Marc von Hobe | Forschungszentrum Juelich (original) (raw)

Papers by Marc von Hobe

Laboratory experiments, field measurements and modelling studies are being carried out in the fra... more Laboratory experiments, field measurements and modelling studies are being carried out in the frame of the EU-project RECONCILE (Reconciliation of essential process parameters for an enhanced predictability of arctic stratospheric ozone loss and its climate interactions) to reach the following objectives 1. Reach quantitative understanding of the chemistry leading to polar ozone depletion 2. Improve our understanding of polar stratospheric cloud (PSC) microphysics and heterogeneous chlorine activation 3. Quantify the flux and mixing of air through the vortex edge and its impact on polar and mid-latitude ozone 4. Incorporate the improved parameterisations into a Chemistry Climate Model (CCM) to generate more reliable predictions for the future evolution of the ozone layer and its effects and sensitivities to climate, and test the improved CCM simulations against observed trends. We present a compact overview of the project activities and show first results. Particular emphasis will be given to very recent trace gas and particle measurements from a field campaign with the high-flying aircraft M55-Geophysica from Kiruna, Sweden, between 17. January and 19. March. Flights were carried out in the cold activated polar vortex. During some flights, the Geophysica sampled air inside synoptic-scale PSCs.

International Journal of Environmental Analytical Chemistry, 2008

... DOI: 10.1080/03067310701642081 Marc Von Hobe a ... The actual sample volume is determined by ... more ... DOI: 10.1080/03067310701642081 Marc Von Hobe a ... The actual sample volume is determined by the flow rate at STP measured by a mass flow controller MFC1 (Tylan, Eching, Germany) and the time that the eight-port valve V8 (Valco, Houston, TX) is in the 'sampling' position. ...

Journal of Geophysical Research, 2002

1] The spatial and temporal variability of the global fluxes of carbonyl sulfide (COS) is discuss... more 1] The spatial and temporal variability of the global fluxes of carbonyl sulfide (COS) is discussed together with possible implications for total column atmospheric COS loading. The input of COS into the atmosphere is calculated as the sum of all known direct sources of COS plus the conversion of carbon disulfide (CS 2 ) and dimethyl sulfide (DMS) to COS by atmospheric oxidation processes. Recent models are used to predict COS, CS 2 , and DMS release from the oceans and COS uptake by soils, plants, and oceans. This forward approach to constructing global integrated COS fluxes has a large associated range of uncertainty. The best guess global annual-integrated COS net flux estimate does not differ from zero within the range of estimated uncertainty, consistent with the observed absence of long-term trends in atmospheric COS loading. Interestingly, the hemispheric time-dependent monthly fluxes are very close in phase for the Northern and Southern Hemispheres. The monthly variation of the Northern Hemisphere flux seems to be driven primarily by high COS vegetation uptake in summer, while the monthly variation of the Southern Hemisphere flux appears to be driven mostly by high oceanic fluxes of COS, CS 2 , and DMS in summer. Citation: Kettle, A. J., U. Kuhn, M. von Hobe, J. Kesselmeier, and M. O. Andreae, Global budget of atmospheric carbonyl sulfide: Temporal and spatial variations of the dominant sources and sinks,

Biogeosciences, 2005

COS uptake by trees, as observed under dark/light changes and under application of the plant horm... more COS uptake by trees, as observed under dark/light changes and under application of the plant hormone abscisic acid, exhibited a strong correlation with the CO 2 assimilation rate and the stomatal conductance. As the uptake of COS occurred exclusively through the stomata we compared experimentally derived and re-evaluated deposition velocities (V d ; related to stomatal conductance) for COS and CO 2 . We show that V d of COS is generally significantly larger than that of CO 2 . We therefore introduced this attribute into a new global estimate of COS fluxes into vegetation. The new global estimate of the COS uptake based on available net primary productivity data (NPP) ranges between 0.69-1.40 Tg a −1 . However, as a COS molecule is irreversibly split in contrast to CO 2 which is released again by respiration processes, we took into account the Gross Primary Productivity (GPP) representing the true CO 2 leaf flux the COS uptake has to be related to. Such a GPP based deposition estimate ranged between 1.4-2.8 Tg a −1 (0.73-1.50 Tg S a −1 ). We believe that in order to obtain accurate global COS sink estimates such a GPP-based estimate corrected by the different deposition velocities of COS and CO 2 must be taken into account.

