Monitoring the dry deposition of SO2 in the Netherlands

A micrometeorological gradient method has been applied to monitor SO2 dry deposition fluxes on a routine basis. Results of a feasibility study during 1987 to 1989 at a rural site in the Netherlands demonstrated that this method can be used to measure dry deposition flux and deposition velocity of SO2 directly and that routine application in a monitoring station is possible. However, about 76% of the collected measuring cycles (36 - 39 minutes) had to be rejected for flux calculations due to fluctuations in turbulence intensities, low concentration levels and instrument failures. From the data a clear diurnal pattern was found for the flux and deposition velocity. A yearly average flux of 22 kg SO2 ha-1y-1, an average deposition velocity of 0.7 cm s-1 were obtained. A strong relation between deposition velocity and wind speed was found, indicating that the deposition is controlled by aerodynamic properties. The measurements suggest very low surface resistance values due to surface wetness in an alkaline agricultural environment. The uncertainty in surface resistance was found to be very large.