Hartwig Gernandt

Halogen Occultation Experiment ozone channel validation

The HALogen Occultation Experiment (HALOE) instrument on UARS observes vertical profiles of ozone and other gases of interest for atmospheric chemistry using the solar occultation technique. A broadband radiometer in the 9.6-μm band is used for ozone measurements. Version 17 ozone retrieved by HALOE is intercompared successfully with about 400 profiles of other sounders, including ozonesondes, lidars, balloons, rocketsondes, and other satellites. Usually, the HALOE data are within the error range of the correlative measurements between about 100 and 0.03 mbar atmospheric pressure. Between about 30 and 1 mbar, HALOE agrees typically within 5%, with a tendency to be low. In the first year of data, larger errors sometimes occur in the lower stratosphere due to the necessary correction for Pinatubo aerosol effects, but these differences do not exceed 20%. The data show internal consistency for sunrise and sunset events at the same locations. Some examples of observed ozone distributions, including polar regions, are given.

Severe chemical ozone loss in the Arctic during the winter of 1995-96

Severe stratospheric ozone depletion is the result of perturbations of chlorine chemistry owing to the presence of polar stratospheric clouds (PSCs) during periods of limited exchange of air between the polar vortex and midlatitudes and partial exposure of the vortex to sunlight. These conditions are consistently encountered over Antarctica during the austral spring. In the Arctic, extensive PSC formation occurs only during the coldest winters, when temperatures fall as low as those regularly found in the Antarctic,,. Moreover, ozone levels in late winter and early spring are significantly higher than in the corresponding austral season,,, and usually strongly perturbed by atmospheric dynamics. For these reasons, chemical ozone loss in the Arctic is difficult to quantify. Here we use the correlation between CH4 and O3 in the Arctic polar vortex to discriminate between changes in ozone concentration due to chemical and dynamical effects. Our results indicate that 120–160 Dobson units (DU) of ozone were chemically destroyed between January and March 1996—a loss greater than observed in Antarctica in 1985, when the ‘ozone hole’ was first reported,. This loss outweighs the expected increase in total ozone over the same period through dynamical effects, leading to an observed net decrease of about 50 DU. This ozone loss arises through the simultaneous occurrence of extremely low Arctic stratospheric temperatures, and large stratospheric chlorine loadings. Comparable depletion is likely to recur because stratospheric cooling, and elevated chlorine concentrations, are expected to persist for several decades.

Ozone depletion in and below the Arctic vortex for 1997

The winter 1996/97 was quite unusual with late vortex formation and polar stratospheric cloud (PSC) development and subsequent record low temperatures in March. Ozone depletion in the Arctic vortex is determined using ozonesondes. The diabatic cooling is calculated with PV-theta mapped ozone mixing ratios and the large ozone depletions, especially at the center of the vortex where most PSC existence was predicted, enhances the diabatic cooling by up to 80%. The average vortex chemical ozone depletion from January 6 to April 6 is 33, 46, 46, 43, 35, 33, 32 and 21 % in air masses ending at 375, 400, 425, 450, 475, 500, 525, and 550 K (about 14–22 km). This depletion is corrected for transport of ozone across the vortex edge calculated with reverse domain-filling trajectories. 375 K is in fact below the vortex, but the calculation method is applicable at this level with small changes. The column integrated chemical ozone depletion amounts to about 92 DU (21%), which is comparable to the depletions observed during the previous four winters.

Possible impact of polar stratospheric processes on mid-latitude vertical ozone distributions

Abstract Balloon-borne ozone soundings have also been performed as part of European Arctic Stratospheric Ozone Experiment (EASOE) at Lindenberg, Hohenpeissenberg and Athens. Differences between these stations will be discussed with regard to the vertical ozone distributions and the features of stratospheric circulation during winter. The results give some evidence on the impact of chemically-disturbed air from the Arctic stratosphere to mid-latitudes from January until the middle of February during EASOE-winter in 1992. Ozone depletion is significantly pronounced at greater altitudes in mid-latitudes as it develops in polar latitudes.

