A numerical evaluation of chamber methods for determining gas fluxes

Mathematical simulations of nitrous oxide (N2O) flux from homogeneous soil into above-surface chambers have been done for both a closed type of chamber in which soil air is statically collected and an open type of chamber in which ambient air is dynamically drawn across the soil surface. Results indicate that chamber-measured fluxes over land surfaces may be subject to considerable uncertainty, due in part to concentration gradient changes within the soil profile that are a function of the type and the size of the chamber. Assessment of the uncertainties in chamber flux determinations are reported. For reasonable parameters closed-chamber flux values may be underestimated by as much as 55%. Data-analysis procedures are described that can improve the flux estimates. Use of open chambers may yield better flux estimates than closed chambers because of less disturbance to the natural gas concentration profile within the soil. An application to N2O flux measurements over water also is included.