Two-year record of carbonaceous fraction in ambient PM2.5 over a forested location in central India: temporal characteristics and estimation of secondary organic carbon

Fine particles (PM2.5), collected on multiple filter substrates, were measured over Van Vihar National Park in Bhopal, central India, for 2 years (01 January 2012–31 December 2013). A thermal-optical carbon analyser (DRI 2001A) was used to measure carbon fractions on punches of quartz fibre filters. Total carbon contributed 38% to PM2.5 mass, of which pyrolysis corrected OC and EC contributed 27% and 11%, respectively. The seasonal averages of OC during the study duration were 22.5 ± 12.3 μg m−3 (post-monsoon) > 20 ± 12.3 μg m−3 (winter) > 8.3 ± 5.6 μg m−3 (pre-monsoon) > 6.4 ± 4.0 μg m−3 (monsoon). Daytime and nighttime OC and EC concentrations were not statistically different from one another, suggesting that local sources like traffic and diurnal meteorology driven effects did not influence these concentrations much. The value of mean OC/EC ratio was 4.2 ± 3.3 during the study period and was highest (6.1 ± 3.4) during the monsoon season. OC and EC concentrations were strongly correlated (r = 0.9), suggesting common primary sources for these species. SOC (secondary organic carbon) estimated by the EC tracer method constituted 31% of the total OC mass. Although SOC concentrations were low during the monsoons, its contribution to OC was maximum during this season (41% and 53% on average during 2012 and 2013, respectively), suggesting aqueous phase heterogeneous processing of SOC. Meteorological parameters and SOC exhibited no association, while only a weak correlation was observed between SO42− and SOC (r = 0.36), providing no clear indication on the likely role of SO42−-catalysed SOC formation.