Abstract. The international research project RECONCILE has addressed central questions regarding polar ozone depletion, with the objective to quantify some of the most relevant yet still uncertain physical and chemical processes and thereby improve prognostic modelling capabilities to realistically predict the response of the ozone layer to climate change. This overview paper outlines the scope and the general approach of RECONCILE, and it provides a summary of observations and modelling in 2010 and 2011 that have generated an in many respects unprecedented dataset to study processes in the Arctic winter stratosphere. Principally, it summarises important outcomes of RECONCILE including (i) better constraints and enhanced consistency on the set of parameters governing catalytic ozone destruction cycles, (ii) a better understanding of the role of cold binary aerosols in heterogeneous chlorine activation, (iii) an improved scheme of polar stratospheric cloud (PSC) processes that includes heterogeneous nucleation of nitric acid trihydrate (NAT) and ice on non-volatile background aerosol leading to better model parameterisations with respect to denitrification, and (iv) long transient simulations with a chemistry-climate model (CCM) updated based on the results of RECONCILE that better reproduce past ozone trends in Antarctica and are deemed to produce more reliable predictions of future ozone trends. The process studies and the global simulations conducted in RECONCILE show that in the Arctic, ozone depletion uncertainties in the chemical and microphysical processes are now clearly smaller than the sensitivity to dynamic variability.

Abstract. We analyze polar stratospheric cloud (PSC) signatures in airborne MIPAS-STR (Michelson Interferometer for Passive Atmospheric Sounding – STRatospheric aircraft) observations in the spectral regions from 725 to 990 and 1150 to 1350 cm−1 under conditions suitable for the existence of nitric acid trihydrate (NAT) above northern Scandinavia on 11 December 2011. The high-resolution infrared limb emission spectra of MIPAS-STR show a characteristic “shoulder-like” signature in the spectral region around 820 cm−1, which is attributed to the ν2 symmetric deformation mode of NO3− in β-NAT. Using radiative transfer calculations involving Mie and T-Matrix methods, the spectral signatures of spherical and aspherical particles are simulated. The simulations are constrained using collocated in situ particle measurements. Simulations assuming highly aspherical spheroids with aspect ratios (AR) of 0.1 or 10.0 and a lognormal particle mode with a mode radius of 4.8 µm reproduce the observed spectra to a high degree. A smaller lognormal mode with a mode radius of 2.0 µm, which is also taken into account, plays only a minor role. Within the scenarios analyzed, the best overall agreement is found for elongated spheroids with AR  =  0.1. Simulations of spherical particles and spheroids with AR  =  0.5 and 2.0 return results very similar to each other and do not allow us to reproduce the signature around 820 cm−1. The observed “shoulder-like” signature is explained by the combination of the absorption/emission and scattering characteristics of large highly aspherical β-NAT particles. The size distribution supported by our results corresponds to ∼ 9 ppbv of gas-phase equivalent HNO3 at the flight altitude of ∼ 18.5 km. The results are compared with the size distributions derived from the in situ observations, a corresponding Chemical Lagrangian Model of the Stratosphere (CLaMS) simulation, and excess gas-phase HNO3 observed in a nitrification layer directly below the observed PSC. The presented results suggest that large highly aspherical β-NAT particles involved in denitrification of the polar stratosphere can be identified by means of passive infrared limb emission measurements.

Abstract.We report on the retrieval of PAN (CH 3 C(O)OONO 2 ) in the upper tropical troposphere from limb measurements by the remote-sensor MIPAS-STR on board the Russian high altitude research aircraft M55-Geophysica. The measurements were performed close to Araçatuba, Brazil, on 17 February 2005. The retrieval was made in the spectral range 775-820 cm −1 where PAN exhibits its strongest feature but also more than 10 species interfere. Especially trace gases such as CH 3 CCl 3 , CFC-113, CFC-11, and CFC-22, emitting also in spectrally broad not-resolved branches, make the processing of PAN prone to errors. Therefore, the selection of appropriate spectral windows, the separate retrieval of several interfering species and the careful handling of the water vapour profile are part of the study presented.The retrieved profile of PAN has a maximum of about 0.14 ppbv at 10 km altitude, slightly larger than the lowest reported values (<0.1 ppbv) and much lower than the highest reported in the literature (0.65 ppbv). Besides the NO y constituents measured by MIPAS-STR (HNO 3 , ClONO 2 , HO 2 NO 2 , PAN), the in situ instruments aboard the Geophysica provide simultaneous measurements of NO, NO 2 , and the sum NO y . Comparing the sum of in-situ and remotely derived NO+NO 2 +HNO 3 +ClONO 2 +HO 2 NO 2 +PAN with total Correspondence to: M. Höpfner (michael.hoepfner@imk.fzk.de) NO y a deficit of 30-40% (0.2-0.3 ppbv) in the troposphere remains unexplained whereas the values fit well in the stratosphere.

