Numerical evaluation of contributions of pollutant sources extracted by constrained oblique rotational factor analysis for precipitation data. Extraction of features of precipitations at Hyogo and Akita areas

A statistical method developed by our group, ''the constrained oblique rotational factor analysis'' enables feature reduction and grouping of the pollutant sources in precipitations from their chemical compositions. A study applied to the precipitations has indicated the necessity of the numerical evaluation of contributions of the extracted sources. In this paper, a procedure for the numerical evaluation is demonstrated; and, as one application, the precipitations collected in Hyogo prefecture are compared with those in Akita prefecture. The analysis has proposed that an ion-balance check of the extracted pollutant source gives a simple criterion about how many pollutant sources should be extracted. As a result, the causes of lowering of the pH of the precipitation of these two areas are explained by two sources: 1) an acidic source with sulfuric acid predominant brought over the Sea of Japan from China by winter monsoons; and 2) another acidic source with sulfuric acid and nitric acid predominant emitted domestically. The seasonal changes of acidity of the precipitation of Japan depend upon the amounts of these two contributions.