Carbonaceous aerosols (CA) strongly impact regional and global climate through their light-absorbing and scattering properties, yet their effects remain uncertain in dust-influenced regions. We investigated the optical properties, source contributions, and radiative impacts of CA at two climatically distinct regions in northwestern India: an arid region (AR, Jodhpur; post-monsoon) and a semi-arid region (SAR, Kota; winter). Mean absorption & Aring;ngstr & ouml;m exponent (AAE) values were comparable between the two regions (AR: 1.416 +/- 0.173; SAR: 1.395 +/- 0.069), but temporal cluster analysis revealed source-specific variability, with lower AAE during traffic-dominated periods (similar to 1.30) and elevated AAE during solid fuel and biomass combustion (1.68 in AR and 1.52 in SAR). While equivalent BC (eBC) levels were higher in AR with a relatively uniform liquid-fuel contribution (BClf = 80.06 +/- 1.98 %), the mass absorption cross- of BC (MAC(BC)) in SAR was similar to 4.5X greater, driven by local solid fuel combustion and transported biomass burning emissions (BCsf = 34.61 +/- 6.88 %). Mie modelling indicated higher SSA in AR due to higher contribution of mineral dust, in contrast to SAR, where carbonaceous aerosols caused stronger absorption, forward scattering, and higher imaginary refractive index (k(OBD)). Although absorption enhancement (E-lambda) was slightly higher in AR (similar to 1.11 vs. similar to 0.99), SAR aerosols nearly doubled the warming potential (Delta RFE), with RFE values of similar to 0.87 W/m(2) in SAR versus similar to 0.43 W/m(2) in AR. These findings highlight strong source-specific and site-specific variability in aerosol absorption and radiative, emphasizing the need to integrate region-specific parameters into climate models and air quality assessments for data-scarce arid and semi-arid South Asian environments.

Biomass burning is a major source of carbonaceous aerosols that significantly influences the Earth's radiation balance. However, the spectral light absorption properties of biomass burning aerosols (BBAs), particularly the contribution of brown carbon (BrC), remain poorly constrained due to reliance on laboratory measurements that may not accurately represent real-world atmospheric conditions. To address this limitation, we developed an unmanned aerial vehicle (UAV) based-platform for direct in-situ measurements of BBAs in the ambient atmosphere over the rural North China Plain. This approach reduces biases inherent to laboratory chamber experiments and enables a more realistic characterization of BBAs absorption properties. Our measurements revealed that the absorption & Aring;ngstr & ouml;m exponent (AAE) for typical residential biomass burning was 3.70 +/- 0.04 under smoldering conditions and 1.50 +/- 0.08 under flaming conditions. Variations in AAE were driven primarily by combustion conditions and smoke humidity rather than fuel type. Additionally, field-observed OC/EC ratios were up to ten times higher than those reported in laboratory chamber studies, resulting in systematically lower mass absorption cross-sections. This finding suggests that the BBAs light absorption and radiative forcing estimates in the North China Plain may be systematically overestimated by chamber-based studies. Notably, under smoldering conditions, BrC absorption at 375 nm was up to 6.6 times greater than that of black carbon (BC) once mass emissions are considered, emphasizing that strategies aiming at reducing smoldering combustion could be particularly effective in mitigating the ultraviolet radiative effects of BBAs. Our results demonstrate that ambient atmospheric measurements are essential for accurately constraining BBAs absorption properties and their climate impacts.

The climate models of the Intergovernmental Panel on Climate Change list black carbon (BC) as an important contributor to global warming based on its radiative forcing (RF) impact. Examining closely these models, it becomes apparent that they might underpredict significantly the direct RF for BC, largely due to their assumed spherical BC morphology. Specifically, the light absorption and direct RF of BC agglomerates are enhanced by light scattering between their constituent primary particles as determined by the Rayleigh-Debye-Gans theory interfaced with discrete dipole approximation and recent relations for the refractive index and lensing effect. The light absorption of BC is enhanced by about 20% by the multiple light scattering between BC primary particles regardless of the compactness of their agglomerates. The resulting light absorption agrees very well with the observed absorption aerosol optical depth of BC. ECHAM-HAM simulations accounting for the realistic BC morphology and its coatings reveal high direct RF = 3-5 W/m2 in East, South Asia, sub-Sahara, western Africa, and the Arabian peninsula. These results are in agreement with satellite and AERONET observations of RF and indicate a regional climate warming contribution by 0.75-1.25 degrees C, solely due to BC emissions.

