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 bulk wet deposition concentrations of strong acids, SO4, NO3, NH4, total nitrogen (N-tot), total phosphorus (P-tot), Ca, K, Mg, Na, Cl, and total organic carbon (TOC). PMF produces strictly nonnegative factors, optimally based on error estimates of data values, with almost no rotational ambiguity. The application of PMF to environmental data is outlined: handling of outliers and missing values, determination of error estimates, interpretation of results. The results are displayed in different 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 factors are characterized by the following five key elements: strong acids (H+), nitrogen compounds (N), Cl, TOC and P. Likely main sources for factors are discussed. A high degree of neutralization is observed at all inland stations. Only at four stations the acidity-related substances-SO4 and NO3-are mainly explained by the H-factor. The neutralization caused by the Estonian oil-shale industry is detected at one station. The N-factor is the major anthropogenic factor associating acidic anions SO4 and NO3 together with NH4. Some features of the factors H and N seem to be connected with degradation processes during the collection period of one month. The marine source creates a well-defined Cl-factor at five stations. The annual cycle of the TOC-factor and its association with Ca and K could be connected to airborne particulate matter, such as soil dust. The seasonal behaviour and elemental concentrations of the P-factor suggest a biological origin: pollen, spores, plant debris. The anion-cation balance is shown for all factors and it is mostly good.