Characterisation of vertical BrO distribution during events of enhanced tropospheric BrO in Antarctica, from combined remote and in-situ measurements

Tropospheric BrO was measured by a ground-based remote-sensing spectrometer at Halley in Antarctica in spring 2007, and BrO was measured by satellite-borne remote-sensing spectrometers using similar spectral regions and similar Differential Optical Absorption Spectroscopy (DOAS) analyses. Near-surface BrO was simultaneously measured in situ at Halley by Chemical Ionisation Mass Spectrometer (CIMS), and in an earlier year near-surface BrO was measured at Halley over a long path by a ground-based DOAS spectrometer. During enhancement episodes, total amounts of tropospheric BrO from the ground-based remote-sensor were similar to those from space, but if we assume that the BrO was confined to the mixed layer they were very much larger than values measured by either near-surface technique. This large apparent discrepancy can be resolved if substantial amounts of BrO were in the free troposphere during most enhancement episodes. Amounts observed by the ground-based remote sensor at different elevation angles, and their formal inversions to vertical profiles, demonstrate that much of the BrO was indeed often in the free troposphere. This is consistent with the ~5 day lifetime of Bry and with the enhanced BrO observed during some Antarctic blizzards

Ozone profiles in the high-latitude stratosphere and lower mesosphere measured by the Improved Limb Atmospheric Spectrometer (ILAS)-II: comparison with other satellite sensors and ozonesondes

A solar occultation sensor, the Improved Limb Atmospheric Spectrometer (ILAS)-II, measured 5890 vertical profiles of ozone concentrations in the stratosphere and lower mesosphere and of other species from January to October 2003. The measurement latitude coverage was 54–71°N and 64–88°S, which is similar to the coverage of ILAS (November 1996 to June 1997). One purpose of the ILAS-II measurements was to continue such high-latitude measurements of ozone and its related chemical species in order to help accurately determine their trends. The present paper assesses the quality of ozone data in the version 1.4 retrieval algorithm, through comparisons with results obtained from comprehensive ozonesonde measurements and four satellite-borne solar occultation sensors. In the Northern Hemisphere (NH), the ILAS-II ozone data agree with the other data within ±10% (in terms of the absolute difference divided by its mean value) at altitudes between 11 and 40 km, with the median coincident ILAS-II profiles being systematically up to 10% higher below 20 km and up to 10% lower between 21 and 40 km after screening possible suspicious retrievals. Above 41 km, the negative bias between the NH ILAS-II ozone data and the other data increases with increasing altitude and reaches 30% at 61–65 km. In the Southern Hemisphere, the ILAS-II ozone data agree with the other data within ±10% in the altitude range of 11–60 km, with the median coincident profiles being on average up to 10% higher below 20 km and up to 10% lower above 20 km. Considering the accuracy of the other data used for this comparative study, the version 1.4 ozone data are suitably used for quantitative analyses in the high-latitude stratosphere in both the Northern and Southern Hemisphere and in the lower mesosphere in the Southern Hemisphere

Polar tropospheric ozone depletion events observed in the International Geophysical Year of 1958

The Royal Society expedition to Antarctica established a base at Halley Bay, in support of the International Geophysical Year of 1957–1958. Surface ozone was measured during 1958 only, using a prototype Brewer-Mast sonde. The envelope of maximum ozone was an annual cycle from 10 ppbv in January to 22 ppbv in August. These values are 35% less at the start of the year and 15% less at the end than modern values from Neumayer, also a coastal site. This may reflect a general increase in surface ozone since 1958 and differences in summer at the less windy site of Halley, or it may reflect ozone loss on the inlet together with long-term conditioning. There were short periods in September when ozone values decreased rapidly to near-zero, and some in August when ozone values were rapidly halved. Such ozone-loss episodes, catalysed by bromine compounds, became well-known in the Artic in the 1980s, and were observed more recently in the Antarctic. In 1958, very small ozone values were recorded for a week in midwinter during clear weather with light winds. The absence of similar midwinter reductions at Neumayer, or at Halley in the few measurements during 1987, means we must remain suspicious of these small values, but we can find no obvious reason to discount them. The dark reaction of ozone and seawater ice observed in the laboratory may be fast enough to explain them if the salinity and surface area of the ice is sufficiently amplified by frost flowers

The Brewer–Dobson circulation in the stratosphere and mesosphere – Is there a trend?

