M. von Hobe

Global budget of atmospheric carbonyl sulfide: Temporal and spatial variations of the dominant sources and sinks

[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.

Validation of the Aura Microwave Limb Sounder ClO measurements

We assess the quality of the version 2.2 (v2.2) ClO measurements from the Microwave Limb Sounder (MLS) on the Earth Observing System Aura satellite. The MLS v2.2 ClO data are scientifically useful over the range 100 to 1 hPa, with a single- profile precision of similar to 0.1 ppbv throughout most of the vertical domain. Vertical resolution is similar to 3-4 km. Comparisons with climatology and correlative measurements from a variety of different platforms indicate that both the amplitude and the altitude of the peak in the ClO profile in the upper stratosphere are well determined by MLS. The latitudinal and seasonal variations in the ClO distribution in the lower stratosphere are also well determined, but a substantial negative bias is present in both daytime and nighttime mixing ratios at retrieval levels below (i. e., pressures larger than) 22 hPa. Outside of the winter polar vortices, this negative bias can be eliminated by subtracting gridded or zonal mean nighttime values from the individual daytime measurements. In studies for which knowledge of lower stratospheric ClO mixing ratios inside the winter polar vortices to better than a few tenths of a ppbv is needed, however, day - night differences are not recommended and the negative bias must be corrected for by subtracting the estimated value of the bias from the individual measurements at each affected retrieval level.

Assessing the flux of different volatile sulfur gases from the ocean to the atmosphere

Reduced sulfur gases and optical properties were measured in the upper ocean during an Atlantic Meridional Transect cruise of the RRS James Clark Ross from England to the Falkland Islands in 1998. Sea surface concentrations of carbonyl sulfide (COS) exhibited a pronounced diel variation at low latitudes but no diel cycle at mid and high latitudes possibly because of oceanic fronts and the longer hydrolysis time constant in cold water. The highest COS concentrations were observed in coastal and upwelling areas. CS2 concentrations varied less than those of COS, and the highest concentrations were found in the North African upwelling area with decreasing concentrations both northward and southward of this maximum. Dimethylsulfide (DMS) had the largest concentrations in the upper ocean, but methyl mercaptan (CH3SH) was an important reduced sulfur species in some coastal and upwelling areas. This may be associated with high bacterial growth rates. Colored dissolved organic matter (CDOM) fluorescence and absorbance also exhibit the highest values in coastal and upwelling areas and the lowest values in the southern subtropical gyre. The results suggest that carbon disulfide (CS2) outgassed from the oceans and oxidized in the atmosphere may represent a larger source of COS to the atmosphere than the direct flux of COS across the sea-air interface.

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)

Dark production: A significant source of oceanic COS

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 � 3 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 � 2 d � 1 . 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, a350, 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 a350 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 � 1 to the annual COS flux from the ocean to the atmosphere.