Characterization of atmospheric Black Carbon over a semi-urban site of Southeast India: Local sources and long-range transport

Continuous measurements of Black Carbon (BC) mass concentration using an advanced multi wavelength Aethalometer (AE-33) during January-December 2016 were carrried out, for the first time, at the premises of KL University, Vijayawada (16.44°N, 80.62°E, 30 m amsl), a semi-urban site in the southeast India. The estimates of the percentage contribution of Biomass Burning (BB%) aerosols to total BC concentration is utilised to understand the contribution of anthropogenic activities over the observational site. In this context, the diurnal and seasonal variation of BC mass concentrations and BB (%) contributions have been investigated and the possible sources are examined. A significant diurnal variation in BC concentration is observed with two prominent peaks, one occuring in the morning hours (around 0800-0900 IST) and the other occuring in the evening hours (around 2000 IST). Annually, the maximum BC concentration is observed during November (6.88 ± 1.56 μg m^-3) and minimum during August (1.7 ± 0.52 μg m^-3) over this location. On a seasonal scale, mean BC mass concentration is highest during winter (DJF, 4.68 ± 1.32 μg m^-3) followed by post-monsoon (SON, 4.52 ± 2.36 μg m^-3), summer (MAM, 2.44 ± 0.8 μg m^-3) and lowest during monsoon (JJA, 1.83 ± 0.61 μg m^-3). The higher BB (%) contribution is observed during summer (25.21 ± 7.02%), while the contribution is minimum in monsoon season (16.83 ± 4.15%). Examination of the spectral properties of aerosol absorption in terms of Ångström exponent (α) indicates that the major contributor of BC over this location is biomass burning (α ~ 1.66 ± 0.06). This is supported by the higher number of fire counts (obtained from Moderate Resolution Imaging Spectroradiometer-MODIS onboard Terra satellite) in and around the study region during winter and summer revealing the higher contribution of BB aerosols. The concentration weighted trajectories (CWT) analysis (based on 5-day airmass back trajectories arriving the receptor site at an altitude of 500 m AGL) reveals that synoptic airmasses arriving from the continental regions located northeast of the observational sites mainly influence the BC properties over Vijayawada.