Soot particles, consisting of black carbon (BC), organic carbon (OC), inorganic salts, and trace elements, are emitted into the atmosphere during incomplete combustion. Accurate measurements of atmospheric BC are important as BC particles cause adverse health effects and impact the climate. <br><br> Unfortunately, the accurate measurement of the properties and mass concentrations of BC particles remains difficult. The Single Particle Soot Photometer (SP2) can contribute to improving this situation by measuring the mass of refractory BC in individual particles as well as its mixing state. <br><br> Here, the results of the first detailed SP2 intercomparison, involving 6 SP2s from 6 different research groups, are presented, including the most evolved data products that can presently be calculated from SP2 measurements. <br><br> It was shown that a detection efficiency of almost 100% down to 1 fg BC per particle can readily be achieved, and that this limit can be pushed down to &sim;0.2 fg BC with optimal SP2 setup. Number and mass size distributions of BC cores agreed within &plusmn;5% and &plusmn;10%, respectively, in between the SP2s, with larger deviations in the range below 1 fg BC. <br><br> The accuracy of the SP2's mass concentration measurement depends on the calibration material chosen. The SP2 has previously been shown to be equally sensitive to fullerene soot and ambient BC from sources where fossil fuel was dominant and less sensitive to fullerene soot than to Aquadag. Fullerene soot was therefore chosen as the standard calibration material by the SP2 user community; however, many data sets rely solely on Aquadag calibration measurements. The difference in SP2 sensitivity was found to be almost equal (fullerene soot to Aquadag response ratio of &sim;0.75 at 8.9 fg BC) for all SP2s. This allows the calculation of a fullerene soot equivalent calibration curve from a measured Aquadag calibration, when no fullerene soot calibration is available. It could be shown that this approach works well for all SP2s over the mass range of 1–10 fg. This range is suitable for typical BC mass size distributions in the ambient air far from sources. <br><br> The number size distribution of purely scattering particles optically measured by the 6 SP2s also agreed within 15%. Measurements of the thickness of non-refractory coatings (i.e. product from α-pinene ozonolysis) on the BC particles, relying on BC mass optical size and on an additional particle position measurement, also compared well (within &plusmn;17%). The estimated coating thickness values were consistent with thermo-optical analysis of OC and elemental carbon (EC) content, though absolutely accurate values cannot be expected given all the assumptions that have to be made regarding refractive index, particle morphology, etc. <br><br> This study showed that the SP2 provides accurate and reproducible data, but also that high data quality is only achieved if the SP2 is carefull...

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Clouds play an important role in Arctic amplification. This term represents the recently observed enhanced warming of the Arctic relative to the global increase of near-surface air temperature. However, there are still important knowledge gaps regarding the interplay between Arctic clouds and aerosol particles, and surface properties, as well as turbulent and radiative fluxes that inhibit accurate model simulations of clouds in the Arctic climate system. In an attempt to resolve this so-called Arctic cloud puzzle, two comprehensive and closely coordinated field studies were conducted: the Arctic Cloud Observations Using Airborne Measurements during Polar Day (ACLOUD) aircraft campaign and the Physical Feedbacks of Arctic Boundary Layer, Sea Ice, Cloud and Aerosol (PASCAL) ice breaker expedition. Both observational studies were performed in the framework of the German Arctic Amplification: Climate Relevant Atmospheric and Surface Processes, and Feedback Mechanisms (AC) project. They took place in the vicinity of Svalbard, Norway, in May and June 2017. ACLOUD and PASCAL explored four pieces of the Arctic cloud puzzle: cloud properties, aerosol impact on clouds, atmospheric radiation, and turbulent dynamical processes. The two instrumented Polar 5 and Polar 6 aircraft; the icebreaker Research Vessel (R/V) Polarstern; an ice floe camp including an instrumented tethered balloon; and the permanent ground-based measurement station at Ny-Ålesund, Svalbard, were employed to observe Arctic low- and mid-level mixed-phase clouds and to investigate related atmospheric and surface processes. The Polar 5 aircraft served as a remote sensing observatory examining the clouds from above by downward-looking sensors; the Polar 6 aircraft operated as a flying in situ measurement laboratory sampling inside and below the clouds. Most of the collocated Polar 5/6 flights were conducted either above the R/V Polarstern or over the Ny-Ålesund station, both of which monitored the clouds from below using similar but upward-looking remote sensing techniques as the Polar 5 aircraft. Several of the flights were carried out underneath collocated satellite tracks. The paper motivates the scientific objectives of the ACLOUD/PASCAL observations and describes the measured quantities, retrieved parameters, and the applied complementary instrumentation. Furthermore, it discusses selected measurement results and poses critical research questions to be answered in future papers analyzing the data from the two field campaigns.

