F. Stroh

An overview of the SOLVE/THESEO 2000 campaign

[1]xa0Between November 1999 and April 2000, two major field experiments, the Stratospheric Aerosol and Gas Experiment (SAGE) III Ozone Loss and Validation Experiment (SOLVE) and the Third European Stratospheric Experiment on Ozone (THESEO 2000), collaborated to form the largest field campaign yet mounted to study Arctic ozone loss. This international campaign involved more than 500 scientists from over 20 countries. These scientists made measurements across the high and middle latitudes of the Northern Hemisphere. The main scientific aims of SOLVE/THESEO 2000 were to study (1) the processes leading to ozone loss in the Arctic vortex and (2) the effect on ozone amounts over northern midlatitudes. The campaign included satellites, research balloons, six aircraft, ground stations, and scores of ozonesondes. Campaign activities were principally conducted in three intensive measurement phases centered on early December 1999, late January 2000, and early March 2000. Observations made during the campaign showed that temperatures were below normal in the polar lower stratosphere over the course of the 1999–2000 winter. Because of these low temperatures, extensive polar stratospheric clouds (PSC) formed across the Arctic. Large particles containing nitric acid trihydrate were observed for the first time, showing that denitrification can occur without the formation of ice particles. Heterogeneous chemical reactions on the surfaces of the PSC particles produced high levels of reactive chlorine within the polar vortex by early January. This reactive chlorine catalytically destroyed about 60% of the ozone in a layer near 20 km between late January and mid-March 2000, with good agreement being found between a number of empirical and modeling studies. The measurements made during SOLVE/THESEO 2000 have improved our understanding of key photochemical parameters and the evolution of ozone-destroying forms of chlorine.

Understanding the kinetics of the ClO dimer cycle

Among the major factors controlling ozone loss in the polar vortices in winter/spring is the kinetics of the ClO dimer catalytic cycle. Here, we propose a strategy to test and improve our understanding of these kinetics by comparing and combining information on the thermal equilibrium between ClO and Cl 2O2, the rate of Cl 2O2 formation, and the Cl2O2 photolysis rate from laboratory experiments, theoretical studies and field observations. Concordant with a number of earlier studies, we find considerable inconsistencies of some recent laboratory results with rate theory calculations and stratospheric observations of ClO and Cl 2O2. The set of parameters for which we find the best overall consistency – namely the ClO/Cl 2O2 equilibrium constant suggested by Plenge et al. (2005), the Cl 2O2 recombination rate constant reported by Nickolaisen et al. (1994) and Cl 2O2 photolysis rates based on absorption cross sections in the range between the JPL 2006 assessment and the laboratory study by Burkholder et al. (1990) – is not congruent with the latest recommendations given by the JPL and IUPAC panels and does not represent the laboratory studies currently regarded as the most reliable experimental values. We show that the incorporation of new Pope et al. (2007) Cl 2O2 absorption cross sections into several models, combined with best estimates for other key parameters (based on either JPL and IUPAC evaluations or on our study), results in severe model underestimates of observed ClO and observed ozone loss rates. This finding suggests either the existence of an unknown process that drives the partitioning of ClO and Cl 2O2, or else some unidentified problem with either the laboratory study or numerous measurements of atmospheric ClO. Our mechanistic understanding of the ClO/Cl 2O2 system is grossly lacking, with severe implications for our ability to simulate both present and future polar ozone depletion. Correspondence to: M. von Hobe (m.von.hobe@fz-juelich.de)

A test of our understanding of the ozone chemistry in the Arctic polar vortex based on in situ measurements of ClO, BrO, and O3 in the 1994/1995 winter

We present an analysis of in situ measurements of ClO, BrO, O3, and long-lived tracers obtained on a balloon flight in the Arctic polar vortex launched from Kiruna, Sweden, 68°N, on February 3, 1995. Using the method of tracer correlations, we deduce that the air masses sampled at an altitude of 21 km (480 K potential temperature), where a layer of enhanced ClO mixing ratios of up to 1150 parts per trillion by volume was observed, experienced a cumulative chemical ozone loss of 1.0±0.3 ppmv between late November 1994 and early February 1995. This estimate of chemical ozone loss can be confirmed using independent data sets and independent methods. Calculations using a trajectory box model show that the simulations underestimate the cumulative ozone loss by approximately a factor of 2, although observed ClO and BrO mixing ratios are well reproduced by the model. Employing additional simulations of ozone loss rates for idealized conditions, we conclude that the known chlorine and bromine catalytic cycles destroying odd oxygen with the known rate constants and absorption cross sections do not quantitatively account for the early winter ozone losses infered for air masses observed at 21 km.

