADIBAL). Even in the case of datasets obtained simultaneously, such comparisons are likely to be flawed when the targeted stratospheric compound is spatially variable along the instrument line of sight, as already investigated for stratospheric chemical species (Berthet et al, 2007). The effect of comparing these different observation techniques is more apparent when observations are close to the top of the plume around 18 km, where the sulphate aerosols are sparse and not continuously detected.…”

Validation of NO<sub>2</sub> and NO from the Atmospheric Chemistry Experiment (ACE)

“…Payan et al, 1999;Rivière et al, 2002) in contradiction with our current knowledge of polar chemistry. As demonstrated by Berthet et al (2007), such non-zero values can be attributed to effects of NO 2 local inhomogeneities present at higher altitudes along the lines-of-sight of these instruments and mainly resulting from perturbed dynamical situations. In such cases, the validity of the spatial homogeneity hypothesis inherent in remotesensing methods can be ruled out, consequently affecting the retrievals of the lower part of the vertical profile.…”

“…Above 23.6 km, NO 2 concentrations measured by SPIRALE sharply increase and the disagreement between both instruments is reduced to less than 50%. Note that the REPROBUS CTM simulates a profile with a similar gradient above 23.6 km (Berthet et al, 2007). The latter is more precise so it is used in the comparisons.…”

OClO slant column densities derived from GOMOS averaged transmittance measurements

“…The flight occurred in January 2006 in Kiruna and the measurements were done during the balloon ascent inside the polar vortex. The OClO concentration profile from the SALOMON measurements has been studied by comparisons with the results of the model described in Berthet et al (2007). In particular, they have shown that the OClO product from SALOMON is in acceptable agreement with results from chemistry transport model calculations.…”

“…A few studies have covered the subject (Riviere et al, 2003, Riviere et al, 2004, Berthet et al, 2007, Tetard et al, 2009 but all have concluded that uncertainties in the understanding of these interactions persist. It is therefore critical to monitor simultaneously OClO and NO 2 .…”