The estimates of radiative forcing of black carbon (BC) remain great uncertainty, largely due to variations in the absorption enhancement of BC by mixing with organic and inorganic coatings in ambient aerosols. We applied a two-step solvent treatment method that experimentally removed coating materials in aerosol samples to determine the BC absorption enhancement. Aerosol samples were collected at Mt. Tai and a severely polluted urban area (Jinan) in North China Plain (NCP). The mass absorption cross- (MAC) of BC aerosols was determined before and after the coating removal. Three thermal-optical protocols, NIOSH, EUSAAR and IMPROVE, were tested for determining of BC mass and MAC. The EUSAAR protocol gave the optimal values of BC mass concentrations and MAC. The MAC for decoated BC was 3.8 +/- 0.9 and 3.8 +/- 0.1 m(2) g(-1) (Average and 1SD) at 678 nm wavelength at the urban area and Mt. Tai, respectively, and it was consistent with the theoretical calculation for pure BC. The MAC for ambient aerosol samples was enhanced to 7.4 +/- 2.6 and 7.8 +/- 2.7 m(2) g(-1) at Jinan and Mt. Tai respectively. Non - BC coatings could enhance the MAC (E-MAC) by a factor of 2 at both the polluted urban area and mountain summit. The light absorption of BC may be rapidly enhanced from air pollution in severely polluted area, and then it remains relatively constant for aged aerosols at Mt. Tai. Climate model is recommended for amplifying BC absorption by a factor of 2 in East Asia and other areas with intense industrialization and urbanization. (C) 2018 Elsevier B.V. All rights reserved.

A reliable assessment of the optical properties of atmospheric black carbon is of crucial importance for an accurate estimation of radiative forcing. In this study we investigated the spatio-temporal variability of the mass absorption cross- (MAC) of atmospheric black carbon, defined as light absorption coefficient (sigma(ap)) divided by elemental carbon mass concentration (m(EC)). sigma(ap) and m(EC) have been monitored at supersites of the ACTRIS network for a minimum period of one year. The 9 rural background sites considered in this study cover southern Scandinavia, central Europe and the Mediterranean. sigma(ap) was determined using filter based absorption photometers and m(EC) using a thermal-optical technique. Homogeneity of the data-set was ensured by harmonization of all involved methods and instruments during extensive intercomparison exercises at the European Center for Aerosol Calibration (ECAC). Annual mean values of sigma(ap) at a wavelength of 637 nm vary between 0.66 and 1.3 Mm(-1) in southern Scandinavia, 3.7-11 Mm(-1) in Central Europe and the British Isles, and 2.3-2.8 Mm(-1) in the Mediterranean. Annual mean values of mEC vary between 0.084 and 0.23 mu g m(-3) in southern Scandinavia, 0.28 -1.1 in Central Europe and the British Isles, and 0.22-0.26 in the Mediterranean. Both sigma(ap) and mEC in southern Scandinavia and Central Europe have a distinct seasonality with maxima during the cold season and minima during summer, whereas at the Mediterranean sites an opposite trend was observed. Annual mean MAC values were quite similar across all sites and the seasonal variability was small at most sites. Consequently, a MAC value of 10.0 m(2) g(-1) (geometric standard deviation = 133) at a wavelength of 637 nm can be considered to be representative of the mixed boundary layer at European background sites, where BC is expected to be internally mixed to a large extent. The observed spatial variability is rather small compared to the variability of values in previous literature, indicating that the harmonization efforts resulted in substantially increased precision of the reported MAC. However, absolute uncertainties of the reported MAC values remain as high as +/- 30-70% due to the lack of appropriate reference methods and calibration materials. The mass ratio between elemental carbon and non-light-absorbing matter was used as a proxy for the thickness of coatings around the BC cores, in order to assess the influence of the mixing state on the MAC of BC. Indeed, the MAC was found to increase with increasing values of the coating thickness proxy. This provides evidence that coatings do increase the MAC of atmospheric BC to some extent, which is commonly referred to as lensing effect. (C) 2016 The Authors. Published by Elsevier Ltd.