Uptake and reaction of HOI and IONO2 on frozen and dry NaCl/NaBr surfaces and H2SO4

Abstract

The uptake and reaction of HOI and with dry and frozen NaCl/NaBr salt surfaces was investigated IONO 2 using a coated-wall Ñow tube reactor coupled to a mass spectrometer for gas-phase analysis. HOI and IONO 2 react with both surfaces to form the di-halogens IBr and ICl, which are released into the gas phase. On mixed Cl~/Br~surfaces in which the concentration of Cl~greatly exceeds that of Br~, both HOI and react IONO 2 to form ICl or IBr. ICl reacts further with Br~to form the observed gas-phase product, IBr. Formation and release of ICl is only observed once the Br~concentration has been chemically depleted. The sum of IBr ] ICl in the gas phase was found to remain constant during the reaction, and to account for all of the HOI/IONO 2 taken up, indicating no loss of iodine to the surface, and a catalytic role of iodine in the activation and release of bromine and chlorine from mixed salt surfaces. The uptake coefficient of HOI on a frozen, mixed salt surface of similar composition to sea-water at 243 K was [10~2. Similar results (c [ 10~2) were obtained for the uptake of HOI onto dry, mixed salt surfaces of similar composition at 298 and 243 K. Lower limits for the accommodation coefficient of HOI on frozen salt (243 K), dry salt (253 K) and (253 K) of 0.05, 0.12 and H 2 SO 4 0.3, respectively were obtained. The implications of these results for tropospheric halogen chemistry are brieÑy discussed.

Evaluated kinetic and photochemical data for atmospheric chemistry: Volume V – heterogeneous reactions on solid substrates

“…The results of and Holmes et al (2001) were obtained in different temperature ranges and concur that the uptake coefficient is large. A parameterisation, γ =7×10 −5 exp(1300/T ) generates uptake coefficients that are consistent with both datasets.…”

“…A parameterisation, γ =7×10 −5 exp(1300/T ) generates uptake coefficients that are consistent with both datasets. At the lower temperatures covered by , the uptake is observed to be irreversible, whereas Holmes et al (2001) observe partially reversible adsorption at 243 K. From the experimental datasets it is not obvious how HOI reacts with the ice surface. Both studies observed I 2 as product and Holmes et al (2001) suggest that this may arise from self-reaction of HOI (or reaction of HOI with IONO 2 impurity) on the ice surface.…”

“…At the lower temperatures covered by , the uptake is observed to be irreversible, whereas Holmes et al (2001) observe partially reversible adsorption at 243 K. From the experimental datasets it is not obvious how HOI reacts with the ice surface. Both studies observed I 2 as product and Holmes et al (2001) suggest that this may arise from self-reaction of HOI (or reaction of HOI with IONO 2 impurity) on the ice surface. It is possible that on a pure ice surface, HOI will adsorb reversibly unless high concentrations and low temperatures result in formation of a thermodynamically stable phase (e.g.…”

Inorganic bromine in the marine boundary layer: a critical review

“…Wachsmuth et al (2002) investigated uptake of HOBr on NaBr aerosol particles and found a mass accommodation coefficient of α = 0.6. Uptake of HOI on bromide has also been studied (Mössinger and Cox, 2001;Holmes et al, 2001). …”

“…Several attempts have been made to measure BrO in the mbl outside the polar regions, both at a clean remote site (HAW99b-G) and also at sites that are more polluted to varying degrees (MHD96-G, MHD97a-G, MHD97b-G, MHD98-G, TEN97-G, WEY96a-G, WEY96b-G, CRT00-G). The results from these LP-DOAS measurements indicate ambient concentrations around or below the DL of the respective instruments (about 1 to 3 pmol/mol) (Alicke, 1997;Allan, 1998;Hönninger, 1999;.…”