ALBERT

Description: Remote sensing measurements from the Cryogenic Infrared Spectrometers and Telescope for the Atmosphere – New Frontiers (CRISTA-NF) during a flight on 29 July 2006 are presented. This flight is part of the AMMA-SCOUT-O3 measurement campaign, where CRISTA-NF was deployed on the high-flying research aircraft M55-Geophysica. The flight path was located over Italy and the Mediterranean Sea and crossed over the subtropical jet twice. Measurements of temperature, and the volume mixing ratios of water vapor (H2O), ozone (O3), nitric acid (HNO3) and peroxyacetyl nitrate (PAN) are available with a vertical resolution of up to 500 m between about 6 to 21 km altitude. CRISTA-NF observes these trace gases simultaneously and provides a quasi-2-D view of the transition region between the troposphere and the stratosphere. The observation of these different trace gases allows to determine tropospheric and stratospheric air masses. As expected, higher abundances are found where the main source of the trace gases is located: in the stratosphere for O3 and in the troposphere for H2O and PAN. Tracer-tracer correlations between O3 and PAN are used to identify the mixed tropospheric and lowermost stratospheric air at the subtropical jet and around the thermal tropopause north of the jet. An intrusion of stratospheric air into the troposphere associated with the subtropical jet is found in the CRISTA-NF observations. The observations indicate that the intrusion is connected to a tropopause fold which is not resolved in the ECMWF analysis data. The intrusion was reproduced in a simulation with the Chemical Lagrangian Model of the Stratosphere (CLaMS). The CLaMS simulation shows, that the lowermost stratospheric air masses in the intrusion where transported along the the subtropical jet. The tropospheric air masses around the intrusion originate from the vicinity of the Asian monsoon anticyclone. This work discusses the nature of the observed processes at the subtropical jet based on the CRISTA-NF observations and the CLaMS simulation.

Description: The photolysis rate constant of dichlorine peroxide (ClOOCl, ClO dimer) JClOOCl is a critical parameter in catalytic cycles destroying ozone (O3) in the polar stratosphere. In the atmospherically relevant wavelength region (λ 〉 310 nm), significant discrepancies between laboratory measurements of ClOOCl absorption cross sections and spectra cause a large uncertainty in JClOOCl. Previous investigations of the consistency of published JClOOCl with atmospheric observations of chlorine monoxide (ClO) and ClOOCl have focused on the photochemical equilibrium between ClOOCl formation and photolysis, and thus could only constrain the ratio of JClOOCl over the ClOOCl formation rate constant krec. Here, we constrain the atmospherically effective JClOOCl independent of krec, using ClO measured in the same air masses before and directly after sunrise during an aircraft flight that was part of the RECONCILE field campaign in the winter 2010 from Kiruna, Sweden. Over sunrise, when the ClO/ClOOCl system comes out of thermal equilibrium and the influence of the ClO recombination reaction is negligible, the increase in ClO concentrations is significantly faster than expected from JClOOCl based on the absorption spectrum proposed by Pope et al. (2007), but does not warrant cross sections larger than recently published values by Papanastasiou et al. (2009). In particular, the existence of a significant ClOOCl absorption band longwards of 420 nm is not supported by our observations. The observed night-time ClO would not be consistent with a ClO/ClOOCl thermal equilibrium constant significantly higher than the one proposed by Plenge et al. (2005).

Description: Chlorine activation in the Arctic is investigated by examining different parameterizations for uptake coefficients on stratospheric aerosols, high-resolution in-situ measurements and vortex-wide satellite observations. The parameterizations for heterogeneous chemistry on liquid aerosols are most sensitive to temperature with the reaction rates doubling for every 1 K increase in temperature. However, differences between the currently available parameterizations are negligible. For Nitric Acid Trihydrate particles (NAT) the major factors of uncertainty are the number density of nucleated particles and different parameterizations for heterogeneous chemistry. These two factors induce an uncertainty that covers several orders of magnitude on the reaction rate. Nonetheless, since predicted reaction rates on liquid aerosols always exceed those on NAT, the overall uncertainty for chlorine activation is small. In-situ observations of ClOx from Arctic winters in 2005 and 2010 are used to evaluate the heterogeneous chemistry parameterizations. The conditions for these measurements proved to be very different between those two winters with HCl being the limiting reacting partner for the 2005 measurements and ClONO2 for the 2010 measurements. Modeled levels of chlorine activation are in very good agreement with the in-situ observations and the surface area provided by Polar Stratospheric Clouds (PSCs) has only a limited impact on modeled chlorine activation. This indicates that the parameterizations give a good representation of the processes in the atmosphere. Back-trajectories started on the location of the observations in 2005 indicate temperatures on the threshold for PSC formation, hence the surface area is mainly provided by the background aerosol. Still, the model shows additional chlorine activation during this time-frame, providing cautionary evidence for chlorine activation even in the absence of PSCs. Vortex-averaged satellite observations by the MLS instrument also show no definite connection between chlorine activation and PSC formation. The inter -and intra-annual variability of vortex-average HCl and HNO3 based on MLS observations is examined for the Arctic winters 2004/2005 to 2010/2011. These observations show that removal of HCl and HNO3 from the gas-phase are not correlated. HNO3 loss exhibits great inter-annual variability depending on prevailing temperatures while HCl loss is continuous through December without considerable inter- or intra-annual variability. Only the recovery of HCl in late winter depends on the level of denitrification. Hence, the occurrence of HNO3 containing PSC particles does not seem to have a significant effect on the speed of initial chlorine activation on a vortex-wide scale.

