| Popis: |
Urban sampling sites have been specifically excluded from recent large networks for measuring chemical composition of precipitation. Because information on precipitation composition in urban areas is needed for a variety of current applications, it is useful to summarize present knowledge. Most of the available information is based on samples of bulk precipitation, collected by continuously open collectors. This method is now widely acknowledged to be of limited value because of its poor sampling characteristics for dry deposition. For many ions, particularly those residing on large airborne particles, urban bulk sampling yielded considerably higher concentrations than found in samples collected in ‘wet-only’ samplers. Spatial variability of ionic concentrations in urban areas is expressed in terms of the sample standard deviation of site precipitation-weighted means, as a percentage of the overall urban mean. Median values of the most major ions were near 30%; half of the available measurements were between 20 and 47%. Differences between urban and nearby rural concentrations of ions in precipitation were often reported. Those ions with some tendency to occur in higher concentrations in cities included Na+, Mg2+, Ca2+, NO3−, SO42− and Cl−. These differences should be viewed with caution, however, because they were usually based on one or two, possibly unrepresentative, rural sampling sites. Seasonal variations of urban [H+] were different in Europe and the U.S. Highest concentrations occurred in winter in Europe, but in summer in the U.S. There is a pressing need for additional urban precipitation chemistry monitoring and research. |