Journal of Geophysical Research, 2001

Carbonyl sulfide (COS) in air and dissolved in seawater was determined during a cruise in August ... more Carbonyl sulfide (COS) in air and dissolved in seawater was determined during a cruise in August 1999 in the Sargasso Sea in the northwest Atlantic Ocean. Dissolved concentrations at the sea surface displayed only a weak diel cycle with a mean of 8.6 ± 2.8 pmol dm À3 owing to low abundance of photochemical precursors and high temperatures causing rapid hydrolysis. Depth profiles measured over the oceanic mixed layer revealed significant vertical gradients of COS concentration with higher values at the surface, suggesting that the rate of photochemical production at the surface exceeds the rate of vertical mixing. The mean atmospheric mixing ratio was 486 ± 40 ppt, and calculated sea-air fluxes ranged from 0.03 to 0.8 g COS km À2 d À1 . COS dark production, estimated from the predawn COS concentration at the surface and the hydrolysis constant, contributed significantly to the total amount of COS produced. A strong temperature dependence of the COS dark production rate q was found by comparing previously published values. The data further indicate an approximately first-order relationship between q and chromophoric dissolved organic matter (CDOM) absorbance at 350 nm, a 350 , which is used as a proxy for the CDOM content of the water but is likely to covary with other parameters, such as biological activity, that could also affect COS dark production. Together with known functions for COS hydrolysis and solubility, the parameterization of dark production as a function of temperature and a 350 allows for the prediction of COS concentrations and saturation ratios as a function of physical and optical seawater properties in the absence of photoproduction. This is used to estimate a lower limit of 0.056 Tg COS yr À1 to the annual COS flux from the ocean to the atmosphere.

Atmospheric Environment, 1999

Carbonyl sulphide (COS) concentrations in air and dissolved in seawater were determined during a ... more Carbonyl sulphide (COS) concentrations in air and dissolved in seawater were determined during a cruise in summer 1997 in the northeast Atlantic Ocean. Seawater characteristics and meteorological parameters were monitored. Dissolved COS concentrations throughout the entire cruise exhibited a strong diel cycle with maximum concentrations in late afternoon and minimum concentrations at sunrise. This is in good agreement with the theory that COS is photochemically produced from dissolved organic matter during the day and removed by hydrolysis. The overall mean concentration of dissolved COS was 23.6 pmol dm\; the daily mean varied according to water mass characteristics and was highest in upwelling areas owing to increased dissolved organic matter. Atmospheric COS concentrations, varying from about 450 to 800 pptv with an average value of 657 pptv, showed some dependency on air mass history and local COS #ux from the sea surface. The exchange of COS between the ocean and the atmosphere depended on dissolved COS concentrations and wind speed. The daily integrated #ux was always from the sea into the air, and the average was 1.3$0.8 g COS km\ d\. The collected data were used to test a mixed layer box model and a one-dimensional model, both describing the behaviour of COS in seawater. We found that the one-dimensional model simulates the data more closely, especially during the night.

Atmospheric Chemistry and Physics Discussions, 2002

A simple inverse model is proposed to deduce hemisphere-integrated COS flux based on published ti... more A simple inverse model is proposed to deduce hemisphere-integrated COS flux based on published time series of total column COS. The global atmosphere is divided into two boxes representing the Northern and Southern Hemispheres, and the total column COS data from several stations are used to deduce hemispheric COS loadings.

Volatile organic sulfur compounds from an Atlantic Meridional Transect of the RRS James Clark Ros... more Volatile organic sulfur compounds from an Atlantic Meridional Transect of the RRS James Clark Ros are presented together with optical data and upper ocean chlorophyl data. The latitudinal distribution of DMS and CH3SH is consistent with a biological source with low values in oligotrophic areas and high values in coastal regions. The DMS/CH3SH ratio shows systematic variation between oligotrophic and coastal areas and seems to correlate inversely with absorbance and fluorescence of colored dissolved organic matter. The latitudinal distribution of COS and CS2 conform with existing models based on known sources and sinks of these compounds. A system of photochemical provinces is developed for these compounds based on a previously published scheme. An estimate of the global distribution of these provinces is made based on data archives and remotely sensed data.

