AS2.12

The Arctic aerosol and trace gas regime features strong seasonal differences. The haze season in winter is dominated by long-range transported mid-latitude anthropogenic emissions, while the cleaner summer season is characterized by more local and natural trace gas and aerosol sources. Aerosols and trace gases have been shown to be important for the Arctic radiative balance, inducing an overall net positive radiative forcing through direct radiation interactions.

Aerosols and trace gases are fundamentally different between seasons in terms of chemical composition and microphysical properties. In winter, accumulation mode particles - with a concentration between 100 and 300 cm^-3- composed mainly of sulfate, sodium and organics occur, and trace gases relevant for aerosol formation have very low concentrations. In summer, the aerosol number concentration is highly variable reaching from a few to thousands per cubic centimeter, in case of new particle formation. Their size distribution contains nucleation, Aitken and accumulation modes. Trace gases become more abundant, particularly in the marginal ice zone where marine microbial activity emits dimethylsulfide, which leads to the formation of the trace gases sulfuric acid and methanesulfonic acid, which in turn form secondary aerosol mass.

The transition seasons, i.e. spring and autumn, have been studied much less in terms of aerosol and trace gas chemical and microphysical properties in the past , except for ozone depletion events in spring where halogenated trace gases are predominantly involved. The transition between the two aerosol regimes is relatively short. Focusing on spring, the season is characterized by the arrival of warmer and moister air masses from the south, which transport different aerosols and trace gases up north. Cloud formation and precipitation en route have a strong impact on the aerosol number concentrations and size distribution, as well as on the chemical composition due to heterogeneous chemistry in cloud droplets.

Here, we present first results from observations of warm air mass intrusions reaching the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition in mid-April 2020. The period before arrival was characterized by persistent northerly winds, hence aged and dry Arctic air masses, where a very stable accumulation mode composed of sulfate and organics with traces of halogens was measured. With the arrival of southerly air masses, the size distribution started featuring several modes, which increased and decreased in diameter and concentration. Moreover, the chemical composition was significantly changed, featuring methanesulfonic acid from algal blooms in the north Atlantic.

How to cite: Schmale, J., Dada, L., Beck, I., Jokinen, T., Quélélever, L., and Laurila, T.: Impact of warm air mass intrusions on atmospheric chemistry and microphysics – Observations during MOSAiC, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4137, https://doi.org/10.5194/egusphere-egu21-4137, 2021.

Aircraft observations from two Arctic field campaigns are used to derive ice surface characteristics and make recommendations for the parametrisation of surface heat and moisture exchange over sea ice and the marginal ice zone (MIZ). The observations were gathered in the Barents Sea and Fram Strait as part of the Aerosol–Cloud Coupling And Climate Interactions in the Arctic (ACCACIA) project, and off the south-east coast of Greenland as part of the Iceland-Greenland Seas Project (IGP). Estimates of roughness lengths for momentum (z₀), heat (z_0T) and moisture (z_0q) are derived from turbulent wind velocity, temperature and humidity measurements; while sea ice concentration is derived from albedo measurements. The two data sets cover a range of sea ice characteristics, being much rougher in general during IGP than during ACCACIA. Large fluxes of heat and moisture were observed in the vicinity of the MIZ during both field campaigns. We show that z_0T and z_0q over 100 % sea ice (z_0Ti and z_0qi) vary as a function of a roughness Reynolds number (R*; which itself is a function of z₀ and wind stress), with a peak at the transition between the aerodynamically smooth (R*<0.135) and aerodynamically rough (R*>2.5) regimes. One of the few theory-based parameterisations available for z_0Ti and z_0qi (that of Andreas et al., 1987) reproduces these peaks, in contrast to the simple treatments currently employed in two leading numerical weather and climate prediction models – the Met Office Unified Model (MetUM) and the Integrated Forecast System (IFS) – which do not. The MetUM and IFS schemes perform adequately in smooth conditions, but greatly overestimate heat and moisture exchange in rough conditions. We develop a new parameterisation for heat and moisture exchange as a function of sea ice concentration, which blends the Andreas et al. (1987) scheme over sea ice with exchange over the ocean. This new parameterisation performs much better than the current MetUM and IFS schemes for the rough conditions observed during IGP, at least halving the bias and root-mean-square errors in sensible and latent heat fluxes; and is also marginally better for the comparatively smooth conditions observed during ACCACIA, suggesting further evaluation is warranted. However, it should be noted that representing heat and moisture exchange over sea ice is currently limited by the variability in z₀ over 100 % sea ice, which is unrepresented in weather and climate models.

How to cite: Elvidge, A., Renfrew, I., Brooks, I., Srivastava, P., Yelland, M., and Prytherch, J.: Observations of surface heat and moisture exchange in the marginal sea ice and implications for model parameterization, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11175, https://doi.org/10.5194/egusphere-egu21-11175, 2021.

