Evaluation of the flux gradient technique for measurement of ozone surface fluxes over snowpack at Summit, Greenland

摘要

A multi-step procedure for investigating ozone surface fluxes over polar snowby the tower gradient method was developed and evaluated. These measurementswere then used to obtain five months (April–August 2004) of turbulent ozoneflux data at the Summit research camp located in the center of the Greenlandice shield. Turbulent fluxes were determined by the gradient methodincorporating tower measurements of (a) ozone gradients measured bycommercial ultraviolet absorption analyzers, (b) ambient temperaturegradients using aspirated thermocouple sensors, and (c) wind speed gradientsdetermined by cup anemometers. All gradient instruments were regularlyinter-compared by bringing sensors or inlets to the same measurement height.The developed protocol resulted in an uncertainty on the order of 0.1 ppbvfor 30-min averaged ozone gradients that were used for the ozone fluxcalculations. This protocol facilitated a lower sensitivity threshold for theozone flux determination of ∼8 × 10μg m s, respectively ∼0.01 cm s forthe ozone deposition velocity for typical environmental conditionsencountered at Summit. Uncertainty in the 30-min ozone exchange measurements(evaluated by the Monte Carlo statistical approach) was on the order of10 cm s. This uncertainty typically accounted to~20–100% of the ozone exchange velocities that were determined.These measurements are among the most sensitive ozone depositiondeterminations reported to date. This flux experiment allowed formeasurements of the relatively low ozone uptake rates encountered for polarsnow, and thereby the study of their environmental and spring-versus-summerdependencies.