Ship engines in the open ocean and Arctic typically combust heavy fuel oil (HFO), resulting in light-absorbing particulate matter (PM) emissions that have been attributed to black carbon (BC) and conventional, soluble brown carbon (brC). We show here that neither BC nor soluble brC is the major light-absorbing carbon (LAC) species in HFO-combustion PM. Instead, "tar brC" dominates. This tar brC, previously identified only in open-biomass-burning emissions, shares key defining properties with BC: it is insoluble, refractory, and substantially absorbs visible and near-infrared light. Relative to BC, tar brC has a higher Angstrom absorption exponent (AAE) (2.5-6, depending on the considered wavelengths), a moderately-high mass absorption efficiency (up to 50% of that of BC), and a lower ratio of sp 2-to sp 3-bonded carbon. Based on our results, we present a refined classification of atmospheric LAC into two sub-types of BC and two sub-types of brC. We apply this refined classification to demonstrate that common analytical techniques for BC must be interpreted with care when applied to tar-containing aerosols. The global significance of our results is indicated by field observations which suggest that tar brC already contributes to Arctic snow darkening, an effect which may be magnified over upcoming decades as Arctic shipping continues to intensify.

Abstract. Accurate measurement of black carbon (BC) mass concentrations in snow and ice is crucial for the assessment of climatic impacts. However, it is difficult to compare methods used to assess BC levels in the literature as they are not the same. The single particle soot photometer (SP2) method appears to be one of the most suitable to measure low concentrations of BC in snow and ice. In this paper, we evaluated a method for the quantification of refractory BC (rBC) in snow and ice samples coupling the SP2 with the APEX-Q nebulizer. The paper reviews all the steps of rBC determination, including SP2 calibration, correction for rBC particle aerosolization efficiency (75 ± 7% using the APEX-Q nebulizer), and treatment of the samples. In addition, we compare the SP2 method and the thermal–optical method – Sunset organic carbon (OC) / elemental carbon (EC) aerosol analyzer with EUSAAR2 protocol – using snow and firn samples with different characteristics from the Greenland Summit, the French Alps, the Caucasus, and the Himalayas. Careful investigation was undertaken of analytical artifacts that potentially affect both methods. The SP2-based rBC quantification may be underestimated when the SP2 detection range does not cover correctly the existing size distribution of the sample. Thermal–optical EC measurements can be underestimated by low filtration efficiency of quartz fiber filter before analysis or dust properties (concentration and type), and overestimated by pyrolyzed OC artifacts during EC analysis. These results underline the need for careful assessment of the analytical technique and procedure for correct data interpretation.

Abstract. Black carbon (BC) contributes to Arctic warming, yet sources of Arctic BC and their geographic contributions remain uncertain. We interpret a series of recent airborne (NETCARE 2015; PAMARCMiP 2009 and 2011 campaigns) and ground-based measurements (at Alert, Barrow and Ny-Ålesund) from multiple methods (thermal, laser incandescence and light absorption) with the GEOS-Chem global chemical transport model and its adjoint to attribute the sources of Arctic BC. This is the first comparison with a chemical transport model of refractory BC (rBC) measurements at Alert. The springtime airborne measurements performed by the NETCARE campaign in 2015 and the PAMARCMiP campaigns in 2009 and 2011 offer BC vertical profiles extending to above 6 km across the Arctic and include profiles above Arctic ground monitoring stations. Our simulations with the addition of seasonally varying domestic heating and of gas flaring emissions are consistent with ground-based measurements of BC concentrations at Alert and Barrow in winter and spring (rRMSE < 13 %) and with airborne measurements of the BC vertical profile across the Arctic (rRMSE = 17 %) except for an underestimation in the middle troposphere (500-700 hPa).Sensitivity simulations suggest that anthropogenic emissions in eastern and southern Asia have the largest effect on the Arctic BC column burden both in spring (56 %) and annually (37 %), with the largest contribution in the middle troposphere (400-700 hPa). Anthropogenic emissions from northern Asia contribute considerable BC (27 % in spring and 43 % annually) to the lower troposphere (below 900 hPa). Biomass burning contributes 20 % to the Arctic BC column annually.At the Arctic surface, anthropogenic emissions from northern Asia (40-45 %) and eastern and southern Asia (20-40 %) are the largest BC contributors in winter and spring, followed by Europe (16-36 %). Biomass burning from North America is the most important contributor to all stations in summer, especially at Barrow.Our adjoint simulations indicate pronounced spatial heterogeneity in the contribution of emissions to the Arctic BC column concentrations, with noteworthy contributions from emissions in eastern China (15 %) and western Siberia (6.5 %). Although uncertain, gas flaring emissions from oilfields in western Siberia could have a striking impact (13 %) on Arctic BC loadings in January, comparable to the total influence of continental Europe and North America (6.5 % each in January). Emissions from as far as the Indo-Gangetic Plain could have a substantial influence (6.3 % annually) on Arctic BC as well.