The Brewer–Dobson circulation brings tropospheric air, accompanied by CFCs and greenhouse gases, into the stratosphere. Many models predict an increased circulation associated with an increase in greenhouse gases, such as that since the 1960s. A recent observation supports this: the rate at which total ozone increases in Antarctica during early winter is consistent with the descent and convergence that are part of the Brewer–Dobson circulation; at 65°S the rate doubled between the 1960s and 1990s. Another recent observation may also support this: the decrease in temperature since 1960 in the Antarctic mid-winter lower stratosphere is much less than the decrease calculated from the greenhouse effect of increased H2O, suggesting less CH4 oxidation in the 1970s; this could be caused by an increase in Brewer–Dobson circulation during the 1970s. An important paradox may be resolved by an increase in Brewer–Dobson circulation: the decrease in tropical cold-point temperature since the 1960s conflicts with the increase in mid-latitude H2O in the lower stratosphere, if it represents an increase at tropical entry of H2O; the conflict could be resolved if dehydration during stratospheric entry is incomplete and the circulation has increased

Possible descent across the 'Tropopause' in Antarctic winter

Descent of air from stratosphere to troposphere in Antarctic winter is proposed to be feasible, because of forcing from above by subsidence plus wave-breaking, together with suction from below to resupply the katabatic winds which flow down the slopes of the Antarctic Plateau in the boundary layer. In Antarctic winter, there is no real tropopause to prevent such descent, hence the quotes in the title: the temperature profile is often that of a radiative equilibrium atmosphere. Such descent would be important because the dryness and low precipitation over the Antarctic Plateau would be less altered during global warming, because there would be an alternative fractionation pathway for H2O18 and HDO in Antarctic ice-cores, and because the ozone budget in the unpolluted troposphere of the southern hemisphere would be significantly different. Each of these features could have a major impact on climate or on the study of climate

A review of stratospheric H2O and NO2

Twenty years ago there were large disagreements between instruments measuring stratospheric H2O and NO2, and there were no reliable long-term records. Now, there is greatly improved agreement between techniques, there are 20-year records of profiles of stratospheric H2O and of total NO2 at single sites, there is a qualified H2O record extending back 40 years and 12-year records of total NO2 from many sites, and there are reliable global measurements from satellites to form the basis of climatologies. This excellent progress is marred by a discrepancy between the observed trend in lower stratospheric H2O and temperatures at the tropical cold point, and by a possible discrepancy between the observed trend in total NO2 and the trend in the source of NO2. The second discrepancy would be resolved by a trend in the residual circulation in the stratosphere, in the same way as variability in the residual circulation was responsible for variability in the trend in H2O in the upper stratosphere in the 1990s

Antarctic ozone loss in 1979-2010: first sign of ozone recovery

A long-term ozone loss time series is necessary to understand the evolution of ozone in Antarctica. Therefore, we construct the time series using ground-based, satellite and bias-corrected multi-sensor reanalysis (MSR) data sets for the period 1989–2010. The trends in ozone over 1979–2010 are also estimated to further elucidate its evolution in the wake of decreasing halogen levels in the stratosphere. Our analysis with ground-based observations shows that the average ozone loss in the Antarctic is about −33 to −50% (−90 to −155 DU (Dobson Unit)) in 1989–1992, and then stayed at around −48% (−160 DU). The ozone loss in the warmer winters (e.g. 2002 and 2004) is lower (−37 to −46%), and in the very cold winters (e.g. 2003 and 2006) it is higher (−52 to −55%). These loss estimates are in good agreement with those estimated from satellite observations, where the differences are less than ±3%. The ozone trends based on the equivalent effective Antarctic stratospheric chlorine (EEASC) and piecewise linear trend (PWLT) functions for the vortex averaged ground-based, Total Ozone Mapping Spectrometer/Ozone Monitoring Instrument (TOMS/OMI), and MSR data averaged over September–November exhibit about −4.6 DU yr−1 over 1979–1999, corroborating the role of halogens in the ozone decrease during the period. The ozone trends computed for the 2000–2010 period are about +1 DU yr−1 for EEASC and +2.6 DU yr−1 for the PWLT functions. The larger positive PWLT trends for the 2000–2010 period indicate the influence of dynamics and other basis functions on the increase of ozone. The trends in both periods are significant at 95% confidence intervals for all analyses. Therefore, our study suggests that Antarctic ozone shows a significant positive trend toward its recovery, and hence, leaves a clear signature of the successful implementation of the Montreal Protocol

