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Properties of atmospheric black carbon (BC) particles were characterized during a field experiment at a rural background site (Melpitz, Germany) in February 2017. BC absorption at a wavelength of 870 nm was measured by a photoacoustic extinctiometer, and BC physical properties (BC mass concentration, core size distribution and coating thickness) were measured by a single-particle soot photometer (SP2). Additionally, a catalytic stripper was used to intermittently remove BC coatings by alternating between ambient and thermo-denuded conditions. From these data the mass absorption cross section of BC (MACBC) and its enhancement factor (EMAC) were inferred for essentially water-free aerosol as present after drying to low relative humidity (RH). Two methods were applied independently to investigate the coating effect on EMAC : a correlation method (MACBC,ambient vs. BC coating thickness) and a denuding method (MACBC,ambient vs. MACBC,denuded). Observed EMAC values varied from 1.0 to 1.6 (lower limit from denuding method) or ∼1.2 to 1.9 (higher limit from correlation method), with the mean coating volume fraction ranging from 54 % to 78 % in the dominating mass equivalent BC core diameter range of 200–220 nm. MACBC and EMAC were strongly correlated with coating thickness of BC. By contrast, other potential drivers of EMAC variability, such as different BC sources (air mass origin and absorption Ångström exponent), coating composition (ratio of inorganics to organics) and BC core size distribution, had only minor effects. These results for ambient BC measured at Melpitz during winter show that the lensing effect caused by coatings on BC is the main driver of the variations in MACBC and EMAC , while changes in other BC particle properties such as source, BC core size or coating composition play only minor roles at this rural background site with a large fraction of aged particles. Indirect evidence suggests that potential dampening of the lensing effect due to unfavorable morphology was most likely small or even negligible. [ABSTRACT FROM AUTHOR]

The mass concentration of black carbon (BC) particles in the atmosphere has traditionally been quantified with two methods: as elemental carbon (EC) concentrations measured by thermal-optical analysis and as equivalent black carbon (eBC) concentrations when BC mass is derived from particle light absorption coefficient measurements. Over the last decade, ambient measurements of refractory black carbon (rBC) mass concentrations based on laser-induced incandescence (LII) have become more common, mostly due to the development of the Single-Particle Soot Photometer (SP2) instrument. In this work, EC and rBC mass concentration measurements from field campaigns across several background European sites (Paris, Bologna, Cabauw and Melpitz) have been collated and examined to identify the similarities and differences between BC mass concentrations measured by the two techniques. All EC concentration measurements in PM2.5 were performed with the EUSAAR-2 thermal-optical protocol. All rBC concentration measurements were performed with SP2s calibrated with the same calibration material as recommended in the literature. The median ratio between observed rBC and EC mass concentrations was 0.92, when considering all data points from all five campaigns, and the corresponding geometric standard deviation (GSD) was 1.5. The minimal and maximal observed values of median rBC to EC mass concentration ratios on single campaign level were 0.53 and 1.29, respectively. This shows that substantial systematic bias between these two quantities occurred during some campaigns, which also contributes to the large overall GSD. On single campaign level, the relative spread of individual rBC to EC mass concentration ratios was typically between a factor of 1.2 and 1.3 (1 GSD), which indicates fairly good precision of both methods. Despite considerable variability of BC properties and sources across the whole data set, it was not possible to clearly assign reasons for discrepancies to one or the other method, both known to have their own specific limitations and uncertainties. However, differences in the particle size range covered by these two methods were identified as one likely reason for discrepancies. In particular, rBC to EC mass concentration ratios were found to be systematically less than unity, despite applying a correction for small BC cores that remain undetected by the SP2. This was observed when the rBC mass size distribution was shifted towards smaller modal diameter, which occurred during traffic emission dominated episodes. Overall, the high correlation between rBC and EC mass concentrations indicates that both methods essentially quantify the same property of atmospheric aerosols, whereas systematic differences in measured absolute values by up to a factor of 2 can occur. This finding for the level of agreement between two current state-of-the-art techniques has important implications for studies based on BC mass concentration measurements, for example for the interpretation of uncertainties of inferred BC mass absorption coefficient values, which are required for modelling the radiative forcing of BC. Homogeneity between BC mass determination techniques is very important also towards a routine BC mass measurement for air quality or human health regulations. [ABSTRACT FROM AUTHOR]

