One　hundred　and　forty　seasonal　PM2.5　samples　were　collected　from　January　2010　to　January　2011　at　one　urban　site　and　one　suburban　site　simultaneously　in　a　Chinese　megacity,　Shanghai,　to　study　the　concentrations　and　seasonal　variation　of　secondary　organic　aerosols　(SOA).　Concentrations　of　water-soluble　organic　carbon　(WSOC)　were　determined　together　with　organic　and　elemental　carbons.　Thirteen　organic　tracers,　including　the　tracer　for　biomass　burning　and　tracers　for　SOA　from　isoprene,　alpha-pinene,　beta-caryophyllene　and　toluene,　were　measured.　EC-based　method,　WSOC-based　method,　tracer-based　method　and　PMF　modeling　were　used　to　estimate　the　seasonal　contributions　of　secondary　organic　carbon　(SOC)　in　Shanghai,　and　the　results　from　the　different　methods　were　compared　and　evaluated.　Biomass　burning　was　the　major　contributor　to　the　measured　WSOC　in　the　autumn　sampling　period,　while　SOA　was　the　major　contributor　in　the　other　seasons.　The　concentrations　of　the　SOA　tracers　in　summer　were　obviously　higher　than　that　in　other　seasons.　It　was　found　that　SOC　estimated　with　the　tracer-based　method　accounted　for　only　a　small　part　of　the　SOC　from　the　WSOC-based　method　in　Shanghai,　especially　for　the　winter　and　spring　sampling　periods.　PMF　results　showed　that　a　large　part　of　the　SOC　was　associated　with　sulfate　and　nitrate　but　not　with　the　SOA　tracers.　(C)　2013　Elsevier　Ltd.　All　rights　reserved.