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.
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.
Within the framework of the Network for the Detection of Stratospheric Change (NDSC), an intercomparison campaign of ground‐based zenith‐sky viewing UV‐visible spectrometers was held at the Andøya ...Rocket Range (69°N, 16°E) at Andenes, Norway, from February 12 to March 8, 2003. The chosen site is classified as a complementary NDSC site. Eight groups from seven countries participated in the campaign which focused on the measurements of slant columns of NO2, BrO, and OClO. This first campaign publication concentrates on measurements of the NO2 slant columns. Different analysis criteria were investigated during the campaign. These included the use of fitting parameters as chosen by each group to provide what they considered to be optimized retrievals. Additional sets of parameters, imposed for all the groups, were also used, including the wavelength interval, absorption cross sections, and species fitted. Each instrument's results were compared to the measurements of selected reference instruments, whose choice was based on a technique combining regression analysis and examination of the residuals with solar zenith angle. Considering the data obtained during the whole campaign for solar zenith angles between 75° and 95°, all instruments agreed within 5% in the case of NO2 with imposed analysis parameters in the 425–450 nm region. Measurements agree less well when retrieving the NO2 slant columns in the 400–418 nm region or when using parameters optimized by each investigator for their instrument.
Spectroscopic measurements of BrO using direct sun and zenith sky viewing geometries are combined in an optimal estimation retrieval algorithm to obtain tropospheric and stratospheric columns of BrO. ...Twenty‐two twilight periods are investigated over Arrival Heights, Antarctica (77.8°S, 166.7°E) during the polar spring period of 2002. This paper presents the first tropospheric and stratospheric BrO column retrievals from UV‐visible ground‐based measurements for a polar location. A direct comparison is made between stratospheric columns retrieved at 80°, 84°, and 88° solar zenith angles (SZA) from the spectroscopic measurements and those calculated by the SLIMCAT three‐dimensional chemical transport model. The ground‐based column BrO observations are consistent with a SLIMCAT stratospheric Bry loading of 21.2 parts per trillion at 20 km. SLIMCAT reproduces the observed sunrise column BrO increase but does not match the sunset observations, which display less variation. The significant warming of the Antarctic polar stratosphere in 2002 led to highly variable stratospheric columns being observed. The observed column BrO decreased with the transition from vortex to extravortex air on 21 September but did not change much following the return of the vortex on 12 October. For the tropospheric column, an almost normal distribution consistent with a “background” of 0.3 ± 0.3 × 1013 molecules cm−2 is observed from the ground (80°, 84°, and 88° for both sunrise and sunset). A statistically significant “bromine explosion” event (at the 2σ level) was detected at the end of October with a tropospheric column of 1.8 ± 0.1 × 1013 molecules cm−2. The measured tropospheric columns are compared with the tropospheric Model of Atmospheric Transport and Chemistry–Max Planck Institute for Chemistry version model. The tropospheric BrO sunrise column observations can only be explained with an additional bromine source other than decomposition of CH3Br and downward transport of long‐lived bromine from the stratosphere. A comparison with the spaceborne Global Ozone Monitoring Experiment (GOME) found the total columns observed from the ground to be 16–25% smaller than the total columns observed by GOME for SZAs between 80° and 88°.
Spectroscopic measurements of BrO using direct sun and zenith‐sky viewing geometries are combined in an optimal estimation retrieval algorithm to obtain tropospheric and stratospheric columns of BrO. ...Seventy‐two twilight periods are investigated over Lauder, New Zealand (45.0°S, 169.7°E), between March 2001 and April 2003. A direct comparison between tropospheric and stratospheric columns retrieved at 80°, 84°, and 87° solar zenith angles (SZAs) from the spectroscopic measurements and those calculated by the three‐dimensional chemical transport model SLIMCAT shows good agreement. The stratospheric Bry loading of 21 pptv from the SLIMCAT calculations is consistent with the ground‐based measurements. The seasonal and diurnal variation of the stratospheric BrO columns evident from the ground‐based measurement retrievals is well described by the SLIMCAT model. The tropospheric column retrievals illustrate a high variability with a mean value of 0.2 pptv if the troposphere is assumed to be well mixed. An upper limit of 0.9 pptv is established for the ubiquitous BrO tropospheric column at 80° under cloud free conditions.
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 + O2 to the upper limit of current recommendations. However, other possible causes of the discrepancy at high latitudes cannot be ruled out.
Ground‐based measurements of BrO slant column densities (SCDs) were performed using zenith sky DOAS (Differential Optical Absorption Spectroscopy) during autumn (February to May) and spring (August ...to October) of 1995 at Arrival Heights (77.8°S, 166.7°E).
In both August and September, single episodes of sudden large BrO column enhancement (of magnitude 3.5 and 3.2 × 1014 molec. cm−2 respectively) were observed. The episode in August did not coincide with changes of other stratospheric parameters (OClO, NO2 and temperature). Furthermore, the diurnal variation in the SCD during these events was indicative of a tropospheric rather than a stratospheric absorber. The tropospheric BrO mixing ratios deduced from the data are similar to those observed by ground‐based measurements in the Arctic boundary layer (∼30 ppt).
Simultaneous balloon soundings, one during each of the two events, showed statistically significant (2 σ) tropospheric ozone depletion between 0.5 and 2 km in August and 1.5 and 2.8 km in September. Our results strongly suggest that halogen catalysed boundary layer ozone depletion not only occurs in the Arctic but also in Antarctica. This has the implication that Arctic Haze and anthropogenic influence is unlikely as a cause for this phenomenon.
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