Aerosol-trace gases interactions and their role in air quality control of Delhi city (India)

Abstract

Atmospheric dust is considered to be the major cause of poor air quality due to its contribution to high particulate levels, but their interaction with the acidic gases helps in controlling the level of SO2 and NO2 through ambient neutralization reactions. In the present study, the interaction of acidic gases such as SO2 and NO2 with alkaline dust was investigated during October, 2013–July, 2014 at a site named as Babarpur located at the Trans-Yamuna region of Delhi. The concentration of SO2 ranged from 10 to 170 μg/m3 with an average of 36 μg/m3 while that of NO2 ranged from 15 to 54 μg/m3 with an average of 26 ± 8 μg/m3. The results were observed to be well within the National Ambient Air Quality Standard (NAAQS) limits prescribed by the Central Pollution Control Board (CPCB). The average concentrations of SO2 during day and night time were recorded as 31 ± 18 and 43 ± 53 μg/m3 respectively while the mean concentrations of NO2 during day and night time were recorded as 26 ± 7 and 27 ± 12 μg/m3 respectively. A positive correlation between SO42− and NO3− was also observed indicating their secondary aerosol formation. In aerosol phase, average concentrations of SO42− during day and night time were 3.9 ± 0.3 and 6.5 ± 2.3 μg/m3 respectively while that of NO3− were 9.5 ± 1.5 and 7.3 ± 0.5 μg/m3 respectively. Molar ratios of Ca2+/SO42−, NH4+/SO42−, and NH4+/NO3− were observed as 8, 5, and 1.7 during daytime and 1.5, 0.4, and 0.8 during nighttime respectively. Such molar ratios confirmed high concentrations of sulphate (SO4)2− and low concentrations of nitrate (NO3−) during night time, thereby indicating different pathway of aerosol formation during day and night time. Surface morphology and elemental composition of aerosol samples showed various oval, globular, and platy shapes where the diameter varied from few nm to ~5 μm depending on their precursors. There were certain shapes like grossularite, irregular aggregate, grape-like, triangular, and flattened which indicate the crustal origin of aerosols and their possible role in SO2 and NO2 adsorption.