Aerosol particles can contribute to the Arctic amplification (AA) by direct and indirect radiative effects. Specifically, black carbon (BC) in the atmosphere, and when deposited on snow and sea ice, has a positive warming effect on the top-of-atmosphere (TOA) radiation balance during the polar day. Current climate models, however, are still struggling to reproduce Arctic aerosol conditions. We present an evaluation study with the global aerosol-climate model ECHAM6.3-HAM2.3 to examine emission-related uncertainties in the BC distribution and the direct radiative effect of BC. The model results are comprehensively compared against the latest ground and airborne aerosol observations for the period 2005–2017, with a focus on BC. Four different setups of air pollution emissions are tested. The simulations in general match well with the observed amount and temporal variability in near-surface BC in the Arctic. Using actual daily instead of fixed biomass burning emissions is crucial for reproducing individual pollution events but has only a small influence on the seasonal cycle of BC. Compared with commonly used fixed anthropogenic emissions for the year 2000, an up-to-date inventory with transient air pollution emissions results in up to a 30 % higher annual BC burden locally. This causes a higher annual mean all-sky net direct radiative effect of BC of over 0.1 W m -2 at the top of the atmosphere over the Arctic region (60–90 ∘ N), being locally more than 0.2 W m -2 over the eastern Arctic Ocean. We estimate BC in the Arctic as leading to an annual net gain of 0.5 W m -2 averaged over the Arctic region but to a local gain of up to 0.8 W m -2 by the direct radiative effect of atmospheric BC plus the effect by the BC-in-snow albedo reduction. Long-range transport is identified as one of the main sources of uncertainties for ECHAM6.3-HAM2.3, leading to an overestimation of BC in atmospheric layers above 500 hPa, especially in summer. This is related to a misrepresentation in wet removal in one identified case at least, which was observed during the ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) summer aircraft campaign. Overall, the current model version has significantly improved since previous intercomparison studies and now performs better than the multi-model average in the Aerosol Comparisons between Observation and Models (AEROCOM) initiative in terms of the spatial and temporal distribution of Arctic BC. [ABSTRACT FROM AUTHOR]

Although aerosols in the Arctic have multiple and complex impacts on the regional climate, their removal due to deposition is still not well quantified. We combined meteorological, aerosol, precipitation, and snowpack observations with simulations to derive information about the deposition of sea salt components and black carbon (BC) from November 2011 to April 2012 to the Arctic snowpack at two locations close to Ny-Ålesund, Svalbard. The dominating role of sea salt and the contribution of dust for the composition of atmospheric aerosols were reflected in the seasonal composition of the snowpack. The strong alignment of the concentrations of the major sea salt components in the aerosols, the precipitation, and the snowpack is linked to the importance of wet deposition for transfer from the atmosphere to the snowpack. This agreement was less strong for monthly snow budgets and deposition, indicating important relocation of the impurities inside the snowpack after deposition. Wet deposition was less important for the transfer of nitrate, non-sea-salt sulfate, and BC to the snow during the winter period. The average BC concentration in the snowpack remains small, with a limited impact on snow albedo and melting. Nevertheless, the observations also indicate an important redistribution of BC in the snowpack, leading to layers with enhanced concentrations. The complex behavior of bromide due to modifications during sea salt aerosol formation and remobilization in the atmosphere and in the snow were not resolved because of the lack of bromide measurements in aerosols and precipitation. [ABSTRACT FROM AUTHOR]

Black carbon (BC) aerosol represents one of the most important anthropogenic emissionproducts in the Arctic and has an important impact on the radiative budget due to its stronglight absorption and scattering behavior. The radiative forcing of BC does not depend only onthe total atmospheric load, but it is also influenced by its vertical distribution from theupper atmosphere down to the snow. BC particles in a high altitude absorb andscatter a significant part of the incoming solar radiation causing dimming at thesurface, which finally leads to local cooling of the near-surface air temperature.Locally produced and long-traveled BC particles are deposited on snow and icesurfaces by dry and wet removal from the atmosphere. The resulting reduction of thespectral surface albedo leads to a positive radiative forcing and a warming of thenear-surface air. To quantify the two opposite surface radiative effects of atmospheric BCand BC in snow, a sensitivity study based on measured BC concentrations wasperformed. An atmospheric radiative transfer model, the library for radiative transfer (libRadtran),was iteratively coupled with the Two-streAm Radiative TransfEr in Snow (TARTES) snowmodel. Two measured atmospheric BC mass concentration vertical profiles are considered.BC data from the Arctic Research of the Composition of the Troposphere fromAircraft and Satellites (ARCTAS) campaign represent a rather polluted spring season,whereas the Arctic CLoud Observations Using airborne measurements during polarDay (ACLOUD) campaign the more pristine summer season. On the basis of theradiative transfer simulations using the measured BC profiles vertical profiles of thesolar radiative forcing by BC in the atmosphere and in snow are presented. Thediurnal cycle of BC radiative forcing is analyzed to estimate the impact of the solarzenith angle (SZA) and the daily mean radiative forcing. The simulations show thatfor 30 ng g−1 BC deposited in fresh snow and a maximum noon SZA of 60˚ thewarming effect of BC suspended in snow dominates the radiative forcing at thesurface leading to a positive surface radiative forcing of 2.2 W m−2. The daily meanvalue is about 1.7 W m−2. The surface cooling by atmospheric BC is significantlylower in the range of -0.25 W m−2. However, in the atmosphere, a warming ofBC containing layers of 0.4 K per day was calculated for the polluted ARCTASBC profile. For the clean conditions of ACLOUD BC only warms by 0.05 K perday. This work is funded by the German Research Foundation (Deutsche Forschungsgemeinschaft,DFG) – Project Number 268020496 – within the Transregional Collaborative ResearchCenter TRR 172 on "Arctic Amplification (AC)3". [ABSTRACT FROM AUTHOR]