[1] Black carbon (BC), the product of incomplete combustion of fossil fuels and biomass (called elemental carbon (EC) in atmospheric sciences), was quantified in 12 different materials by 17 laboratories from different disciplines, using seven different methods. The materials were divided into three classes: (1) potentially interfering materials, (2) laboratory-produced BC-rich materials, and (3) BC-containing environmental matrices (from soil, water, sediment, and atmosphere). This is the first comprehensive intercomparison of this type (multimethod, multilab, and multisample), focusing mainly on methods used for soil and sediment BC studies. Results for the potentially interfering materials (which by definition contained no fire-derived organic carbon) highlighted situations where individual methods may overestimate BC concentrations. Results for the BC-rich materials (one soot and two chars) showed that some of the methods identified

Abstract. Atmospheric brown carbon (BrC) is a collective term for light absorbing organic compounds in the atmosphere. While the identification of BrC and its formation mechanisms is currently a central effort in the community, little is known about the atmospheric removal processes of aerosol BrC. As a result, we report on a series of laboratory studies of photochemical processing of BrC in the aqueous phase, by direct photolysis and OH oxidation. Solutions of ammonium sulfate mixed with glyoxal (GLYAS) or methylglyoxal (MGAS) are used as surrogates for a class of secondary BrC mediated by imine intermediates. Three nitrophenol species, namely 4-nitrophenol, 5-nitroguaiacol and 4-nitrocatechol, were investigated as a class of water-soluble BrC originating from biomass burning. Photochemical processing induced significant changes in the absorptive properties of BrC. The imine-mediated BrC solutions exhibited rapid photo-bleaching with both direct photolysis and OH oxidation, with atmospheric half-lives of minutes to a few hours. The nitrophenol species exhibited photo-enhancement in the visible range during direct photolysis and the onset of OH oxidation, but rapid photo-bleaching was induced by further OH exposure on an atmospheric timescale of an hour or less. To illustrate the atmospheric relevance of this work, we also performed direct photolysis experiments on water-soluble organic carbon extracted from biofuel combustion samples and observed rapid changes in the optical properties of these samples as well. Overall, these experiments indicate that atmospheric models need to incorporate representations of atmospheric processing of BrC species to accurately model their radiative impacts.

International audienceThe concentrations of sulfate, black carbon (BC) and other aerosols in the Arctic are characterized by high values in late winter and spring (so-called Arctic Haze) and low values in summer. Models have long been struggling to capture this seasonality and especially the high concentrations associated with Arctic Haze. In this study, we evaluate sulfate and BC concentrations from eleven different models driven with the same emission inventory against a comprehensive pan-Arctic measurement data set over a time period of two years (2008–2009). The set of models consisted of one Lagrangian particle dispersion model, four chemistry-transport models (CTMs), one atmospheric chemistry-weather forecast model and five chemistry-climate models (CCMs), of which two were nudged to meteorological analyses and three were running freely. The measurement data set consisted of surface measurements of equivalent BC (eBC) from five stations (Alert, Barrow, Pallas, Tiksi and Zeppelin), elemental carbon (EC) from Station Nord and Alert and aircraft measurements of refractory BC (rBC) from six different campaigns. We find that the models generally captured the measured eBC/rBC and sulfate concentrations quite well, compared to past comparisons. However, the aerosol seasonality at the surface is still too weak in most models. Concentrations of eBC and sulfate averaged over three surface sites are underestimated in winter/spring in all but one model (model means for January-March underestimated by 59 and 37% for BC and sulfate, respectively), whereas concentrations in summer are overestimated in the model mean (by 88 and 44% for July–September), but with over- as well as underestimates present in individual models. The most pronounced eBC underestimates, not included in the above multi-site average, are found for the station Tiksi in Siberia where the measured annual mean eBC concentration is three times higher than the average annual mean for all other stations. This suggests an underestimate of BC sources in Russia in the emission inventory used. Based on the campaign data, biomass burning was identified as another cause of the modelling problems. For sulfate, very large differences were found in the model ensemble, with an apparent anti-correlation between modeled surface concentrations and total atmospheric columns. There is a strong correlation between observed sulfate and eBC concentrations with consistent sulfate/eBC slopes found for all Arctic stations, indicating that the sources contributing to sulfate and BC are similar throughout the Arctic and that the aerosols are internally mixed and undergo similar removal. However, only three models reproduced this finding, whereas sulfate and BC are weakly correlated in the other models. Overall, no class of models (e.g., CTMs, CCMs) performed better than the others and differences are independent of model resolution

