J.-U. Grooß

Chlorine activation and ozone depletion in the Arctic vortex: Observations by the Halogen Occultation Experiment on the Upper Atmosphere Research Satellite

Chlorine-catalyzed ozone destruction is clearly observed during austral spring in the Antarctic lower stratosphere. While high concentrations of ozone-destroying ClO radicals have likewise been measured during winter in the Arctic stratosphere, the chemical ozone depletion there is more difficult to quantify. Here we present observations of the Halogen Occultation Experiment on the Upper Atmosphere Research Satellite in the vortex region of the Arctic lower stratosphere during the winter and spring months of 1991/1992, 1992/1993, 1993/1994, and 1994/1995. All February measurements indicate an almost complete conversion of the otherwise main chlorine reservoir species HCl to chemically more reactive forms. Using CH4 as a chemically conserved tracer, we show that significant chemical ozone loss occurred in the Arctic vortex region during all four winters. The deficit in column ozone was about 60 and 50 Dobson units (DU) in the winters 1991/1992 and 1993/1994, respectively. During the two winters of 1992/1993 and 1994/1995 a severe chemical loss in lower-stratospheric ozone took place, with local reductions of the mixing ratios by over 50% and a loss in the column ozone of the order of 100 DU.

Impact of the vertical velocity scheme on modeling transport in the tropical tropopause layer

[1] To assess the impact of the vertical velocity scheme on modeling transport in the tropical tropopause layer (TTL), 3 month backward trajectories are initialized in the TTL for boreal winter and summer 2002. The calculations are done in either a kinematic scenario with pressure tendency as the vertical velocity or in a diabatic scenario with cross-isentropic velocity deduced from various diabatic heating rates due to radiation (clear sky, all sky) and latent, diffusive and turbulent heating. This work provides a guideline for assessing the sensitivity of trajectory and chemical transport model (CTM) results on the choice of the vertical velocity scheme. We find that many transport characteristics, such as time scales, pathways and dispersion, crucially depend on the vertical velocity scheme. The strongest tropical upwelling results from the operational European Centre for Medium-Range Weather Forecasts kinematic scenario with the time scale for ascending from 340 to 400 K of 1 month. For the ERA-Interim kinematic and total diabatic scenarios, this time scale is about 2 months, and for the all-sky scenario it is as long as 2.5 months. In a diabatic scenario, the whole TTL exhibits mean upward motion, whereas in a kinematic scenario, regions of subsidence occur in the upper TTL. However, some transport characteristics robustly emerge from the different scenarios, such as an enhancement of residence times between 350 and 380 K and a strong impact of meridional in-mixing from the extratropics on the composition of the TTL. Moreover, an increase of meridionally transported air from the summer hemisphere into the TTL (maximum for boreal summer) is found as an invariant feature among all the scenarios.

A reevaluation of the ozone budget with HALOE UARS data : no evidence for the ozone deficit

Recently, additional ozone production mechanisms have been proposed to resolve the ozone deficit problem, which arises from greater ozone destruction than production in several photochemical models of the upper stratosphere and lower mesosphere. A detailed ozone model budget analysis was performed with simultaneous observations of O3, HCl, H2O, CH4, NO, and NO2 from the Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite (UARS) under conditions with the strongest photochemical control of ozone. The results indicate that an ozone deficit may not exist. On the contrary, the use of currently recommended photochemical parameters leads to insufficient ozone destruction in the model.

HALOE observations of the vertical structure of chemical ozone depletion in the Arctic vortex during winter and early spring 1996-1997

We discuss observations by the Halogen Occultation Experiment on the Upper Atmosphere Research Satellite in the lower stratosphere in the Arctic vortex during winter and spring 1996-1997. Using HF as a chemically conserved tracer, we identify chemical ozone depletion and chlorine activation, despite variations caused by dynamical processes. For the Arctic vortex region, significant chemical ozone loss (up to two thirds around 475 K potential temperature) due to extensive activation of the inorganic chlorine reservoir is deduced, as observed similarly for previous winters. Chemical reductions in column ozone of up to 70-80 Dobson units (DU) in the lower stratosphere are calculated. Both chlorine activation and ozone loss inside the vortex, however, are more variable than observed in previous years.

