Black carbon physical and optical properties across northern India during pre-monsoon and monsoon seasons
Authors: Brooks James , Liu Dantong , Allan James D. , Williams Paul I. , Haywood Jim , Highwood Ellie J. , Kompalli Sobhan K. , Babu S. Suresh , Satheesh Sreedharan K. , Turner Andrew G. , Coe Hugh 
Published: October 2019

Abstract. Black carbon (BC) is known to have major impacts on both climate and human
health and is therefore of global importance, particularly in regions
close to large populations that have strong sources. The size-resolved
mixing state of BC-containing particles was characterised using a single-particle soot photometer (SP2). The study focusses on the Indo-Gangetic
Plain (IGP) during the pre-monsoon and monsoon seasons. Data presented are
from the UK Facility for Airborne Atmospheric Measurements BAe-146 research
aircraft that performed flights during the pre-monsoon (11 and
12 June) and monsoon (30 June to 11 July) seasons of
2016. Over the IGP, BC mass concentrations were greater (1.95 µg m−3)
compared to north-west India (1.50 µg m−3) and north-east India
(0.70 µg m−3) during the pre-monsoon season. Across northern India, two
distinct BC modes were recorded; a mode of small BC particles (core diameter
<0.16 µm and coating thickness <50 nm) and a mode
of moderately coated BC (core diameter <0.22 µm and coating
thickness of 50–200 nm). The IGP and north-east India locations exhibited
moderately coated black carbon particles with enhanced coating thicknesses,
core sizes, mass absorption cross sections, and scattering enhancement values
compared to much lower values present in the north-west. The coating
thickness and mass absorption cross section increased with altitude (13 %)
compared to those in the boundary layer. As the monsoon arrived across the
region, mass concentration of BC decreased over the central IGP and
north-east locations (38 % and 28 % respectively), whereas for the
north-west location BC properties remained relatively consistent.
Post-monsoon onset, the coating thickness, core size, mass absorption cross
section, and scattering enhancement values were all greatest over the central
IGP much like the pre-monsoon season but were considerably reduced over both
north-east and north-west India. Increases in mass absorption cross section
through the atmospheric column were still present during the monsoon for the
north-west and central IGP locations, but less so over the north-east due to
lack of long-range transport aerosol aloft. Across the Indo-Gangetic Plain
and north-east India during the pre-monsoon and monsoon seasons, solid-fuel (wood
burning) emissions form the greatest proportion of BC with moderately coated
particles. However, as the monsoon develops in the north-east there was a
switch to small uncoated BC particles indicative of traffic emissions, but
the solid-fuel emissions remained in the IGP into the monsoon. For both
seasons in the north-west, traffic emissions form the greatest proportion of
BC particles. Our findings will prove important for greater understanding of the BC
physical and optical properties, with important consequences for the
atmospheric radiative forcing of BC-containing particles. The findings will
also help constrain the regional aerosol models for a variety of
applications such as space-based remote sensing, chemistry transport
modelling, air quality, and BC source and emission inventories.