K. Kreher

Global ozone trends in potential vorticity coordinates using TOMS and GOME intercompared against the Dobson network: 1978–1998

Global total column ozone trends were calculated by applying a least squares regression model to five homogenized satellite data sets from November 1978 to December 1998. Drifts and offsets in the satellite data were removed through comparisons of coincident satellite overpass and ground-based Dobson spectrophotometer measurements and through intersatellite comparisons. The satellite data were remapped into a potential vorticity coordinate (equivalent latitude) before zonal means were calculated. This remapping preserves steep meridional gradients in ozone and ozone trends across the winter polar vortex boundary and thereby reveals statistically significant negative trends just poleward of the Antarctic vortex boundary (∼70°S equivalent latitude) during May (−0.51±0.42% yr−1 (2σ)), June (−0.65±0.44% yr−1) and July (−0.85±0.44% yr−1). This feature is indistinct in trends derived as a function of geographic latitude most likely as a result of the smoothing by zonal averaging across steep ozone gradients. Comparison with similar earlier analysis (November 1978 to May 1991) indicates that Antarctic ozone trends have weakened (from −3.71±1.80% yr−1 to −2.95±0.40% yr−1), most likely as a result of saturation, while Arctic ozone trends are now much larger (from −1.05±0.96%yr−1 to −1.93±0.40% yr−1) following severe wintertime Arctic ozone depletion in recent years. Midlatitude negative trends calculated over the longer time period are also slightly reduced.

Intercomparison of BrO measurements from ERS-2 GOME, ground-based and balloon platforms

The consistency of BrO column amounts derived from GOME spectra and from correlative ground-based and balloon measurements performed in 1998-1999 during the Third European Stratospheric Experiment on Ozone (THESEO) has been investigated. The study relies on W-visible observations at several mid- and high latitude ground-based stations in both hemispheres, complemented by balloon-borne solar occultation profile measurements and 3D chemical transport model simulations. Previous investigations have reported GOME BrO columns systematically larger than those deduced from balloon, suggesting BrO being present, possibly ubiquitously, in the free troposphere. The robustness of this hypothesis has been further tested based on the presently available correlative data set. It is shown that when accounting for the BrO diurnal variation and the solar zenith angle dependency of the sensitivity of correlative data to the troposphere, measurements from all platforms are consistent with the presence of a tropospheric BrO background of 1-3 ~10’~ mole&m’ extending over mid- and high

Comparison of measurements and model calculations of stratospheric bromine monoxide

Ground-based zenith sky UV-visible measurements of stratospheric bromine monoxide (BrO) slant column densities are compared with simulations from the SLIMCAT three-dimensional chemical transport model. The observations have been obtained from a network of 11 sites, covering high and midlatitudes of both hemispheres. This data set gives for the first time a near-global picture of the distribution of stratospheric BrO from ground-based observations and is used to test our current understanding of stratospheric bromine chemistry. In order to allow a direct comparison between observations and model calculations, a radiative transfer model has been coupled to the chemical model to calculate simulated slant column densities. The model reproduces the observations in general very well. The absolute amount of the BrO slant columns is consistent with a total stratospheric bromine loading of 20 ± 4 ppt for the period 1998-2000, in agreement with previous estimates. The seasonal and latitudinal variations of BrO are well reproduced by the model. In particular, the good agreement between the observed and modeled diurnal variation provides strong evidence that the BrO-related bromine chemistry is correctly modeled. A discrepancy between observed and modeled BrO at high latitudes during events of chlorine activation can be resolved by increasing the rate constant for the reaction BrO + ClO → BrCl + O 2 to the upper limit of current recommendations. However, other possible causes of the discrepancy at high latitudes cannot be ruled out.

Ozone loss episodes in the free Antarctic troposphere, suggesting a possible climate feedback

Sudden loss of tropospheric ozone well above the boundary layer was observed on three occasions at two coastal sites in Antarctica in spring 1995. Back trajectories show that the air sampled the boundary layer near the northern edge of the sea ice (1000 km from the coast) between 3 and 5 days previously. Enhanced BrO observed over sea ice in spring suggests that such ozone loss is common offshore, hence it may cause a small climate effect whose sign would create positive feedback, if sea ice reduced during warming.

The retrieval of profile and chemical information from ground-based UV-visible spectroscopic measurements

Abstract An algorithm has been developed to retrieve altitude information at different diurnal stages for trace gas species by combining direct-sun and zenith-sky UV-visible differential slant column density (DSCD) measurements. DSCDs are derived here using differential optical absorption spectroscopy. Combining the complementary zenith-sky measurements (sensitive to the stratosphere) with direct-sun measurements (sensitive to the troposphere) allows this vertical distinction. Trace gas species such as BrO and NO 2 have vertical profiles with strong diurnal dependence. Information about the diurnal variation is simultaneously retrieved with the altitude distribution of the trace gas. The retrieval is a formal optimal estimation profile retrieval, allowing a complete assessment of information content and errors.

Glyoxal observations in the global marine boundary layer

Glyoxal is an important intermediate species formed by the oxidation of common biogenic and anthropogenic volatile organic compounds such as isoprene, toluene, and acetylene. Although glyoxal has been shown to play an important role in urban and forested environments, its role in the open ocean environment is still not well understood, with only a few observations showing evidence for its presence in the open ocean marine boundary layer (MBL). In this study, we report observations of glyoxal from 10 field campaigns in different parts of the worlds oceans. These observations together represent the largest database of glyoxal in the MBL. The measurements are made with similar instruments that have been used in the past, although the open ocean values reported here, average of about 25 parts per trillion by volume (pptv) with an upper limit of 40 pptv, are much lower than previously reported observations that were consistently higher than 40 pptv and had an upper limit of 140 pptv, highlighting the uncertainties in the differential optical absorption spectroscopy method for the retrieval of glyoxal. Despite retrieval uncertainties, the results reported in this work support previous suggestions that the currently known sources of glyoxal are insufficient to explain the average MBL concentrations. This suggests that there is an additional missing source, more than a magnitude larger than currently known sources, which is necessary to account for the observed atmospheric levels of glyoxal. Therefore, it could play a more important role in the MBL than previously considered.

