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Observations of peroxynitric acid (HO2NO2) and nitric acid (HNO3) were made during a 4 month period of Antarctic winter darkness at the coastal Antarctic research station, Halley. Mixing ratios of HNO3 ranged from instrumental detection limits to ∼8 parts per trillion by volume (pptv), and of HO2NO2 from detection limits to ∼5 pptv; the average ratio of HNO3 : HO2NO2 was 2.0(± 0.6):1, with HNO3 always present at greater mixing ratios than HO2NO2 during the winter darkness. An extremely strong association existed for the entire measurement period between mixing ratios of the respective trace gases and temperature: for HO2NO2, R2 = 0.72, and for HNO3, R2 = 0.70. We focus on three cases with considerable variation in temperature, where wind speeds were low and constant, such that, with the lack of photochemistry, changes in mixing ratio were likely to be driven by adsorption/desorption mechanisms alone. We derived enthalpies of adsorption (ΔHads) for these three cases. The average ΔHads for HNO3 was −42 ± 7 kJ mol−1 and for HO2NO2 was −56 ± 3 kJ mol−1; these values are extremely close to laboratory-derived values. This exercise demonstrates (i) that adsorption to/desorption from the snow pack should be taken into account when addressing budgets of boundary layer HO2NO2 and HNO3 at any snow-covered site, and (ii) that Antarctic winter can be used as a~natural "laboratory in the field" for testing data on physical exchange mechanisms.

CHABLIS (Chemistry of the Antarctic Boundary Layer and the Interface with Snow) was a collaborative UK research project aimed at probing the detailed chemistry of the Antarctic boundary layer and the exchange of trace gases at the snow surface. The centre-piece to CHABLIS was the measurement campaign, conducted at the British Antarctic Survey station, Halley, in coastal Antarctica, from January 2004 through to February 2005. The campaign measurements covered an extremely wide range of species allowing investigations to be carried out within the broad context of boundary layer chemistry. Here we present an overview of the CHABLIS campaign. We provide details of the measurement location and introduce the Clean Air Sector Laboratory (CASLab) where the majority of the instruments were housed. We describe the meteorological conditions experienced during the campaign and present supporting chemical data, both of which provide a context within which to view the campaign results. Finally we provide a brief summary of highlights from the measurement campaign. Unexpectedly high halogen concentrations profoundly affect the chemistry of many species at Halley throughout the sunlit months, with a secondary role played by emissions from the snowpack. This overarching role for halogens in coastal Antarctic boundary layer chemistry was completely unanticipated, and the results have led to a step-change in our thinking and understanding.

Observations of the unique chemical environment over snow and ice in recent decades, particularly in the polar regions. have stimulated increasing interest in the boundary layer processes that mediate exchanges between the ice/snow interface and the atmosphere. This paper provides a review of the underlying concepts and examples from recent field studies in polar boundary layer meteorology, which will generally apply to atmospheric flow over snow and ice surfaces. It forms a companion paper to the chemistry review papers in this special issue of ACP that focus on processes linking halogens to the depletion of boundary layer ozone in coastal environments, mercury transport and deposition, snow photochemistry, and related snow physics. In this context, observational approaches, stable boundary layer behavior, the effects of a weak or absent diurnal cycle, and transport and mixing over the heterogeneous surfaces characteristic of coastal ocean environments are of particular relevance.

A one-dimensional chemical transport model has been developed to investigate the vertical gradients of bromine and iodine compounds in the Antarctic coastal boundary layer (BL). The model has been applied to interpret recent year-round observations of iodine and bromine monoxides (IO and BrO) at Halley Station, Antarctica. The model requires an equivalent I atom flux of similar to 10(10) molecule cm(-2) s(-1) from the snowpack in order to account for the measured IO levels, which are up to 20 ppt during spring. Using the current knowledge of gas-phase iodine chemistry, the model predicts significant gradients in the vertical distribution of iodine species. However, recent ground-based and satellite observations of IO imply that the radical is well-mixed in the Antarctic boundary layer, indicating a longer than expected atmospheric lifetime for the radical. This can be modelled by including photolysis of the higher iodine oxides (I2O2, I2O3, I2O4 and I2O5), and rapid recycling of HOI and INO3 through sea-salt aerosol. The model also predicts significant concentrations (up to 25 ppt) of I2O5 in the lowest 10 m of the boundary layer. Heterogeneous chemistry involving sea-salt aerosol is also necessary to account for the vertical profile of BrO. Iodine chemistry causes a large increase (typically more than 3-fold) in the rate of O-3 depletion in the BL, compared with bromine chemistry alone. Rapid entrainment of O-3 from the free troposphere appears to be required to account for the observation that on occasion there is little O-3 depletion at the surface in the presence of high concentrations of IO and BrO. The halogens also cause significant changes to the vertical profiles of OH and HO2 and the NO2/NO ratio. The average Hg-0 lifetime against oxidation is also predicted to be about 10 h during springtime. An important result from the model is that very large fluxes of iodine precursors into the boundary layer are required to account for the observed levels of IO. The