Description: Significant reductions in stratospheric ozone occur inside the polar vortices each spring when chlorine radicals produced by heterogeneous reactions on cold particle surfaces in winter destroy ozone mainly in two catalytic cycles, the ClO dimer cycle and the ClO/BrO cycle. Chlorofluorocarbons (CFCs), which are responsible for most of the chlorine currently present in the stratosphere, have been banned by the Montreal Protocol and its amendments, and the ozone layer is predicted to recover to 1980 levels within the next few decades. During the same period, however, climate change is expected to alter the temperature, circulation patterns and chemical composition in the stratosphere, and possible geo-engineering ventures to mitigate climate change may lead to additional changes. To realistically predict the response of the ozone layer to such influences requires the correct representation of all relevant processes. The European project RECONCILE has comprehensively addressed remaining questions in the context of polar ozone depletion, with the objective to quantify the rates of some of the most relevant, yet still uncertain physical and chemical processes. To this end RECONCILE used a broad approach of laboratory experiments, two field missions in the Arctic winter 2009/10 employing the high altitude research aircraft M55-Geophysica and an extensive match ozone sonde campaign, as well as microphysical and chemical transport modelling and data assimilation. Some of the main outcomes of RECONCILE are as follows: (1) vortex meteorology: the 2009/10 Arctic winter was unusually cold at stratospheric levels during the six-week period from mid-December 2009 until the end of January 2010, with reduced transport and mixing across the polar vortex edge; polar vortex stability and how it is influenced by dynamic processes in the troposphere has led to unprecedented, synoptic-scale stratospheric regions with temperatures below the frost point; in these regions stratospheric ice clouds have been observed, extending over 〉106km2 during more than 3 weeks. (2) Particle microphysics: heterogeneous nucleation of nitric acid trihydrate (NAT) particles in the absence of ice has been unambiguously demonstrated; conversely, the synoptic scale ice clouds also appear to nucleate heterogeneously; a variety of possible heterogeneous nuclei has been characterised by chemical analysis of the non-volatile fraction of the background aerosol; substantial formation of solid particles and denitrification via their sedimentation has been observed and model parameterizations have been improved. (3) Chemistry: strong evidence has been found for significant chlorine activation not only on polar stratospheric clouds (PSCs) but also on cold binary aerosol; laboratory experiments and field data on the ClOOCl photolysis rate and other kinetic parameters have been shown to be consistent with an adequate degree of certainty; no evidence has been found that would support the existence of yet unknown chemical mechanisms making a significant contribution to polar ozone loss. (4) Global modelling: results from process studies have been implemented in a prognostic chemistry climate model (CCM); simulations with improved parameterisations of processes relevant for polar ozone depletion are evaluated against satellite data and other long term records using data assimilation and detrended fluctuation analysis. Finally, measurements and process studies within RECONCILE were also applied to the winter 2010/11, when special meteorological conditions led to the highest chemical ozone loss ever observed in the Arctic. In addition to quantifying the 2010/11 ozone loss and to understand its causes including possible connections to climate change, its impacts were addressed, such as changes in surface ultraviolet (UV) radiation in the densely populated northern mid-latitudes.

Description: Remote sensing measurements from the Cryogenic Infrared Spectrometers and Telescope for the Atmosphere – New Frontiers (CRISTA-NF) during a flight on 29 July 2006 are presented. This flight is part of the AMMA-SCOUT-O3 measurement campaign, where CRISTA-NF was deployed on the high-flying research aircraft M55-Geophysica. The flight path was located over Italy and the Mediterranean Sea and crossed over the subtropical jet twice. Measurements of temperature, and the volume mixing ratios of water vapor (H2O), ozone (O3), nitric acid (HNO3) and peroxyacetyl nitrate (PAN) are available with a vertical resolution of up to 500 m between about 6 to 21 km altitude. CRISTA-NF observes these trace gases simultaneously and provides a quasi-2D view of the transition region between the troposphere and the stratosphere. The observation of these different trace gases allows to determine the origin of air masses in the stratosphere or troposphere. As expected, higher abundances are found where the main source of the trace gases is located: in the stratosphere for O3 and in the troposphere for H2O and PAN. Tracer-tracer correlations between O3 and PAN are used to identify mixed tropospheric and lowermost stratospheric air at the subtropical jet and around the thermal tropopause north of the jet. An intrusion of stratospheric air into the troposphere associated with the subtropical jet is found in the CRISTA-NF observations. The observations indicate that the intrusion is connected to a tropopause fold which is not resolved in the ECMWF analysis data. The intrusion was reproduced in a simulation with the Chemical Lagrangian Model of the Stratosphere (CLaMS). This work discusses the nature of the observed processes at the subtropical jet based on the CRISTA-NF observations and the CLaMS simulation.

