ALBERT

Description: Transport of anthropogenic aerosol into the Arc- tic in the spring months has the potential to affect regional climate; however, modeling estimates of the aerosol direct radiative effect (DRE) are sensitive to uncertainties in the mixing state of black carbon (BC). A common approach in previous modeling studies is to assume an entirely exter- nal mixture (all primarily scattering species are in separate particles from BC) or internal mixture (all primarily scat- tering species are mixed in the same particles as BC). To provide constraints on the size-resolved mixing state of BC, we use airborne single-particle soot photometer (SP2) and ultrahigh-sensitivity aerosol spectrometer (UHSAS) mea- surements from the Alfred Wegener Institute (AWI) Polar 6 flights from the NETCARE/PAMARCMIP2015 campaign to estimate coating thickness as a function of refractory BC (rBC) core diameter and the fraction of particles contain- ing rBC in the springtime Canadian high Arctic. For rBC core diameters in the range of 140 to 220 nm, we find av- erage coating thicknesses of approximately 45 to 40 nm, re- spectively, resulting in ratios of total particle diameter to rBC core diameters ranging from 1.6 to 1.4. For total par- ticle diameters ranging from 175 to 730 nm, rBC-containing particle number fractions range from 16% to 3%, respec- tively. We combine the observed mixing-state constraints with simulated size-resolved aerosol mass and number dis- tributions from GEOS-Chem–TOMAS to estimate the DRE with observed bounds on mixing state as opposed to assuming an entirely external or internal mixture. We find that the pan-Arctic average springtime DRE ranges from

Description: The vertical distribution of black carbon (BC) par- ticles in the Arctic atmosphere is one of the key parameters controlling their radiative forcing and thus role in Arctic cli- mate change. This work investigates the presence and prop- erties of these light-absorbing aerosols over the High Cana- dian Arctic ( 〉 70 degree N). Airborne campaigns were performed as part of the NETCARE project (Network on Climate and Aerosols: Addressing Key Uncertainties in Remote Canadian Environments) and provided insights into the variability of the vertical distributions of BC particles in summer 2014 and spring 2015. The observation periods covered evolutions of cyclonic disturbances at the polar front, which favoured the transport of air pollution into the High Canadian Arctic, as otherwise this boundary between the air masses largely im- pedes entrainment of pollution from lower latitudes. A total of 48 vertical profiles of refractory BC (rBC) mass concen- tration and particle size, extending from 0.1 to 5.5 km altitude were obtained with a Single-Particle Soot Photometer (SP2). Generally, the rBC mass concentration decreased from spring to summer by a factor of 10. Such depletion was as- sociated with a decrease in the mean rBC particle diameter, from approximately 200 to 130 nm at low altitude. Due to the very low number fraction, rBC particles did not substantially contribute to the total aerosol population in summer. The analysis of profiles with potential temperature as ver- tical coordinate revealed characteristic variability patterns within specific levels of the cold and stably stratified, dome- like, atmosphere over the polar region. The associated his- tory of transport trajectories into each of these levels showed that the variability was induced by changing rates and effi- ciencies of rBC import. Generally, the source areas affecting the polar dome extended southward with increasing potential temperature (i.e. altitude) level in the dome. While the lower dome was mostly only influenced by low-level transport from sources within the cold central and marginal Arctic, for the mid-dome and upper dome during spring it was found that a cold air outbreak over eastern Europe caused intensified northward transport of air from a corridor over western Rus- sia to central Asia. This sector was affected by emissions from gas flaring, industrial activity and wildfires. The devel- opment of transport caused rBC concentrations in the second lowest level to gradually increase from 32 to 49 ng m

