Chemical characteristics of PM2.5 at a source region of biomass burning emissions: Evidence for secondary aerosol formation

•Striking diurnal variability has been observed in PM2.5 mass and chemical species.•Secondary organic and inorganic aerosol formation were evident in all periods.•Meteorology played important role in secondary aerosol formation over IGP.•Scattering species (SO42−, NO3−, OC) in PM2.5 were always dominant (∼50%).•Absorbing species (EC fraction) in PM2.5 increases (4-10%) from October to March.

A systematic study on the chemical characteristics of ambient PM2.5, collected during October-2011 to March-2012 from a source region (Patiala: 30.2°N, 76.3°E; 250 m amsl) of biomass burning emissions in the Indo-Gangetic Plain (IGP), exhibit pronounced diurnal variability in mass concentrations of PM2.5, NO3−, NH4+, K+, OC, and EC with ∼30-300% higher concentrations in the nighttime samples. The average WSOC/OC and SO42−/PM2.5 ratios for the daytime (∼0.65, and 0.18, respectively) and nighttime (0.45, and 0.12, respectively) samples provide evidence for secondary organic and SO42− aerosol formation during the daytime. Formation of secondary NO3− is also evident from higher NH4NO3 concentrations associated with lower temperature and higher relative humidity conditions. The scattering species (SO42− + NO3− + OC) contribute ∼50% to PM2.5 mass during October-March whereas absorbing species (EC) contribute only ∼4% in October-February and subsequently increases to ∼10% in March, indicating significance of these species in regional radiative forcing.