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Zwart HJMA ; Hogenkamp JEM ; Mennen MG ; Erisman JW

107 p in Dutch   1994

Toon Nederlands

English Abstract
This report deals with the performance of a monitoring system for continuous determination of SO2 and NO2 dry deposition fluxes to forests. The system consists of several meteorological instruments, gas monitors, a computer and data acquisition equipment. Fluxes of SO2 and NO2 are determined according to the gradient method and with the eddy correlation technique (only for NO2). The performance was studied from December 1991 to August 1992 both in the laboratory and in the field, at the heathland site Elspeetsche Veld next to an SO2 dry deposition system for low vegetation. The results of these experiments were used to determine the reliability of the forest system and to estimate the error in measured deposition fluxes. For SO2 the system works well. The SO2 monitors and meteorological instruments are reliable for monitoring. The average random error in SO2 fluxes and deposition velocities is 50%, based on 4-hour average gradients, and systematic errors are generally limited to 20%. For NO2 the system is less suitable. The NO2 monitors frequently show technical problems and require a lot of maintenance. The random error in the NO2 fluxes determined with the gradient method is 50% to 250% for 4-hour average gradients. Systematic errors are caused by reactions involving O3, NO and NO2, and by interferences of O3, PAN and H2O on the signal of the NO2 monitor. Nevertheless, these fluxes show approximately the same diurnal course as those obtained from a parameterization, which is based on the assumption that the surface exchange of NO2 is mainly determined by stomatal behaviour. NO2 fluxes determined with the eddy correlation were systematically underestimated. This method must be further investigated. After the performance study the system was installed at the site Speulder forest (in November 1992), where it was operated, further improved and extended with instruments for determination of NH3 fluxes.


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( 1994-03-31 )