Black carbon physical and optical properties across northern India during pre-monsoon and monsoon seasons

Black carbon (BC) is known to have major impacts on both climate and human health, so is therefore of global importance, particularly so in regions close to large populations that have strong sources. The physical properties and mixing state of black carbon containing particles are important determinants in these effects but information is often lacking, particularly in some of the most important regions of the globe. Detailed analysis into the vertical and horizontal BC optical and physical properties across northern India has been carried out using airborne in-situ measurements. 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 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 (11th and 12th June) and monsoon (30th June to 11th July) seasons of 2016. Over the Indo-Gangetic Plain, BC mass concentrations were greater (1.95 µg/m3) compared to north-west India (1.50 µg/m3) and north-east India (0.70 µg/m3) during the pre-monsoon. 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 50–200 nm). The Indo-Gangetic Plain 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 the boundary layer. As the monsoon arrived across the region, mass concentration of BC decreased over the central Indo-Gangetic Plain and north-east locations (38 % and 28 % respectively), except for the north-west location where 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 Indo-Gangetic Plain much like the pre-monsoon 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 Indo-Gangetic Plain 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, 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 on the atmospheric radiative forcing of BC-containing particles. It will also be useful for BC source and emission inventory studies in the future.