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78 CHAPTER 2. ATMOSPHERE AND REMOTE SENSING 2.5.3 Modeling organic surface films on Atmospheric Aerosol Particles and their Influence on Chemistry Linda Smoydzin, Roland von Glasow Abstract Organic material from the ocean’s surface can be incorporated into sea salt aerosol particles often producing a surface film on the aerosol. Such an organic coating can reduce the mass transfer between the gas phase and the aerosol phase influencing sea salt chemistry in the marine atmosphere. Figure 2.41: Time evolution of total bromine and chlorine gas phase mixing ratios and HOBr and HOCl aqueous phase concentrations at 50 m altitude for three days (midnight day 1 - midnight day 3). The different lines refer to different cases where the fractions of the organic constituents which react with ozone and OH are varied. The accommodation coefficient is reduced by one order of magnitude for all cases. Red: 50% OA, 50% ORG, green: 47% OA 52% ORG, blue: 45% OA, 55% ORG, black: base case without organic coatings. Background Recent field measurements have shown that the organic mass fraction in marine particles can be up to 60% especially in regions with high biological activity. However, it is not possible to determine which organic species are present on sea salt aerosols. Methods and results To investigate the importance of surfactants for atmospheric chemistry in the marine boundary layer (MBL) the onedimensional numerical model MISTRA is used. Uncertainties regarding the magnitude of uptake reduction, the concentrations of organic compounds in sea salt aerosols and the oxidation rate of the organics are considered to analyse the possible influence of organic surfactants on gas and liquid phase chemistry. Organic surface films can be destroyed by reaction with atmospheric oxidants like ozone or OH. As long as a complete coating on the sea salt particle is present the accommodation coefficient is reduced. By assuming destruction rates for the organic coating (which comprises of oleic acid (OA)) based on laboratory measurements we saw a rapid destruction of the organic coatings within the first meters of the MBL. Larger organic initial concentrations lead to a longer lifetime of the coating but lead also to an unrealistically strong decrease of ozone concentrations as the organic film is destroyed by reaction with ozone. The lifetime of the film can be increased by either assuming smaller reactive uptake coefficients for ozone on oleic acid or by assuming that a part of the organic surfactants (ORG) react with OH. By variing the oleic acid/ORG fraction sensitivity studies were perfomed. It can be shown that gas phase halogen concentrations decrease. Chlorine concentrations decrease stronger than Bromine concentrations (∆Cl(tot):10-20%, ∆Br(tot):9%). For the aqueous phase we see an increase in bromine (∆HOBr:30%) and a decrease in chlorine concentrations (∆HOCl:40-50%). Overall aqueous phase chemistry is affected stronger than gas phase chemistry. Funding DFG-Emmy Noether Junior Research group Marhal GL 353/1-2 Main publication Smoydzin & von Glasow [2006]
2.5. MARHAL - MODELING OF MARINE AND HALOGEN CHEMISTRY 79 2.5.4 Importance of the surface reaction OH + Cl − on sea salt aerosol for the chemistry of the marine boundary layer - a model study Roland von Glasow Abstract The implications for the chemistry of the marine boundary layer (MBL) of the reaction of OH with Cl − on the surface of sea salt aerosol producing gas phase Cl2 and particulate OH − have been investigated with a numerical model. They were found to be very minor in contradiction to previous suggestions in the literature. Figure 2.42: Temporal evolution of the most important gas phase compounds for scenario “remote MBL”. The difference between the “cases” is how the reaction rate of the surface reaction is calculated as explained in paranthesis. Case 1 (no surface reaction) - black, solid line; case 2 (“best guess”) - red, dashed line; case 3 (no gas phase diffusion limitation) - blue, dotted line; case 4 (γ = 1, with gas phase diffusion limitation) - blue, solid line; case 5 (γ = 1, no gas phase diffusion limitation) - green, dash-dotted line. Note, that most lines except for Cl2 and Cl overlap. The abscissa is time since model start in minutes. Background The reaction OH + Cl − −→ OH − + Cl2 had been suggested by Laskin et al. (2003) to play a major role in the sulfur cycle in the marine boundary layer. The aqueous phase oxidation of SO2 is strongly pH dependent, and relevant mainly in fresh, non-acidified sea salt aerosol particles. This pathway for sulfur oxidation would gain in importance if alkalinity would be produced in the particles as is the case in the aforementioned reaction. Furthermore, it was suggested that the gas phase product, Cl2, might be relevant for the photochemistry of the MBL. Methods and results Based on literature data a new “best estimate” for the rate coefficient of the surface reaction was deduced and applied in the box-model vesrion of MISTRA. Its importance for the chemistry of the MBL has been investigated under conditions typical for the pristine MBL of the Southern Ocean, the remote MBL, and marine regions influenced by polluted outflow from the continent. The results showed that the additional sulfate production by this reaction is less than 1%, therefore having only a minor impact on sulfate production. Even though the gas phase concentration of Cl2 increased strongly in the model (see Figure), the concentration of Cl radicals increased by less than 5% for the “best estimate” case and the impact on O3 and other compounds is negligible. Therefore it was concluded that - at least under the investigated conditions - this reactions is of minor importance for the chemistry of the MBL and the marine sulfur cycle. A very interesting feature of the acidification of large sea salt particles that was predicted with the model is a two-stage acidification of large fresh sea salt aerosol. This effect is caused by the rapid change in particle pH due to uptake of acids and production of acidity during the production of sulfate and the related change in pH-dependent reactions rates. Funding DFG: Emmy Noether Junior Research Group MarHal GL 353/1-2 Main publication von Glasow [2006b]
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Institut für Umweltphysik und Arbe
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4 CONTENTS 2.4.10 Satellite monitor
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184 CHAPTER 7. BIBLIOGRAPHY Wang, P
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7.5. INVITED TALKS 193 Invited Talk
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