A comparison of bulk and wet-only deposition at two adjacent sites in Melle (Belgium)

During 9 months, the weekly bulk and wet-only precipitation depositions in an urbanised region of Flanders (Belgium) were compared at two sites with a different height and separated by 1 km. The amount of rainfall at the two sites was similar, and the difference in ion deposition between the two sites was generally less than 5%. While the amount of rainfall measured was almost the same for both collector types, bulk deposition was significantly (p<0.02) higher than the wet deposition of all ions other than H+ and NH4+. Averaged for both sites, bulk deposition was 129% (K+), 84% (Ca2+), 51% (Cl−), 50% (Mg2+), 46% (Na+), 32% (SO42−), 27% (NO3−), 17% (F−), and 11% (NH4+) higher than wet-only deposition. The acidity of bulk samples was significantly (p<0.06) lower than the acidity of wet-only samples. Bulk NH4+ concentrations were only significantly (p<0.002) higher than wet-only concentrations at one site because of the sensor-related, delayed closing of the wet-only lid at the second site. Although dry deposition significantly contributed to bulk precipitation measurements, bulk deposition exceeded the wet acidifying deposition of NO3−, NH4+, and SO42− by less than 25%.