Long term trends and potential source sectors of trace gases over megacity Kolkata

Mahajan, Shubham (2019) Long term trends and potential source sectors of trace gases over megacity Kolkata. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Trace gases are considered as principal components of air pollution indices and can lead to severe effects on ecosystems even with small increments in concentration. Concerning air quality, the current situation in India is quite severe owing to rapid urbanization and industrialization. This is especially the case for megacity Kolkata, which has recently been declared the 2nd most polluted city in India. To address the issue, this study utilizes total column measurements of 5 trace gases (NO₂, SO₂, O₃, CO, HCHO) for 14 years (2005-2018) over Kolkata using OMI and MOPITT satellite instruments and further analyzed using Mann-Kendall, Theil-Sen and ANOVA Bonferroni tests. Trends, transport pathways and potential sources of these gases are studied, and associations with existing emission inventories like ECCAD-CEDS are investigated. O₃, NO₂, HCHO and SO₂ total column showed an increasing trend (5% - 40%) over Kolkata over the last 14 years (2005-2018). The total column of SO2 and NO2 showed strong seasonality with the highest column (NO2: 0.22 ± 0.01, SO2: 0.19 ± 0.02) during the winter season probably due to increased residential and fossil fuel use for domestic heating. Backward trajectory analysis showed the possibility of O₃ transport from the Bay of Bengal, and Central India probably due to phytoplankton metabolism, and photochemical processing of these biogenic VOCs during transport. While SO₂ is mostly sourced from Indo-Gangetic Plain and Bay of Bengal possibly due to thermal power plants, regional open biomass burning and shipping. HCHO total column measured over Kolkata is significantly correlated (r=0.93, p<0.01) with anthropogenic emission inventory. SO₂ total column shows a moderate correlation with emission inventory while CO total column shows the opposite trend, possibly suggesting a dominant contribution from regional rather than local sources. CO total column decreases significantly till 2014 (p=0.025, Q= -0.45), but a later increase disrupts this trend. The overall decreasing trend in CO total column (-3%) needs further work to understand their potential source sectors.

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