Journal of Geophysical Research, 2001

In 1985, Farman et al. discovered the near complete disappearance of the stratospheric ozone laye... more In 1985, Farman et al. discovered the near complete disappearance of the stratospheric ozone layer over Antarctica in spring. This 'Ozone Hole' took the atmospheric research community by surprise as it could not be explained by the known catalytic cycles removing ozone in the stratosphere. McElroy et al. (1986) and Molina and Molina (1987) seemed to have solved the enigma by proposing two new catalytic cycles -- the ClO-BrO-cycle and the ClO dimer cycle -- that could rapidly destroy ozone at cold temperatures and high zenith angles. Subsequent work describing the kinetics of these cycles as well as stratospheric observations of chlorine and bromine compounds supported their theory and led to atmospheric chemistry models reproducing observed ozone loss reasonably well. Today, more than 20 years after the discovery of the ozone hole and the ratification of the Montreal Protocol, a new laboratory study (Pope et al., 2007) -- suggesting much smaller absorption cross sections and hence photolysis rates of the ClO dimer -- seriously calls into question our understanding of how ozone is destroyed in the spring polar stratosphere. With the new cross sections, both the dimer cycle and the ClO-BrO-cycle run much slower, and observations of neither chlorine compounds nor ozone loss are reproduced by model simulations (von Hobe et al., 2007): the known catalytic cycles cannot cause an ozone hole. Obviously, this also calls into question our ability to predict future polar ozone depletion. In search for an explanation, we discuss possible shortcomings of the Pope et al. experiment that could lead to an underestimation of the dimer absorption and examine various new chemical processes for their likelihood to influence chlorine partitioning and cause significant ozone loss in the atmosphere and at the same time go undetected in laboratory based kinetic studies. A strategy is presented for designing the tests needed to unambiguously confirm or rule out proposed solutions to the dilemma. Farman, J.C. et al., Nature 315, 207, 1985. McElroy, M.B. et al., Nature 321, 759, 1986. Molina, L.T. and Molina, M.J., J. Phys. Chem. 91, 433, 1987. Pope, F.D.et al., J. Phys. Chem. A 111, 4322, 2007. von Hobe, M. et al., Atmos. Chem. Phys. 7, 3055, 2007.

Journal of Physical Chemistry A, 2005

The bond strength of chlorine peroxide (ClOOCl) is studied by photoionization mass spectrometry. ... more The bond strength of chlorine peroxide (ClOOCl) is studied by photoionization mass spectrometry. The experimental results are obtained from the fragmentation threshold yielding ClO + , which is observed at 11.52 ( 0.025 eV. The O-O bond strength D°is derived from this value in comparison to the first ionization energy of ClO, yielding D°2 98 ) 72.39 ( 2.8 kJ mol -1 . The present work provides a new and independent method to examine the equilibrium constant K eq for chlorine peroxide formation via dimerization of ClO in the stratosphere. This yields an approximation for the equilibrium constant in the stratospheric temperature regime between 190 and 230 K of the form K eq ) 1.92 × 10 -27 cm 3 molecules -1 × exp(8430 K/T). This value of K eq is lower than current reference data and agrees well with high altitude aircraft measurements within their scattering range. Considering the error limits of the present experimental results and the resulting equilibrium constant, there is agreement with previous works, but the upper limit of current reference values appears to be too high. This result is discussed along with possible atmospheric implications.

Geophysical Research Letters, 2006

1] During a flight of the M55 Geophysica into the Arctic polar vortex on 7 March 2005, ozone, hal... more 1] During 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/À17) DU below 19 km on 7 March.

Atmospheric Chemistry and Physics, 2005

In-situ measurements of ClO and its dimer carried out during the SOLVE II/VINTERSOL-EUPLEX and EN... more In-situ measurements of ClO and its dimer carried out during the SOLVE II/VINTERSOL-EUPLEX and ENVISAT Validation campaigns in the Arctic winter 2003 suggest that the thermal equilibrium between the dimer formation and dissociation is shifted significantly towards the monomer compared to the current JPL 2002 recommendation. Detailed analysis of observations made in thermal equilibrium allowed to re-evaluate the magnitude and temperature dependence of the equilibrium constant. A fit of the JPL format for equilibrium constants yields K EQ =3.61×10 −27 exp(8167/T ), but to reconcile the observations made at low temperatures with the existing laboratory studies at room temperature, a modified equation, K EQ =5.47×10 −25 (T /300) −2.29 exp(6969/T ), is required. This format can be rationalised by a strong temperature dependence of the reaction enthalpy possibly induced by Cl 2 O 2 isomerism effects. At stratospheric temperatures, both equations are practically equivalent. Using the equilibrium constant reported here rather than the JPL 2002 recommendation in atmospheric models does not have a large impact on simulated ozone loss. Solely at large zenith angles after sunrise, a small decrease of the ozone loss rate due to the ClO dimer cycle and an increase due to the ClO-BrO cycle (attributed to the enhanced equilibrium ClO concentrations) is observed, the net effect being a slightly stronger ozone loss rate.