Abstract. Vertical redistribution of HNO 3 through large HNO 3 -containing particles associated with polar stratospheric clouds (PSCs) plays an important role in the chemistry of the Arctic winter stratosphere. During the RECON-CILE (Reconciliation of essential process parameters for an enhanced predictability of Arctic stratospheric ozone loss and its climate interactions) campaign, apparently very large NAT (nitric acid trihydrate) particles were observed by the airborne in situ probe FSSP-100 . Our analysis shows that the FSSP-100 observations associated with the flight on 25 January 2010 cannot easily be explained assuming compact spherical NAT particles due to much too short growing time at temperatures below the existence temperature of NAT (T NAT ). State-of-the-art simulations using CLaMS (Chemical Lagrangian Model of the Stratosphere; Grooß et al., 2014) suggest considerably smaller particles. We consider the hypothesis that the simulation reproduces the NAT particle masses in a realistic way, but that real NAT particles may have larger apparent sizes compared to compact spherical particles, e.g. due to non-compact morphology or aspheric shape. Our study focuses on the consequence that such particles would have reduced settling velocities compared to compact spheres, altering the vertical redistribution of HNO 3 . Utilising CLaMS simulations, we investigate the impact of reduced settling velocities of NAT particles on vertical HNO 3 redistribution and compare the results with observations of gas-phase HNO 3 by the airborne Fourier transform spectrometer MIPAS-STR associated with two REC-ONCILE flights. The MIPAS-STR observations confirm conditions consistent with denitrification by NAT particles for the flight on 25 January 2010 and show good agreement with the simulations within the limitations of the comparison. Best agreement is found if settling velocities between 100 and 50 % relative to compact spherical particles are considered (slight preference for the 70 % scenario). In contrast, relative settling velocities of 30 % result in too weak vertical HNO 3 redistribution. Sensitivity simulations considering temperature biases of ±1 K and multiplying the simulated nucleation rates by factors of 0.5 and 2.0 affect the comparisons to a similar extent, but result in no effective improvement compared to the reference scenario. Our results show that an accurate knowledge of the settling velocities of NAT particles is important for quantitative simulations of vertical HNO 3 redistribution.

In the winter of 1992/1993 the airborne Michelson interferometer for passive atmospheric sounding (MIPAS‐FT) was operated on board a German research aircraft (Transall C‐160) to record infrared emission spectra of the atmosphere inside and outside the Arctic vortex. Measurements were made during four campaigns between December 4, 1992, and March 29, 1993, in the European Arctic as well as over central and southern Europe (82°N–37.5°N). We present the retrieved zenith column amounts of the stratospheric trace gases ClONO2, HNO3, and O3 of this period. Inside the polar vortex, the column amounts of ClONO2 and HNO3 were considerably enhanced already in early December, up to 3.1×1015 cm−2 and 2.7×1016 cm−2, respectively. Around the end of January, low ClONO2 (1×1015 cm−2) and high HNO3 column amounts (up to 3.7×1016 cm−2) were observed inside the vortex, whereas a highly variable “collar” of ClONO2 had developed at the vortex edge. During March, after temperatures had been above the threshold for polar stratospheric clouds (PSCs) for several weeks, we measured lower HNO3 (below 2.5×1016 cm−2) and very high ClONO2 column amounts (up to 6×1015 cm−2) inside the vortex. Thus a major part of the reactive chlorine had been converted into ClONO2, and the potential for rapid ozone depletion was reduced markedly in the region observed. On March 10, when the polar vortex extended southward to the Mediterranean, ClONO2 column amounts as high as 4.6×1015 cm−2 were observed at 40°N. At the end of March, considerable amounts of ClONO2 (up to 3.4×1015 cm−2) were measured also far outside the vortex.