The summertime high Arctic is an extremely low-aerosol region, where even small inputs of particles can have significant impacts on cloud formation and therefore on the surface energy budget. The relative importance of new particle formation from gas phase precursors and primary sea spray production in this region remains uncertain, as does the role of atmospheric transport. We made direct, time-resolved composition measurements of Aitken mode (~20-60 nm diameter) aerosol over the high Arctic pack ice in August-September 2018, including during an intense Aitken mode formation event on August 30-31. The event particles contained both primary sea spray materials (sodium, potassium, and polysaccharide-like organics) and secondary components (non-sea-salt sulfate, methanesulfonic acid, non-sea-salt iodine, and secondary organics), most of which could be quantified on the basis of analytical standards. The composition is consistent with primary sea spray that had been atmospherically processed, and the aerosol size distribution dynamics imply the action of a process by which larger atmospheric particles or aggregates broke up to form smaller particles. Hypotheses to explain the results will be discussed.

How to cite: Lawler, M., Saltzman, E., Karlsson, L., Zieger, P., Salter, M., Baccarini, A., Schmale, J., and Leck, C.: An Aitken mode aerosol formation event in the high Arctic: evidence for aggregate breakup, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13650, https://doi.org/10.5194/egusphere-egu21-13650, 2021.

Above freezing temperatures and melting surface snow have occurred at Summit Station (3250 m asl), atop the Greenland Ice Sheet, only five times in the last 800 years, including once in 2012 and twice 2019 (June 12; July 29-31). Such events are linked to southerly advection of continental air that cross the North Atlantic as atmospheric rivers (ARs). The specific mechanisms that are responsible for these rare events appear to be varied and complex. While the 2012 event was supported by anomalous cloud forcing caused by thin, liquid-bearing clouds, the two events in 2019 occurred under both clear and cloudy conditions. The net surface radiation measured during the 2019 events was actually similar between clear (~47 W m^-2) and cloudy (~52 W m^-2) conditions, and, surprisingly, these values are unremarkable for afternoon conditions in summer at Summit Station.

Observations from the ICECAPS-ACE project at Summit Station (including radiative and turbulent fluxes, surface skin temperature, snow pit stratigraphy) allow a process-level analysis of the mechanisms that transfer energy from the AR events into local melting. By combining the measurements with a finite-volume diffusion model of the sub-surface temperature field, we find that a concentration of energy in the surface layer caused by converging fluxes toward the surface from both directions (upward from within the snowpack and downward from the atmosphere) led to initiation of both of the 2019 melt events. Thus, coupling between the atmosphere and the snowpack, and the timing of atmospheric fluctuations, appear important, and are suggestive that preconditioning of the snowpack from events prior to the day of melt may be a factor. Both sensible and latent heat fluxes were relatively small during these melt events while several regimes of commonly occurring radiative processes were observed. Under cloudy conditions, longwave cloud radiative forcing played a role, while under clear skies, lower surface albedo was sufficient to make up the difference of the absence of cloud forcing. For example, during the July 2019 event, the surface snow albedo decreased significantly from 0.86 to 0.80, which facilitated greater absorption of solar radiation. These findings are supportive of the notion that longwave processes are triggers of melt while shortwave processes persist melt. he co-dependent roles of the radiative and subsurface heat fluxes during the 2019 events suggest that the rarity of melt at Summit Station may be explained by preconditioning processes, and that a particular sequence of events over several days leading up to melt may be important.

How to cite: Walden, V., Guy, H., Cox, C., Neff, W., Neely, R., and Shupe, M.: Mechanisms of multiple, anomalous melt events at Summit Station, Greenland in summer 2019, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13119, https://doi.org/10.5194/egusphere-egu21-13119, 2021.

On December 19-21, 2018, an atmospheric river hit the French-Italian Concordia station, located at Dome C, East Antarctic Plateau, 3 269 m above sea level. It induced an extreme surface warming (+ 15°C in 3 days), combined with high specific humidity (multiplied by 3 in 3 days) and a strong isotopic anomaly in water vapor (+ 15 ‰ for δ18O). The isotopic composition of water vapor monitored during the event may be explained by (1) the isotopic signature of long-range water transport, and by (2) local moisture uptake during the event. In this study we quantify the influence of each of these processes.

To estimate the isotopic composition of water vapor advected by long-range transport, we perform back-trajectories with the FLEXible PARTicle dispersion model FLEXPART. We retrieve meteorological conditions along different trajectories between the moisture uptake area and Concordia, and use them to compute isotopic fractionation during transport with the mixed cloud isotope model MCIM. While intermediate conditions along the trajectory do not seem to have a major impact on the final isotopic composition (less than 0.1 ‰), the latter appears sensitive to surface conditions (temperature, pressure and relative humidity) in the moisture uptake area (±5.1 ‰). As the event is characterized by the presence of liquid water clouds above Concordia, we show additional sensitivity tests exploring the impact of mixed phase clouds on the water vapor isotopic composition.

