Dry deposition to forests—On the use of data from clearings

To quantify dry deposition rates to forest canopies normally requires the use of a tower erected so that measurements can be made above the treetops, as well as often within the canopy. However, wet deposition protocols call for deployment of instruments in forest clearings. The question arises as to whether forest clearing measurements can be used to estimate dry deposition rates to the surrounding forest canopy. A trial program was implemented in the 1980s, to explore this possibility. Two nearby sites were set up at Oak Ridge, Tennessee, as part of the Dry Deposition Inferential Method network, now a component of the Atmospheric Integrated Research Monitoring Network (AIRMoN). One site was a tower extending to about 35 m above the floor of a mixed deciduous forest, with trees about 27 m tall. The second site was in the forest clearing used for measurements under the protocols of the National Atmospheric Deposition Program (NADP). The data indicate that clearing concentrations of SO2, SO42−, and HNO3 are consistently and significantly different from those above the forest, as measured at the nearby tower. In addition, turbulence data obtained in the clearing do not lead to estimates of deposition velocities like those derived from the tower instrumentation. The data indicate that the ratio Ff/Fc (where Ff is the dry deposition rate to the forest canopy and Fc is that of the same chemical to the clearing) varies with the chemical species of interest, and with the time of year in a manner that suggests a direct linkage with the leaf area index of the canopy. On the average, Ff/Fc appears to be slightly above 2.0 for SO2 and HNO3, and slightly below 2.0 for SO42−.