Conclusions

Two approaches for identification of source locations and preferred transport pathways of atmospheric particulate trace elements and aerosol species have been investigated. Both approaches are based on combining chemical data with calculated air parcel backward trajectories. The two methods have been applied to four multi-species multi-annual concentration time series measured at sites in Finland, Norway, and Israel. It has been found that the methods agree well with each other and correctly identify known emission sources. We have shown that the use of an `ideal source' test can provide supplementary information about real and `ghost' sources arising from highly correlated probability/concentration values. A nonparametric bootstrap test has been developed in order to estimate the statistical significance of the probability maps.

As for the relative merits of the PSCF and CF methods, we feel that they complement each other. The PSCF method works with probabilities, the CF method with concentrations. CF maps may be more readily understood and easier to interpret, in particular by workers outside the field. However, when one looks for common source regions for two or more sites, the PSCF method has the advantage that working with probabilities provides the same comparison base for all sites. Another advantageous feature of the PSCF method is that it allows one to examine the smearing of a real source by means of an ideal source test. We may conclude that application of trajectory statistics methods can provide in many cases acceptable qualitative information about source regions.