Description: Decreasing sea ice and increasing marine navigability in northern latitudes have changed Arctic ship traffic patterns in recent years and are predicted to increase annual ship traffic in the Arctic in the future. Development of effective regulations to manage environmental impacts of shipping requires an understanding of ship emissions and atmospheric processing in the Arctic environment. As part of the summer 2014 NETCARE (Network on Climate and Aerosols) campaign, the plume dispersion and gas and particle emission factors of effluents originating from the Canadian Coast Guard icebreaker Amundsen operating near Resolute Bay, NU, Canada, were investigated. The Amundsen burned distillate fuel with 1.5 wt% sulfur. Emissions were studied via plume intercepts using the Polar 6 aircraft measurements, an analytical plume dispersion model, and using the FLEXPART-WRF Lagrangian particle dispersion model. The first plume intercept by the research aircraft was carried out on 19 July 2014 during the operation of the Amundsen in the open water. The second and third plume intercepts were carried out on 20 and 21 July 2014 when the Amundsen had reached the ice edge and operated under ice-breaking conditions. Typical of Arctic marine navigation, the engine load was low compared to cruising conditions for all of the plume intercepts. The measured species included mixing ratios of CO2, NOx, CO, SO2, particle number concentration (CN), refractory black carbon (rBC), and cloud condensation nuclei (CCN). The results were compared to similar experimental studies in mid-latitudes. Plume expansion rates were calculated using the analytical model and found to be D0.75+0.81, 0.93+0.37, and 1.19+0.39 for plumes 1, 2, and 3, respectively. These rates were smaller than prior studies conducted at mid-latitudes, likely due to polar boundary layer dynamics, including reduced turbulent mixing compared to mid- latitudes. All emission factors were in agreement with prior observations at low engine loads in mid-latitudes. Ice-breaking increased the NOx emission factor from EFNOx 43.1+15.2 to 71.6+9.68 and 71.4+4.14 g kg-diesel-1 for plumes 1, 2, and 3, likely due to changes in combustion temperatures. The CO emission factor was EFCO 137+120, 12.5+3.70 and 8.13+1.34 g kg-diesel-1 for plumes 1, 2, and 3. The rBC emission factor was EFrBC D0.202+0.052 and 0.202+0.125 g kg-diesel-1 for plumes 1 and 2. The CN emission factor was reduced while ice-breaking from EFCN 2.41+0.47 to 0.45+0.082 and 0.507+0.037+1016 kg-diesel+1 for plumes 1, 2, and 3. At 0.6% supersaturation, the CCN emission factor was comparable to observations in mid-latitudes at low engine loads with EFCCN D3.03+0.933, 1.39+0.319, and 0.650+0.136+1014 kg-diesel-1 for plumes 1, 2, and 3.

Description: Size-resolved and vertical profile measurements of single particle chemical composition (sampling altitude range 50–3000 m) were conducted in July 2014 in the Cana- dian high Arctic during an aircraft-based measurement cam- paign (NETCARE 2014). We deployed the single parti- cle laser ablation aerosol mass spectrometer ALABAMA (vacuum aerodynamic diameter range approximately 200– 1000 nm) to identify different particle types and their mix- ing states. On the basis of the single particle analysis, we found that a significant fraction (23 %) of all analyzed parti- cles (in total: 7412) contained trimethylamine (TMA). Two main pieces of evidence suggest that these TMA-containing particles originated from emissions within the Arctic bound- ary layer. First, the maximum fraction of particulate TMA occurred in the Arctic boundary layer. Second, compared to particles observed aloft, TMA particles were smaller and less oxidized. Further, air mass history analysis, associated wind data and comparison with measurements of methane- sulfonic acid give evidence of a marine-biogenic influence on particulate TMA. Moreover, the external mixture of TMA- containing particles and sodium and chloride (“Na/Cl-”) containing particles, together with low wind speeds, sug- gests particulate TMA results from secondary conversion of precursor gases released by the ocean. In contrast to TMA- containing particles originating from inner-Arctic sources, particles with biomass burning markers (such as levoglucosan and potassium) showed a higher fraction at higher al- titudes, indicating long-range transport as their source. Our measurements highlight the importance of natural, marine inner-Arctic sources for composition and growth of summer- time Arctic aerosol.