1-D profiles and time series from an idealised atmospheric boundary layer model are presented, which show agreement with boundary layer measurements of polar NOx. Diffusion models are increasingly being used as the framework for studying tropospheric air chemistry dynamics. Models based on standard boundary layer diffusivity profiles have an intrinsic behaviour that is not necessarily intuitive, due to the variation of turbulent diffusivity with height. The simple model presented captures the essence of the evolution of a trace gas released at the surface, and thereby provides both a programming and a conceptual tool in the analysis of observed trace gas evolution. A time scale inherent in the model can be tuned by fitting model time series to observations. This scale is then applicable to the more physically simple but chemically complex zeroth order or box models of chemical interactions.

During springtime in the polar regions, unique photochemistry converts inert halide salt ions (e.g. Br-) into reactive halogen species (e.g. Br atoms and BrO) that deplete ozone in the boundary layer to near zero levels. Since their discovery in the late 1980s, research on ozone depletion events (ODEs) has made great advances; however many key processes remain poorly understood. In this article we review the history, chemistry, dependence on environmental conditions, and impacts of ODEs. This research has shown the central role of bromine photochemistry, but how salts are transported from the ocean and are oxidized to become reactive halogen species in the air is still not fully understood. Halogens other than bromine (chlorine and iodine) are also activated through incompletely understood mechanisms that are probably coupled to bromine chemistry. The main consequence of halogen activation is chemical destruction of ozone, which removes the primary precursor of atmospheric oxidation, and generation of reactive halogen atoms/oxides that become the primary oxidizing species. The different reactivity of halogens as compared to OH and ozone has broad impacts on atmospheric chemistry, including near complete removal and deposition of mercury, alteration of oxidation fates for organic gases, and export of bromine into the free troposphere. Recent changes in the climate of the Arctic and state of the Arctic sea ice cover are likely to have strong effects on halogen activation and ODEs; however, more research is needed to make meaningful predictions of these changes.

The Antarctic research stations of Dome C and Halley lie at similar latitudes (∼75°S) and are thus subject to similar diurnal variation of solar radiation at the top of the atmosphere. However, the response of the atmospheric boundary layer to this diurnally varying forcing differs greatly at the two stations. At Dome C during summer there is a strong diurnal cycle in near-surface temperature and wind speed, and a shallow (∼350 m) convective boundary layer is observed to grow in response to diurnal heating. At Halley, diurnal variations in temperature and wind speed are smaller than those at Dome C, and there is no clear diurnal variability in boundary layer depth. Analysis of the summertime surface energy budget for both stations indicates that the main reason for the different diurnal variability at the two stations is the greater partitioning of available energy into latent heat flux at the warmer Halley station. We argue that the diurnally varying convective boundary layer observed at Dome C will not be typical of the whole of the East Antarctic plateau.

Since March 2003, measurements of surface ozone have been made at the British Antarctic Survey Clean Air Sector Laboratory (CASLab) at Halley station in coastal Antarctica. Detailed measurements of boundary layer meteorology, as well as standard meteorological parameters, are also measured at the CASLab. Combining these data allows us to probe the transport pathway of air masses during ozone depletion events (ODEs). ODEs were observed at Halley on several occasions during Antarctic spring 2003. On some occasions, extremely rapid loss of ozone was observed (loss of 16 ppbv in 1 min on one occasion), which was associated with regional-scale transport. For each such event during 2003, the air mass originated in the southern Weddell Sea, an area of vigorous sea-ice production. On other occasions the development of the event and its recovery were strongly associated with the build-up and decline of a stable boundary layer. In these cases, air masses had had recent contact with a nearby open water lead where sea-ice production is known to occur. The data presented here are entirely consistent with the idea that halogens responsible for ozone loss are derived during new sea-ice formation from an associated surface such as brine slush or frost flowers.

