Search

Iodine compounds were measured above, below and within the sea ice of the Weddell Sea during a cruise in 2009, to make progress in elucidating the mechanism of local enhancement and volatilisation of iodine. I2 mixing ratios of up to 12.4 pptv were measured 10 m above the sea ice, and up to 31 pptv was observed above surface snow on the nearby Brunt Ice Shelf – large amounts. Atmospheric IO of up to 7 pptv was measured from the ship, and the average sum of HOI and ICl was 1.9 pptv. These measurements confirm the Weddell Sea as an iodine hotspot. Average atmospheric concentrations of CH3I, C2H5I, CH2ICl, 2-C3H7I, CH2IBr and 1-C3H7I were each 0.2 pptv or less. On the Brunt Ice Shelf, enhanced concentrations of CH3I and C2H5I (up to 0.5 and 1 pptv respectively) were observed in firn air, with a diurnal profile that suggests the snow may be a source. In the sea ice brine, iodocarbons concentrations were over 10 times those of the sea water below. The sum of iodide + iodate was depleted in sea ice samples, suggesting some missing iodine chemistry. Flux calculations suggest I2 dominates the iodine atom flux to the atmosphere, but models cannot reconcile the observations and suggest either a missing iodine source or other deficiencies in our understanding of iodine chemistry. The observation of new particle formation, consistent with the model predictions, strongly suggests an iodine source. This combined study of iodine compounds is the first of its kind in this unique region of sea ice rich in biology and rich in iodine chemistry.

We present surface layer measurements made over Hudson Bay seaice during February/March 2008 from the COBRA (Impact of combined iodine and bromine release on the Arctic atmosphere) experiment which formed part of the International OASIS (Ocean-Atmosphere-SeaIce-Snowpack) IPY programme. All components of the local surface energy balance were measured and it was defined by net radiative cooling throughout most of the day, mainly balanced by the conductive heat flux from the warmer sea water to the cooler seaice at the surface, and a small net radiative warming for a few hours after midday. Unique ground-level ozonefluxes were measured by eddy covariance and deposition velocities ranged from +0.5 mm s−1 (deposition) to −1.5 mm s−1 (emission). Ozone profile measurements suggested ozoneflux divergence within the surface layer. The observed bi-directional fluxes and flux divergence with height reveal the complexity of surface ozonefluxes in the Arctic spring time surface layer, and show that ozone exchange with the seaice surface is best probed using the eddy covariance method alongside frequent or continuous profile measurements. In this study, the local in-situ ozone-halogen photochemistry was identified as weakly controlling the measured ozoneflux, whereas horizontal advection and vertical mixing were considered more important in influencing fluxes. Under these conditions, several measurement sites would be desirable in order to quantify the contribution of advection to the local surface exchange.

Amounts of source gases with stratospheric sinks (CFCs, N2O, CH4) are affected by changes in BrewerDobson circulation. Source gases and their degradation products are important for atmospheric chemistry and climate. With a simple model, we examine how amounts and lifetimes of source gases and products depend on speed of the circulation. Transient results differ from steady-state and stratospheric results differ from those for stratosphere plus troposphere. Increases in speed increase the stratospheric burden of source gases, but reduce products and reduce total burdens and lifetimes of source gases.

Previous work showed a near-continuous increase in stratospheric water vapour between the 1950s and 1990s from a variety of instruments, without recourse to fits between instruments. We reassess the trend from the earliest, the UK frost-point hygrometer, 1954–1976. An error in previous work omitted to transform values from ppmm to ppmv. When corrected, they fit more convincingly with measurements by later frost-point hygrometers. Minor instrument changes between the 1950s and 1970s do not introduce a potential bias to the trend but do increase its error. If the full 1970s data are included, the trend becomes 2.1 ± 0.8%/year (two-sigma).

