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.