Abstract. Although black carbon (BC) is one of the key atmospheric particulate components driving climate change and air quality, there is no agreement on the terminology that considers all aspects of specific properties, definitions, measurement methods, and related uncertainties. As a result, there is much ambiguity in the scientific literature of measurements and numerical models that refer to BC with different names and based on different properties of the particles, with no clear definition of the terms. The authors present here a recommended terminology to clarify the terms used for BC in atmospheric research, with the goal of establishing unambiguous links between terms, targeted material properties and associated measurement techniques.

[1] The rapid temperature increase of 1°C over mainland Europe since 1980 is considerably larger than the temperature rise expected from anthropogenic greenhouse gas increases. Here we present aerosol optical depth measurements from six specific locations and surface irradiance measurements from a large number of radiation sites in Northern Germany and Switzerland. The measurements show a decline in aerosol concentration of up to 60%, which have led to a statistically significant increase of solar irradiance under cloud-free skies since the 1980s. The measurements confirm solar brightening and show that the direct aerosol effect had an approximately five times larger impact on climate forcing than the indirect aerosol and other cloud effects. The overall aerosol and cloud induced surface climate forcing is $+1 W m À2 dec À1 and has most probably strongly contributed to the recent rapid warming in Europe. Citation: Ruckstuhl, C., et al.

[1] During summer of 2004, about 2.7 million hectare of boreal forest burned in Alaska, the largest annual area burned on record, and another 3.1 million hectare burned in Canada. This study explores the impact of emissions from these fires on light absorbing aerosol concentration levels, aerosol optical depths (AOD), and albedo at the Arctic stations Barrow (Alaska), Alert (Canada), Summit (Greenland), and Zeppelin/Ny Å lesund on Spitsbergen (Norway). The Lagrangian particle dispersion model FLEXPART was run backward from these sites to identify periods that were influenced by forest fire pollution plumes. It is shown that the fires led to enhanced values of particle light absorption coefficients (s ap ) at all of these sites. Barrow, about 1000 km away from the fires, was affected by several fire pollution plumes, one leading to spectacularly high 3-hour mean s ap values of up to 32 Mm À1 , more than the highest values measured in Arctic Haze. AOD measurements for a wavelength of 500 nm saturated but were estimated at above 4-5 units, unprecedented in the station records. Fire plumes were transported through the atmospheric column over Summit continuously for 2 months, during which all measured AOD values were enhanced, with maxima up to 0.4-0.5 units. Equivalent black carbon concentrations at the surface at Summit were up to 600 ng m À3 during two major episodes, and Alert saw at least one event with enhanced s ap values. FLEXPART results show that Zeppelin was located in a relatively unaffected part of the Arctic. Nevertheless, there was a 4-day period with daily mean s ap > 0.3 Mm À1 , the strongest episode of the summer half year, and enhanced AOD values. Elevated concentrations of the highly source-specific compound levoglucosan positively confirmed that biomass burning was the source of the aerosols at Zeppelin. In summary, this paper shows that boreal forest fires can lead to elevated concentrations of light absorbing aerosols throughout the entire Arctic. Enhanced AOD values suggest a substantial impact of these plumes on radiation transmission in the Arctic atmosphere. During the passage of the largest fire plume, a pronounced drop of the albedo of the snow was observed at Summit. We suggest that this is due to the deposition of light absorbing particles on the snow, with further potentially important consequences for the Arctic radiation budget.

[1] Large sets of filtered actinometer, filtered pyrheliometer and Sun photometer measurements have been carried out over the past 30 years by various groups at different Arctic and Antarctic sites and for different time periods. They were examined to estimate ensemble average, long-term trends of the summer background aerosol optical depth AOD(500 nm) in the polar regions (omitting the data influenced by Arctic haze and volcanic eruptions). The trend for the Arctic was estimated to be between À1.6% and À2.0% per year over 30 years, depending on location. No significant trend was observed for Antarctica. The time patterns of AOD(500 nm) and Å ngström's parameters a and b measured with Sun photometers during the last 20 years at various Arctic and Antarctic sites are also presented. They give a measure of the large variations of these parameters due to El Chichon, Pinatubo, and Cerro Hudson volcanic particles, Arctic haze episodes most frequent in winter and spring, and the transport of Asian dust and boreal smokes to the Arctic region. Evidence is also shown of marked differences between the aerosol optical parameters measured at coastal and high-altitude sites in Antarctica. In situ optical and chemical composition parameters of aerosol particles measured at Arctic and Antarctic sites are also examined to achieve more complete information on the multimodal size distribution shape parameters and their radiative properties. A characterization of aerosol radiative parameters is also defined by plotting the daily mean values of a as a function of AOD(500 nm), separately for the two polar regions, allowing the identification of different clusters related to fifteen aerosol classes, for which the spectral values of complex refractive index and single scattering albedo were evaluated. Citation: Tomasi, C., et al. (2007), Aerosols in polar regions: A historical overview based on optical depth and in situ observations,

Asix-channel Sun photometer has been calibrated by means of two different methods: Langley plots and standard irradiance lamps. A 4-month calibration campaign was carried out at a high mountain site, Jungfraujoch (3580 m above sea level), in the Swiss Alps. Calibration constants V(0)(λ) determined on clear and stable days by means of a refined Langley-plot technique scatter by less than 0.25% (rms) for wavelengths outside of strong gaseous absorption bands. Inside the 0.94-µm water-vapor absorption band, the V(0)(λ) values retrieved by means of modified Langley plots scatter by 1.0% (rms). Repeated calibrations of the Sun photometer by means of irradiance standard lamps were performed at the World Radiation Center in Davos. The comparison of both methods ranges from perfect agreement to a deviation of 4.9% for the different channels. A discussion of the errors introduced by both methods shows that the Langley-plot calibration, when performed under very clear atmospheric conditions, is superior. However, by means of the standard-lamp calibrations a temporal degradation of the instrument's response up to 4.6% per year was found, implying that a single calibration campaign as done here is not sufficient. Thus we recommend the use of a combination of both methods for maintaining an accurate calibration.