Carbonaceous aerosols in Indian Himalayan atmosphere: seasonal abundance, altitudinal variation and source analysis

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The present study reports carbonaceous aerosol concentrations determined in total suspended particles (TSP) from three mountainous sites viz. Devasthal Peak, Manora Peak and Bhimtal and one low-land Tarai urban site at Pantnagar in Kumaon region of Uttarakhand, western Himalaya. The concentrations of organic carbon (OC) and elemental carbon (EC) were estimated following IMPROVE_thermal optical reflectance protocol. Results showed that TSP and OC/EC concentrations varied significantly and decreased with increasing altitudes in two seasons. Seasonal/(diurnal) analysis indicated TSP and OC/EC concentrations at mountainous sites exhibited higher values in summer/(day-time), respectively while tarai site had higher values in winter/(night-time). Higher summer/(day-time) values at mountainous sites could be associated with local emissions along with vertical movement of atmospheric boundary layer resulting uplift of air pollutants due to convective motions generated in valley region. OC and EC were noted to be strongly correlated (correlation coefficient > 0.8) indicated similar emissions source and atmospheric processes. The ratios of OC/EC were observed in the range of 2.4 to 9.3, 2.1 to 8.6, 1.9 to 4.3 and 1.8 to 3.2 for Devasthal, Manora peak, Bhimtal and Pantnagar, respectively, suggested mixed sources of biomass burning, coal combustion and vehicular emissions. The large variations in OC/EC ratios at studied sites suggested presence of secondary organic carbon (SOC). Mountainous sites experienced higher % SOC contributions (41 – 45%) to total OC in winter than low-land tarai urban site could be due favourable conditions for condensation of photo-oxidation products of organic species and biogenic VOCs emissions.