Heterogeneous uptake of gaseous hydrogen peroxide by Gobi and Saharan dust aerosols: a potential missing sink for H2O2 in the troposphere

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 H2 O2 onto dust can be an important loss process for this species which has been, until now, poorly constrained.