SOURCE IDENTIFICATION OF BULK WET DEPOSITION IN FINLAND BY POSITIVE MATRIX FACTORIZATION

A new variant of factor analysis (positive matrix factorization, PMF) is applied to a Finnish data set (18 years, 15 locations) of monthly b ulk wet deposition concentrations of strong acids, SO4, NO3, NH4, tota l nitrogen (N-tot), total phosphorus (P-tot), Ca, K, Mg, Na, Cl, and t otal organic carbon (TOC). PMF produces strictly nonnegative factors, optimally based on error estimates of data values, with almost no rota tional ambiguity. The application of PMF to environmental data is outl ined: handling of outliers and missing values, determination of error estimates, interpretation of results. The results are displayed in dif ferent ways: (1) seasonal profiles of factors; (2) factor compositions by absolute value; (3) factor compositions scaled by their importance in explaining the variation of data. For most compounds 90-95% of the total weighted variation is explained by four factors. Each of the 15 data matrices is analysed with four factors. Different types of facto rs are characterized by the following five key elements: strong acids (H+), nitrogen compounds (N), Cl, TOC and P. Likely main sources for f actors are discussed. A high degree of neutralization is observed at a ll inland stations. Only at four stations the acidity-related substanc es-SO4 and NO3-are mainly explained by the H-factor. The neutralizatio n caused by the Estonian oil-shale industry is detected at one station . The N-factor is the major anthropogenic factor associating acidic an ions SO4 and NO3 together with NH4. Some features of the factors H and N seem to be connected with degradation processes during the collecti on period of one month. The marine source creates a well-defined Cl-fa ctor at five stations. The annual cycle of the TOC-factor and its asso ciation with Ca and K could be connected to airborne particulate matte r, such as soil dust. The seasonal behaviour and elemental concentrati ons of the P-factor suggest a biological origin: pollen, spores, plant debris. The anion-cation balance is shown for all factors and it is m ostly good.