Long-term trend in Black Carbon mass concentration over Central IGP
location: Understanding the implied change in radiative forcing
Abstract
Black carbon (BC) has several direct, indirect, semi-direct, and
microphysical effects on the Earth’s climate system. Analyses of the
decade-long measurement of BC aerosols at Varanasi (from 2009 to 2021)
was done to understand its impact on radiative balance. General studies
suggest that the daily BC mass concentration (mean of 9.18±6.53 µg m–3)
ranges from 0.07 to 46.23 µg m–3 and show a strong interannual and
intra-annual variation over the 13-year study period. Trend analyses
suggest that the interannual variability of BC shows significant
decreasing trend (-0.47 µg m–3 yr-1) over the station. The decreasing
trend is 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
radiative forcing caused specifically by BC (BC-ARF) at the top of the
atmosphere (TOA), surface (SUR), and within the atmosphere (ATM) is
found to be 10.3 ± 6.4, -30.1 ± 18.9, and 40.5 ± 25.2 Wm−2,
respectively. BC-ARF shows strong interannual variability with a
decreasing trend at the TOA (–0.47 Wm–2 yr-1) and ATM ((–1.94 Wm–2
yr-1) forcing, while it showed an increasing trend at the SUR (1.33
Wm–2 yr-1). To identify the potential source sectors and the transport
pathways of BC aerosols, concentrated weighted trajectories (CWT) and
potential source contribution function (PSCF) analyses have been
conducted over the station. These analyses revealed that the primary
source of pollution at Varanasi originate from the upper IGP, lower IGP,
and central India.