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100 CHAPTER 2. ATMOSPHERE AND REMOTE SENSING 2.6.2 Ozone Depletion Events in the Polar Boundary Layer in Spring: A Model Study Participating scientists Matthias Piot and Roland von Glasow Abstract For more than 20 years, events with almost complete loss of ozone have been observed in the Arctic in spring. The importance of several species and meteorological parameters for these depletions have been investigated with sensitivity studies using a numerical box model. Figure 2.56: Schematic depiction of the most important processes included in the MISTRA model, applied for Arctic conditions. Fluxes over ice and sea salt production are switched ON/OFF, depending on the air composition we study. Background Reactive halogens play a major role in polar ozone depletion events (ODE): the reaction of bromine atoms with ozone, followed by the self-reaction of bromine oxides (BrO) represents a catalytic loss mechanism for ozone in the polar boundary layer (PBL). However, the triggering of the so-called ”bromine explosion” remains unclear. I present model results where prescribed bromine or chlorine fluxes are used to reproduce ODEs. I investigated the importance of several compounds for the chemistry of the PBL as well as meteorological parameters and focused on species which influence the occurrence of an ODE. This study allows us to better understand which species are important in the process of depleting ozone. Funding DFG: Emmy Noether Junior Research Group MarHal GL 353/1-1 Methods and results I used the chemical and microphysical model MISTRA in the lagrangian box-model mode to study the mechanisms influencing these observed depletions in the polar boundary layer. My sensitivity studies consisted of a set of four-day runs where I changed initial mixing ratios or fluxes (or both) of 19 different species (including halogens, NOx, NOy, DMS, H2O2, HCHO...) and compared the results with base runs. The influence of temperature and humidity have also been examined. High HCHO concentrations result via increases in HO2 in a redistribution of bromine species involving the aerosol phase, slowing ODEs down. Elevated DMS leads to an increase in HCHO, therefore also slowing ODEs down. By a similar production of HOx, hydrogen peroxyde (H2O2) also oxidizes Brx compounds inducing an ODE slow down. The presence of both Cl2 and Br2 in the air may lead to an unexpected decrease in the destruction process, depending on their relative mixing ratios. Increasing the mixing ratio of ethane by a factor of 2 reduces the ozone mixing ratio from a partial ODE to a major ODE. For cases with low Br, ethene emphasizes the ozone loss. With high Br, it reduces the destruction process. An increase in HONO led to different eventual ozone mixing ratios, depending on its use by Brx-Clx compounds. A change in temperature or humidity modifies the liquid water content of aerosols and therefore the uptake coefficients. The recycling of halogens via the liquid phase is modified. Outlook/Future work This sensitivity study will help us investigate the potential triggers like frost flowers for the bromine explosion, using MIS- TRA in the lagrangian 1D mode. Main publication Piot and von Glasow, presentations at EGU, Vienna, 2005 and OASIS workshop, Toronto, 2005
2.6. MARHAL - MODELING OF MARINE AND HALOGEN CHEMISTRY 101 2.6.3 Modeling Organic Films on Atmospheric Aerosol Particles and their Influence on Cloud Microphysics and Chemistry Participating scientists Linda Smoydzin, Roland von Glasow Abstract It is well known that organic material from the ocean’s surface can be incorporated into sea salt aerosols and that they often produce surface films. The relevance of these films for the gas phase and sea salt chemistry as well as for potential changes in the microphysical properties in cloud condensation nuclei is investigated. Figure 2.57: Daily course of aqueous phase concentration of HCl and HNO3 (black: organic free case, red: case with organics) Background In the last years more and more field experiments took place to characterize the chemical composition of sea salt aerosols but due to the large number and complexity of organic compounds it is only possible to determine functional groups present in the aerosol. Despite these uncertainties the measurements have shown that the organic mass fraction in fresh sea salt aerosols might be large enough to have a significant influence on cloud microphysics and atmospheric chemistry. It is assumed that surface-active organic matter of biogenic origin is enriched in the oceanic surface layer and gets into the atmosphere by bubble bursting. Surface active organic matter present in sea salt aerosols can lead to a decrease of the aerosol’s surface tension which reduces the mass transfer between the gas phase and liquid phase as well as water uptake into the aerosol. Funding DFG: Emmy Noether Junior Research Group MarHal GL 353/1-1 Methods and results For studying the effect of organic surfactants a one-dimensional numerical model which contains a microphysics scheme and a detailed description of chemistry in the gas phase, in aerosol particles and in cloud droplets is used. Chemical composition and hygroscopic properties of aerosols are important aspects when regarding droplet formation and droplet growth. With simple assumptions simulating an organic film on the aerosol we tested how strong the influence of surfactants on cloud microphysics in our model is. The decrease in water uptake and a decreased solubility of the aerosol due to the organic mass fraction leads to an increase in the number of small aerosol particles. The observed changes in the aerosol size distribution, however, were small and might be negligible when considering droplet growth and cloud cover. Regarding the influence of organic matter on chemistry it is assumed that sea salt aerosols emitted from the ocean contain fatty acids which are known to be film forming compounds. The organic coating which hinders mass transfer between the gas and liquid phase is destroyed by reaction of the fatty acid with ozone. In fig. 2.57 the daily course of HNO3 and HCl for a three days model run is shown. Measurements have shown that an average organic mass fraction of 5-10% can be assumed. Although the organic mass fraction in this model run is less than 5% the uptake into the liquid phase is decreased as can be seen in lower concentrations in the case where organics were present in the aerosol. Coupled with the lower uptake into the liquid phase might be a change in the aerosol’s pH which also changes the chemical properties of the aerosol. Outlook/Future work Further investigation needs to be done to verify the uncertainties regarding the question of how strong the influence of organics really is and which chemical processes are affected exactly.
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Institut für Umweltphysik und Arbe
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Contents 1 Introduction/Overview of
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CONTENTS 7 3.2.1 Glaciological pilo
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Introduction In Heidelberg, environ
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Overview Summary Atmospheric resear
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2.3. RADIATIVE TRANSFER 45 2.3 Radi
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170 CHAPTER 5. SMALL-SCALE AIR-SEA
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Forschungsstelle “Radiometrie”
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7.3. PHD THESES 217 PhD Theses Baye
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7.5. INVITED TALKS 221 Invited Talk
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7.5. INVITED TALKS 223 Pundt, I. 20
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Institut für Umweltphysik Im Neuen
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