Dry deposition of peroxyacetyl nitrate (PAN): Determination of its deposition velocity at night from measurements of the atmospheric PAN and (222)Radon concentration gradient

During the field campaign POPCORN (Photooxidant Formation by Plant Emitted Compounds and OH-Radicals in North-Eastern Germany) in August 1994 we measured the nighttime deposition velocities of PAN above a field. These are the first absolute measurements of PAN deposition velocities in the field, The deposition velocities were derived using a novel method, which uses measurements of the gradients of PAN and Rn-222 and of the emission rates of Rn-222 from the soil. A unique data set of about 250 field measurements of the PAN deposition velocity at night was thus obtained: The deposition velocity at night proved to be highly variable with an average of 0.54 cm/s and a standard deviation of 0.94 cm/s. Recent presumptions by Shepson et al. [1992] that the PAN deposition velocity is strongly reduced with increasing relative humidity could not be confirmed by OUT measurements.