Atmospheric polycyclic aromatic hydrocarbons (PAHs) from post-harvest biomass burning emissions in the Indo-Gangetic Plain: isomer ratios and temporal trends

Abstract

Atmospheric concentrations of particulate polycyclic aromatic hydrocarbons (PAHs) and their isomer ratios have been studied for two distinct biomass burning emissions (post-harvest burning of paddy-residue in Oct-Nov and wheat-residue burning during April-May) in the Indo-Gangetic Plain (IGP). The mass concentrations of PM_2.5 (Av: 246 µ g m^-3), OC (92 µ g m^-3), EC (7 µ g m^-3) and ΣPAHs (40 ng m^-3) are significantly higher from the paddy-residue burning. In contrast, for wheat-residue burning emissions, concentrations of PM_2.5 (53 µ g m^-3), OC (15 µ g m^-3), EC (4 µ g m^-3) and SPAHs (7 ng m^-3) are about 4-5 times lower. The large temporal variability in the concentrations of particulate species and OC/EC ratio (range: 1.9-25.7) is attributed to differences in the two biomass burning emissions and their relative source strength. The mass fraction of EC (Av: 3.1%), associated with the poor combustion efficiency of moist paddy-residue, is significantly lower than that from the wheat-residue burning (EC/PM_2.5 = 7.6%) during dry weather conditions. Furthermore, OC mass fractions from paddy- and wheat-residue burning emissions are 37% and 28% respectively; whereas SPAHs/EC ratios are significantly different, 5.7 and 1.6 mg g^-1, from the two emission sources. The particulate concentrations of 5- and 6-ring isomers (normalized to EC) from paddy-residue burning are about 3-5 times higher than those from the wheat-residue burning emissions. The cross plots of PAHs show distinct differences in isomer ratios from agricultural-waste burning emissions vis-à-vis fossil-fuel combustion.