Ozone Depletion, Chlorine Activation and Water Vapor Observed in Spitsbergen

Ozone depletion, chlorine activation and water vapor observed in Spitsbergen Dissertation zur Erlangung des Grades Dr. rer. nat. der Universitat¨ Bremen vorgelegt von Ingo Wohltmann April 2002 Berichte aus dem Institut fur¨ Umweltphysik – Band 0 herausgegeben von: Dr. Georg Heygster Universitat¨ Bremen, FB 1, Institut fur¨ Umweltphysik, Postfach 33 04 40, D-28334 Bremen URL http://www.iup.physik.uni-bremen.de E-Mail [email protected] Prufer:¨ Prof. Dr. Klaus Kunzi,¨ Prof. Dr. John Burrows c 2001 Ingo Wohltmann Contents 1 Physics and Chemistry of the Atmosphere 13 1.1 Composition of the Atmosphere . 13 1.2 Physics of the Atmosphere . 15 1.3 Dynamics of the Stratosphere . 18 1.4 Ozone Chemistry . 21 1.5 Ozone Distribution . 29 1.6 Chlorine Monoxide and Water Vapor . 31 1.7 Literature . 31 2 Instrument 33 2.1 Overview . 33 2.2 Radiative Transfer . 34 2.3 Absorption Coefficients . 36 2.4 Instrument Description . 40 2.4.1 Quasi Optics . 40 2.4.2 Antenna and Mixer . 42 2.4.3 Intermediate Frequency Chain . 44 2.4.4 Acousto Optical Spectrometer . 45 2.4.5 Bremen Radiometer . 45 2.5 Measurement Principle . 45 2.5.1 Totalpower Method . 46 2.5.2 Reference Beam Method . 47 3 Retrieval 49 3.1 Overview . 49 3.2 Optimal Estimation . 49 3.2.1 Resolution . 51 3.2.2 Errors . 53 3.2.3 Nonlinearities . 54 3.3 Retrieval . 55 3 3.3.1 General Features . 55 3.3.2 Forward Model Features . 59 3.3.3 Moist Weather Conditions . 72 3.4 Water Vapor, Spitsbergen . 73 3.4.1 A Simple Approach . 73 3.4.2 Logarithmic Retrieval . 74 3.4.3 Reference Beam Spectra . 75 3.4.4 Continua and Line Wings . 77 3.4.5 Frequency Shift and Self Broadening . 78 3.A Covariance Matrices . 80 3.B Logarithmic Distribution . 80 4 Ozone Depletion 83 4.1 Overview . 83 4.2 Method . 85 4.3 Implementation . 87 4.4 Meteorological Situation . 89 4.5 Ozone Depletion in 1999/2000 . 94 4.6 Comparison with Other Techniques . 100 4.6.1 Match . 100 4.6.2 Tracer Measurements . 101 4.6.3 Vortex Averaged Measurements . 104 4.6.4 SLIMCAT . 105 4.7 Accuracy of the Heating Rates . 108 4.8 The General Picture . 112 4.9 Chlorine Activation . 115 4.10 Denitrification . 123 4.11 Effect on Ozone Loss . 125 4.12 Ozone Depletion in 2000/2001 . 127 4.A Integration of Loss Rates . 131 4.B Double Convolution . 133 4.C Error Analysis . 133 4.D Convolution of Independent Measurements . 135 5 Water Vapor Columns 137 5.1 Overview . 137 5.2 Theory . 137 5.3 Method . 138 5.4 Correction . 140 5.5 Results . 143 Used Publications Parts of this thesis were published in ¡ Klein, U., Wohltmann, I., Lindner, K. and Kunzi,¨ K. F.: Ozone depletion and chlorine activation in the Arctic winter 1999/2000 observed in Ny- Alesund.˚ J. Geophys. Res., 107, accepted, 2002. 5 Abstract This work discusses measurements of the ground based microwave Radiometer for Atmospheric Measurements (RAM), the retrieval of desired data products and their interpretation. The radiometer is operated at the Arctic station of the Net- work for the Detection of Stratospheric Change (NDSC) at Ny-Alesund,˚ Spitsber- ¢ gen (78.9 ¢ N, 11.9 E). It measures water vapor at 22 GHz, ozone at 142 GHz and chlorine monoxide at 204 GHz. The first part of this work concentrates on the data retrieval. A summary of the most recent status of the retrieval software is given as a reference and to reflect some new additions. A special focus lies on the water vapor retrieval, which proved to be unexpectedly difficult to implement. The second part deals with the determination of ozone loss rates, chlorine activation and denitrification. A method to determine loss rates from the measurements of the RAM has been developed and improved over the last four years. Here, ozone loss calculations for the winters 1999/2000 and 2000/2001 are presented, accompanied by a comparison to other techniques, a mathematical examination of the method and an error analysis, including the effects of different diabatic heating rates, mixing, the definition of the vortex edge, altitude resolution and other aspects. Measurements of chlorine monoxide, denitrification and reservoir gases are presented and examined in the context of the current understanding of ozone chemistry. It is shown by model comparisons that the general aspects of chemistry seem to be well understood. It is also pointed that there remain some considerable uncertainties in measurements as well as in the model assumptions. The third part examines the possibilities of retrieving tropospheric water vapor columns from the measurements of the radiometer. When the tropospheric water vapor bias is measured at two different frequencies, the calculation of the columns is straightforward and an established method. The study here concentrates on the effects of precipitation and clouds and possibilities to correct the occurring errors. 