Abstract
Abstract. The first direct laboratory measurements of gaseous hydrogen peroxide uptake by authentic Gobi and Saharan dust aerosol particles as a function of relative humidity (RH) have been carried out in an entrained aerosol flow tube coupled to a chemical ionization mass spectrometer. Gobi dust shows uptake coefficients, γH2 O2 = (3.33±0.26) ×10−4 at 15% RH rising to γH2 O2 = (6.03±0.42) ×10−4 at 70% RH; the corresponding values for Saharan dust are systematically higher (γH2 O2 = (6.20±0.22)×10−4 at 15% RH rising to γH2 O2 = (9.42±0.41) ×10−4 at 70% RH). High resolution X-ray photoelectron spectroscopy (XPS) measurements of the surface chemical composition of the two mineral dust samples together with published water adsorption isotherms of their principal constituents enables rationalization of these observations, which are relevant to nighttime tropospheric chemistry. A box model study performed by incorporating the experimentally determined data set reveals that uptake of H2O2 onto dust can be an important loss process for this species which has been, until now, poorly constrained.
Highlights
The Gobi desert in northwest China and the Sahara desert in northern Africa are the two most important global sources of mineral dust aerosol with current estimates of between 1000– 3000 Tg/yr being advected into the atmosphere (Tegen and Fung, 1994; Dentener et al, 1996; Luo et al, 2003)
Two types of authentic desert dust aerosols were used: the Saharan dust originates from Cape Verde Islands and is characteristic of mineral dust aerosol transported from the western Sahara, whereas Gobi sand was collected from the Khongoryn Els in Mongolia and was mechanically ground and sieved to ≤1 μm diameter
In passing we note that similar Saharan dust samples, collected from the Cape Verde islands, were used by Hanisch and Crowley (2003): their bulk composition, determined by energy dispersive X-ray spectroscopy, was similar to the surface composition we observed by Xray photoelectron spectroscopy (XPS)
Summary
The Gobi desert in northwest China and the Sahara desert in northern Africa are the two most important global sources of mineral dust aerosol with current estimates of between 1000– 3000 Tg/yr being advected into the atmosphere (Tegen and Fung, 1994; Dentener et al, 1996; Luo et al, 2003). Due to the absence of experimental values for the heterogeneous H2O2 uptake coefficient (γH2O2 ) on mineral dust aerosol, previous work on combined aerosol/gas-phase chemical box models and on global and regional chemistry-transport models was necessarily based on assumptions and estimates. We present a detailed investigation of the heterogeneous chemical kinetics of authentic mineral dust aerosol with gas-phase H2O2 To our knowledge, this is the first laboratory study of the heterogeneous uptake kinetics of gaseous H2O2 by authentic Gobi and Saharan desert dust aerosol particles over a wide range of relative humidity pertinent to the troposphere.
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