A new membrane tube technique (METT) for continuous gas measurements in soils

We present a novel technique for monitoring trace gas concentrations in the air filled pore space of the soil with a high temporal resolution. The method is based on gas diffusing from the air filled pore space into air flowing through an air permeable, hydrophobic, polypropylene tube (Accurel(R) PP V8/2). Gas permeation efficiencies of the membrane tube were determined for NO, N(2)O, CO(2) and (222)Rn in the laboratory. For a length of 1.5 m and flows smaller than 0.8 L min(-1), the permeation efficiency was larger than 96%. The effective diffusion coefficients of NO, N(2)O, CO(2) and (222)Rn in the membrane are 6.2, 6.6, 5.6 and 6.6 times smaller than the corresponding diffusion coefficients in air, respectively. For tubes shorter than 1.5 m the contribution of pressure gradient induced transport into the membrane tube is below 0.9% of concentration gradient induced transport. Profiles of NO and (222)Rn have been measured in the soil of a wheat field. For destructive NO measurements the inlet concentration of the tube was adjusted to the concentrations measured at the outlet whereas for the non-destructive (222)Rn measurements the sampled gas was recycled.