Simulation of ozone depletion in spring 2000 with the Chemical Lagrangian Model of the Stratosphere (CLaMS)

[1]xa0Simulations of the development of the chemical composition of the Arctic stratosphere for spring 2000 are made with the Chemical Lagrangian Model of the Stratosphere (CLaMS). The simulations are performed for the entire Northern Hemisphere on four isentropic levels (400–475 K). The initialization in early February is based on observations made from satellite, balloon and ER-2 aircraft platforms. Tracer-tracer correlations from balloon-borne cryosampler (Triple) and ER-2 measurements, as well as tracer-PV correlations, are used to derive a comprehensive hemispherical initialization of all relevant chemical trace species. Since significant denitrification has been observed on the ER-2 flights, a parameterization of the denitrification is derived from NOy and N2O observations on board the ER-2 aircraft and the temperature history of the air masses under consideration. Over the simulation period from 10 February to 20 March, a chemical ozone depletion of up to 60% was derived for 425–450 K potential temperature. Maximum vortex-averaged chemical ozone loss rates of 50 ppb d−1 or 4 ppb per sunlight hour were simulated in early March 2000 at the 425 and 450 K potential temperature levels. We show comparisons between the measurements and the simulations for the location of the ER-2 flight paths in late February and March and the location of the Triple balloon flight. The simulated tracer mixing ratios are in good agreement with the measurements. It was not possible to reproduce the exact details of the inorganic chlorine compounds. The simulation agrees with ClOx observations on the Triple balloon flight but overestimates for the ER-2 flights. The simulated ozone depletion agrees with estimates from other observations in the 425 and 450 K levels, but is underestimated on the 475 K level.

Severe ozone depletion in the cold Arctic winter 2004-05

[1]xa0During a flight of the M55 Geophysica into the Arctic polar vortex on 7 March 2005, ozone, halogen species, tracers and water vapor were measured. Up to 90% chlorine activation and up to 60% ozone loss were found above 14 km, reflecting the low temperatures and extensive PSC formation prevalent in the Arctic stratosphere over the 2004/05 winter. Observations are generally well reproduced by CLaMS model simulations. The observed levels of active chlorine can only be reproduced by assuming significant denitrification of about 70%. Moderate dehydration up to 0.5 ppm is observed in some locations. We deduce a partial column ozone loss of 62 (+8/−17) DU below 19 km on 7 March.

Polar stratospheric chlorine kinetics from a self‐match flight during SOLVE‐II/EUPLEX

[1]xa0In-situ measurements of ClO made onboard the Geophysica aircraft on 30 January 2003 in the Arctic afford a novel approach to constrain the kinetic parameters governing polar stratospheric chlorine chemistry using atmospheric observations. The self-match flight pattern, i.e. sampling individual air masses twice at different zenith angles, was utilized by simulating the evolution of ClO mixing ratios between two ‘matching’ points using a photochemical model and optimizing the model parameters to fit the observations within a retrieval framework. Our results suggest a ClO/ClOOCl thermal equilibrium constant Keq a factor of 5 smaller and a ratio J/kf a factor of 2 larger than the values based on the JPL recommendations. This concurs with other studies based on observed ClOx partitioning and corroborates that our understanding of stratospheric chlorine chemistry is incomplete, particularly in the light of the most recent laboratory experiments pointing to a J/kf ratio almost an order of magnitude below the JPL recommendation.

Vertical profiles of activated ClO and ozone loss in the Arctic vortex in January and March 2000: In situ observations and model simulations