A suite of 10 autonomous ozone monitoring units, each powered using renewable energy, was developed and built to study surface ozone in Antarctica during the International Polar Year (2007-2009). The monitoring systems were deployed in a network around the Weddell Sea sector of coastal Antarctica with a transect up onto the Antarctic Plateau. The aim was to measure for a full year, thus gaining a much-improved broader view of boundary layer ozone seasonality at different locations as well as of factors affecting the budget of surface ozone in Antarctica. Ozone mixing ratios were measured based on UV photometry using a modified version of the commercial 2B Technologies Inc. Model 202 instrument. All but one of the autonomous units measured successfully within its predefined duty cycle throughout the year, with some differences in performance dependent on power availability and ambient temperature. Mean data recovery after removal of outliers was on average 70% (range 44-83%) and precision varied between 1.5 and 8 ppbv, thus was sufficiently good to resolve year-round the main ozone features of scientific interest. We conclude that, with adequate power, and noting a minor communication problem, our units would be able to operate successfully at ambient temperatures down to -60 degrees C. Systems such as the one described in this paper, or derivatives of it, could therefore be deployed either as local or regional networks elsewhere in the Arctic or Antarctic. Here we present technical information and first results from the experiment.

The majority of tropospheric ozone depletion event (ODE) studies have focussed on time-series measurements, with comparatively few studies of the vertical component. Those that exist have almost exclusively used free-flying balloon-borne ozonesondes and almost all have been conducted in the Arctic. Here we use measurements from two separate Antarctic field experiments to examine the vertical profile of ozone during Antarctic ODEs. We use tethersonde data to probe details in the lowest few hundred meters and find considerable structure in the profiles associated with complex atmospheric layering. The profiles were all measured at wind speeds less than 7 ms(-1), and on each occasion the lowest inversion height lay between 10 m and 40 m. We also use data from a free-flying ozonesonde study to select events where ozone depletion was recorded at altitudes > 1 km above ground level. Using ERA-40 meteorological charts, we find that on every occasion the high altitude depletion was preceded by an atmospheric low pressure system. An examination of limited published ozonesonde data from other Antarctic stations shows this to be a consistent feature. Given the link between BrO and ODEs, we also examine ground-based and satellite BrO measurements and find a strong association between atmospheric low pressure systems and enhanced BrO that must arise in the troposphere. The results suggest that, in Antarctica, such depressions are responsible for driving high altitude ODEs and for generating the large-scale BrO clouds observed from satellites. In the Arctic, the prevailing meteorology differs from that in Antarctica, but, while a less common effect, major low pressure systems in the Arctic can also generate BrO clouds. Such depressions thus appear to be fundamental when considering the broader influence of ODEs, certainly in Antarctica, such as halogen export and the radiative influence of ozone-depleted air masses.

Frost flowers play a role in air-ice exchange in polar regions, contribute to tropospheric halogen chemistry, and affect ice core interpretation. Frost flowers were observed and collected on the Hudson Bay in March 2008. Their specific surface area (SSA) was measured using CH4 adsorption at 77 K. The Brunauer-Emmett-Teller analysis produced SSA values between 63 and 299 cm(2) g(-1) (mean 162 cm(2) g(-1), accuracy and reproducibility 5%). This range is very similar to that of Domine et al. (2005) but our correlation of results with growth time and chemistry reveals the factors responsible for the wide range of SSA values. Longer growth time leads to higher SSA at low temperatures, so frost flowers are more likely to affect total surface area during colder periods. Chemical analysis was performed on frost flower melt and on local seawater and brine. We examined salinity and sulfate and bromide enrichment. The relationship between growth time and salinity varied spatially because of a freshwater plume from a nearby river and of tidal effects at the coast. Enrichment of certain ions in frost flowers, which affects their contribution to atmospheric chemistry, depends heavily on location, growth time, and temperature. No significant enrichment or depletion of bromide was detected. The low surface area index of frost flowers plus their lack of destruction in wind suggest their direct effect on sea salt mobilization and halogen chemistry may be less than previously thought, but their ability to salinate wind-blown snow may increase their indirect importance.

A new climatology of stratospheric BrO profiles based on a parameterization using dynamical and chemical indicators has been developed, with the aim to apply it to the retrieval of tropospheric BrO columns from space nadir measurements. The adopted parameterization is based on three years of output data from the 3-D chemistry transport model BASCOE. The impact of the atmospheric dynamics on the stratospheric BrO distribution is treated by means of Br-y/ozone correlations built from 3-D-CTM model results, while photochemical effects are taken into account using stratospheric NO2 columns as an indicator of the BrO/Br-y ratio. The model simulations have been optimized for bromine chemistry and budget, and validated through comparisons using an extensive data set of ground-based, balloon-borne and satellite limb (SCIAMACHY) stratospheric BrO observations.

