Air pollution issues in China these years drew great attention as well as its high-speed economic growth rate. Megacities are the most vulnerable from Beijing in the north to Guangzhou in the south. Real-time instrumental measurement combined with off-line filter based chemical detection are normally conducted to determine the major species of PM_2.5 particles. As one of the most toxic carcinogens listed in WHO guideline, PAHs impose adverse health effect on human immune system. Although detection methods for measuring low molecular weight PAHs (composed of less than 4 aromatic rings) has been widely studied all around world. However, concentration of high molecular weight of PAHs (composed of 4 or more aromatic rings) have been rarely reported due to their higher hydrophobicity and weaker capability to ionize. In this study, we propose a more convenient way to analyze high molecular weight PAHs by HPLC system hyphenated with atmospheric pressure chemical ionization mass spectrometer. To fill up the all-time blank of large PAHs in air pollutant study, we further apportion PM_2.5 species into different sources by using one-year Beijing and PRD four sites chemical data. The comparison of source profiles of two districts would unveil details of PM_2.5 origins. In addition, the differences or similarities could help to enhance the understanding of particle formation in different meteorology condition and to give some hints for air pollution control.