RESUMO
Environmental measurement programs are often undertaken with the assumption that measurements at a given location will be comparable to others that would be observed at the same time in the immediate vicinity, but this assumption has seldom been tested. This paper does so. We discuss here the precision of atmospheric concentration measurements of persistent organic pollutants (POPs) near the North American Great Lakes-measurements that we have been conducting since 1994. We report the relative percent differences between the measured values for 100-200 duplicate samples, and through our use of surrogate (recovery) standards, we have separated the analytical error from the sampling error for the target compounds. The error contributions we calculated were on the order of 5% for the analytical error and 20% for the sampling error, suggesting that the latter is the greatest hindrance to increased precision. In a comparison of relative percent differences for measurements among different atmospheric phases, we observed the highest errors for precipitation samples, with an average median of 35 ± 3, which is more than for vapor-phase samples (27 ± 3) or particle-phase samples (27 ± 2). We suggest that sampling errors are principally the result of inaccuracies in measuring the sample volume and possibly the result of spatial heterogeneity of the atmosphere.