Geophysical Research Letters, 2008

Atmospheric Chemistry and Physics, 2007

Among the major factors controlling ozone loss in the polar vortices in winter/spring is the kine... more 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 2 O 2 , the rate of Cl 2 O 2 formation, and the Cl 2 O 2 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 2 O 2 . The set of parameters for which we find the best overall consistency -namely the ClO/Cl 2 O 2 equilibrium constant suggested by , the Cl 2 O 2 recombination rate constant reported by and Cl 2 O 2 photolysis rates based on absorption cross sections in the range between the JPL 2006 assessment and the laboratory study by -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. Cl 2 O 2 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 2 O 2 , or else some unidentified problem with either the laboratory study or numerous measurements of atmospheric ClO. Our mechanistic understanding of the ClO/Cl 2 O 2 system is grossly lacking, with severe implications for our ability to simulate both present and future polar ozone depletion.

Atmospheric Chemistry and Physics Discussions, 2007

Strong perturbations of the Arctic stratosphere during the winter 2002/2003 by planetary waves le... more Strong perturbations of the Arctic stratosphere during the winter 2002/2003 by planetary waves led to enhanced stretching and folding of the vortex. On two occasions the vortex in the lower stratosphere split into two secondary vortices that re-merged after some days. As a result of these strong disturbances the role of transport in and out of the vortex was stronger

Physical Chemistry Chemical Physics, 2009

UV/Vis absorption spectra of ClOOCl isolated in neon matrices were measured in the wavelength ran... more UV/Vis absorption spectra of ClOOCl isolated in neon matrices were measured in the wavelength range 220-400 nm. The purity of the trapped samples was checked by infrared and UV/Vis matrix spectroscopy as well as low-temperature Raman spectroscopy. At wavelengths below 290 nm, the results agree with the UV spectrum recently published by Pope et al. [J. Phys. Chem. A, 2007, 111, 4322-4332]. However, the observed absorption in the long wavelength tail of the spectrum-relevant for polar stratospheric ozone loss-is substantially higher than reported by Pope et al. Our results suggest the existence of a ClOOCl electronic state manifold leading to an absorption band similar to those of the near UV spectrum of Cl(2). The differences to previous studies can be accounted for quantitatively by contributions to the reported absorption spectra caused by impurities. The observed band in the long wavelength tail is supported by several high-level ab initio calculations. However, questions arise concerning absolute values of the ClOOCl cross sections, an issue that needs to be revisited in future studies. With calculated photolysis rates based on our spectrum scaled to previous cross sections at the peak absorption, the known polar catalytic ozone-destruction cycles to a large extent account for the observed ozone depletion in the spring polar stratosphere.

Atmospheric Chemistry and Physics Discussions, 2011

The photolysis frequency of dichlorine peroxide (ClOOCl) JClOOCl is a critical parameter in catal... more The photolysis frequency of dichlorine peroxide (ClOOCl) JClOOCl is a critical parameter in catalytic cycles destroying ozone in the polar stratosphere. In the atmospherically relevant wavelength region, published laboratory measurements of ClOOCl absorption cross sections and spectra are not in good agreement, resulting in significant discrepancies in JClOOCl. Previous investigations of the consistency with atmospheric observations of 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 rate constant of the ClO recombination reaction krec. Here, we constrain the atmospherically effective JClOOCl independent of krec using ClO data sampled in the same air masses before and directly after sunrise. Over sunrise, when the ClO/ClOOCl system comes out of thermal equilibrium and the influence of the ClO recombination reaction is negligible, the rise in ClO concentration 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 effectively ruled out by our observations. Additionally, the night-time ClO observations show that the ClO/ClOOCl thermal equilibrium constant can not be significantly higher than the one proposed by Plenge et al. (2005).

During the RECONCILE campaign in the Arctic winter 2009/10, an active Match experiment was perfor... more During the RECONCILE campaign in the Arctic winter 2009/10, an active Match experiment was performed sampling the same air masses up to three times during two consecutive flights of the high-altitude research aircraft M55-Geophysica from Kiruna (67.83 N, 20.42 E). The first flight was westbound and its flightpath designed to resample the air masses from the outbound leg during the return to Kiruna with a time difference of up to 3 hours. Another match was attempted during a second flight 72 hours later when the air masses had moved into the Geophysica's range again. Flightplans were designed using trajectory calculations driven by ECMWF wind fields. In situ measurements of N2O and NOy revealed strong gradients inside the vortex thus allowing us to examine the accuracy of such trajectory calculations with wind fields in different spatial and temporal resolution.

