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76 CHAPTER 2. ATMOSPHERE AND REMOTE SENSING 2.5.1 Modeling iodide – iodate speciation in atmospheric aerosol Susanne Pechtl, Guy Schmitz (Faculté des Sciences Appliquées, Universitée Libre de Bruxelles, Belgium), Roland von Glasow Abstract The speciation of iodine in atmospheric aerosol is currently poorly understood. Models predict negligible iodide concentrations, but accumulation of iodate in aerosol, both of which is not confirmed by recent measurements. An updated aqueous phase iodine chemistry scheme for use in atmospheric models was developed, which improves the agreement with measurements significantly. HIO 3 IO ICl 2 - IClBr - ICl HOCl HOBr HOI Cl - H + Br - H + IBr 2 - HI HIO 2 IBr I - H + H2O2 HOCl HOBr H + HIO2 H + HSO 3 - DOM? HOCl SO 3 - I 2 HOBr H + O 3 H HOI + IBr ICl H + IO 3 - HSO 3 - I - aerosol gas phase Figure 2.39: Scheme of aqueous phase iodine chemistry as implemented in the marine boundary layer model MISTRA. Additional reactions compared to earlier schemes are highlighted in red. Background Although during recent years progress has been made regarding atmospheric iodine chemistry, several aspects are still poorly understood, one of which is the speciation of iodine in atmospheric aerosol. Current models of atmospheric chemistry predict that the aerosol iodide (I− ) concentration is negligible, while iodate (IO − 3 ) is believed to be inert and thus accumulate in particles. In contrast, observational data provide evidence for a non-negligible iodide content in aerosol samples. During two extended ship cruises in the Atlantic Ocean, highly variable I − /IO − 3 ratios were found. Methods and results An updated aqueous phase iodine chemistry scheme (Figure 2.39) was developed for the marine boundary layer model MISTRA, which includes chemistry in the gas and aerosol phase as well as aerosol microphysics. Model sensitivity studies show that iodate can be reduced in acidic aerosol by inorganic reactions, i.e., iodate does not necessarily accumulate in particles. Furthermore, the transformation of particulate iodide to volatile iodine species likely has been overestimated in previous model studies due to negligence of collision-induced upper limits for the reaction rates. However, inorganic reaction cycles still do not seem to be sufficient to reproduce the observed range of iodide-iodate speciation. Therefore, the effects of the recently suggested reaction of HOI with dissolved organic matter (DOM) to produce I − was also investigated. If this reaction is fast enough to compete with the inorganic mechanism, it would not only directly lead to enhanced iodide concentrations but, indirectly via speed-up of iodate reduction cycles, also to a decrease in iodate. Hence, organic iodine chemistry combined with inorganic reaction cycles seems to be able to reproduce observations. The presented chemistry is highly dependent on pH and thus offers an explanation for the large observed variability of the iodide-iodate speciation in atmospheric aerosol. Outlook/Future work The existence of organic forms of iodine has to be re-confirmed in further measurements and its consequences for iodine speciation has to be investigated. Funding DFG: Emmy Noether Junior Research Group MarHal GL 353/1-2 Main publication Pechtl et al. [2006a]
2.5. MARHAL - MODELING OF MARINE AND HALOGEN CHEMISTRY 77 2.5.2 The Potential Importance of Frost Flowers for Ozone Depletion Events - A Model Study Matthias Piot, Roland von Glasow Abstract For more than 20 years, events with almost complete loss of ozone have been observed in the Arctic in spring. We performed model studies with the one-dimensional model MISTRA to investigate the potential role of frost flowers (FF) in this depletion of tropospheric ozone. Figure 2.40: Left: Schematic depiction of the most important processes included in the Arctic version of MISTRA. The boundary layer is denoted as BL, open lead as OL, and the free troposphere as FT. Aerosol and gas phase chemistry are calculated in all layers. (1): Deposition process; (2): Br2/BrCl re-emission from the snowpack. Right: typical gas phase O3 during an ODE in the BL. 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. We present a sensitivity study where meteorological as well as chemical parameters are assessed. This study helps us better understand the conditions favorable for the development of an Ozone Depletion Event (ODE). Methods and results We used MISTRA in a “Lagrangian mode” where a model column of 2000 m height moves across a pre-defined sequence of surfaces: snow, FF, and open lead (see Fig. 2.40-left), with FF aerosols being produced during the FF section. We show that a major ozone depletion event can be satisfactorily reproduced if the recycling on snow of deposited halogens into gas phase Br2/BrCl is considered (see ozone mixing ratio in Fig. 2.40-right). This cycling maintains sufficiently high levels of bromine to deplete ozone down to few nmol mol −1 within four days. We also assessed the influence of different combinations of open lead/frost flowers on the chemistry of the moving column. Results showed noticeable modifications affecting the composition of aerosols and the deposition velocities due to variation of the humidity in the air. In addition, we studied the effects of modified temperature of either the frost flower field or the ambient airmass. A warmer FF field increases the relative humidity and the aerosol deposition rate. The deposition/re-emission process is larger, inducing more reactive bromine in the gas phase, and a stronger ozone depletion. A decrease of 1 K in airmass temperature shows that the aerosol uptake capacity substantially increases, leading to enhanced uptake of acids from the gas phase: the bromine explosion accelerates and O3 mixing ratios decreases. Recent studies have suggested the important role of the precipitation of calcium carbonate (CaCO3) out of the brine layer for the possible acidification of the liquid phase by acid uptake. Our investigation showed that this precipitation is a crucial process for the timing of the bromine explosion in aerosols. Finally, we investigated the release of Br2 potentially produced by heterogeneous reactions directly on frost flowers. In this case, we obtained unlikely results for aerosol compositions and deposition rates on snow compared to observations made in the Arctic. Funding DFG-Emmy Noether Junior Research group Marhal GL 353/1-2.
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7.5. INVITED TALKS 193 Invited Talk
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