Assessment of equivalent black carbon variations and its source apportionment over Varanasi, Indo-Gangetic Basin

Abstract

In this study, the temporal variation of Equivalent Black Carbon (eBC) and its source apportionment is studied using a yearlong (Dec. 2020-Nov. 2021) multiwavelength Aethalometer (AE-33 model) measurements over Varanasi, located in the central Indo-Gangetic Basin (IGB). Results suggest that mean mass concentrations of eBC vary in the range between 0.46 � 0.13 to 11.22 � 5.09 ?g m?3 with an annual mean value of ?3.57 � 2.39 ?g m?3 during the study period. A strong temporal variation in eBC and its components i.e., eBCff (eBC from fossil fuel), and eBCbb (eBC from biomass burning) are found which shows a large variation on different temporal scales with an average value during winter (6.21 � 3.56 ?g m?3), summer (5.09 � 3.61 ?g m?3), monsoon season (1.52 � 1.03 ?g m?3), and post-monsoon (3.75 � 2.68 ?g m?3). The diurnal variation of eBC shows two different maxima between 07:00-08:00 a.m. and 08:00-10:00 p.m. An inverse relationship between eBC concentration and all meteorological parameters (temperature, wind speed, and boundary layer height) is found except relative humidity. The concentration of eBC increases with respect to RH (up to 70 %) suggesting hygroscopic growth while for higher RH (>70 %) value, eBC concentration decreases and indicates the possible wet scavenging processes in the atmosphere. Source apportionment of eBC using the �Aethalometer Model� reveals that eBCff is dominant over eBCbb in total eBC loading during the study period. Cluster analysis of HYSPLIT (Hybrid Single Particle Lagrangian Integrated Trajectory) model computed five days airmass back-trajectory suggests that airmass reached at Varanasi passes through a highly dense fire count region over the northwestern IGB and surrounding which could be the most responsible for the black carbon loading over the study region.