Cinétique de réaction hétérogène d'importance troposphérique sur des substrats modèles et sur des aérosols

The first part of this study consists in the investigation of the evaporation and the condensation rate of H2O in the presence of different types of ice in a low pressure flow reactor. The second part of the present work reports the study of chlorine activation on different types of frozen salt substrates in a low pressure flow reactor. The third part concerns the construction and testing of a laminar flow reactor at atmospheric pressure in order to study bromine and chlorine activation reactions on different types of sea salt and model aerosols at different relative humidity. The kinetics of H2O condensation (kc) and the evaporation flux (Jev) of H2O on ice was studied in the range 130 to 210 K using pulsed-valve and steady state admission techniques in a low pressure flow reactor. The uptake coefficient γ was measured for different types of ice, namely, Condensed (C), Bulk (B), Single Crystal (SC), Snow (S) and Cubic ice (K). The negative temperature dependence of γ for C, B, SC and S ice reveals a precursor-mediated adsorption/desorption process. The non-Arrhenius behavior of the rate of condensation kc manifests itself in a discontinuity in the range 170 to 190 K depending on the type of ice and is consistent with a precursor model of H2O condensation/evaporation. The average of the energy of sublimation Δ0S is (12.0 ± 1.4) kcal/mol for C, B, S, SC ice and is identical within experimental uncertainty between 136 and 210 K. The same is true for the entropy of sublimation ΔSS. In contrast, both γ and the evaporative flux Jev are significantly different for different ices. In the range 130 to 210 K, Jev of H2O ice was significantly smaller than the maximum theoretically allowed value. This corroborates γ values significantly smaller than unity in the specified T range. Based on the present kinetic parameters the time to complete evaporation of a small ice particle of radius 1 µm is approximately a factor of 5 larger than previously thought. The HOCl heterogeneous reactions on frozen solutions of sea salt (SS), frozen KCl or NaCl were studied in a low pressure flow reactor in order to measure the uptake coefficient γ and the reaction products. The HOCl sample always contained up to 25% of Cl2O that was studied apart in order to relate the Cl2 produced to the HOCl taken up. The main product formed on NSS frozen solution is gas-phase Cl2 that is sustained for at least one hour in contrast to KCl or NaCl frozen solution that produced a transient Cl2 flow that decreases after 100s. We found that approximately 30% of the HOCl taken up from the HOCl/Cl2O mixture reacts to produce Cl2 on NSS, RSS and KCl frozen solution at 200 K at a HOCl concentration larger than 1011 molecule cm-3. In contrast, HOCl concentrations of a few 1010 molecule cm-3 at 200 K failed to produce additional Cl2 in contrast to pure Cl2O uptake. A single Br2 burst event was also monitored when Cl2O or a HOCl/Cl2O mixture is taken up on fresh frozen NSS solution during the first uptake run at 200 K. Further Cl2O or HOCl/Cl2O uptake on the same sample will not lead to a Br2 pulse. Annealing of a previously exposed frozen NSS solution to 240 K and subsequently cooling did not lead to a Br2 burst event either. The HOBr and HOCl uptake coefficient γ on acidified submicron salt aerosol was measured in an atmospheric flow tube reactor. The interaction time of the trace gas with the aerosol was in the range 15 to 90 s and led to γ values ranging from 10-4 to 10-2. The acidity of the aerosol is essential in order to enable heterogeneous reactions on NaCl, recrystallized sea salt (RSS) and natural sea salt (NSS) aerosols for both HOCl and HOBr. Specifically, HOCl exclusively reacts on acidified NSS aerosols with a γ ranging from 1.8 × 10-3 to 0.4 × 10-3 at a relative humidity between 40 and 85%, respectively. In addition, γ(HOBr) is approximately 10-2 for relative humidity in the range 77 to 90% on NaCl and RSS acidified aerosol. Uptake experiments of HOBr on aqueous H2SO4 as well as on acidified NaCl, RSS or NSS aerosols were performed for rh ranging from 40 to 93%. γ of HOBr on acidified NSS reaches a maximum at rh = 77% and decreases significantly for higher rh in contrast to acidified NaCl and RSS aerosols. This difference in γ is attributed to the presence of an organic phase in NSS aerosols that forms an organic coating at high rh. In addition, preliminary uptake experiment of HNO3 was performed on NaCl and NSS aerosols using the atmospheric flow tube reactor. We were able to perform experiments only from 2.8 to 25% relative humidity. For a relative humidity larger than 25%, all gas phase HNO3 is taken up on the reactor wall and no experiments could be performed. γ of HNO3 in the range (2-3) × 10-2 for NaCl and NSS aerosol, respectively, at 2.8% relative humidity. At rh = 25%, γ(HNO3) is approximately 10-1 for both NaCl and NSS aerosols.

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