Description: Uncertainties of the photolysis cross sections of ClOOCl have long been a limiting factor in our theoretical understanding of the rate of polar stratospheric ozone losses. Previous work suggested that values slightly larger than current recommendations, which are based on laboratory measurements, result in improved agreement between model calculations of polar stratospheric ozone loss rates and observations while at the same time also leading to improved agreement between observations of the diurnal variation of ClO and model calculations of this species. But new laboratory work on the cross sections of ClOOCl suggest that its photolysis under polar stratospheric winter/spring conditions is nearly an order of magnitude slower than what would be required to explain the observations of ozone loss and ClO in the atmosphere and a factor of six slower than a value based on the current recommendations. We show the impact of these new results on our understanding of polar ozone chemistry.For typical Arctic conditions calculated ratios of ClO/ClOx decrease by about a factor of two. The ozone loss rate by the ClO-dimer cycle, so far believed to be the most efficient ozone loss cycle, drops by about a factor of four and the loss rate by the coupled ClO-BrO cycle by nearly a factor of two. Overall ozone loss rates calculated based on the known ozone loss mechanisms drop by a factor of two to three and become much smaller than observations. Also the calculated levels of ClO become much smaller than those observed in the stratosphere. These results suggest that a major fraction of the observed ozone loss in the polar stratosphere is due to a currently unknown mechanism - a major challenge of our fundamental understanding of the polar stratospheric ozone loss process.We will discuss potential new chemistry that would lead to improved agreement between calculations of ozone loss and ClO diurnal variations with in-situ observations in the stratosphere.

Description: Episodic bromine enhancement occurs over first year sea ice zones during spring, with corresponding rapid decreases in surface ozone concentrations. The release of bromine via autocatalytic heterogeneous reactions on sea-salt surfaces, including young sea ice, snow pack and aerosols, is known as a 'bromine explosion'. With the return of sunlight in spring, the rate of bromine photolysis rapidly increases, and the rresulting bromide ions react with ozone to form bromine monoxide (BrO). A frequent stable inversion layer concentrates BrO near the surface and prevents replenishment of ozone from the overlying free troposphere, resulting in rapid ozone depletion.BrO measurements have been made at Arrival Heights using zenith-sky UV- visible differential optical absorption spectroscopy (DOAS) since 1995 and multi-axis (Max-) DOAS since 1998.

Description: Uncertainties of the photolysis cross sections of ClOOCl have long been a limiting factor in our theoretical understanding of the rate of polar stratospheric ozone losses. Previous work suggested that values slightly larger than current recommendations, which are based on laboratory measurements, result in improved agreement be- tween model calculations of polar stratospheric ozone loss rates and observations while at the same time also leading to improved agreement between observations of the diurnal variation of ClO and model calculations of this species. But new laboratory work (Pope et al, 2007) on the cross sections of ClOOCl suggest that its photolysis under polar stratospheric winter/spring conditions is nearly an order of magnitude slower than what would be required to explain the observations of ozone loss and ClO in the atmosphere and a factor of six slower than a value based on the current recommendations. We show what the impact of these new results on our understand- ing of polar ozone chemistry is. In model calculations that are based on the new cross sections and for typical Arctic conditions ratios of ClO/ClOx decrease by about a factor of two. The ozone loss rate by the ClO-dimer cycle, so far believed to be the most efficient ozone loss cycle, drops by about a factor of four and the loss rate by the coupled ClO-BrO cycle by nearly a factor of two. Overall ozone loss rates calculated based on the known ozone loss mechanisms drop by a factor of two to three and become much smaller than ob- servations. Also the calculated levels of ClO become much smaller than those ob- served in the stratosphere. These results demonstrate the tremendous uncertainty of current ozone loss calculations that comes from the broad range of the published cross sections for ClOOCl. In particular they suggest that, if the most recent publica- tion of the cross sections (Pope et al., 2007) is correct, a major fraction of observed polar ozone loss is due to a currently unknown mechanism - clearly are a major chal- lenge of our fundamental understanding of the polar stratospheric ozone loss proc- ess. We will discuss potential chemical mechanisms that would lead to improved agree- ment between calculations of ozone loss based on the new cross sections with in-situ observations of ClO and ozone loss rates in the stratosphere.

Description: The photochemistry of stratospheric ozone in the tropics was intensively investigated during a balloon campaign at Teresina/Brazil in June 2005. Our limb scanning UV/Vis mini-DOAS instrument was deployed on 3 balloon flights on June 14 (MIPAS-B), June 17 (LPMA/DOAS) and June 30 (LPMA/IASI). During one of this flights (MIPAS-B), we could surprisingly detect HONO in the tropical tropopause transition layer (TTL). Observed HONO profiles were almost time independent for three hours around local noon with maximum HONO concentrations of 1.0x 10^9 molec/cm3 at 11 km. The measured UV/vis skylight spectra were analyzed applying the Differential Optical Absorption Spectroscopy (DOAS) method. When combined with 3D radiative-transfer modeling and an optimal estimation inversion technique, atmospheric concentration profiles of the targeted trace-gases can be inferred for each limb scan [Weidner et al., 2005]. Meteorological data and simultaneously measured O3 and NO2 indicate that the measured HONO can be best explained by intense radical photochemistry in the outflow of a meso-scale convective system, which possibly includes lightning produced NOX and HOX and, subsequent, HONO production. However, in order to explain the large amount of detected HONO by known chemical reactions, an elevated Leighton ratio and /or intense HOX photochemistry are necessary.