Atmospheric black carbon aerosol: Long-term characteristics, source apportionment, and trends

Black carbon (BC) aerosols play a very significant role in influencing air quality, climate, and human health. Large uncertainties still exist in BC emissions due to limited observations on the relative source contributions of fossil fuel (ff) combustion and biomass (wood fuel, wf) burning. Our understanding of long-term changes in BC emissions, especially their source apportionment, is sparse and limited. For the first time, BC characteristics, its source apportionment into ff and wf components, and their trends measured using a multi-wavelength aethalometer over an urban location (Ahmedabad) in India covering a 14 year period (2006-2019) are comprehensively investigated. The average contributions of eBC_ff and eBC_wf concentrations to total eBC are 80 % and 20 %, respectively, which highlights the dominance of emissions from fossil fuel combustion processes. A statistically significant increasing trend in eBC and eBC_ff mass concentrations at the rate of 11 % and 29%yr^-1, respectively, and a decreasing trend in eBC_wf concentration at the rate of 36%yr^-1 are detected. The study reveals a significant decrease in biomass (wood fuel) burning emissions over the past decade and an increase in emissions from fossil fuel combustion. However, the rates of increase and decrease in eBC_ff and eBC_wf are different, which indicate that rapid urbanization led to an increase in anthropogenic emissions, whereas an increase in usage of non-polluting fuel led to a decreasing trend in wood burning contribution. During weekdays and weekends, eBC and eBC_ff mass concentrations did not exhibit any statistically significant trends. However, eBC_wf concentration shows a statistically significant decreasing trend during weekdays 34%yr^-1 and weekends 38%yr^-1. Globally, several countries are adopting various strategies and mitigation policies to improve air quality; however, significant gaps exist in their implementation towards achieving cleaner air and less polluted environment. This comprehensive study is relevant for understanding the impact of urbanization and devising better BC emission control policies.