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Summer Research Fellowship Programme of India's Science Academies

DRY DEPOSITION OF AEROSOLS IN INDIAN MEGACITIES

Chinmaya HM

Indian Institute of Technology (Indian School of Mines), Dhanbad, 826004

Dr. Dilip M Chate

Indian Institute of Tropical Metereology, Pune, 411008

ABSTRACT

Wet deposition and dry deposition are the ultimate paths by which trace gases and particles are removed from the atmosphere. Dry deposition is the transport of gaseous and particulate species from the atmosphere onto surfaces in the absence of precipitation. The estimation of dry deposition velocity of particulate matter and trace gas species is an important challenge to be overcome in order to quantify the impact of them on populations and the environment. Dry deposition as a continuous process becomes important in the megacities (urban atmosphere), which is subjected to large inputs of anthropogenic contaminants arising from both stationary (power plants, industries, incinerators, and residential air conditioning) and mobile sources (road traffic).

In this work, we propose to investigate the effect of dry deposition on mass concentrations of fine particulate matter in Pune(18.5203° N, 73.8567° E), Mumbai (19.0760° N, 72.8777° E) and Delhi (28.7041° N, 77.1025° E) during November 2016 and compare the results for these western and northern Indian megacities. We shall also investigate the dry deposition flux of particulate matter with respect to particle sizes of coarse, fine and ultrafine particles in Delhi (November 2017) as a case study. It is observed that coarse particles had a greater deposition velocity of ~0.0036m/s when compared to fine particles, which had deposition velocity of ~0.0002m/s and ultrafine particles with deposition velocity of ~3.569 x 10-5m/s. In still air, gravitational forces on large particles govern the air quality by virtue of their significant dry depositions. These deposition velocities are used to estimate deposition fluxes and airborne mass after elapse of time at breathing height. On an average, it was found that around 88.47% mass concentration of coarse particles settled during a time span of 1 hour due to dry deposition, whereas, only 11.31% mass concentration of fine particles had settled during the same period of time. It was hence observed that the deposition velocity of particles was directly proportional to the square of particle diameter and settled mass concentration directly depended on the settling velocity of depositing species.

Keywords: deposition velocity; anthropogenic contaminants; particulate matter; mass concentrations;

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