Laboratory experiments, field measurements and modelling studies are being carried out in the fra... more Laboratory experiments, field measurements and modelling studies are being carried out in the frame of the EU-project RECONCILE (Reconciliation of essential process parameters for an enhanced predictability of arctic stratospheric ozone loss and its climate interactions) to reach the following objectives 1. Reach quantitative understanding of the chemistry leading to polar ozone depletion 2. Improve our understanding of polar stratospheric cloud (PSC) microphysics and heterogeneous chlorine activation 3. Quantify the flux and mixing of air through the vortex edge and its impact on polar and mid-latitude ozone 4. Incorporate the improved parameterisations into a Chemistry Climate Model (CCM) to generate more reliable predictions for the future evolution of the ozone layer and its effects and sensitivities to climate, and test the improved CCM simulations against observed trends. We present a compact overview of the project activities and show first results. Particular emphasis will be given to very recent trace gas and particle measurements from a field campaign with the high-flying aircraft M55-Geophysica from Kiruna, Sweden, between 17. January and 19. March. Flights were carried out in the cold activated polar vortex. During some flights, the Geophysica sampled air inside synoptic-scale PSCs.

International Journal of Environmental Analytical Chemistry, 2008

... DOI: 10.1080/03067310701642081 Marc Von Hobe a ... The actual sample volume is determined by ... more ... DOI: 10.1080/03067310701642081 Marc Von Hobe a ... The actual sample volume is determined by the flow rate at STP measured by a mass flow controller MFC1 (Tylan, Eching, Germany) and the time that the eight-port valve V8 (Valco, Houston, TX) is in the 'sampling' position. ...

Journal of Geophysical Research, 2002

1] The spatial and temporal variability of the global fluxes of carbonyl sulfide (COS) is discuss... more 1] The spatial and temporal variability of the global fluxes of carbonyl sulfide (COS) is discussed together with possible implications for total column atmospheric COS loading. The input of COS into the atmosphere is calculated as the sum of all known direct sources of COS plus the conversion of carbon disulfide (CS 2 ) and dimethyl sulfide (DMS) to COS by atmospheric oxidation processes. Recent models are used to predict COS, CS 2 , and DMS release from the oceans and COS uptake by soils, plants, and oceans. This forward approach to constructing global integrated COS fluxes has a large associated range of uncertainty. The best guess global annual-integrated COS net flux estimate does not differ from zero within the range of estimated uncertainty, consistent with the observed absence of long-term trends in atmospheric COS loading. Interestingly, the hemispheric time-dependent monthly fluxes are very close in phase for the Northern and Southern Hemispheres. The monthly variation of the Northern Hemisphere flux seems to be driven primarily by high COS vegetation uptake in summer, while the monthly variation of the Southern Hemisphere flux appears to be driven mostly by high oceanic fluxes of COS, CS 2 , and DMS in summer. Citation: Kettle, A. J., U. Kuhn, M. von Hobe, J. Kesselmeier, and M. O. Andreae, Global budget of atmospheric carbonyl sulfide: Temporal and spatial variations of the dominant sources and sinks,

Biogeosciences, 2005

COS uptake by trees, as observed under dark/light changes and under application of the plant horm... more COS uptake by trees, as observed under dark/light changes and under application of the plant hormone abscisic acid, exhibited a strong correlation with the CO 2 assimilation rate and the stomatal conductance. As the uptake of COS occurred exclusively through the stomata we compared experimentally derived and re-evaluated deposition velocities (V d ; related to stomatal conductance) for COS and CO 2 . We show that V d of COS is generally significantly larger than that of CO 2 . We therefore introduced this attribute into a new global estimate of COS fluxes into vegetation. The new global estimate of the COS uptake based on available net primary productivity data (NPP) ranges between 0.69-1.40 Tg a −1 . However, as a COS molecule is irreversibly split in contrast to CO 2 which is released again by respiration processes, we took into account the Gross Primary Productivity (GPP) representing the true CO 2 leaf flux the COS uptake has to be related to. Such a GPP based deposition estimate ranged between 1.4-2.8 Tg a −1 (0.73-1.50 Tg S a −1 ). We believe that in order to obtain accurate global COS sink estimates such a GPP-based estimate corrected by the different deposition velocities of COS and CO 2 must be taken into account.

