Abstract
Abstract. Here we compared the heterogeneous reactions of carbonyl sulfide (OCS) on the surface of α-Al2O3 and MgO and the effect of water on the reactions at 300 K using Knudsen cell–mass spectrometry, diffuse reflectance infrared Fourier transform spectroscopy, and temperature programmed reactions. H2S and CO2 were found to be hydrolysis products of OCS on both α–Al2O3 and MgO at ambient temperature. At low water vapor pressure (<6.8×10−6 Torr), when water vapor pressure in the Knudsen cell reactor increased from 2.3×10−6 to 6.8×10−6 Torr, the initial true uptake coefficient of OCS on α-Al2O3 decreased from 4.70±0.45×10−7 to 3.59±0.34×10−7; while it increased from 5.19±0.49×10−7 to 6.48±0.62×10−7 on MgO under the same conditions. At high relative humidity (0.07–0.67), the observed uptake coefficients of OCS on α-Al2O3 and MgO, which were measured using an in situ DRIFTS, decreased from 4.63±0.22×10−6 to 1.00±0.47×10−6 and from 9.72±0.46×10−5 to 7.68±0.36×10−5, respectively, when RH increased from 0.07 to 0.67 corresponding to 1.7–15.9 Torr of water vapor pressure. In the RH region of 0.17–0.67, the average observed uptake coefficient of OCS on α-Al2O3 and MgO was equal to 8.34±2.19×10−7 and 8.19±0.48×10−5, respectively. The restrictive effect of water on the heterogeneous reaction of OCS on the surface of α-Al2O3 and MgO was found to be related to competitive adsorption between water and OCS molecules; while the promotive effect of water on the heterogeneous reaction of OCS on the surface of MgO at low coverage was ascribed to the formation of surface hydroxyl groups. When the environmental RH was greater than the RH of the monolayer, which occurred readily at the atmospherically relevant humidity in the troposphere, thick water layer formed on the mineral dusts, especially, the basic thick water layer formed on the basic component of mineral dusts may be the primary contributor to the heterogeneous hydrolysis of OCS in the troposphere.
Highlights
Carbonyl sulfide (OCS) is the most abundant sulfur compound in the atmosphere and, in the troposphere, has a rather uniform mixing ratio of approximately 500 pptv (Watts, 2000)
Prior to the uptake experiments, the system was passivated with the feed gas for 150 min until a steady-state of quadrupole mass spectrometer (QMS) signal of OCS (m/z=60) established as the sample was isolated from the gas by the sample cover
relative humidity of one monolayer (RHML), the uptake coefficients of OCS on the sa9m17ples remained higher than that measured in the background exper
Summary
Carbonyl sulfide (OCS) is the most abundant sulfur compound in the atmosphere and, in the troposphere, has a rather uniform mixing ratio of approximately 500 pptv (Watts, 2000). A few studies demonstrated that heterogeneous reactions of OCS can occur on APM, and on mineral oxides including Al2O3, SiO2, Fe2O3, CaO, MgO, MnO2 and the mixture of Fe2O3 and NaCl under ambient conditions (Chen et al, 2007; He et al, 2005; Liu et al, 2006, 2007a, b; Wu et al, 2004, 2005). Our results demonstrate that catalytic hydrolysis of OCS can take place on the surface of both αAl2O3 and MgO under ambient conditions and that adsorbed water on mineral dust influences this reaction greatly. The mechanism and the atmospheric implications for the effect of water on the heterogeneous reactions of OCS on mineral oxides are discussed
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