Partners and International Organizations
(English)
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Universität Heidelberg (D), LCSR/CNRS Orléans (F), NILU (N), University of Cambridge (UK), University of East Anglia (UK), Universität Wuppertal (D)
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Abstract
(English)
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The project focusses on the tropospheric abundance, reaction cycles, loss processes and effect of reactive halogen species RHS such as Cl, Br, I, ClO, BrO, IO and others in the troposphere, in particular in relation to the total oxidation capacity of the marine and continental troposphere. The present effort emphasizes the study of heterogeneous reactions of those species as well as on some other species thought to be reactive as well and not studied heretofore. In the interest of obtaining fundamental chemical kinetic information on the heterogeneous reactivity the interfacial kinetic studies are performed on model substrates simulating atmospheric aerosols.
Some of the results on the heterogeneous reaction kinetics obtained are the following:
HONO + HBr on Ice and on H2SO4:
The activation of HBr proceeds through NOBr, both on ice as well as liquid and frozen H2SO4. On ice there are two forms of HBr: one reactive and one unreactive towards HONO. The H2SO4 concentration window for significant activation of HONO is much wider for HBr (10-60wt% H2SO4) than for HCl (55-62.5wt%).
N2O5 + HBr on Ice; N2O5 + HCl on Ice.
On ice only 50% of HBr taken up is reactive towards N2O5. N2O5 oxidizes bromide (HBr) to Br2 and is reduced to HONO. The hydrolysis of N2O5 to HNO3 is competitive with the title reactions, but more so for N2O5 + HCl because of its lower reaction probability relative to N2O5 + HBr.
NO3 on Ice and H2SO4/H2O.
NO3 free radical is unreactive towards ice and supercooled sulfuric acid at low temperatures.
NO3 + HX (X=Cl, Br, I) on Ice
The reaction of NO3 with HCl to yield free atomic chlorine does not take place on ice at low temperatures whereas the analogous reaction with HBr and HI does take place depending on the amount of adsorbed HBr and HI.
HOBr on NaCl, KBr at ambient temperature.
On model NaCl and KBr substrates HOBr undergoes primary reactions leading to activation of inorganic halide (chlorides, bromides). In addition HOBr undergoes a decomposition reaction on ionic, that is polar surfaces leading to Br2. For atmospheric conditions at ambient temperature an initial uptake coefficient of 0.01 for NaCl and 0.25 for KBr substrates is determined. The branching ratio for the heterogeneous reaction between HOBr and NaCl is about 7:1 at the aerosol loading typical of the remote marine boundary layer. For HOBr the reaction HOBr(g) + KBr(s) -> Br2(g) + KOH(s) is competitive with photolysis.
Molecular Halogens interacting with Solid Salts
On NaCl substrates, neither Cl2, Br2 nor BrCl show any signs of reactivity (expected) or of a surface residence time. On KBr substrates, Cl2 undergoes an efficient exchange reaction (C12 + KBr -> BrCl + KCl) without showing any surface residence time, whereas Br2 does not react but shows a long surface residence time on the time scale of a few milliseconds.
ClNO2 + KBr, ClONO2 + NaCl at ambient Temperature.
ClNO2 interacting with KBr is characterized by an unmeasurably small surface residence time of less than 20 us matching is low reactive uptake coefficient. ClONO2 interacting with NaCl is characterized by a long surface residence time of 0.7 ms matching its high reactivity towards NaCl. At least for these two examples the surface residence time ts scales with the reactive uptake coefficient.
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