Journal of Geophysical Research, 2001

Carbonyl sulfide (COS) in air and dissolved in seawater was determined during a cruise in August ... more Carbonyl sulfide (COS) in air and dissolved in seawater was determined during a cruise in August 1999 in the Sargasso Sea in the northwest Atlantic Ocean. Dissolved concentrations at the sea surface displayed only a weak diel cycle with a mean of 8.6 ± 2.8 pmol dm À3 owing to low abundance of photochemical precursors and high temperatures causing rapid hydrolysis. Depth profiles measured over the oceanic mixed layer revealed significant vertical gradients of COS concentration with higher values at the surface, suggesting that the rate of photochemical production at the surface exceeds the rate of vertical mixing. The mean atmospheric mixing ratio was 486 ± 40 ppt, and calculated sea-air fluxes ranged from 0.03 to 0.8 g COS km À2 d À1 . COS dark production, estimated from the predawn COS concentration at the surface and the hydrolysis constant, contributed significantly to the total amount of COS produced. A strong temperature dependence of the COS dark production rate q was found by comparing previously published values. The data further indicate an approximately first-order relationship between q and chromophoric dissolved organic matter (CDOM) absorbance at 350 nm, a 350 , which is used as a proxy for the CDOM content of the water but is likely to covary with other parameters, such as biological activity, that could also affect COS dark production. Together with known functions for COS hydrolysis and solubility, the parameterization of dark production as a function of temperature and a 350 allows for the prediction of COS concentrations and saturation ratios as a function of physical and optical seawater properties in the absence of photoproduction. This is used to estimate a lower limit of 0.056 Tg COS yr À1 to the annual COS flux from the ocean to the atmosphere.

Atmospheric Environment, 1999

Carbonyl sulphide (COS) concentrations in air and dissolved in seawater were determined during a ... more Carbonyl sulphide (COS) concentrations in air and dissolved in seawater were determined during a cruise in summer 1997 in the northeast Atlantic Ocean. Seawater characteristics and meteorological parameters were monitored. Dissolved COS concentrations throughout the entire cruise exhibited a strong diel cycle with maximum concentrations in late afternoon and minimum concentrations at sunrise. This is in good agreement with the theory that COS is photochemically produced from dissolved organic matter during the day and removed by hydrolysis. The overall mean concentration of dissolved COS was 23.6 pmol dm\; the daily mean varied according to water mass characteristics and was highest in upwelling areas owing to increased dissolved organic matter. Atmospheric COS concentrations, varying from about 450 to 800 pptv with an average value of 657 pptv, showed some dependency on air mass history and local COS #ux from the sea surface. The exchange of COS between the ocean and the atmosphere depended on dissolved COS concentrations and wind speed. The daily integrated #ux was always from the sea into the air, and the average was 1.3$0.8 g COS km\ d\. The collected data were used to test a mixed layer box model and a one-dimensional model, both describing the behaviour of COS in seawater. We found that the one-dimensional model simulates the data more closely, especially during the night.

Atmospheric Chemistry and Physics Discussions, 2002

A simple inverse model is proposed to deduce hemisphere-integrated COS flux based on published ti... more A simple inverse model is proposed to deduce hemisphere-integrated COS flux based on published time series of total column COS. The global atmosphere is divided into two boxes representing the Northern and Southern Hemispheres, and the total column COS data from several stations are used to deduce hemispheric COS loadings.

Volatile organic sulfur compounds from an Atlantic Meridional Transect of the RRS James Clark Ros... more Volatile organic sulfur compounds from an Atlantic Meridional Transect of the RRS James Clark Ros are presented together with optical data and upper ocean chlorophyl data. The latitudinal distribution of DMS and CH3SH is consistent with a biological source with low values in oligotrophic areas and high values in coastal regions. The DMS/CH3SH ratio shows systematic variation between oligotrophic and coastal areas and seems to correlate inversely with absorbance and fluorescence of colored dissolved organic matter. The latitudinal distribution of COS and CS2 conform with existing models based on known sources and sinks of these compounds. A system of photochemical provinces is developed for these compounds based on a previously published scheme. An estimate of the global distribution of these provinces is made based on data archives and remotely sensed data.