[1]xa0In situ observations of ClO mixing ratios obtained from a balloonborne instrument launched in Kiruna on 27 January 2000 and on 1 March 2000 are presented. ClO mixing ratios and quasi-simultaneously observed ozone loss are compared to model simulations performed with the Chemical Lagrangian Model of the Stratosphere (CLaMS). ClO mixing ratios are simulated initializing the model simulations for early winter conditions. Sensitivity studies are performed to explore the impact of the surface area of the background aerosol, of denitrification, and of the recently reported kinetics of the ClO self-reaction [Bloss et al., 2001] on simulated ClO. For 27 January 2000, model simulations agree with rate constants reported by Bloss et al. [2001], whereas for 1 March 2000 simulations employing rate constants reported by Bloss et al. [2001] and by Sander et al. [2000] reproduce the ClO measurements. The impact of uncertainties arising from accumulated errors along the calculated backward trajectories and uncertainties within temperatures derived from the UK Met Office are also studied. For both flights, simulated ClO show a good overall agreement with measured ClO within uncertainties arising from accumulated errors along air parcel histories. We find a layer of low ClO mixing ratios < 100 pptv between 600 and 620 K for the flight on 27 January 2000 and between 525 and 550 K on 1 March 2000. For this layer, measured ClO is substantially lower than simulated ClO. Potential causes are discussed, but the discrepancy remains unexplained at present. Furthermore, for 1 March 2000, an overall agreement is found between model simulations and measurements by the HALOE instrument of HCl and NOx (=NO + NO2) for all altitudes considered. We conclude that denitrification occurred up to a potential temperature of ≈550 K (≈24 km altitude) on 1 March 2000. Finally, model simulations show that between late January and 1 March, a significant ozone loss of about 0.8–1.8 ppmv is derived between 425 and 490 K of potential temperature in agreement with measured ozone loss and correlated with the enhanced ClO. For 1 March 2000, 77 ± 10 DU is obtained as an estimate of the loss in column ozone.

Cryogenic infrared spectrometers and telescopes for the atmosphere: new frontiers

The new airborne CRyogenic Infrared Spectrometers and Telescope for the Atmosphere experiment (CRISTA-New Frontiers) succeeds the CRISTA satellite instrument operated twice during NASA space shuttle flights in November 1994 (STS 66) and August 1997 (STS 85). The first mission of the instrument will take place aboard the high altitude research aircraft M55-Geophysica in a campaign in the tropics in 2005/06. CRISTA-NF is a limb-scanning instrument measuring thermal emissions of various atmospheric trace gases (e.g. water vapor, ozone, chlorofluorocarbons), clouds and aerosols in the mid-infrared spectral region. The incoming radiation entering the optics through a Herschel telescope is analyzed by two Ebert-Fastie grating spectrometers with moderate spectral resolution and finally registered by cryogenic semiconductor-detectors. The optical system is integrated into a compact cryostat which reaches temperatures down to 10K by cooling with supercritical helium. This allows fast measurements and provides good signal-to-noise ratio. A narrow vertical field of view (200m) results in high vertical resolution which is neccessary for the analysis of small scale dynamic processes especially in the upper troposphere and lower stratosphere. This paper gives a scientific motivation, some remarks on the measurement technique and an overview of instrument design and technology.

Midlatitude ClO during the maximum atmospheric chlorine burden: in situ balloon measurements and model simulations

Chlorine monoxide (ClO) plays a key role in stratospheric ozone loss processes at midlatitudes. We present two balloon-borne in situ measurements of ClO conducted in northern hemisphere midlatitudes during the period of the maximum of total inorganic chlorine loading in the atmosphere. Both ClO measurements were conducted on board the TRIPLE balloon payload, launched in November 1996 in Léon, Spain, and in May 1999 in Aire sur l’Adour, France. For both flights a ClO daylight and night-time vertical profile was derived over an altitude range of approximately 15–35 km. ClO mixing ratios are compared to model simulations performed with the photochemical box model version of the Chemical Lagrangian Model of the Stratosphere (CLaMS). Simulations along 24-hour backward trajectories were performed to study the diurnal variation of ClO in the midlatitude lower stratosphere. Model simulations for the flight launched in Aire sur l’Adour 1999 show an excellent agreement with the ClO measurements. For the flight launched in Léon 1996, an overall good agreement is found, whereas the flight is characterized by a more complex dynamical situation due to a possible mixture of vortex and non-vortex air. We note that for both flights at solar zenith angles greater than 86 –87 simulated ClO mixing ratios are higher than observed ClO mixing ratios. However, the present findings indicate that no substantial uncertainties exist in midlatitude chlorine chemistry of the stratosphere. Correspondence to: B. Vogel (b.vogel@fz-juelich.de)

Studies of the mechanism of the cluster formation in a thermally sampling atmospheric pressure ionization mass spectrometer.

In this study a thermally sampling atmospheric pressure ionization mass spectrometer is described and characterized. The ion transfer stage offers the capability to sample cluster ions at thermal equilibrium and during this transfer fundamental processes possibly affecting the cluster distribution are also readily identified. Additionally, the transfer stage combines optional collision-induced dissociation (CID) analysis of the cluster composition with thermal equilibrium sampling of clusters. The performance of the setup is demonstrated with regard to the proton-bound water cluster system. The benefit of the studied processes is that they can help to improve future transfer stages and to understand cluster ion reactions in ion mobility tubes and high-pressure ion sources. In addition, the instrument allows for the identification of fragmentation and protonation reactions caused by CID.