SPATIAL VARIABILITY OF URBAN PRECIPITATION CHEMISTRY AND DEPOSITION - STATISTICAL ASSOCIATIONS BETWEEN CONSTITUENTS AND POTENTIAL REMOVAL PROCESSES OF PRECURSOR SPECIES

The precipitation chemistry of Greater Manchester, a Metropolitan County in the northwest of England, has been examined for small scale spatial variability using a network of 18 bulk precipitation collectors. Significant spatial variability was found for concentrations of non-marine SO42-, NO3-, NH4+, Ca2+ and H+ ions. The statistical associations between the data were investigated using correlation, partial correlation and principal components analyses. It was found that zero-order correlation coefficients were inadequate for the interpretation of the data and that the computation of first, and higher order partial correlation coefficients was necessary in order to explain the interrelationships between the data and their spatial variability. The statistical associations between the data suggest relationships between Ca2+ and non-marine SO42-, and NO3- and NH4+ in precipitation which are discussed in terms of their possible precursor species. Potential source effects were examined in conjunction with atmospheric removal processes. The dry deposition of SO4 particles, rather than the dry deposition of SO2, may explain the spatial variability of non-marine SO42-. The erosion of CaSO4 formed from the reaction of SO2 with CaCO3 on urban surfaces with subsequent resuspension is thought to be the basis of the relationship between Ca2+ and non-marine SO42- concentrations in precipitation. The wet and dry deposition of CaCO3 particles from local sources may be partially responsible for the spatial variability of H+, and dry deposition and scavenging of NH3, in conjunction with the predominant wind direction may explain the spatial variability of NO3- and NH4+ ions. Ammonia is thought to originate from sources both outside the study area and within it.