Carbonaceous and inorganic species in atmospheric aerosols during wintertime over urban and high-altitude sites in North India

Abstract

Synchronous sampling of bulk-aerosols, carried out during wintertime from the two strategically located sites in North India, reveals that total suspended particulates (TSP) over an urban site (Hisar: 29.2°N 75.7°E; 219 m asl) ranged from 67 to 396 µg m^-3; in contrast, TSP at Manora Peak (a high-altitude station: 29.4°N 79.5°E; 1950 m asl) was relatively low (range: 13.7 to 42.7 µg m^-3). At Hisar, on average, water-soluble ionic species (WSIS, range: 14.1 to 78.3 µg m^-3) contribute nearly one-fourth by weight to TSP, with dominant contribution from SO₄ ^2-, NO₃ ^- and NH₄ ⁺. The time series analysis over a span of 30 days shows somewhat uniform distribution of organic carbon/elemental carbon (OC/EC) ratio centering around 8.5 ± 2.2 at this urban site; and the water-soluble organic components (WSOC range: 6.7 to 42.0 µg m^-3) account for 11.5 % to the TSP concentration. Both WSOC and OC exhibit significant positive correlation with water-soluble K⁺ (r = 0.88 and 0.79 respectively), suggesting their dominant contribution from biomass burning. At Manora Peak, the chemical composition of ambient aerosols show characteristically lower abundances of WSIS (range: 2.0 to 9.9 µg m^-3) and WSOC (range: 1.4 to 6.0 µg m^-3); together they account for one-third of the TSP. The characteristic low abundances of OC (range: 2.8 to 6.9 µg C m^-3) and EC (range: 0.34 to 1.4 µg C m^-3) at this high-altitude site and their significant correlation with K⁺ and SO₄ ^2- suggest contribution from long-range transport of anthropogenic species. This study represents a first comprehensive data set for documenting the chemical characteristics of ambient aerosols and source apportionment of EC, OC, WSIS and mineral dust over urban and high-altitude sites in north India, an important data set required for the south Asian region. If the observed OC/EC ratios far greater than ~2 (unlike reported values in the literature for urban sites) and the semi-empirical estimates of secondary OC are typical of the annual average abundances in the ambient aerosols over north India, then the temporal and regional analyses of primary and secondary OC using the existing emission models require reassessment for this region.