“…With the help of the PCA-MLR technique, Ma et al (2011) found that pyrogenic sources (including CC for residential heating), coke oven, and unidentified sources contributed 44, 39, and 17 %, respectively, to the total PAHs in winter Beijing. By using factor analysis with nonnegative constraints, Tian et al (2009) indentified that coal-fired boiler emission (56 %), CC for residential heating (33 %), and VE (11 %) were the main sources of PAHs in winter Dalian. Okuda et al (2010) concluded that CC for residential heating, oil combustion, gasoline VE, BB, waste incineration, and others contributed 26, 22, 19, 13, 13, and 6 %, respectively, to the total PAHs in winter Xi'an.…”