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

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 PM2.5 (Av: 246 μg m−3), OC (92 μg m−3), EC (7 μg 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 PM2.5 (53 μg m−3), OC (15 μg m−3), EC (4 μg m−3) and ΣPAHs (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/PM2.5 = 7.6%) during dry weather conditions. Furthermore, OC mass fractions from paddy- and wheat-residue burning emissions are 37% and 28% respectively; whereas ΣPAHs/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.