Showing papers by "Franz Rohrer published in 2002"

Free radicals and fast photochemistry during BERLIOZ

Abstract: The free radicals OH, HO2, RO2, and NO3 are known to be the driving force for most chemical processes in the atmosphere. Since the low concentration of the above radicals makes measurements particularly difficult, only relatively few direct measurements of free radical concentrations have been reported to date. We present a comprehensive set of simultaneous radical measurements performed by Laser Induced Fluorescence (LIF), Matrix Isolation — Electron spin Resonance (MI-ESR), Peroxy Radical Chemical Amplification (PERCA), and Differential Optical Absorption Spectroscopy (DOAS) during the BERLIner OZonexperiment (BERLIOZ) during July and August of 1998 near Berlin, Germany. Most of the above radical species were measured by more than one technique and an intercomparison gave good agreement. This data set offered the possibility to study and quantify the role of each radical at a rural, semi-polluted site in the continental boundary layer and to investigate interconnections and dependencies among these free radicals. In general (box) modelled diurnal profiles of the different radicals reproduced the measurements quite well, however measured absolute levels are frequently lower than model predictions. These discrepancies point to disturbing deficiencies in our understanding of the chemical system in urban air masses. In addition considerable night-time peroxy radical production related to VOC reactions with NO3 and O3 could be quantified.

Tritiated water vapor in the stratosphere: Vertical profiles and residence time

Abstract: [1] The historic and partly unpublished measurements of the tritium content in stratospheric water vapor made at the National Center for Atmospheric Research between 1975 and 1983 are reanalyzed. The resulting vertical profiles of the T content, mainly at 32°N latitude, show little variation with altitude above 20 km but a strong decay with time. This decay is a consequence of the large T injections into the stratosphere by the atmospheric tests of high-yield thermonuclear devices prior to 1963 and seems to proceed with a single e-fold time of 5.12 years. Correcting for the radioactive decay of HTO within the stratosphere and for a temporal increase in stratospheric H2O, we obtain a decay time for stratospheric HTO of 7.7 ± 2.0 years. This decay time, which is solely due to the transport of HTO into the troposphere, is much longer than the age of stratospheric air at these altitudes or the accepted values for stratospheric residence times. The differences are discussed and resolved by interpreting the HTO decay time as the Eigentime of the longest-lived mode of the stratospheric transport equations. This Eigentime should provide a useful constraint in modeling stratospheric transport.

Actinic Radiation and Photolysis Processes in the Lower Troposphere: Effect of Clouds and Aerosols

Abstract: Within the German Tropospheric Research Program (TFS) a series of projects were performed focussing on aspects of radiation transfer and the effects of UV-radiation on air chemistry. The individual projects covered laboratory investigations, instrument development for photolysis processes as well as field studies of actinic radiation and comparison to model calculations. One and three-dimensional models were tested against field campaign data. The results confirm the improvement of measurement technology achieved through deployment of new techniques like spectroradiometry that offer a wider range of investigations than was previously attainable using chemical actinometry or fixed wavelength filter radiometry. Reasonable agreement was also found between measurements and models for a few selected and well defined cloudy conditions. On the other hand, using simple stratiform geometry models yielded significant deviations between measurement and model in both directions particularly in the case of high zenith angles and with high aerosol load. Further tools both for experimental investigations and for model calculations were developed within the framework of the Troposphere Research Program (TFS) and deficiencies were identified demanding further investigations when broken clouds and more complex cloud layers prevail.