Total 360 samples (of 8 h each) of PM2.5 were collected from six sampling sites for summer and winter seasons in Kanpur city, India. The collected PM2.5 mass was subjected to chemical speciation for: (1) ionic species (NH+4, SO2-4, NO -3, and Cl-), (2) carbon contents (EC and OC), and (3) elemental contents (Ca, Mg, Na, K, Al, Si, Fe, Ti, Mn, V, Cr, Ni, Zn, Cd, Pb, Cu, As, and Se). Primary and secondary components of PM2.5 were assessed from speciation results. The influence of marine source to PM 2.5 was negligible, whereas the contribution of crustal dust was significant (10% in summer and 7% in winter). A mass reconstruction approach for PM2.5 could distinctly establish primary and secondary components of measured PM2.5 as: (1) Primary component (27% in summer and 24% in winter): crustal, elemental carbon, and organic mass, (2) Secondary component (45% in summer and 50% in winter): inorganic and organic mass, and (3) others: unidentified mass (27% in summer and 26% in winter). The secondary inorganic component was about 34% in summer (NH+4: 9%; SO 2-4: 16%; NO-3: 9%) and 32% in winter (NH+4: 8%; SO2+4: 13%; NO-3: 11%). The secondary organic component was 12% in summer and 18% in winter. In conclusion, secondary aerosol formation (inorganic and organic) accounted for significant mass of PM2.5 (about 50%) and any particulate control strategy should also include control of primary precursor gases. © 2010 American Association for Aerosol Research.