One-decade trend analysis of stratospheric BrO over Harestua (60°N) and Lauder (45°S) reveals a decline

A trend analysis is performed of stratospheric BrO from ground-based UV-visible observations at Harestua (60°N, 11°E) and Lauder (45°S, 170°E) from 1995 through 2005. At both stations, a positive trend of about +2.5% per year is found for the 1995-2001 period, while a negative trend of about -1% per year is obtained between 2001 and 2005. Given a mean age of air of about 4 ± 1 years, the decline in stratospheric bromine since 2002 follows the decline of tropospheric organic bromine observed since the second half of 1998, as a result of the Montreal Protocol. These findings confirm that the impact of the Montreal Protocol restrictions on brominated substances have now reached the stratosphere. From our study, we have also derived a contribution of 6 ± 4 ppt of the brominated very short-lived substances and inorganic bromine tropospheric sources to the total bromine loading.

Ground-Based DOAS Measurements of Stratospheric Trace Gases at Two Antarctic Stations during the 2002 Ozone Hole Period

Abstract Compared to recent years, the development of the Antarctic ozone hole in 2002 showed very unusual dynamical features. The midwinter polar vortex was one of the smallest observed during the past decade. Driven by planetary waves, the vortex showed a strong asymmetry in early spring. A large air mass separated in late September, leaving what was previously a small vortex even smaller. Furthermore, stratospheric temperatures exceeded the polar stratospheric cloud (PSC) threshold earlier than in previous years, leading to a decrease in halogen activation by heterogeneous surface reactions. Ground-based observation of stratospheric trace gases in austral spring of 2001 and 2002 using passive Differential Optical Absorption Spectroscopy (DOAS) observations of zenith-scattered sunlight in the UV and visible wavelength region (320–650 nm) are presented. Using DOAS measurements of ozone, NO2, BrO, and OClO at two different Antarctic sites, Neumayer Station (70°S, 8°W) and Arrival Heights (78°S, 167°E), th...

Ozone and temperature profiles measured above Kiruna inside, at the edge of, and outside the Arctic Polar Vortex in February and March 1997

Ozone depletion above Kiruna (67.9°N, 21.1°E), Sweden, was investigated using daily ozone and temperature measurements by ozonesondes between 1 February and 25 March 1997. Using UKMO Ertels potential vorticity (EPV) and wind fields, three dynamically distinct regions were defined on a grid of isentropic surfaces viz.: the polar vortex boundary region characterized by steep EPV gradients, the area poleward of the boundary region (inside the polar vortex), and the area equatorward of the boundary region (outside the polar vortex). Due to dynamically induced displacements of the vortex, measurements were made in all three regions. By calculating the isentropic EPV at each measurement point and comparing it with the values defining the equatorward and poleward edges of the vortex boundary region, all ozone and temperature measurements could be binned according to their position relative to the vortex edge. Since the data outside the polar vortex were highly variable, mean ozone profiles and their standard deviations were calculated and compared only for the two other regions. To investigate whether differences between these mean profiles were indicative of ozone loss, the temporal evolution of ozone mixing ratios measured along several isentropic surfaces was examined, taking into account the diabatic descent of airmasses. Finally, ozone loss rates were calculated for six potential temperature surfaces and loss rates of up to 0.63ppm/month were found inside the Arctic vortex at surfaces descending from approximately 475 K (1 February) to 460 K (25 March).

Precursors to Particles (P2P) at Cape Grim 2006: Campaign Overview

Environmental context. Understanding the role of clouds in assessing the impact of climate change is a challenging issue. It is thought that plankton and seaweed contribute to the formation of clouds by emitting gases that lead to the particle production necessary for cloud formation. Macroalgae (kelp) at Mace Head, Ireland, produce large quantities of iodine when exposed to sunlight at low tide and this iodine results in the rapid production of particles. Cape Grim, Tasmania, also has large colonies of kelp and the role of Bull Kelp (Durvillaea potatorum) in particle production was assessed. Abstract. Iodine emissions from coastal macroalgae have been found to be important initiators for nucleation events at Mace Head, Ireland. The source of this iodine is the large beds of the brown kelp Laminaria digitata, which are significantly exposed at low tide. On the coast around Cape Grim, Tasmania, there are beds of the brown kelp Durvillaea potatrum. The Precursors to Particles 2006 (P2P 2006) campaign at the Cape Grim Baseline Air Pollution Station in late summer (February) 2006 focused on the role of this local kelp in providing precursor gases to particle formation. Durvillaea potatorum does not produce iodated precursor gases at the levels observed at Mace Head. IO was measured at 0.5 ± 0.3 ppt, while OIO was below detection limits (9 ppt). The dominant atmospheric iodated species was methyl iodide and the average concentration measured at the Cape Grim Station was 1.5 ± 0.3 pptv in baseline conditions, but showed significant variation in discrete samples collected immediately above the ocean surface. Nucleation events were not detected at the Cape Grim Station, except for one period where the plume of a local bushfire interacted with air of marine origin. The passage of four fronts did not result in nucleation bursts and measurements on the beach 94 m below the Cape Grim Station suggested that Durvillaea potatorum was only a weak source of new particles.