Use of the modified bowen-ratio technique to measure fluxes of trace gases

The modified Bowen-ratio (MBR) technique is a micrometeorological method that can be used to estimate air-surface exchange rates, providing differences in concentrations between two heights can be resolved. Application of the MBR method requires that fluxes and gradients for at least one scalar entity be measured in order to directly compute the exchange coefficient k, which is assumed to be applicable for all scalars. Direct measurements of fluxes (eddy correlation) and gradients for CO2 and H2O were made during three experiments. Measurements were made at the forest floor of a boreal forest, over a lake surface, and at the forest floor of a leafless deciduous forest. Data from these experiments were used to examine the equality of H2O and CO2 exchange coefficients. The mean difference between the exchange coefficients for CO2 and H2O was not found to be statistically different from zero. Uncertainties in the determination of k associated with gradient measurements and sampling strategies are evaluated. The MBR technique is then used to assess the air-surface exchange rates of gas-phase mercury. Although the acceptably good agreement between the exchange coefficients for CO2 and H2O does not prove they are equal to k for Hg, greater confidence is assumed in the determination of Hg fluxes than just using k for heat.