Atmospheric Boundary Layer and the Vertical Distribution of Black Carbon over Plain and Hilly Terrain of North East India using In Situ Measurements

Abstract

The Atmospheric Boundary Layer (ABL) has always played a critical role in determining global aerosol dispersion and distribution in the atmosphere. Absorbing aerosols constituting of black carbon (BC) influences our atmosphere in several ways which includes absorption of the incoming solar radiation. There have been several studies on the spatio – temporal heterogeneity of aerosols over the North Eastern Region (NER) of India but very few studies exist on the vertical distribution of aerosols till date. To fill this gap, Microaethalometer (MA), Optical Particle Counter (OPC) and Dr. Pisharoty Radiosonde (sonde) has been deployed using tethered balloon up to 1 kilometre altitude on campaign mode for the first time in NER of India for characterization of airborne aerosols and meteorological parameters. The experiment was conducted successfully over a high altitude station in Meghalaya, Umiam (25.67 oN, 91.91o E, 1040 m amsl) during winter, pre monsoon and post monsoon seasons of the year 2019 and over 3 stations viz., Dhubri (26.02° N, 89.97° E, 31 metres), Guwahati (26.10° N, 91.60° E, 55 metres), and Dibrugarh (27.47° N, 94.91° E, 108 metres) along the Brahmaputra valley in Assam, India during the winter and pre monsoon seasons of the year 2021. Meteorological balloons were also launched simultaneously 4-5 times per day in each of these 3 stations for one day during the time of tethered balloon launch. Distinct diurnal variability has been observed in the vertical profiles of BC and meteorological parameters throughout the day. In the absence of incoming solar radiation or weak turbulence i.e. during the morning, evening and night time the ABL height (ABLH) measured by sonde varied from 75 – 200 m, 50 – 150 m, 100 – 275 m and 50 – 225 m during winter season over Umiam, Dhubri, Guwahati and Dibrugarh respectively. While during pre monsoon season, ABLH varied from 50 – 200 m, 75 – 250 m, 125 – 250 m and 50 to 250 m respectively over the 4 stations. Near surface BC concentration (BCC) was found to be high within the ABL during morning, evening and night time over all the stations. During winter and pre monsoon, maximum BCC reached up to approximately 9 µg/m3, 30.98 µg/m3, 22.16 µg/m3, 13.01 µg/m3 and 5 µg/m3, 9.65 µg/m3, 13.6 µg/m3, 6.4 µg/m3 respectively over Umiam, Dhubri, Guwahati and Dibrugarh near to the surface. ABLH goes above 1 km during day time, where the rapid development of a well mixed vertical profile of BC was observed during the day over the study sites. The vertical profiles of BC also showed multiple elevated layers in some stations which closely followed the vertical profiles of the meteorological parameters.