Northern midlatitude stratospheric ozone dilution in spring modeled with simulated mixing

Measurements have shown a substantial decrease in Northern midlatitude ozone, which has only partially been explained by chemical models. The large ozone depletions determined for the Arctic vortex in recent winters will ultimately spread out and dilute the midlatitudes and thus contribute to the observed decrease. Here we have followed the ozone-depleted air inside the Arctic vortex in 1995 and 1997 during April and May with high-resolution reverse domain-filling (RDF) trajectory calculations. The resulting average midlatitude (30°–60°N) stratospheric ozone dilution in May is 2.9% and 2.6% of the 1979 column ozone in 1995 and 1997, respectively, or about 40% of the observed depletion. Nearly realistic mixing was introduced by a regridding procedure between successive 7-day long RDF calculations. Low-resolution grid point models give too much mixing, causing an overestimate of the calculated dilution. A recovery of about 12% of the midlatitude dilution in May 1997 is calculated with a photochemical box model, but is not included in the number given above.

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)

ML-CIRRUS - The airborne experiment on natural cirrus and contrail cirrus with the high-altitude long-range research aircraft HALO

AbstractThe Midlatitude Cirrus experiment (ML-CIRRUS) deployed the High Altitude and Long Range Research Aircraft (HALO) to obtain new insights into nucleation, life cycle, and climate impact of natural cirrus and aircraft-induced contrail cirrus. Direct observations of cirrus properties and their variability are still incomplete, currently limiting our understanding of the clouds’ impact on climate. Also, dynamical effects on clouds and feedbacks are not adequately represented in today’s weather prediction models.Here, we present the rationale, objectives, and selected scientific highlights of ML-CIRRUS using the G-550 aircraft of the German atmospheric science community. The first combined in situ–remote sensing cloud mission with HALO united state-of-the-art cloud probes, a lidar and novel ice residual, aerosol, trace gas, and radiation instrumentation. The aircraft observations were accompanied by remote sensing from satellite and ground and by numerical simulations.In spring 2014, HALO performed 16 f...

High-latitude, summertime NOx activation and seasonal ozone decline in the lower stratosphere: Model calculations based on observations by HALOE on UARS

Between May and September a continuous decrease of ozone concentrations is observed in the lower stratosphere at high latitudes in the northern hemisphere. Low local ozone concentrations are correlated with high concentrations of NO and NO2, and HCl. A detailed photochemical box model and a two-dimensional chemical model initialized by the Halogen Occultation Experiment (HALOE) data are used to calculate ozone destruction rates between 20 and 31 km altitude for different situations during the observational periods in mid and late summer. The largest ozone destruction rates are computed for ozone-rich midlatitude air masses that are transported to high latitudes reaching low Sun, but 24 hours per day sunlight conditions. It is shown that the observed summertime low ozone concentrations and much of the seasonal course of ozone is due to catalytic ozone destruction by NO and NO2, which become the main odd nitrogen compounds under these conditions.

Ozone Chemistry during the 2002 Antarctic Vortex Split

Abstract In September 2002, the Antarctic polar vortex was disturbed, and it split into two parts caused by an unusually early stratospheric major warming. This study discusses the chemical consequences of this event using the Chemical Lagrangian Model of the Stratosphere (CLaMS). The chemical initialization of the simulation is based on Halogen Occultation Experiment (HALOE) measurements. Because of its Lagrangian nature, CLaMS is well suited for simulating the small-scale filaments that evolve during this period. Filaments of vortex origin in the midlatitudes were observed by HALOE several times in October 2002. The results of the simulation agree well with these HALOE observations. The simulation further indicates a very rapid chlorine deactivation that is triggered by the warming associated with the split of the vortex. Correspondingly, the ozone depletion rates in the polar vortex parts rapidly decrease to zero. Outside the polar vortex, where air masses of midlatitude origin were transported to the ...

Mixing and Chemical Ozone Loss during and after the Antarctic Polar Vortex Major Warming in September 2002

Abstract The 3D version of the Chemical Lagrangian Model of the Stratosphere (CLAMS) is used to study the transport of CH4 and O3 in the Antarctic stratosphere between 1 September and 30 November 2002, that is, over the time period when unprecedented major stratospheric warming in late September split the polar vortex into two parts. The isentropic and cross-isentropic velocities in CLAMS are derived from ECMWF winds and heating/cooling rates calculated with a radiation module. The irreversible part of transport, that is, mixing, is driven by the local horizontal strain and vertical shear rates with mixing parameters deduced from in situ observations. The CH4 distribution after the vortex split shows a completely different behavior above and below 600 K. Above this potential temperature level, until the beginning of November, a significant part of vortex air is transported into the midlatitudes up to 40°S. The lifetime of the vortex remnants formed after the vortex split decreases with the altitude with v...