Description: Motivated by increasing levels of open ocean in the Arctic summer and the lack of prior altitude-resolved studies, extensive aerosol measurements were made during 11 flights of the NETCARE July 2014 airborne campaign from Resolute Bay, Nunavut. Flights included vertical profiles (60 to 3000m above ground level) over open ocean, fast ice, and boundary layer clouds and fogs. A general conclusion, from observations of particle numbers between 5 and 20 nm in diameter (N5

Description: Aerosol particles impact the Arctic climate system both directly and indirectly by modifying cloud properties, yet our understanding of their vertical distribution, chemical composition, mixing state, and sources in the summertime Arctic is incomplete. In situ vertical observations of particle properties in the high Arctic combined with modelling analy- sis on source attribution are in short supply, particularly dur- ing summer. We thus use airborne measurements of aerosol particle composition to demonstrate the strong contrast be- tween particle sources and composition within and above the summertime Arctic boundary layer. In situ measure- ments from two complementary aerosol mass spectrometers, the Aircraft-based Laser Ablation Aerosol Mass Spectrom- eter (ALABAMA) and an Aerodyne high-resolution time- of-flight aerosol mass spectrometer (HR-ToF-AMS), are pre- sented alongside black carbon measurements from an single particle soot photometer (SP2). Particle composition anal- ysis was complemented by trace gas measurements, satel- lite data, and air mass history modelling to attribute parti- cle properties to particle origin and air mass source regions. Particle composition above the summertime Arctic bound- ary layer was dominated by chemically aged particles, con- taining elemental carbon, nitrate, ammonium, sulfate, and organic matter. From our analysis, we conclude that the pres- ence of these particles was driven by transport of aerosol and precursor gases from mid-latitudes to Arctic regions. Specifically, elevated concentrations of nitrate, ammonium, and organic matter coincided with time spent over vegeta- tion fires in northern Canada. In parallel, those particles were largely present in high CO environments (〉 90 ppbv ). Ad- ditionally, we observed that the organic-to-sulfate ratio was enhanced with increasing influence from these fires. Besides vegetation fires, particle sources in mid-latitudes further in- clude anthropogenic emissions in Europe, North America, and East Asia. The presence of particles in the Arctic lower free troposphere, particularly sulfate, correlated with time spent over populated and industrial areas in these regions. Further, the size distribution of free tropospheric particles containing elemental carbon and nitrate was shifted to larger diameters compared to particles present within the boundary layer. Moreover, our analysis suggests that organic matter, when present in the Arctic free troposphere, can partly be identified as low molecular weight dicarboxylic acids (ox- alic, malonic, and succinic acid). Particles containing dicar- boxylic acids were largely present when the residence time of air masses outside Arctic regions was high. In contrast particle composition within the marine boundary layer was largely driven by Arctic regional processes. Air mass history modelling demonstrated that alongside primary sea spray particles, marine biogenic sources contributed to secondary aerosol formation via trimethylamine, methanesulfonic acid, sulfate, and other organic species. Our findings improve our knowledge of mid-latitude and Arctic regional sources that influence the vertical distribution of particle chemical com- position and mixing state in the Arctic summer.