7 Introduction Microwave radiometers built in Bremen, Germany, have been measuring stratospheric trace gases for almost a decade now. Looking back, a wealth of measurements and scientific results has resulted from our research activity. First ozone measurements have already been conducted in 1992, and continuous observations ¢ began in 1994 in Ny-Alesund,˚ Spitsbergen (79 ¢ N, 12 E). In the following years, more and more instruments have been added. In the moment, ozone profiles, chlorine monoxide profiles, tropospheric water vapor columns and stratospheric water vapor profiles are measured. Ny-Alesund˚ is part of the primary Arctic station of the Network for the Detection of Stratospheric Change (NDSC), where the instruments are operated in cooperation with the Alfred Wegener Institute of Polar and Marine Research (AWI) in Potsdam, Germany, and the Institute of Environmental Physics (IUP) in Bremen. Main goal of the NDSC is the identification of changes in the ozone layer and the examination of their causes. Therefore, a continuous series of ozone measurements has been recorded at Ny-Alesund˚ and used to infer ozone depletion. Equally relevant are the measurements of chlorine monoxide, which is an important species in the chemistry of ozone depletion and can be used to test the understanding of the underlying chemical mechanisms. Chlorine monoxide is obtained by only half a dozen of instruments in the Arctic, with very limited spatial and temporal coverage, making observations very valuable. A similar situation exists for water vapor, which is important for the radiation balance in the stratosphere and the trend in ozone depletion. ¢ The story is continued with new radiometers at Bremen (53 ¢ N, 8 E), Merida,´ ¢ ¢ ¢ Venezuela (8 ¢ N, 71 W), and Summit, Greenland (72 N, 38 W). The radiometer in Bremen is measuring ozone at mid latitudes since 1999, while the instrument in Venezuela will be one of the first radiometers measuring trace gases in the tropics and is located in more than 4500 m altitude to reduce the influence of water vapor. The extremely dry location of Summit in the Arctic, situated in more than 3000 m altitude, is ideally suited for the observation of rarely measured species that only show up at high frequencies, where the atmosphere is nearly opaque under humid conditions. For example, the first continuous measurements of N2O and HNO3 in the Arctic will be carried out at Summit. 9 Summit Ny−Alesund Merida Bremen Microwave radiometers measure the thermally induced radiation of atmospheric trace gases and deduce profile information from the pressure broadened form of the emission lines. A great advantage to other instruments is the possibility to measure in the polar night and the high temporal resolution of the measurements. The instruments are also almost independent of weather conditions. Hence, a continuous measurement series for ozone and water vapor and regular observations of chlorine monoxide provide the opportunity of long term trend analyses and continuing observations of Arctic ozone loss. Purpose of this work is the retrieval and interpretation of the numerous data products of the radiometers, with a special focus on ozone depletion. The determination of anthropogenic ozone depletion has been a key topic in atmospheric sci- ences in the last decade. This development was initiated in 1985, when Farman et al. discovered a strong ozone deficit in the southern polar region in spring, which was apparently caused by anthropogenic influences. In the nineties, a similar phe- nomenon could also be observed in the Arctic. In the following, the political and public interest induced by the harmful effects of increasing ultraviolet radiation and the impact of ozone on the climate change catalyzed a tremendous amount of scientific research activity. In an unprecedented event in environmental politics, this led to a worldwide ban on chlorofluorocarbons, which were made responsible for the ozone decline. A detailed outline of the history of ozone depletion is given in the first chapter of this thesis, which leads us to a summary of the contents of this work. The basic principles of stratospheric physics and chemistry will be explained in the first chapter, while the second introductory chapter contains a description of the instrument and the fundamentals of radiative transfer. The third chapter deals with the retrieval of desired data products from the measured spectra. A summary of the most recent status of the retrieval software is given as a reference and to reflect some new additions not published until yet. Special attention is given to the new water vapor retrieval, a species that proved to be unexpectedly difficult to implement. Although no validated profiles could be obtained yet, some promising developments are shown. The detailed review of the physical model and of the changes in the retrieval code hopefully will lay the foundation for a successful retrieval in the future.

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