Mercury isotopes of atmospheric particle bound mercury for source apportionment study in urban Kolkata, India

The particle bound mercury (PBM) in urban-industrial areas is mainly of anthropogenic origin, and is derived from two principal sources: Hg bound to particulate matter directly emitted by industries and power generation plants, and adsorption of gaseous elemental mercury (GEM) and gaseous oxidized mercury (GOM) on air particulates from gas or aqueous phases. Here, we measured the Hg isotope composition of PBM in PM10 samples collected from three locations, a traffic junction, a waste incineration site and an industrial site in Kolkata, the largest metropolis in Eastern India. Sampling was carried out in winter and monsoon seasons between 2013-2015. The objective was to understand whether the isotope composition of the PBM represents source composition. The PBM collected from the waste burning site showed little mass independent fractionation (MIF) (Delta Hg-199 = + 0.12 to -0.11 parts per thousand), similar to the signature in liquid Hg and Hg ores around the world with no seasonal variations. Samples from the industrial site showed mostly negative MDF and MIF (delta Hg-202 = -1.34 to -3.48 % and Delta Hg-199 = + 0.01 to -0.31 parts per thousand). The MDF is consistent with PBM generated by coal combustion however, the MIF is 0.15 parts per thousand more negative compared to the Hg isotope ratios in Indian coals. The traffic junction PBM is probably not produced in situ, but has travelled some distances from nearby industrial sources. The longer residence time of this PBM in the atmosphere has resulted in-aerosol aqueous photoreduction. Thus, the MIF displays a larger range (Delta Hg-199 = + 0.33 to -0.30 parts per thousand) compared to the signature from the other sites and with more positive values in the humid monsoon season. Different Hg isotopic signature of PBM in the three different sampling locations within the same city indicates that both source and post emission atmospheric transformations play important roles in determining isotopic signature of PBM.