Knudsen cell measurements can be used to determine reaction probabilities of gases on solid surfaces under dry condtions. These kinetic measurements are needed in order to quantify heterogeneous atmospheric processes for input in global atmospheric models. The flow of gas through a cell at low pressures is measured in the presence and absence of a reactive surface, in our experiments we are interested in the uptake of gases on components of mineral dust aerosol. The particles (e.g. CaCO₃, alpha-Fe₂O₃, kaolinite…) are placed inside of a sample holder. A plunger with a cover at end is used to isolate the metal oxide particles from the gas. The flow of gas through the cell is monitored with a UTI-Detector II quadrupole mass spectrometer (QMS). Initially, the cover is in place so that the reactive particles are not exposed. After a steady-state flow is established through the low pressure cell, the cover is lifted up to expose the metal oxide particles. The design of the Knudsen cell is such that the volume of the cell stays constant when the metal oxide particles are exposed. Therefore, no volume corrections need to be made and the reaction probability, gamma. A diagram of our multi-sample holder Knudsen cell reactor is shown below.