Description: Abstract. The springtime composition of the Arctic lower troposphere is to a large extent controlled by the transport of midlatitude air masses into the Arctic. In contrast, pre- cipitation and natural sources play the most important role during summer. Within the Arctic region sloping isentropes create a barrier to horizontal transport, known as the polar dome. The polar dome varies in space and time and exhibits a strong influence on the transport of air masses from mid- latitudes, enhancing transport during winter and inhibiting transport during summer. We analyzed aircraft-based trace gas measurements in the Arctic from two NETCARE airborne field campaigns (July 2014 and April 2015) with the Alfred Wegener Insti- tute Polar 6 aircraft, covering an area from Spitsbergen to Alaska (134 to 17◦ W and 68 to 83◦ N). Using these data we characterized the transport regimes of midlatitude air masses traveling to the high Arctic based on CO and CO2 mea- surements as well as kinematic 10 d back trajectories. We found that dynamical isolation of the high Arctic lower tro- posphere leads to gradients of chemical tracers reflecting dif- ferent local chemical lifetimes, sources, and sinks. In par- ticular, gradients of CO and CO2 allowed for a trace-gas- based definition of the polar dome boundary for the two mea- surement periods, which showed pronounced seasonal differences. Rather than a sharp boundary, we derived a transi- tion zone from both campaigns. In July 2014 the polar dome boundary was at 73.5◦ N latitude and 299–303.5 K potential temperature. During April 2015 the polar dome boundary was on average located at 66–68.5◦ N and 283.5–287.5 K. Tracer–tracer scatter plots confirm different air mass prop- erties inside and outside the polar dome in both spring and summer. Further, we explored the processes controlling the recent transport history of air masses within and outside the polar dome. Air masses within the springtime polar dome mainly experienced diabatic cooling while traveling over cold sur- faces. In contrast, air masses in the summertime polar dome were diabatically heated due to insolation. During both sea- sons air masses outside the polar dome slowly descended into the Arctic lower troposphere from above through ra- diative cooling. Ascent to the middle and upper troposphere mainly took place outside the Arctic, followed by a north- ward motion. Air masses inside and outside the polar dome were also distinguished by different chemical compositions of both trace gases and aerosol particles. We found that the fraction of amine-containing particles, originating from Arc- tic marine biogenic sources, is enhanced inside the polar dome. In contrast, concentrations of refractory black carbon are highest outside the polar dome, indicating remote pollu- tion sources. Synoptic-scale weather systems frequently disturb the transport barrier formed by the polar dome and foster ex- change between air masses from midlatitudes and polar re- gions. During the second phase of the NETCARE 2014 measurements a pronounced low-pressure system south of Resolute Bay brought inflow from southern latitudes, which pushed the polar dome northward and significantly affected trace gas mixing ratios in the measurement region. Mean CO mixing ratios increased from 77.9 ± 2.5 to 84.9 ± 4.7 ppbv between these two regimes. At the same time CO2 mix- ing ratios significantly decreased from 398.16 ± 1.01 to 393.81 ± 2.25 ppmv. Our results demonstrate the utility of applying a tracer-based diagnostic to determine the polar dome boundary for interpreting observations of atmospheric composition in the context of transport history.

Description: Despite the potential importance of black carbon (BC) for radiative forcing of the Arctic atmosphere, ver- tically resolved measurements of the particle light scatter- ing coefficient (σsp ) and light absorption coefficient (σap ) in the springtime Arctic atmosphere are infrequent, espe- cially measurements at latitudes at or above 80◦ N. Here, re- lationships among vertically distributed aerosol optical prop- erties (σap, σsp and single scattering albedo or SSA), par- ticle microphysics and particle chemistry are examined for a region of the Canadian archipelago between 79.9 and 83.4◦ N from near the surface to 500 hPa. Airborne data collected during April 2015 are combined with ground- based observations from the observatory at Alert, Nunavut and simulations from the Goddard Earth Observing Sys- tem (GEOS) model, GEOS-Chem, coupled with the TwO- Moment Aerosol Sectional (TOMAS) model (collectively GEOS-Chem–TOMAS; Kodros et al., 2018) to further our knowledge of the effects of BC on light absorption in the Arctic troposphere. The results are constrained for σsp less than 15 Mm−1, which represent 98 % of the observed σsp, be- cause the single scattering albedo (SSA) has a tendency to be lower at lower σsp, resulting in a larger relative contribution to Arctic warming. At 18.4 m2 g−1, the average BC mass ab- sorption coefficient (MAC) from the combined airborne and Alert observations is substantially higher than the two aver- aged modelled MAC values (13.6 and 9.1 m2 g−1) for two different internal mixing assumptions, the latter of which is based on previous observations. The higher observed MAC value may be explained by an underestimation of BC, the presence of small amounts of dust and/or possible differences in BC microphysics and morphologies between the obser- vations and model. In comparing the observations and simulations, we present σap and SSA, as measured, and σap/2 and the corresponding SSA to encompass the lower modelled MAC that is more consistent with accepted MAC values. Me- dian values of the measured σap, rBC and the organic com- ponent of particles all increase by a factor of 1.8 ± 0.1, going from near-surface to 750 hPa, and values higher than the sur- face persist to 600 hPa. Modelled BC, organics and σap agree with the near-surface measurements but do not reproduce the higher values observed between 900 and 600 hPa. The dif- ferences between modelled and observed optical properties follow the same trend as the differences between the mod- elled and observed concentrations of the carbonaceous com- ponents (black and organic). Model-observation discrepan- cies may be mostly due to the modelled ejection of biomass burning particles only into the boundary layer at the sources. For the assumption of the observed MAC value, the SSA range between 0.88 and 0.94, which is significantly lower than other recent estimates for the Arctic, in part reflecting the constraint of σsp 〈 15 Mm−1. The large uncertainties in measuring optical properties and BC, and the large differ- ences between measured and modelled values here and in the literature, argue for improved measurements of BC and light absorption by BC and more vertical profiles of aerosol chemistry, microphysics and other optical properties in the Arctic.

