The simultaneous near surface measurements of aerosol scattering and absorption coefficients over different environments (Ahmedabad, urban and Gurushikhar, a high altitude remote site) in western India were conducted to estimate SSA and investigate the importance of SSA in aerosol radiative forcing. The surface SSA (0.79, 0.85) is lower than the column SSA (0.92, 0.95) as emission sources for black carbon aerosols (absorbing in nature) are abundant near the surface (Ahmedabad, Gurushikhar). The atmospheric warming over the urban region estimated using column SSA is a factor of 3 lower ( similar to 18 Wm(-2)) than that of the warming ( similar to 52 Wm(-2)) estimated utilising surface SSA. The significant difference in atmospheric warming arises due to the differences in the SSA as aerosol optical depth (a measure of column concentration of aerosols) is the same. Surface and column SSA are comparatively higher over the high altitude remote site as the abundance of absorbing aerosols is less over a non-source region. In addition, the differences between surface and column SSA are less (< 9%) resulting in comparable aerosol radiative forcing estimates. This study highlights the differences that can arise in aerosol radiative effects due to the differences in SSA as a function of altitude (surface vs. column) and environment (urban vs. remote), thereby providing regional bounds on aerosol radiative forcing which can further be used in climate assessment studies.

Wildfires inject large amounts of black carbon (BC) particles into the atmosphere, which can reach the lowermost stratosphere (LMS) and cause strong radiative forcing. During a 14-month period of observations on board a passenger aircraft flying between Europe and North America, we found frequent and widespread biomass burning (BB) plumes, influencing 16 of 160 flight hours in the LMS. The average BC mass concentrations in these plumes (similar to 140 ng.m(-3), standard temperature and pressure) were over 20 times higher than the background concentration (similar to 6 ng.m(-3)) with more than 100-fold enhanced peak values (up to similar to 720 ng.m(-3)). In the LMS, nearly all BC particles were covered with a thick coating. The average mass equivalent diameter of the BC particle cores was similar to 120 nm with a mean coating thickness of similar to 150 nm in the BB plume and similar to 90 nm with a coating of similar to 125 nm in the background. In a BB plume that was encountered twice, we also found a high diameter growth rate of similar to 1 nm.h(-1) due to the BC particle coatings. The observed high concentrations and thick coatings of BC particles demonstrate that wildfires can induce strong local heating in the LMS and may have a significant influence on the regional radiative forcing of climate.

Temporal and seasonal variabilities in black carbon (BC) mass concentrations, equivalent BC from fossil fuel (BCff) and wood burning (BCwb) are investigated using multiwavelength aethalometer measurements made over urban (Ahmedabad) and high altitude remote (Gurushikhar) sites in western India during 2015-2016. BC, BCff and BCwb mass concentrations exhibit strong diurnal variation over Ahmedabad compared to Gurushikhar. Annual mean contribution of BCff to total BC mass concentration is estimated to be 80 and 72% respectively over Ahmedabad and Gurushikhar, which indicates the dominance of fossil fuel emissions. To delineate the impact of BC aerosols on the Earth atmosphere radiation budget aerosol radiative forcing due to composite aerosols, and BC aerosols alone is estimated. Maximum atmospheric forcing due to BC is observed during December (15 Wm(-2)) and November (8 Wm(-2)) over Ahmedabad and Gurushikhar respectively, because BC mass concentration is highest in the respective months. Surface composite forcing is higher during postmonsoon (-27 Wm(-2)) and premonsoon (-16 Wm(-2)) over Ahmedabad and Gurushikhar respectively due to high aerosol optical depths. BC aerosols contribute similar or equal to 60% to the shortwave atmospheric forcing. The present study shows that the large spatial and temporal variation in BC mass concentrations over an urban and high altitude remote site can produce significant regional variabilities in aerosol radiative effects.