Vertical resolution of oversampled limb-sounding measurements from satellites and aircraft

Measurements of trace gases by remote sensors which observe the atmospheric limb have a natural verticalresolution of 2–. With a typical field of view this becomes 3–. Oversampling, combined with acceptance of a degradation in random error, improves the verticalresolution. We investigate the improvement by means of Backus–Gilbert trade-off curves. Sacrificing a factor 10 in random error results in resolutions of for measurements of weak absorption or emission, and for strong pressure-broadened absorption. Weighting function calculations are technically more difficult for airborne than for satellitelimb-sounders; oversampled functions cause more technical difficulty

Increased stratospheric greenhouse gases could delay recovery of the ozone hole and of ozone loss at southern mid-latitudes

Stratospheric H2O is increasing, and may be responsible for a large part of the observed cooling of the lower stratosphere. Further cooling will lead to more PSCs in the edge of the Antarctic stratospheric vortex in spring, though not in the vortex core which already becomes cold enough for near-continuous PSCs. A new diagnostic of mixing, plus measurements of H2O, show that the vortex edge is weakly mixed with the core until late in the spring. This isolation will allow any increase in PSCs to result in continued severe ozoneloss, despite reduced chlorine due to the Montreal Protocol. The isolated edge region is half the area of the ozonehole. It frequently passes over southern South America late enough in the spring for major UV damage, and in summer the broken-up ozonehole contributes to significant hemisphere-wide ozone loss

The equilibrium constant of NO2 with N2O4 and the temperature dependence of the visible spectrum of NO2: A critical review and the implications for measurements of NO2 in the polar stratosphere

Measurements of stratospheric NO2 by ground-based visible spectrometers rely on laboratory measurements of absorption cross-sections. We review low-temperature laboratory measurements, which disagree by amounts claimed to be significant. Our recalculation of their errors shows that in general disagreements are not significant and that errors in the ratios of cross-sections at low to room temperature are between ±3% and ±8.8%. Of these errors, up to ±3.5% was contributed by errors in the equilibrium constant,Kp, in those measurements where the pressure was above 0.1 mbar. We review measurements and calculations ofKp, which were accurate to ±5% from 300 to 233 K. Each method was potentially flawed. For example, infrared measurements of the partial pressure of NO2 ignored the dependence of absorption on total pressure. From thermodynamic theory, formulae forKpcan be derived from expressions for the variation of heat capacity with temperature. Contrary to common belief, coefficients in the formulae used by spectroscopists were not derived from the thermodynamic quantities. Rather, they were fitted to measurements or to calculations. Hence, they are empirical and it is dangerous to extrapolate below 233 K, the lowest temperature of the measurements. There are no measurements of NO2 cross-sections below 230 K. Extrapolation of these cross-sections to analysis of measurements of NO2 at the low temperatures of the Arctic and Antarctic stratosphere is also dangerous. For satisfactory analysis of polar spectra, the NO2 cross-sections should be measured at temperatures down to 190 K with a relative accuracy of ±1%. This difficult experiment would need a cell of minimum length 32 m whose length can be adjusted. Because their effects are circular, many errors cannot be removed simply. Although circular errors also arise in the measurements ofKpand of the infrared spectrum, their weights differ from those in the visible spectrum. The optimum experiment might therefore simultaneously measure the visible and infrared spectra andKp