Identification of source nature and seasonal variations of arctic aerosol by positive matrix factorization

Week-long samples of airborne particulate matter were obtained at Alert, No rthwest Territories, Canada, between 1980 and 1991. The concentrations of 2 4 particulate constituents have some strong, persistent seasonal variations that depend on the transport from their sources. In order to explore the n ature of the cyclical variation of the different processes that give rise t o the measured concentrations, the observations were arranged into both a t wo-way matrix and a three-way data array. For the latter the three modes co nsist of chemical constituents, weeks within a year, and years. The two-way bilinear model and a three-way trilinear model were used to fit the data a nd a new data analysis technique, positive matrix factorization (PMF), has been used to obtain the solutions. PMF utilizes the error estimates of the observations to provide an optimal pointwise scaling data array far weighti ng, which enables it to handle missing data, a common occurrence in environ mental measurements. It can also apply nonnegative constraints to the facto rs. Five factors have been obtained that reproduce the data quite well for both two-way and three-way analyses. Each factor represents a probable sour ce with a compositional profile and distinctive seasonal variations. Specif ically, there are (i) an acid photochemical factor typified by Br-, H+, and SO42- and characterized by a concentration maximum around April, or shortl y after polar sunrise; (ii) a soil factor representing by Si, Al, and Ca an d having its main seasonal maximum in September and October; (iii) an anthr opogenic factor dominated by SO42- together with metallic species like Pb, Zn, V, As, Sb, Se, In, etc., peaking from December to April; (iv) a sea sal t factor consisting mainly of Cl, Na, and K with maximum concentrations dur ing the period from October to April: and (v) a biogenic factor characteriz ed by methane-sulfonate and having a primary maximum at May and a secondary maximum in August. The results obtained by both two-way and three-way PMF analyses are generally consistent with one another. However, there are diff erences because of additional constraints on the solution imposed by the th ree-way analysis. The results also help to confirm the hypotheses regarding the origins of the Arctic aerosol.