The uptake of HO2 on meteoric smoke analogues
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Abstract
Abstract The kinetics of heterogeneous HO 2 uptake onto meteoric smoke particles (MSPs) has been studied in the laboratory using analogues of MSP aerosol entrained into a flow tube. The uptake coefficient, γ , was determined on synthetic amorphous olivine (MgFeSiO 4 ) to be (6.9 ± 1.2) × 10 −2 at a relative humidity (RH) of 10%. On forsterite (Mg 2 SiO 4 ), γ = (4.3 ± 0.4) × 10 −3 at RH = 11.6% and (7.3 ± 0.4) × 10 −2 at RH = 9.9% on fayalite (Fe 2 SiO 4 ). These results indicate that Fe plays a more important mechanistic role than Mg in the removal of HO 2 from the gas phase. Electronic structure calculations show that Fe atoms exposed at the particle surface provide a catalytic site where HO 2 is converted to H 2 O 2 via an Eley‐Rideal mechanism, but this does not occur on exposed surface Mg atoms. The impact of this heterogeneous process in the middle atmosphere was then investigated using a whole atmosphere chemistry‐climate model which incorporates a microphysical treatment of MSPs. Using a global MSP production rate from meteoric ablation of 44 t/day, heterogeneous uptake (with γ = 0.2) on MSPs significantly alters the HO x budget in the nighttime polar vortex. This impact is highly latitude dependent and thus could not be confirmed using currently available satellite measurements of HO 2 , which are largely unavailable at latitudes greater than 70°.
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