Heterogeneous Interaction of Isoprene with Natural Gobi Dust

Mineral dust is a major component of atmospheric aerosol that can impact both the climate and the oxidative capacity of the atmosphere. It has been recently suggested that heterogeneous processes on natural mineral dusts can act as a sink of volatile organic compounds (VOCs) and possibly as a sources of oxygenated volatile organic compounds (OVOCs). More precisely, literature findings suggest that compounds such as isoprene (ISP), the highest emitted biogenic volatile organic compound (BVOC) in the atmosphere, could be involved in heterogeneous processes on atmospheric particles and could account for OVOC formation. Hence, within this framework, the adsorption of ISP on Gobi dust was investigated in the temperature ( T ) and relative humidity (RH) ranges of 253–358 K and <0.01–10%, respectively, using zero air as the bath gas. The kinetic measurements were performed using a novel experimental setup equipped with a selected-ion flow-tube mass spectrometer (SIFT-MS) for gas-phase monitoring. The initial uptake coefficient, γ 0, of ISP was measured as a function of several parameters (ISP mass, temperature, and RH). γ 0 is evidenced as inversely dependent upon the RH and dependent upon the temperature according to the empirical equations γ 0 = 2.7 × 10 –10 /(0.005 + RH 1.44 ) and γ 0 = (5.30 ± 1.5) × 10 –7 exp[(1376 ± 133)/ T ], respectively. Furthermore, adsorption isotherms of ISP were determined, and the results were simulated with the Langmuir adsorption model to obtain the partitioning constant, K Lin, as a function of the RH and temperature according to the equations K Lin (296 K) = 0.028/(0.604 + RH 1.54 ) and K Lin = (1.392 ± 0.33) × 10 –7 exp[(3732 ± 140)/ T ], respectively. The effect of an increasing temperature on the reversible fraction of ISP adsorption on Gobi dust was recorded. The atmospheric lifetime of the heterogeneous loss of ISP onto Gobi dust was calculated for forested environments and found to be several years.