Black carbon (BC) is a dominant aerosol light absorber, and its brown carbon (BrC) coating can enhance absorption and lead to uncertainties concerning the radiative forcing estimation. This study investigates the mass absorption cross- of equivalent BC (MAC(eBC)) during a long-term field measurement (2013-2017) at a rural Central European site. The MAC enhancement factor (E-abs) and the contribution of BrC coatings to the absorption coefficient (B-abs) were estimated by combining different approaches. The annual mean B-abs and MAC(eBC) values decreased slightly over the measurement period associated with change in the submicron aerosol size distribution. Regardless of the wavelength, B-abs exhibited clear seasonal and diurnal variations, with higher values in winter when a higher absorption Angstromexponent (1.4) was observed due to the local biomass burning (BB). In contrast, MACeBC did not have a distinct temporal trend at 600 nm (7.84 +/- 2.79 m(2) g(-1)), while it showed a seasonal trend at 370 nm with higher values in winter (15.64 +/- 4.77 m(2) g(-1)). During this season, E-abs_(660) was 1.18 +/- 0.27 and did not exhibit any clear wavelength dependence, despite the influence of BB. During the study period, BrC-attributed absorption was observed in 31% of the samples, with a contribution of up to 40% of total Babs. In summer, the E-abs_(660) increased to 1.59 +/- 0.60, when a larger BC coating could be formed by secondary aerosol fractions. During this season, MAC(eBC)_ (660) and E-abs_(660) showed comparable source profiles that were mainly associated with aged air masses over central Europe, thereby supporting the fact that characteristics of coating materials formed during atmospheric aging are a major factor driving the MAC(eBC)_(660) measured at the regional background site. Further field investigations of the composition of BC coatings would help to better understand and estimate uncertainties related to the radiative effect of aerosols. (C) 2021 The Authors. Published by Elsevier B.V.

Mass absorption cross- of black carbon (MAC(BC)) describes the absorptive cross- per unit mass of black carbon, and is, thus, an essential parameter to estimate the radiative forcing of black carbon. Many studies have sought to estimate MAC(BC) from a theoretical perspective, but these studies require the knowledge of a set of aerosol properties, which are difficult and/or labor-intensive to measure. We therefore investigate the ability of seven data analytical approaches (including different multivariate regressions, support vector machine, and neural networks) in predicting MAC(BC) for both ambient and biomass burning measurements. Our model utilizes multi-wavelength light absorption and scattering as well as the aerosol size distributions as input variables to predict MAC(BC) across different wavelengths. We assessed the applicability of the proposed approaches in estimating MAC(BC) using different statistical metrics (such as coefficient of determination (R-2), mean square error (MSE), fractional error, and fractional bias). Overall, the approaches used in this study can estimate MAC(BC) appropriately, but the prediction performance varies across approaches and atmospheric environments. Based on an uncertainty evaluation of our models and the empirical and theoretical approaches to predict MAC(BC), we preliminarily put forth support vector machine (SVM) as a recommended data analytical technique for use. We provide an operational tool built with the approaches presented in this paper to facilitate this procedure for future users.

The Tibetan Plateau (TP) is one of the world's most sensitive areas for climate change, but the lack of information on light-absorption by aerosols limits the understanding of climate forcing feedbacks. Here, the contributions of black carbon (BC) and brown carbon (BrC) to light absorption and radiative forcing were investigated. Absorption Angstrom exponents (alpha), mass absorption cross sections (MAC), and absorption coefficients (b(abs)) for selected wavelengths were measured for a year of aerosol samples collected at Lulang on the southeast TP. Aerosol absorption at all wavelengths was strongest in the pre-monsoon when levoglucosan, a biomass burning indicator, was elevated. The contributions of BC, BrC, and dust to b(abs) were decoupled. Results showed that dust contributed 8.5% to the total light absorption at 405 nm and 3.9% 808 nm. A two-component model indicated that BC and BrC contributed 48.7% and 44.0% to total b(abs) at 405 nm but BrC had a smaller effect at middle visible wavelengths. Elevated b(abs,non-dust,BC) and b(abs,non-dust,BrC) and a high alpha(BrC) but low alpha(aerosol) values in the pre-monsoon were attributed to biomass burning, which produces not only BrC but also BC which has a much lower a value. Average non-dust MACs for BC and BrC at 405 nm were 6.1 +/- 2.8 and 0.72 +/- 0.55 m(2) g(-1), respectively. Nonparametric statistical tests showed that the MAC(non-dustBC) was relatively constant but MAC(non-dust, BrC) was more variable. In addition, BrC was correlated with non-dust b(abs,BC) and MAC(non-dust,BC) in winter and the pre-monsoon, implying BrC and BC shared sources in those two seasons, but lower correlations in the monsoon and post-monsoon suggest that a mixture of sources impacted BrC (e.g., biogenic emission, secondary formation, etc.). Finally, the relative contributions of BrC to BC for radiative forcing from 405 to 808 nm were 29.4 +/- 9.5% with no remarkable seasonal differences, confirming the importance of BrC to light absorption in the near UV throughout the year. As a result, the BrC absorption is an important additional factor which needs to be considered in atmospheric models, although the atmospheric heating by BC seems to be a larger climate forcer in this region.