Small anomalies in ice-shelf surface temperature correlate with measured microtopography. Clear-sky thermal infrared (TIR) images of the Brunt Ice Shelf, Antarctica, frequently show persistent patterns of anomalous snow surface temperatures. The anomalous signatures appear as stripes orientated along the ice flowline and are of the order of 5 K in magnitude. The positional persistence of the stripes suggests a topographic mechanism for their formation. In order to test this hypothesis, the TIR stripes are compared to a digital terrain model (DTM) derived from a kinematic global positioning system survey of the ice shelf. Ridges and valleys are seen in the DTM; the ridges correspond to the warmer TIR stripes, the valleys to the colder areas. In order to investigate the mechanism that couples elevation with thermal signature, two comparable but contrasting sets of clear-sky infrared images are presented, along with surface meteorological data. The first shows strong TIR stripes, whilst the second, despite similar snow- and air-temperature profiles, shows a weaker signature and smaller sensible-heat flux, H. Two possible mechanisms are presented which explain the TIR signature: surface elevation mapping onto the vertical air-temperature profile or, alternatively, enhanced surface sensible-heat flux on elevated areas. At present, there is insufficient information to resolve this uncertainty.

Redistribution of snow by the wind can drive spatial and temporal variations in snow accumulation that may affect the reconstruction of paleoclimate records from ice cores. In this paper we investigate how spatial variations in snow accumulation along a 13 km transect across Lyddan Ice Rise, Antarctica, are related to wind-borne snow redistribution. Lyddan Ice Rise is an approximately two-dimensional ridge which rises about 130 m above the surrounding ice shelves. Local slopes on its flanks never exceed 0.04. Despite this very smooth profile, there is a pronounced gradient in snow accumulation across the feature. Accumulation is highest on the ice shelf to the east ( climatologically upwind) of the ice rise and decreases moving westward, with the lowest accumulation seen to the west ( climatologically downwind) of the ice rise crest. Superimposed on this broad-scale gradient are large ( 20-30%), localized variations in accumulation on a scale of around 1 km that appear to be associated with local variations in surface slope of less than 0.01. The broad-scale accumulation gradient is consistent with estimates of wind-borne redistribution of snow made using wind speed observations from three automatic weather stations. The small-scale variability in accumulation is reproduced quite well using a snow transport model driven by surface winds obtained from an airflow model, providing that both the wind shear and static stability of the upwind flow are taken into account. We conclude that great care needs to be exercised in selecting ice core sites in order to avoid the possibility of blowing snow transport confounding climate reconstructions.

It has been an underlying assumption in many studies that near-surface layers imaged by ground-penetrating radar (GPR) can be interpreted as depositional markers or isochrones. It has been shown that GPR layers can be approximately reproduced from the measured electrical properties of ice, but these material layers are generally narrower and more closely spaced than can be resolved by typical GPR systems operating in the range 50-400 MHz. Thus GPR layers should be interpreted as interference patterns produced from closely spaced and potentially discontinuous material layers, and should not be assumed to be interpretable as precise markers of isochrones. We present 100 MHz GPR data from Lyddan Ice Rise, Antarctica, in which near-surface (< 50 m deep) layers are clearly imaged. The growth of the undulations in these layers with depth is approximately linear, implying that, rather than resulting from a pattern of vertical strain rate, they do correspond to some pattern of snowfall variation. Furthermore, comparison of the GPR layers with snow-stake measurements suggests that around 80% of the rms variability in mean annual accumulation is present in the GPR layers. The observations suggest that, at least in this case, the GPR layers do approximate isochrones, and that patterns of snow accumulation over Lyddan Ice Rise are dominated by extremely persistent spatial variations with only a small residual spatial variability. If this condition is shown to be widely applicable it may reduce the period required for measurements of surface elevation change to be taken as significant indications of mass imbalance.