Arctic Study of Tropospheric Aerosol and Radiation (ASTAR) 2000: Arctic haze case study

The ASTAR 2000 (Arctic Study of Tropospheric Aerosol and Radiation) campaign ran from 12 March until 25 April 2000 with extensive flight operations in the vicinity of Svalbard (Norway) from Longyearbyen airport (78.25°N, 15.49°E). It was a joint Japanese (NIPR Tokyo)–German (AWI Bremerhaven/Potsdam) airborne measurement campaign using AWI aircraft POLAR 4 (Dornier 228-101). Simultaneous ground-based measurements were done at the international research site Ny-Ålesund (78.95°N, 11.93°E) in Svalbard, at the German Koldewey station, at the Japanese Rabben station and at the Scandinavian station at Zeppelin Mountain (475 m above sea level). During the campaign 19 profiles of various aerosol properties were measured. In general, the Arctic spring aerosol in the vicinity of Svalbard had significant temporal and vertical variability. A strong haze event occurred between 21 and 25 March in which the optical depth from ground-based observation was 0.18, which was significantly greater than the background value of 0.06. Airborne measurements on 23 March during this haze event showed a high aerosol layer with an extinction coefficient of 0.03 km−1 or more up to 3 km and a scattering coefficient from 0.02 in the same altitude range. From the chemical analyses of airborne measurements, sulfate, soot and sea salt particles were dominant, and there was a high mixing ratio of external soot particles in some layers during the haze event, whereas internal mixing of soot in sulfate was noticeable in some layers for the background condition. We argue that the high aerosol loading is due to direct transport from anthropogenic source regions. In this paper we focus on the course of the haze event in detail through analyses of the airborne and ground-based results.

Validation of ILAS Version 3.10 ozone with ozonesonde measurements

Ozone (O3) measurements made with the Improved Limb Atmospheric Spectrometer (ILAS) onboard the Advanced Earth Observing Satellite (ADEOS) were validated with correlative ozonesonde measurements conducted at five stations, Andoya, Kiruna and Yakutsk in the Northern Hemisphere, and Neumayer and Syowa in the Southern Hemisphere. The ILAS Version 3.10 O3 vertical profiles were compared with 79 correlative ozonesonde measurements that were made within 500 km and 3 hours in distance and time differences, respectively. The comparisons indicate that ILAS O3 typically has an accuracy within 20% between 12 and 35 km. The precision of the ILAS O3 is estimated to be ±10–25% between 12 and 20 km, ±5–7% between 20 and 30 km, and ±5% between 30 and 40 km.

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.

Arctic and Antarctic ozone layer observations: chemical and dynamical aspects of variability and long-term changes in the polar stratosphere

The altitude dependent variability of ozone in the polar stratosphere is regularly observed by balloon-borne ozonesonde observations at Neumayer Station (70°S) in the Antarctic and at Koldewey Station (79°N)in the Arctic. The reasons for observed seasonal and interannual variability and long-term changes are discussed. Differences between the hemispheres are identified and discussed in light of differing dynamical and chemical conditions. Since the mid- 1980s, rapid chemical ozone loss has been recorded in the lower Antarctic stratosphere during the spring season. Using coordinated ozone soundings in some Arctic winters, similar chemical ozone loss rates have been detected related to periods of low temperatures. The currently observed cooling trend of the stratosphere, potentially caused by the increase of anthropogenic greenhouse gases, may further strengthen chemical ozone removal in the Arctic. However, the role of internal climate oscillations in observed temperature trends is still uncertain. First results of a 10000 year integration of a low order climate model indicate significant internal climate variability. on decadal time scales, that may alter the effect of increasing levels of greenhouse gases in the polar stratosphere.

Long‐term climate variability in a simple, nonlinear atmospheric model

A nonlinear, baroclinic, hemispheric, low-order model of the atmosphere with nonzonal orographic and zonal thermal forcings has been constructed. The model is used to investigate the long-term climate variability by running it over 1100 years. The model runs show a chaotic behavior in a realistic parameter range. With and without a seasonal cycle in the thermal forcing, the model generates decadal climate variations which are of the same order as interannual variations. The maximum variability is found in a broad range of periods between 3 and 44 years. Empirical orthogonal function analysis reveals that these fluctuations are predominantly caused by the interaction between the orographically excited standing wave and the mean zonal flow. The computed power spectra of the principal component time series stress the importance of the high-frequency transients in long-term climate variability.

Temporal development of Mt. Pinatubo aerosols as observed by lidar and sun photometer at Ny‐Ålesund, Spitsbergen

Since summer 1991 multiwavelength lidar and sun photometer observations of the Pinatubo aerosol layer are performed at the Arctic NDSC station (Koldewey-Station) in Ny-Alesund, Spitsbergen. The height integrated backscatter coefficient and the optical density decrease exponentially in time with time constants of 0.89±0.39 and 0.94±0.37 years, respectively. Their wavelength dependence indicates a decrease in median particle radii between 1992 and 1993. It is shown that the observations are consistent with results of parameter-free model calculations. The calculations are based on the assumption that gravitational sedimentation is the dominant process for volcanic aerosol removal in the Arctic stratosphere.