Over the last 30 years, Antarctic mid-tropospheric temperatures in winter have increased by 0.5 K per decade, the largest regional tropospheric warming observed. Over this period, amounts of polar stratospheric cloud(PSC) have also increased, as rising CO2 concentrations cooled the stratosphere. By imposing an idealisation of these increases in PSC within the radiation scheme of an atmosphere-only general circulation model, we find that they could have contributed to the observed warming. The present generation of global climate models do not properly represent PSCs, and so these results demonstrate the need to improve the representation of PSCs. Copyright. (C) 2009 Royal Meteorological Society and Crown Copyright

The source of bromine that drives polar boundary layer ozone depletion events (ODEs) is still open to some debate. While ODEs are generally noted to form under conditions of a shallow stable boundary layer, observations of depleted air under high wind conditions are taken as being transport-related. Here we report observations from Antarctica in which an unusually large cloud of BrO formed over the Weddell Sea. The enhanced BrO was observed over Halley station in coastal Antarctica, providing an opportunity to probe the conditions within an active 'bromine explosion' event. On this occasion, enhanced BrO and depleted boundary layer ozone coincided with high wind speeds and saline blowing snow. We derive a simple model to consider the environmental conditions that favour ODEs and find two maxima, one at low wind/stable boundary layer and one at high wind speeds with blowing snow. Modelling calculations aiming to reproduce the wider regional or global impacts of ODEs, either via radiative effects or as a halogen source, will also need to account for high wind speed mechanisms.

Iodine species in the troposphere are linked to ozone depletion and new particle formation. In this study, a full year of iodine monoxide (IO) columns retrieved from measurements of the SCIAMACHY satellite instrument is presented, coupled with a discussion of their uncertainties and the detection limits. The largest amounts of IO are found near springtime in the Antarctic. A seasonal variation of iodine monoxide in Antarctica is revealed with high values in springtime, slightly less IO in the summer period and again larger amounts in autumn. In winter, no elevated IO levels are found in the areas accessible to satellite measurements. This seasonal cycle is in good agreement with recent ground-based measurements in Antarctica. In the Arctic region, no elevated IO levels were found in the period analysed. This implies that different conditions with respect to iodine release exist in the two Polar Regions. To investigate possible release mechanisms, comparisons of IO columns with those of tropospheric BrO, and ice coverage are described and discussed. Some parallels and interesting differences between IO and BrO temporal and spatial distributions are identified. Overall, the large spatial coverage of satellite retrieved IO data and the availability of a long-term dataset provide new insight about the abundances and distributions of iodine compounds in the troposphere.

We present results of multiple regressions of the leading mode of atmospheric variability at southern high latitudes: the Southern Annular Mode (SAM). It is regressed against indices with large interannual variability, and one of several trend indices in order to determine which trend term gives the optimum fit. We use SAM in sea-level pressure from station data in order to provide a long time series, from 1957 to 2005. The regression indices are stratospheric volcanic aerosol, solar activity, the quasi-biennial oscillation (QBO), the El Nino-Southern Oscillation, together with either a linear trend, or the effective equivalent stratospheric chlorine (EESC) that depletes polar ozone, or ozone mass deficit (OMD) in the Antarctic vortex. We find a statistically significant signal for volcanic aerosol, and, when the solar and QBO indices are represented by their product, a highly significant response to the product. We find a significant linear trend in SAM, but there is a major increase in significance using EESC2 and a further increase using OMD. We make no direct attempt to identify cause and effect, but if the trend is due to human influence then ozone loss is at least 9 times more likely the principle cause of the trend in SAM than greenhouse gases, although we do not exclude greenhouse gases making a smaller contribution. Monthly and seasonal regressions show a maximum correlation with OMD between December and May (summer and autumn), consistent with previous work on stratospheric change as a cause of change in the troposphere.

