Fine particulate matter (PM2.5/PM1.0) in Beijing, China: Variations and chemical compositions as well as sources

•Annual mean PM1.0 and PM2.5 were 35.49 and 66.58 µg/m3 in urban Beijing.•Growth of sulfate, nitrate, and ammonium plays an enhancement role in polluted days.•Notable discrepancy of chemical species is observed between PM1 and PM2.5 in spring.•Regional transportation from surroundings facilitates PM increase in haze episodes.•Secondary and vehicular sources become more important to fine PM loading in Beijing.