Journal of Geophysical Research, 2001

In 1985, Farman et al. discovered the near complete disappearance of the stratospheric ozone laye... more In 1985, Farman et al. discovered the near complete disappearance of the stratospheric ozone layer over Antarctica in spring. This 'Ozone Hole' took the atmospheric research community by surprise as it could not be explained by the known catalytic cycles removing ozone in the stratosphere. McElroy et al. (1986) and Molina and Molina (1987) seemed to have solved the enigma by proposing two new catalytic cycles -- the ClO-BrO-cycle and the ClO dimer cycle -- that could rapidly destroy ozone at cold temperatures and high zenith angles. Subsequent work describing the kinetics of these cycles as well as stratospheric observations of chlorine and bromine compounds supported their theory and led to atmospheric chemistry models reproducing observed ozone loss reasonably well. Today, more than 20 years after the discovery of the ozone hole and the ratification of the Montreal Protocol, a new laboratory study (Pope et al., 2007) -- suggesting much smaller absorption cross sections and hence photolysis rates of the ClO dimer -- seriously calls into question our understanding of how ozone is destroyed in the spring polar stratosphere. With the new cross sections, both the dimer cycle and the ClO-BrO-cycle run much slower, and observations of neither chlorine compounds nor ozone loss are reproduced by model simulations (von Hobe et al., 2007): the known catalytic cycles cannot cause an ozone hole. Obviously, this also calls into question our ability to predict future polar ozone depletion. In search for an explanation, we discuss possible shortcomings of the Pope et al. experiment that could lead to an underestimation of the dimer absorption and examine various new chemical processes for their likelihood to influence chlorine partitioning and cause significant ozone loss in the atmosphere and at the same time go undetected in laboratory based kinetic studies. A strategy is presented for designing the tests needed to unambiguously confirm or rule out proposed solutions to the dilemma. Farman, J.C. et al., Nature 315, 207, 1985. McElroy, M.B. et al., Nature 321, 759, 1986. Molina, L.T. and Molina, M.J., J. Phys. Chem. 91, 433, 1987. Pope, F.D.et al., J. Phys. Chem. A 111, 4322, 2007. von Hobe, M. et al., Atmos. Chem. Phys. 7, 3055, 2007.

Journal of Physical Chemistry A, 2005

The bond strength of chlorine peroxide (ClOOCl) is studied by photoionization mass spectrometry. ... more The bond strength of chlorine peroxide (ClOOCl) is studied by photoionization mass spectrometry. The experimental results are obtained from the fragmentation threshold yielding ClO + , which is observed at 11.52 ( 0.025 eV. The O-O bond strength D°is derived from this value in comparison to the first ionization energy of ClO, yielding D°2 98 ) 72.39 ( 2.8 kJ mol -1 . The present work provides a new and independent method to examine the equilibrium constant K eq for chlorine peroxide formation via dimerization of ClO in the stratosphere. This yields an approximation for the equilibrium constant in the stratospheric temperature regime between 190 and 230 K of the form K eq ) 1.92 × 10 -27 cm 3 molecules -1 × exp(8430 K/T). This value of K eq is lower than current reference data and agrees well with high altitude aircraft measurements within their scattering range. Considering the error limits of the present experimental results and the resulting equilibrium constant, there is agreement with previous works, but the upper limit of current reference values appears to be too high. This result is discussed along with possible atmospheric implications.

Geophysical Research Letters, 2006

1] During a flight of the M55 Geophysica into the Arctic polar vortex on 7 March 2005, ozone, hal... more 1] During 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/À17) DU below 19 km on 7 March.

Atmospheric Chemistry and Physics, 2005

In-situ measurements of ClO and its dimer carried out during the SOLVE II/VINTERSOL-EUPLEX and EN... more In-situ measurements of ClO and its dimer carried out during the SOLVE II/VINTERSOL-EUPLEX and ENVISAT Validation campaigns in the Arctic winter 2003 suggest that the thermal equilibrium between the dimer formation and dissociation is shifted significantly towards the monomer compared to the current JPL 2002 recommendation. Detailed analysis of observations made in thermal equilibrium allowed to re-evaluate the magnitude and temperature dependence of the equilibrium constant. A fit of the JPL format for equilibrium constants yields K EQ =3.61×10 −27 exp(8167/T ), but to reconcile the observations made at low temperatures with the existing laboratory studies at room temperature, a modified equation, K EQ =5.47×10 −25 (T /300) −2.29 exp(6969/T ), is required. This format can be rationalised by a strong temperature dependence of the reaction enthalpy possibly induced by Cl 2 O 2 isomerism effects. At stratospheric temperatures, both equations are practically equivalent. Using the equilibrium constant reported here rather than the JPL 2002 recommendation in atmospheric models does not have a large impact on simulated ozone loss. Solely at large zenith angles after sunrise, a small decrease of the ozone loss rate due to the ClO dimer cycle and an increase due to the ClO-BrO cycle (attributed to the enhanced equilibrium ClO concentrations) is observed, the net effect being a slightly stronger ozone loss rate.