Co-located high resolution profiles of acoustic backscatter, wind vector and potential temperature are presented, measured within the stable atmospheric boundary layer over an Antarctic ice shelf. Acoustic profiles from a monostatic acoustic radar (Sodar) indicate complex structure within the boundary layer, whilst wind and temperature profiles from a tethersonde show corresponding bands of differing stability. Internal waves and fossil convection are shown to invalidate attempts to compare backscatter measurements with theoretical estimates based on local wind and temperature gradients, but during ideal conditions, a qualitative agreement is observed.

Data from a monostatic acoustic radar operating at British Antarctic Survey's Halley station on a coastal Antarctic ice shelf show a band of small target echoes at an altitude of 700-1000 m during spring 1991. Statistical analysis of the echo signature show that the targets are spread more evenly in the horizontal than would be expected for a random signature, whilst the local distribution in the vertical is Gaussian. Similar echo signatures have been observed previously, and are attributed to birds, bats or insects: the Antarctic data are also consistent with bird targets, but the case is not proven. No birds have been observed directly, but at 700 in they would be barely visible to the naked eye. The nearest nesting area with suitably large numbers of birds (Antarctic petrels, Thalassoica antarctica) is a few hundred kilometres away. Estimates of velocity and target density imply that the Halley site would need to be specially favoured by the colony for their acoustic signature to be observed in such numbers, but such might be the case due to the presence of perennial coastal open water to the west of the station.

The surface climatology of Coats Land, Antarctica, is described through observations from automatic weather stations, from Halley station, from upper air soundings and from satellite remote sensing. Coats Land consists of the Brunt Ice Shelf and the adjoining continent to the South. The topography of this region is typical of much of the Antarctic coastal fringes: a modest slope (5% at most) and relative uniformity across the slope. A basic climatology broken into site and season is presented. In winter, and to an extent in the equinoctial seasons, the region clearly divides into two dynamical regimes. Over the ice shelf winds are usually from the east or occasionally from the west, whereas over the continental slopes winds are from the east to south quadrant. Over the ice shelf the surface layer is about 10 K colder, in terms of potential temperature, than on the continent, and is also more stable than on the steeper parts of the slope. Motivated by case studies. three criteria are developed to select a subset of the data that are katabatic in the sense that the flow is believed to be primarily due to a downslope buoyancy forcing. On the continental slope, the Criteria pick out a coherent Subset of the data that are tightly clustered in wind speed and wind direction. Typical katabatic winds are from 10degrees to the east of the fall line and 7.5 ms(-1) at the steepest part of the slope (5.1 ms(-1) higher up). They are rarely more than 15 ms(-1) in this region; hence their description as ordinary, in contrast with those extraordinary katabatic regimes that have been the focus of previous studies. The katabatic flow remains close to adiabatic as it moves down the slope, and is relatively dry near the slope foot. We estimate the flow to be primarily katabatic at most 40-50% of the time, although it may appear to be katabatic, from wind speed and wind direction characteristics, some 60-70% of the time. There is no coherent katabatic-flow signature on the ice shelf.

It has been shown that NOx is produced photochemically within the snowpack of polar regions. If emitted to the atmosphere, this process could be a major source of NOx in remote snowcovered regions. We report here on measurements made at the German Antarctic station, Neumayer, during austral summer 1999, aimed at detecting and quantifying emissions of NOx from the surface snow. Gradients of NOx were measured, and fluxes calculated using local meteorology measurements. On the 2 days of flux measurements, the derived fluxes showed continual release from the snow surface, varying between similar to0 and 3x10(8) molecs/cm(2)/s. When not subject to turbulence, the variation was coincident with the uv diurnal cycle, suggesting rapid release once photochemically produced. Scaling the diurnal average of Feb. 7th (1.3x10(8) molecs/cm(2)/s) suggests an annual emission over Antarctica of the order 0.0076TgN.

We have used micrometeorological data collected at Halley Research Station, Antarctica, to estimate monthly totals of snow sublimation. Direct sublimation from the snow surface is calculated using bulk-transfer formulae, while the sublimation of blowing snow is estimated using a model for suspended-particle number density and individual particle sublimation rates. During the winter months, sublimation losses are negligible, but between November and March sublimation removes around 25% of the snowfall. Surface sublimation and sublimation of blowing snow make roughly equal contributions to this total. Estimates of sublimation using micrometeorological data agree well with estimates made from daily snow-stake measurements.