Surface concentration of black carbon (BC) is a key factor for the understanding of the impact of anthropogenic pollutants on human health. The majority of Italian cities lack long-term measurements of BC concentrations since such a metric is not regulated by EU legislation. This work attempts a long-term (2001-2017) inference of equivalent black carbon (eBC) concentrations in the city of Rome (Italy) based on sun-photometry data. To this end, aerosol light absorption coefficients at the surface are inferred from the columnar aerosol aerosol light absorption coefficient records from the Rome Tor Vergata AERONET sun-photometer. The main focus of this work is to rescale aerosol light absorption columnar data (AERONET) to ground-level BC data. This is done by using values of mixing layer height (MLH) derived from ceilometer measurements and then by converting the absorption into eBC mass concentration through a mass-to-absorption conversion factor, the Mass Absorption Efficiency (MAE). The final aim is to obtain relevant data representative of the BC aerosol at the surface (i.e., in-situ)-so within the MLH- and then to infer a long-term record of surface equivalent black carbon mass concentration in Rome. To evaluate the accuracy of this procedure, we compared the AERONET-based results to in-situ measurements of aerosol light absorption coefficients (alpha(abs)) collected during some intensive field campaigns performed in Rome between 2010 and 2017. This analysis shows that different measurement methods, local emissions, and atmospheric conditions (MLH, residual layers) are some of the most important factors influencing differences between inferred and measured alpha(abs). As a general result, inferred and measured alpha(abs) resulted to reach quite a good correlation (up to r = 0.73) after a screening procedure that excludes one of the major cause of discrepancy between AERONET inferred and in-situ measured alpha(abs): the presence of highly absorbing aerosol layers at high altitude (e.g., dust), which frequently affects the Mediterranean site of Rome. Long-term trends of inferred alpha(abs), eBC, and of the major optical variables that control aerosol's direct radiative forcing (extinction aerosol optical depth, AOD(EXT), absorption aerosol optical depth, AOD(ABS), and single scattering albedo, SSA) have been estimated. The Mann-Kendall statistical test associated with Sen's slope was used to test the data for long-term trends. These show a negative trend for both AOD(EXT) (-0.047/decade) and AOD(ABS), (-0.007/decade). The latter converts into a negative trend for the alpha(abs) of -5.9 Mm(-1)/decade and for eBC mass concentration of -0.76 mu g/m(3)/decade. A positive trend is found for SSA (+0.014/decade), indicating that contribution of absorption to extinction is decreasing faster than that of scattering. These long-term trends are consistent with those of other air pollutant concentrations (i.e., PM2.5 and CO) in the Rome area. Despite some limitations, findings of this study fill a current lack in BC observations and may bear useful implications with regard to the improvement of our understanding of the impact of BC on air quality and climate in this Mediterranean urban region.

Despite growing evidence of light-absorbing organic aerosols (OAs), their contribution to the Earth's radiative budget is still poorly understood. In this study we derived a new empirical relationship that binds OA single scattering albedo (SSA), which is the ratio of light scattering to extinction, with sulfate + nitrate aerosol optical depth (AOD) and applied this method to estimate OA SSA over the tropical biomass burning regions. This method includes division of the attribution of black carbon (BC) and OA absorption aerosol optical depths from the Aerosol Robotic Network (AERONET) observation and determination of the fine-mode ratio of sea-salt and dust AODs from several atmospheric chemistry models. Our best estimate of OA SSA over the tropical biomass burning regions is 0.91 at 550 nm. Uncertainties associated with observations and models permit a value range of 0.82-0.93. Furthermore, by using the estimated OA SSA and comprehensive observations including AERONET, Moderate Resolution Imaging Spectroradiometer (MODIS) and Multi-angle Imaging Spectroradiometer (MISR), we examined the first global estimate of sulfate + nitrate AOD through a semi-observational approach. The global mean sulfate + nitrate AOD of 0.017 is in the lower range of the values obtained from 21 models participated in AeroCom phase II. The results imply that most aerosol models as well as climate models, which commonly use OA SSA of 0.96-1.0, have so far ignored light absorption by OAs and have overestimated light scattering by sulfate + nitrate aerosols. This indicates that the actual aerosol direct radiative forcing should be less negative than currently believed. (C) 2016 Elsevier Ltd. All rights reserved.

Little is known about the optical significance of light absorbing particulate organic compounds (i.e., brown carbon, BrC), including the importance relative to black carbon (BC) and influence on direct radiative forcing by aerosols. The vertical profile of BrC affects its radiative forcing, yet the distribution of BrC in the free troposphere is largely unknown. In this study, BrC absorption was directly measured in solvent extracts of particulate filters obtained from aircraft sampling over the continental USA. Excluding biomass burning plumes, BrC was observed throughout the tropospheric column (<13km), and its prevalence increased relative to BC with increasing altitude, indicating contributions from secondary sources. Closure analysis showed good agreement between light absorption from BC plus BrC relative to measured total aerosol absorption. A radiative transfer model indicated that BrC absorption reduced top of atmosphere aerosol forcing by similar to 20%, suggesting that it is an important component of direct aerosol radiative forcing. Key Points BrC is prevalent in the troposphere and increases relative to BC with altitude Optical closure is obtained between BrC plus BC and total absorption at 365nm BrC contributes 20% to top of atmosphere absorbing aerosol forcing