Long‐Term Trend in Black Carbon Mass Concentration Over Central Indo‐Gangetic Plain Location: Understanding the Implied Change in Radiative Forcing.

For the first‐time, analysis of a decade long measurement of Black Carbon mass concentration (BC) was carried out at a representative central Indo‐Gangetic Plain (IGP) location, Varanasi (25.30°N, 83.03°E, 79 m asl), from 2009 to 2021 to understand its physical, optical, and radiative impacts. During the 13‐year study period, the daily BC mass concentration was found to vary between 0.07 and 46.23 μg m−3 (mean 9.18 ± 6.53 μg m−3) and showed a strong inter‐annual and intra‐annual variations. The inter‐annual variability of BC showed a significant decreasing trend (−0.47 μg m−3 yr−1), with a maximum during the post‐monsoon (−1.86 μg m−3 yr−1) and minimum during the pre‐monsoon season (−0.31 μg m−3 yr−1). The Black Carbon Aerosol Radiative Forcing (BC‐ARF) at the top of the atmosphere (BC‐ARFT), surface (BC‐ARFS), and within the atmosphere (BC‐ARFA) was found to be 10.3 ± 6.4, −30.1 ± 18.9, and 40.5 ± 25.2 W m−2, respectively. BC‐ARF also showed a strong inter‐annual variability with a decreasing trend for BC‐ARFT (−0.47 W m−2 yr−1) and BC‐ARFA (−1.94 W m−2 yr−1), while it showed an increasing trend for BC‐ARFS (1.33 W m−2 yr−1). Concentrated weighted trajectories (CWT) and potential source contribution function (PSCF) analyses were performed at the station to determine the potential source sectors and transport routes of BC aerosols. These analyses revealed that the long‐range source of BC at Varanasi originates from the upper and lower IGP, central highlands, southern peninsular region, Pakistan, and even from the Central East Asia region. Plain Language Summary: Long‐term measurement of Black Carbon aerosol was done at a representative Central Indo‐Gangetic Plain (IGP) location, Varanasi, India, from 2009 to 2021. Analysis suggests significant interannual and intra‐annual variation of BC mass for the location. The important finding shows a significant decreasing trend in BC mass: the maximum for the post‐monsoon season and the minimum for the pre‐monsoon. Black Carbon‐Aerosol Radiative Forcing at the top of the atmosphere, at the surface, and within the atmosphere was found to be 10.3 ± 6.4, −30.1 ± 18.9, 40.5 ± 25.2 W m−2, respectively. Black Carbon Aerosol Radiative Forcing also showed a decreasing trend and implies an induced deceasing trend in the atmospheric heating rate. According to the concentrated weighted trajectory and potential source contribution function analyses, the upper and lower IGP, central highlands, southern peninsular region, Pakistan, and even the Central East Asia region are potential long‐range sources of BC at the Varanasi. Key Points: Black Carbon is decreasing over the central Indo‐Gangetic Plain (IGP) since 2009: maximum for post‐monsoon and minimum for pre‐monsoon seasonThe Black Carbon‐Aerosol Radiative Forcing (BC‐ARF) shows a decreasing trend over the central IGP on yearly scale since 2009Decreasing trend in BC‐ARF within the atmosphere relates to a decrease in BC mass concentration and suggests reduced atmospheric warming [ABSTRACT FROM AUTHOR]