Geophysical Research Letters, 2008

Atmospheric Chemistry and Physics, 2007

Among the major factors controlling ozone loss in the polar vortices in winter/spring is the kine... more 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 2 O 2 , the rate of Cl 2 O 2 formation, and the Cl 2 O 2 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 2 O 2 . The set of parameters for which we find the best overall consistency -namely the ClO/Cl 2 O 2 equilibrium constant suggested by , the Cl 2 O 2 recombination rate constant reported by and Cl 2 O 2 photolysis rates based on absorption cross sections in the range between the JPL 2006 assessment and the laboratory study by -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. Cl 2 O 2 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 2 O 2 , or else some unidentified problem with either the laboratory study or numerous measurements of atmospheric ClO. Our mechanistic understanding of the ClO/Cl 2 O 2 system is grossly lacking, with severe implications for our ability to simulate both present and future polar ozone depletion.

Atmospheric Chemistry and Physics Discussions, 2007

Strong perturbations of the Arctic stratosphere during the winter 2002/2003 by planetary waves le... more Strong perturbations of the Arctic stratosphere during the winter 2002/2003 by planetary waves led to enhanced stretching and folding of the vortex. On two occasions the vortex in the lower stratosphere split into two secondary vortices that re-merged after some days. As a result of these strong disturbances the role of transport in and out of the vortex was stronger

Physical Chemistry Chemical Physics, 2009

UV/Vis absorption spectra of ClOOCl isolated in neon matrices were measured in the wavelength ran... more UV/Vis absorption spectra of ClOOCl isolated in neon matrices were measured in the wavelength range 220-400 nm. The purity of the trapped samples was checked by infrared and UV/Vis matrix spectroscopy as well as low-temperature Raman spectroscopy. At wavelengths below 290 nm, the results agree with the UV spectrum recently published by Pope et al. [J. Phys. Chem. A, 2007, 111, 4322-4332]. However, the observed absorption in the long wavelength tail of the spectrum-relevant for polar stratospheric ozone loss-is substantially higher than reported by Pope et al. Our results suggest the existence of a ClOOCl electronic state manifold leading to an absorption band similar to those of the near UV spectrum of Cl(2). The differences to previous studies can be accounted for quantitatively by contributions to the reported absorption spectra caused by impurities. The observed band in the long wavelength tail is supported by several high-level ab initio calculations. However, questions arise concerning absolute values of the ClOOCl cross sections, an issue that needs to be revisited in future studies. With calculated photolysis rates based on our spectrum scaled to previous cross sections at the peak absorption, the known polar catalytic ozone-destruction cycles to a large extent account for the observed ozone depletion in the spring polar stratosphere.

Atmospheric Chemistry and Physics Discussions, 2011

The photolysis frequency of dichlorine peroxide (ClOOCl) JClOOCl is a critical parameter in catal... more The photolysis frequency of dichlorine peroxide (ClOOCl) JClOOCl is a critical parameter in catalytic cycles destroying ozone in the polar stratosphere. In the atmospherically relevant wavelength region, published laboratory measurements of ClOOCl absorption cross sections and spectra are not in good agreement, resulting in significant discrepancies in JClOOCl. Previous investigations of the consistency with atmospheric observations of 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 rate constant of the ClO recombination reaction krec. Here, we constrain the atmospherically effective JClOOCl independent of krec using ClO data sampled in the same air masses before and directly after sunrise. Over sunrise, when the ClO/ClOOCl system comes out of thermal equilibrium and the influence of the ClO recombination reaction is negligible, the rise in ClO concentration 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 effectively ruled out by our observations. Additionally, the night-time ClO observations show that the ClO/ClOOCl thermal equilibrium constant can not be significantly higher than the one proposed by Plenge et al. (2005).

During the RECONCILE campaign in the Arctic winter 2009/10, an active Match experiment was perfor... more During the RECONCILE campaign in the Arctic winter 2009/10, an active Match experiment was performed sampling the same air masses up to three times during two consecutive flights of the high-altitude research aircraft M55-Geophysica from Kiruna (67.83 N, 20.42 E). The first flight was westbound and its flightpath designed to resample the air masses from the outbound leg during the return to Kiruna with a time difference of up to 3 hours. Another match was attempted during a second flight 72 hours later when the air masses had moved into the Geophysica's range again. Flightplans were designed using trajectory calculations driven by ECMWF wind fields. In situ measurements of N2O and NOy revealed strong gradients inside the vortex thus allowing us to examine the accuracy of such trajectory calculations with wind fields in different spatial and temporal resolution.