Measurements of blowing snow particle concentration from the second Stable Antarctic Boundary Layer Experiment (STABLE 2) are presented. The measurements, made at Halley Station (75.6 degreesS, 26.7 degreesW) throughout the 1991 austral winter, are supplemented with profile measurements of wind speed, air temperature, and humidity. Threshold wind speeds for blowing snow are shown to be distinctly different for various episodes, often depending strongly on the the availability of loose snow. Blowing snow is measured to occur at Halley between 27 and 37% of the time during winter. Total winter (June, July, August) blowing snow sublimation at Halley is calculated to be around 4.7 mm water equivalent, 3.7% of total accumulation over the same period. Total winter blowing snow mass transport is calculated to be around 5.5 x 10(5) kg per metre width. Measured humidity profiles at Halley show that during winter blowing snow conditions, a layer of near-saturated air forms, causing total sublimation to be less than that for seasonal or limited fetch snow covers. The extent of sublimation is shown to be strongly dependent on wind speed, temperature and fetch.

Because investigations of PAN at higher southern latitudes are very scarce, we measured surface PAN concentrations for the first time in Antarctica. During the Photochemical Experiment at Neumayer (PEAN'99) campaign mean surface PAN mixing ratios of 13 +/- 7 pptv and maximum values of 48 pptv were found. When these PAN mixing ratios were compared to the sum of NOx and inorganic nitrate they were found to be equal or higher. Low ambient air temperatures and low PAN concentrations caused a slow homogeneous PAN decomposition rate of approximately 5 x 10(-2) pptv h(-1) These slow decay rates were not sufficient to firmly establish the simultaneously observed NOx concentrations. In addition, low concentration ratios of [HNO3]/[NOx] imply that the photochemical production of NOx within the snow pack can influence surface NOx mixing ratios in Antarctica. Alternate measurements of PAN mixing ratios at two different heights above the snow surface were performed to derive fluxes between the lower troposphere and the underlying snow pack using calculated friction velocities. Most of the concentration differences were below the precision of the measurements. Therefore, only an upper limit for the PAN flux of +/- 1x10(13) molecules m(-2) s(-1) without a predominant direction can be estimated. However, PAN fluxes below this limit can still influence both the transfer of nitrogen compounds between atmosphere and ice, and the PAN budget in higher southern latitudes. (C) 2000 Elsevier Science Ltd. All rights reserved.

Internal gravity waves are frequently observed in stably stratified regions of the atmospheric boundary layer. In order to determine the statistical influence of such waves on the dynamics of the boundary layer it is necessary to compile information concerning properties of the waves such as frequency of occurrence, propagation, and spectral characteristics. Gravity wave climatologies have been compiled from relatively few locations. In this paper a climatological study of gravity waves, in the period range 1-20 min, propagating in the stably stratified atmospheric boundary layer overlying an Antarctic ice shelf is presented. An extensive set of boundary layer measurements were compiled throughout 1991. Surface pressure fluctuations were recorded from a spatial array of six sensitive microbarographs. Wind and temperature records from an instrumented mast were also available. A beam-steering technique has been used to determine wave parameters from the surface pressure data. The microbarographs detected the presence of internal gravity waves throughout the observational campaign. Root-mean-square pressure values were typically in the region 16-40 mu b, but a significant number of isolated events with amplitudes of up to 180 mu b were also found. Wave properties have been studied in conjunction with the mean wind and temperature profiles in the boundary layer. It was found that most of the wave activity did not originate locally, but from shear layers aloft, or, more commonly, from the katabatic how regime where the ice shelf joins the Antarctic continent.

The occurence of a diurnal variability in measured total oxidised nitrogen (NOy) was observed at Neumayer, Antarctica, (70 degrees 39'8, 8 degrees 15'W) during a recent summer measurement campaign. Minima and maxims occurred in the early morning/early evening respectively, with the amplitude of the cycle around 40% of the daily mean NOy values. Given that this campaign was the first to attempt NOy measurements on the Antarctic continent, it is not presently clear whether this is an Antarctic-wide phenomenon, or local to Neumayer. A similar cycle was observed for HNO3, although HNO3 concentrations and fluctuations are too small to account for all of the NOy variability. In this paper we investigate possible mechanisms that might cause such diurnal signals, focussing on the influence of local meteorology and also of the snowpack. Exchange processes at the air/snow interface appear to dominate the observed NOy variability, although an influence from the changing surface inversion strength exists. These findings have important implications in understanding and hence correctly interpreting ice core nitrate data.

