ALBERT

Description: 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.

Description: Analyses of shallow cores obtained at the European Project for Ice Coring in Antarctica (EPICA) drilling site Kohnen station (75°00′ S, 00°04′ E; 2892 m a.s.l.) on the plateau of Dronning Maud Land reveal the presence of conserved snow dunes in the firn. In situ observations during three dune formation events in the 2005/06 austral summer at Kohnen station show that these periods were characterized by a phase of 2 or 3 days with snowdrift prior to dune formation which only occurred during high wind speeds of 〉10 m s-1 at 2 m height caused by the influence of a low-pressure system. The dune surface coverage after a formation event varied between 5% and 15%, with a typical dune size of (4 ± 2) m × (8 ± 3) m, a maximum height of 0.2 ± 0.1 m and a periodicity length of about 30 m. The mean density within a snow dune varied between 380 and 500 kg m-3, whereas the mean density at the surrounding surface was 330 ± 5 kgm-3. The firn cores covering a time-span of 22 ± 2 years reveal that approximately three to eight events per year occurred, during which snow dunes had been formed and were preserved in the firn.

Description: After aerosol deposition from the atmosphere, black carbon (BC) takes part in the snow albedo feed- back contributing to the modification of the Arctic radiative budget. With the initial goal of quantifying the concentra- tion of BC in the Arctic snow and subsequent climatic im- pacts, snow samples were collected during the research ves- sel (R/V) Polarstern expedition of PASCAL (Physical Feed- backs of Arctic Boundary Layer, Sea Ice, Cloud and Aerosol; Polarstern cruise 106) in the sea-ice-covered Fram Strait in early summer 2017. The refractory BC (rBC) content was then measured in the laboratory of the Alfred Wegener In- stitute with the single particle soot photometer (SP2). Based on the strong observational correlations between both rBC concentration and rBC diameter with snow salinity, we hy- pothesize a salt-induced matrix effect interfering with the SP2 analysis. This paper evaluates the impact of sea salt, based on the measurement of electrical conductivity (κ) in water samples, on rBC measurements made with a SP2 neb- ulizer technique. Under realistic salinity conditions, labora- tory experiments indicated a dramatic six-fold reduction in observed rBC concentration with increasing salinity. In the salinity conditions tested in the present work (salt concen- tration below 0.4 g L−1) the impact of salt on the nebuliza- tion of water droplets might be negligible. However, the SP2 mass detection efficiency systematically decreased with in- creasing salinity, with the smaller rBC particles being preferentially undetected. The high concentration of suspended salt particles and the formation of thick salt coatings on rBC cores caused problems in the SP2 analog-to-digital conver- sion of the signal and incandescence quenching, respectively. Changes to the signal acquisition parameters and the laser power of the SP2 improved the mass detection efficiency, which, nonetheless, stayed below unity. The present work provides evidence that a high concentration of sea salt un- dermines the quantification of rBC in snow performed with the SP2 nebulizer system described here. This interference has not been previously reported and might affect the future such analysis of rBC particles in snow collected, especially over sea ice or coastal regions strongly affected by sea salt deposition.

Description: Properties of atmospheric black carbon (BC) par- ticles were characterized during a field experiment at a ru- ral 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 photome- ter (SP2). Additionally, a catalytic stripper was used to in- termittently remove BC coatings by alternating between am- bient and thermo-denuded conditions. From these data the mass absorption cross section of BC (MACBC) and its en- hancement factor (EMAC) were inferred for essentially water- free aerosol as present after drying to low relative humid- ity (RH). Two methods were applied independently to in- vestigate the coating effect on EMAC: a correlation method (MACBC,ambient vs. BC coating thickness) and a denud- ing method (MACBC, ambient vs. MACBC, denuded). Observed EMAC values varied from 1.0 to 1.6 (lower limit from de- nuding method) or ∼ 1.2 to 1.9 (higher limit from correla- tion 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 con- trast, other potential drivers of EMAC variability, such as dif- ferent 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 parti- cles. Indirect evidence suggests that potential dampening of the lensing effect due to unfavorable morphology was most likely small or even negligible.

Description: Spectral radiance measurements by a digital single-lens reflex camera were used to derive the directional reflectivity of clouds and different surfaces in the Arctic. The camera has been calibrated radiometrically and spectrally to provide accurate radiance measurements with high angular resolution. A comparison with spectral radiance measure- ments with the Spectral Modular Airborne Radiation mea- surement sysTem (SMART-Albedometer) showed an agree- ment within the uncertainties of both instruments (6 % for both). The directional reflectivity in terms of the hemispher- ical directional reflectance factor (HDRF) was obtained for sea ice, ice-free ocean and clouds. The sea ice, with an albedo of ρ = 0.96 (at 530 nm wavelength), showed an al- most isotropic HDRF, while sun glint was observed for the ocean HDRF (ρ = 0.12). For the cloud observations with ρ = 0.62, the cloudbow – a backscatter feature typically for scattering by liquid water droplets – was covered by the cam- era. For measurements above heterogeneous stratocumu- lus clouds, the required number of images to obtain a mean HDRF that clearly exhibits the cloudbow has been estimated at about 50 images (10 min flight time). A representation of the HDRF as a function of the scattering angle only reduces the image number to about 10 (2 min flight time). The measured cloud and ocean HDRF have been com- pared to radiative transfer simulations. The ocean HDRF simulated with the observed surface wind speed of 9 m s−1 agreed best with the measurements. For the cloud HDRF, the best agreement was obtained by a broad and weak cloud- bow simulated with a cloud droplet effective radius of Reff = 4 μm. This value agrees with the particle sizes derived from in situ measurements and retrieved from the spectral radiance of the SMART-Albedometer.

Description: 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 ef- fect on the top-of-atmosphere (TOA) radiation balance dur- ing 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 uncer- tainties in the BC distribution and the direct radiative ef- fect 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 differ- ent setups of air pollution emissions are tested. The simula- tions 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. Com- pared 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 lo- cally 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 ef- fect 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 mis- representation 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 Satel- lites) summer aircraft campaign. Overall, the current model version has significantly improved since previous intercom- parison 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.