We present a multiple regression analysis of time series of the leading modes of variability in high latitudes, the Northern and Southern Annular Modes (NAM and SAM). The potential forcing factors we investigate include solar activity and stratospheric (volcanic) aerosol, the quasi-biennial oscillation, the El Niño-Southern Oscillation (ENSO) and stratospheric ozone concentrations. We investigate levels from the surface to the middle stratosphere and we also conduct a similar analysis of the North Atlantic Oscillation (NAO) in surface pressure. We find a significant trend in SAM at all levels and in surface NAM but not in the NAO. There is some correlation of ENSO with both the SAM and NAM, with warm events being associated with weakening of the polar vortices. Strong volcanic aerosol signals are found with different signs in the two hemispheres. There is a clear solar influence on the NAO but no statistically significant signals of solar forcing are found in either the NAM or SAM. The QBO is not detected except, marginally, in the NAM in winter. However, when a new index, the product of the solar and QBO indices, is used in place of the two factors individually then a good correlation is found throughout the atmosphere in the SAM, and at the lower levels in the winter NAM. The different results for the NAO and surface NAM lead us to conclude, speculatively, that a (solar) stratospheric influence on the troposphere may well be through two different routes. The first of these is the influence of low latitude lower stratospheric heating on the Hadley circulation and mid-latitude eddies and thus on the NAO; the second is modulation, by a combination of solar and QBO forcing, of the polar stratosphere which influences the annular modes in both the northern and southern hemispheres.

Calculations of equivalent length from an artificial advected tracer provide new insight into the isentropic transport processes occurring within the Antarctic stratospheric vortex. These calculations show two distinct regions of approximately equal area: a strongly mixed vortex core and a broad ring of weakly mixed air extending out to the vortex boundary. This broad ring of vortex air remains isolated from the core between late winter and midspring. Satellite measurements of stratospheric H2O confirm that the isolation lasts until at least mid-October. A three-dimensional chemical transport model simulation of the Antarctic ozone hole quantifies the ozone loss within this ring and demonstrates its isolation. In contrast to the vortex core, ozone loss in the weakly mixed broad ring is not complete. The reasons are twofold. First, warmer temperatures in the broad ring prevent continuous polar stratospheric cloud (PSC) formation and the associated chemical processing (i.e., the conversion of unreactive chlorine into reactive forms). Second, the isolation prevents ozone-rich air from the broad ring mixing with chemically processed air from the vortex core. If the stratosphere continues to cool, this will lead to increased PSC formation and more complete chemical processing in the broad ring. Despite the expected decline in halocarbons, sensitivity studies suggest that this mechanism will lead to enhanced ozone loss in the weakly mixed region, delaying the future recovery of the ozone hole.

Photographs of clouds taken with a camera with a large aperture ratio must have a short depth of focus to resolve small droplets. Hence the sampling volume is small, which limits the number of droplets and gives rise to a large statistical error on the number counted. However, useful signals can be obtained with a small aperture ratio, which allows for a sample volume large enough for counting cloud droplets at aircraft speeds with useful spatial resolution. The signal is sufficient to discriminate against noise from a sunlit cloud as background, provided the bandwidth of the light source and camera are restricted, and against readout noise. Hence, in principle, an instrument to sample the size distribution of cloud droplets from aircraft in daylight can be constructed from a simple TV camera and an array of laser diodes, without any components or screens external to the aircraft window.

A SAOZ (System d'Analyse et d'Observations Zenithales) u.v.-visible spectrometer was installed at Faraday, Antarctica, in 1990, with a view to gaining year-round measurements of column ozone. Observations have been validated by comparison with the Faraday Dobson. Agreement between the two instruments is good, with SAOZ measuring 3 ± 2% lower than the Dobson over the comparison period. Both instruments show the same small-scale variation in column ozone. However, there is a notable seasonal signal in the ratio of (SAOZ column ozone)/(Dobson column ozone) which calls into question the validity of the SAOZ winter column ozone measurements made when the solar elevation is too low for making Dobson observations. The problem appears to lie with the calculated air-mass factors (AMFs) used in deriving vertical columns from the SAOZ slant columns. New SAOZ data were derived using empirical AMFs. The qualitative behaviour of winter column ozone is the same for both sets of AMFs, showing an increase to mid-winter, followed by a decrease to the spring minimum, although smaller mid-winter columns are calculated when using empirical AMFs.