We characterized the chemical composition and optical properties of particulate matter (PM) emitted by a marine diesel engine operated on heavy fuel oil (HFO), marine gas oil (MGO), and diesel fuel (DF). For all three fuels, ∼80% of submicron PM was organic (and sulfate, for HFO at higher engine loads). Emission factors varied only slightly with engine load. Refractory black carbon (rBC) particles were not thickly coated for any fuel; rBC was therefore externally mixed from organic and sulfate PM. For MGO and DF PM, rBC particles were lognormally distributed in size (mode at d rBC ≈120 nm). For HFO, much larger rBC particles were present. Combining the rBC mass concentrations with in situ absorption measurements yielded an rBC mass absorption coefficient MAC BC,780 nm of 7.8 ± 1.8 m 2 /g at 780 nm for all three fuels. Using positive deviations of the absorption Ångström exponent (AAE) from unity to define brown carbon (brC), we found that brC absorption was negligible for MGO or DF PM (AAE(370,880 nm) ≈ 1.0 ± 0.1) but typically 50% of total 370-nm absorption for HFO PM. Even at 590 nm, ∼20 of the total absorption was due to brC. Using absorption at 880 nm as a reference for BC absorption and normalizing to organic PM mass, we obtained a MAC OM,370 nm of 0.4 m 2 /g at typical operating conditions. Furthermore, we calculated an imaginary refractive index of (0.045 ± 0.025)( ∕370 nm) −3 for HFO PM at 370 nm> > 660 nm, more than twofold greater than previous recommendations. Climate models should account for this substantial brC absorption in HFO PM. Plain Language SummaryWe characterized the fundamental properties of marine engine exhaust that are relevant to its aerosol-radiation interactions in climate models. In particular, we focussed on "brown carbon" light absorption (i.e., absorption in excess of that expected for the black carbon in canonical soot). We found that brown carbon can increase the direct radiative forcing of heavy-fuel-oil ship exhaust by 18% over snow.

Heavy fuel oil (HFO) particulate matter (PM) emitted by marine engines is known to contain toxic heavy metals, including vanadium (V) and nickel (Ni). The toxicity of such metals will depend on the their chemical state, size distribution, and mixing state. Using online soot-particle aerosol mass spectrometry (SP-AMS), we quantified the mass of five metals (V, Ni, Fe, Na, and Ba) in HFO-PM soot particles produced by a marine diesel research engine. The in-soot metal concentrations were compared to in-PM2.5 measurements by inductively coupled plasma-optical emission spectroscopy (ICP-OES). We found that <3% of total PM2.5 metals was associated with soot particles, which may still be sufficient to influence in-cylinder soot burnout rates. Since these metals were most likely present as oxides, whereas studies on lower-temperature boilers report a predominance of sulfates, this result implies that the toxicity of HFO PM depends on its combustion conditions. Finally, we observed a 4-to-25-fold enhancement in the ratio V:Ni in soot particles versus PM2.5, indicating an enrichment of V in soot due to its lower nucleation/condensation temperature. As this enrichment mechanism is not dependent on soot formation, V is expected to be generally enriched within smaller HFO-PM particles from marine engines, enhancing its toxicity.