Measurements of humidity at a height of 4 m were made at Halley Research Station between 1995 and 1997 using a frost point hygrometer. Relative humidities were usually close to ice saturation, particularly during the winter, but ice supersaturations of up to 20% were frequently measured. Supersaturation with respect to liquid water was rarely observed. During the summer, diurnal variation in temperature drove a daily cycle in relative humidity, although corresponding variations in mixing ratio were comparatively small. It is suggested that the high frequency of ice supersaturations and almost total absence of water supersaturation results from the high abundance of condensation nuclei relative to ice nuclei.

While some people involved in the acoustic remote sensing field are aware of the possibility of receiving dot echoes from nonatmospheric targets, most of the papers available in the scientific literature dealing with this phenomenon associate them to atmospheric targets, such as clusters of water vapor inhomogeneity, thermodynamical processes of condensation and reevaporation of water vapor, anisotropic irregularities localized in thin layers, etc. At present, dot echoes are defined by their appearance on the echogram and are nor differentiated by causative processes. As such, they share similar characteristics, such as being randomly distributed and having a time length that is similar to the time length of the emitted tone. In this paper dot echoes conforming to this definition are investigated through the analysis of the signal in both the time and frequency domain. The timescale of a dot signature along with the configuration of the sodar system provide an upper limit to the size of the targets producing these echoes. The spectral characteristics and the first and second momenta of the echoes are compared with clear-air echoes as well as with echoes produced by pilot balloons released from nearby sodar antennas. The conclusion is that the dot echoes analyzed in this paper are reflections from birds and are not due to atmospheric effects.

Large amplitude, isolated, wave-like phenomena have been observed in the lowest 40 m of the strongly stably stratified atmospheric boundary layer overlying a coastal Antarctic ice shelf. The waves only occur when prevailing wind speeds are low. They always propagate from over the land, with phase speeds exceeding the local mean wind speed. They have wavelengths of the order of 200 m. Several examples are described and a summary of the statistical properties of these waves events is presented.

We present measurements of the components of the energy and mass balance of the snow surface at Halley Research Station, Antarctica. During the winter months, when insolation is small or zero, the surface energy balance is dominated by radiative cooling. This is mostly balanced by a downward transport of atmospheric sensible heat, with an upward conductive flux of heat through the snowpack making a secondary contribution. The average flux of atmospheric latent heat is downward but of negligible importance in the surface energy balance. During the winter, a significant imbalance is seen in the measured energy budget, with insufficient sensible and conductive heat fluxes to balance the radiative cooling. The wintertime surface mass balance is dominated by precipitation. Sublimation of blowing snow makes a small negative contribution to the budget and is observed to be highly dependent on wind speed. It is suggested that this may be an important mechanism for removing surface mass in some parts of Antarctica.

The theory of operation of humidity sensors has not been widely discussed in the literature. In particular, no convincing explanation has been given as to why they respond to relative humidity (RH) rather than to an absolute measure of humidity, such as vapor pressure or mixing ratio, over a wide temperature range. A simple mechanism based on the adhesion of surface water to polarized solids is presented that explains many of the properties of these sensors.

In the Antarctic winter of 1991 an extensive series of micro- meteorological and snow cover measurements were made as part of the second Stable Boundary Layer Experiment (STABLE II) at Halley Station on the Brunt Ice Shelf, Antarctica, These data have been used to validate the DAISY distributed snow model using parameter values deter­ mined by calibration using independent data collected at Halley during the International Geophysical Year 1957/1958. These "effective" para­ meter values are consistent with values determined by other independent field and laboratory experiments on polar snow. Simulation of the STABLE data is successful except during periods of very high stability in the boundary layer. A better simulation of the snow surface tempera­ ture is obtained by using a roughness length of 5 cm for heat and water vapour transport rather than the value of 0.01 cm used for the aero­ dynamic roughness length. This suggests that the magnitude of turbulent heat and mass transfer may be larger than hitherto assumed in modelling the response of polar snow to climate change.