Year-long variability of the fossil fuel and wood burning black carbon components at a rural site in southern Delhi outskirts

This study examines year-long (April 2015 – March 2016) measurements of black carbon (BC) concentrations acquired with a 7-wavelength Aethalometer (ΑE-42) at Panchgaon, in the southern Delhi outskirts. We assess the seasonality of BC concentrations and focus on the quantification of the relative contributions from fossil-fuel combustion (BCff) and wood burning (BCwb) on annual basis. Significant monthly, daily and diurnal variability in both BC components is found due to seasonal changes in the emission sources, dispersion conditions, and boundary-layer dynamics. The annual-mean BC concentration at 880 nm is 7.2 ± 0.3 μg m−3 (range of 0.09–38.6 μg m−3), exhibiting the highest values in winter (9.3 ± 0.7 μg m−3) and the lowest (4.6 ± 0.2 μg m−3) in summer monsoon. BCff (mean of 5.9 ± 0.2 μg m−3) is estimated to account for 81% of the BC mass on annual basis, while the BCwb ranges from 0.55 ± 0.06 μg m−3 in July to 2.7 ± 0.2 μg m−3 in December. The highest BCff/BC fraction is observed in April–June (~86–87% at 880 nm), while the BCwb/BC peaks in October–December (23–29% at 880 nm). The BCff contribution increases in the early-morning hours due to traffic, while the BCwb fraction peaks during the evening and night hours in winter, due to wood and waste material burning for heating. The BC diurnal pattern is highly affected by the mixing-layer height variations, leading to lower concentrations around noon, when the vertical mixing and dilution processes are the strongest. The WRF-Chem model simulations represent rather satisfactorily the BC concentrations and diurnal patterns in summer, but fail to reproduce the BC peaks during late post-monsoon and winter, suggesting the need for refinement of the BC emissions, simulations of local meteorology, and/or boundary-layer dynamics.