Description: Observations addressing effects of aerosol par- ticles on summertime Arctic clouds are limited. An air- borne study, carried out during July 2014 from Resolute Bay, Nunavut, Canada, as part of the Canadian NETCARE project, provides a comprehensive in situ look into some effects of aerosol particles on liquid clouds in the clean environment of the Arctic summer. Median cloud droplet number concentrations (CDNC) from 62 cloud samples are 10 cm−3 for low-altitude cloud (clouds topped below 200 m) and 101 cm−3 for higher-altitude cloud (clouds based above 200m). The lower activation size of aerosol particles is ≤50nm diameter in about 40% of the cases. Particles as small as 20 nm activated in the higher-altitude clouds consis- tent with higher supersaturations (S) for those clouds inferred from comparison of the CDNC with cloud condensation nu- cleus (CCN) measurements. Over 60 % of the low-altitude cloud samples fall into the CCN-limited regime of Mauritsen et al. (2011), within which increases in CDNC may increase liquid water and warm the surface. These first observations of that CCN-limited regime indicate a positive association of the liquid water content (LWC) and CDNC, but no associ- ation of either the CDNC or LWC with aerosol variations. Above the Mauritsen limit, where aerosol indirect cooling may result, changes in particles with diameters from 20 to 100nm exert a relatively strong influence on the CDNC. Within this exceedingly clean environment, as defined by low carbon monoxide and low concentrations of larger parti- cles, the background CDNC are estimated to range between 16 and 160 cm−3, where higher values are due to activation of particles ≤ 50 nm that likely derive from natural sources. These observations offer the first wide-ranging reference for the aerosol cloud albedo effect in the summertime Arctic.

Description: The summertime Arctic lower troposphere is a relatively pristine background aerosol environment dominated by nucleation and Aitken mode particles. Understanding the mechanisms that control the formation and growth of aerosol is crucial for our ability to predict cloud properties and therefore radiative balance and climate. We present an analysis of an aerosol growth event observed in the Canadian Arctic Archipelago during summer as part of the NETCARE project. Under stable and clean atmospheric conditions, withlow inversion heights, carbon monoxide less than 80 ppbv, and black carbon less than 5 ngm-3, we observe growth of small particles, 〈20 nm in diameter, into sizes above 50 nm. Aerosol growth was correlated with the presence of organic species, trimethylamine, and methanesulfonic acid (MSA) in particles ≈80 nm and larger, where the organics are similar to those previously observed in marine settings. MSA-to-sulfate ratios as high as 0.15 were observed during aerosol growth, suggesting an important marine influence. The organic-rich aerosol contributes significantly to particles active as cloud condensation nuclei (CCN, supersaturation D0.6 %), which are elevated in concentration during aerosol growth above background levels ≈100 to ≈220 cm-3. Results from this case study highlight the potential importance of secondary organic aerosol formationand its role in growing nucleation mode aerosol into CCN active sizes in this remote marine environment.