Description: Stratospheric chemistry and denitrification are simulated for the Arctic winter 2009/2010 with the Lagrangian Chemistry and Transport Model ATLAS. A number of sensitivity runs is used to explore the impact of uncertainties in chlorine activation and denitrification on the model results. In particular, the efficiency of chlorine activation on different types of liquid aerosol versus activation on nitric acid trihydrate clouds is examined. Additionally, the impact of changes in reaction rate coefficients, in the particle number density of polar stratospheric clouds, in supersaturation, temperature or the extent of denitrification is investigated. Results are compared to satellite measurements of MLS and ACE-FTS and to in-situ measurements onboard the Geophysica aircraft during the RECONCILE measurement campaign. It is shown that even large changes in the underlying assumptions have only a small impact on the modeled ozone loss, even though they can cause considerable differences in chemical evolution and denitrification. In addition, it is shown that chlorine activation on liquid aerosols alone is able to explain the observed magnitude and morphology of the mixing ratios of active chlorine, reservoir gases and ozone.

Description: Chlorine activation in the Arctic is evaluated by examining the different parameterizations for uptake coefficients on stratospheric aerosols, high-resolution in-situ measurements and vortex-wide satellite observations. The parameterizations for heterogeneous chemistry on liquid aerosols are most sensitive to temperature with the reaction rates doubling for each Kelvin increase in temperature. However, differences between the parameterizations are negligible. For Nitric Acid Trihydrate particles (NAT) the major factors of uncertainty are the number density of nucleated particles and different parameterization choices. These two factors induce an uncertainty that covers several orders of magnitude on the reaction rate. But as predicted reaction rates on liquid aerosols always exceed those on NAT the overall uncertainty is small. In-situ observations of ClOx from Arctic winters in 2005 and 2010 are used to validate the heterogeneous chemistry parameterizations. The ambient conditions for these measurements proved to be very different between those two winters with HCl being the limiting reacting partner for the 2005 measurements and ClONO2 for the 2010 measurements. Modeled levels of chlorine activation are in very good agreement with the in-situ observations and the surface area provided by Polar Stratospheric Clouds (PSCs) has only a limited impact on modeled chlorine activation. This indicates that the parameterizations give a good representation of the processes in the atmosphere. Back-trajectories started on the location of the observations in 2005 indicate temperatures on the threshold for PSC formation, hence the surface area is mainly provided by the background aerosol. Still, the model shows additional chlorine activation during this time-frame, providing cautionary evidence for chlorine activation even in the absence of PSCs. Vortex-averaged satellite observations also show no definite connection between chlorine activation and PSC formation. The inter- and intra-annual variability of vortex-average HCl and HNO3 based on MLS observations is examined for the Arctic winters 2004/2005 to 2010/2011. These observations show that removal of HCl and HNO3 from the gas-phase are not correlated. HNO3 loss exhibits great inter-annual variability depending on prevailing temperatures while HCl loss is continuous through December without considerable inter- or intra-annual variability. Only the recovery of HCl in late in winter depends on the level of denitrification. Hence, the occurrence of HNO3 containing PSC particles does not seem to have a significant effect on the speed of initial chlorine activation on a vortex-wide scale.

Description: The Cryogenic Infrared Spectrometers and Telescope for the Atmosphere – New Frontiers (CRISTA-NF), an airborne infrared limb-sounder, was operated aboard the high-flying Russian research aircraft M55-Geophysica during the Arctic RECONCILE campaign from January to March 2010. This paper describes the calibration process of the instrument and the retrieval algorithm employed and then proceeds to present retrieved trace gas volume mixing ratio cross-sections for one specific flight in this campaign. We are able to resolve the uppermost troposphere/lower stratosphere for several trace gas species for several kilometres below the flight altitude (16 to 19 km) with an unprecedented vertical resolution of 400 to 500 m for the limb-sounding technique. The instrument points sideways with respect to the flight direction. Therefore, the observations are also characterised by a rather high horizontal sampling along the flight track, which provides a full vertical profile every ≈15 km. Assembling the vertical trace gas profiles derived from CRISTA-NF measurements to cross-sections shows filaments of vortex and extra-vortex air masses in the vicinity of the polar vortex. During this campaign, the M55-Geophysica carried further instruments enabling trace gas volume mixing ratios derived from CRISTA-NF to be validated by comparing them with measurements by the in situ instruments HAGAR and FOZAN and observations by MIPAS-STR. This validation suggests that the retrieved trace gas volume mixing ratios are both qualitatively and quantitatively reliable.