One year of aerosol particle observations from Alert, Nunavut shows that new particle formation (NPF) is common during clean periods of the summertime Arctic associated with attendant low condensation sinks and with the presence of methane sulfonic acid (MSA), a product of the atmospheric oxidation of dimethyl sulfide (DMS). The clean aerosol time periods, defined using the distribution of refractory black carbon number concentrations, increase in frequency from June through August as the anthropogenic influence dwindles. During the clean periods, the number concentrations of particles that can act as cloud condensation nuclei (CCN) increase from June through August suggesting that DMS, and possibly other oceanic organic precursors, exert significant control on the Arctic summertime submicron aerosol, a proposition supported by simulations from the GEOS-Chem-TOMAS global chemical transport model with particle microphysics. The CCN increase for the clean periods across the summer is estimated to be able to increase cloud droplet number concentrations (CDNC) by 23-44 cm -3 , comparable to the mean CDNC increase needed to yield the current global cloud albedo forcing from industrial aerosols. These results suggest that DMS may contribute significantly to modification of the Arctic summer shortwave cloud albedo, and they offer a reference for future changes in the Arctic summer aerosol.

Abstract. Number concentrations of ice-nucleating particles (NINP) in the Arctic were derived from ground-based filter samples. Examined samples had been collected in Alert (Nunavut, northern Canadian archipelago on Ellesmere Island), Utqiaġvik, formerly known as Barrow (Alaska), Ny-Ålesund (Svalbard), and at the Villum Research Station (VRS; northern Greenland). For the former two stations, examined filters span a full yearly cycle. For VRS, 10 weekly samples, mostly from different months of one year, were included. Samples from Ny-Ålesund were collected during the months from March until September of one year. At all four stations, highest concentrations were found in the summer months from roughly June to September. For those stations with sufficient data coverage, an annual cycle can be seen. The spectra of NINP observed at the highest temperatures, i.e., those obtained for summer months, showed the presence of INPs that nucleate ice up to −5 ∘C. Although the nature of these highly ice-active INPs could not be determined in this study, it often has been described in the literature that ice activity observed at such high temperatures originates from the presence of ice-active material of biogenic origin. Spectra observed at the lowest temperatures, i.e., those derived for winter months, were on the lower end of the respective values from the literature on Arctic INPs or INPs from midlatitude continental sites, to which a comparison is presented herein. An analysis concerning the origin of INPs that were ice active at high temperatures was carried out using back trajectories and satellite information. Both terrestrial locations in the Arctic and the adjacent sea were found to be possible source areas for highly active INPs.

Abstract. Measurements of the kinetic isotope effect (KIE) for the reactions of light n-alkanes as well as for several unsaturated hydrocarbons, including alkenes, dienes, benzene, and ethyne with OH-radicals are presented. All measured KIEs are positive; that is, molecules containing only C-12 react faster than the C-13 labeled molecules. However, the KIEs for n-alkanes are quite small; between one and four permil. They can be explained mainly by the mass dependence of the collision frequency between the n-alkanes and OH-radicals. KIEs for the reaction of alkenes with OHradicals are considerably higher. They can be explained by a fractionation of 24.5 + 1.1%o for the addition of an OH-radical to a double bond. Inverse dependence on number of carbon atoms and mass dependence of the collision frequencies explain our observations. For benzene the KIE is slightly higher; for ethyne it is somewhat lower than expected from this simple model. For the reaction of many light nonmethane hydrocarbons (NMHC), especially of unsaturated hydrocarbons, with OH-radicals the KIEs are sufficiently large to have significant impact on the isotopic composition of atmospheric NMHC. A small series of stable carbon isotope ratio measurements of atmospheric NMHC were made in the greater Toronto area. Traffic related NMHC emissions were also studied for their stable carbon isotope ratios. From these data it is possible to quantitatively determine the extent of photochemical processing due to OH-radical reactions that the individual NMHC has experienced. Thus such measurements allow quantitative evaluation of the extent of chemical processing the different NMHC have gone through. This also includes the possibility to differentiate between the impact of local sources and regional or large scale transport. It is shown that in combination with concentration measurements isotope ratio measurements are extremely valuable to study the complex interaction between chemical removal mechanisms, mixing, and dilution processes.

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