Acid rain is one of the major concerns on the environmental problems. The value of pH for each rain varies event by event. The presence of several pollutant sources with different origins is expected. Major ions found in precipitation samples are H⁺, Na⁺, K⁺, NH₄⁺, Mg²⁺, Ca²⁺, Cl^-, NO₃^-, SO₄^2-, HCO₃^-, and OH^-. These ions are included into a precipitation as a form of salt. Sea salt particle is an example. Extraction of these independent pollutant sources and evaluation of their contributions to each precipitation sample are important to study pollution mechanisms. For this purpose, we have developed a new chemometrical method called the oblique rotational factor analysis with partially non-negative constraints. A remarkable feature is to exclude hydrogen ion from this constraint to count the neutralization reaction among acidic and alkaline pollutants. This approach enables the evaluation of acidity (role as acid or base) of pollutant source. In this report, basic concept, procedure and the validity of the method are demonstrated using the precipitation data collected at Hyogo, Akita, and Tottori areas in Japan.