Finally, we perform a water vapor mass budget in the boundary layer using observations and simulations from the regional atmospheric model MAR, ran with and without drifting snow. The presence of mixed-phase clouds during the event induced a significant increase in downward longwave radiative fluxes, which led to high turbulent mixing in the boundary layer and to heavy drifting snow (white-out conditions). Using MAR simulations, we show that a significant part of the atmospheric water vapor originates from sublimation of drifting snow particles removed from the snowpack. Consequently, the isotopic signal monitored in water vapor during this atmospheric river event reflects both long-range moisture advection and interactions between the boundary layer and the snowpack. Only specific meteorological conditions driven by the atmospheric river, and their associated intense poleward moisture transport, can explain these strong interactions.

How to cite: Agosta, C., Davrinche, C., Leroy-Dos Santos, C., Berchet, A., Landais, A., Fourré, E., Orsi, A., Prié, F., Amory, C., Favier, V., Fettweis, X., Genthon, C., Kittel, C., Veron, D., and Wille, J.: Isotopic anomalies in water vapor during an atmospheric river event at Dome C, East Antarctic plateau, controlled by large-scale advection and boundary layer processes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13076, https://doi.org/10.5194/egusphere-egu21-13076, 2021.

The Arctic region suffers an extreme vulnerability to climate change, with an increase in surface air temperatures that have reached twice the global rate during several decades (McBean et al., 2005). The role of clouds, and in particular low-levels clouds and fog, in this arctic amplification by regulating the energy transport from and to space has recently gained interest among the scientific community. The NASCENT 2019-2020 campaign (Ny-Ålesund AeroSol Cloud ExperimeNT) based in Ny-Ålesund, Svalbard (79º North) aimed at studying the microphysical and chemical properties of low-level clouds using measurements both at the sea level and at the Zeppelin station (475 m a.s.l.). Specifically, the susceptibility of droplet formation, which has recently been shown to be highly dependent on aerosol levels in European alpine valleys (Georgakaki et al., under review), could strongly vary between the fall to winter months, with pristine-like conditions, and the higher particle concentrations generally found in spring, known as the arctic haze. First results using a scanning mobility particle sizer (SMPS) and a cloud condensation nuclei counter (CCNC) confirmed that aerosol concentrations in the range 10 < D_part [nm] < 500 were approximatively 4-5 times higher during the months of spring 2021 compared to those of fall 2020. In addition, we found relatively low values of the aerosol hygroscopic parameter κ, generally below 0.3, consistently with previous studies in the arctic region (Moore et al., 2011).

Georgakaki, P., Bougiatioti, A., Wieder, J., Mignani, C., Kanji, Z. A., Henneberger, J., Hervo, M., Berne, A. and Nenes, A.: On the drivers of droplet variability in Alpine mixed-phase clouds, , 34, under review.

McBean, G., Alekseev, G., Chen, D., Førland, E., Fyfe, Groisman, J., P. Y., King, R., Melling, H., Voseand, R., Whitfield, P. H.: Arctic climate: past and present. Arctic Climate Impacts Assessment (ACIA), C. Symon, L. Arris and B. Heal, Eds., Cambridge University Press, Cambridge, 21-60, 2005.

Moore, R. H., Bahreini, R., Brock, C. A., Froyd, K. D., Cozic, J., Holloway, J. S., Middlebrook, A. M., Murphy, D. M. and Nenes, A.: Hygroscopicity and composition of Alaskan Arctic CCN during April 2008, Atmospheric Chemistry and Physics, 11(22), 11807–11825, https://doi.org/10.5194/acp-11-11807-2011, 2011.

How to cite: Motos, G., Georgakaki, P., Zieger, P., Wieder, J., Lohmann, U., and Nenes, A.: In-situ observations of aerosol-cloud interactions in Ny-Ålesund, Svalbard, during fall 2019 and spring 2020, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15775, https://doi.org/10.5194/egusphere-egu21-15775, 2021.

During the Arctic winter, local emissions (e.g. from home-heating, traffic, power station or industry plumes) coupled to poor dispersion caused by strong temperature inversions can lead to severe air pollution events. For example, each winter, Fairbanks (Alaska) experiences high abundances of gaseous pollutants and particulate matter (PM), leading to air-quality exceedances. However, there is still limited knowledge on the coupled physico-chemical and dynamical processes that cause wintertime Arctic pollution and aerosol formation under the very cold and low light conditions, and where levels of oxidants such as ozone at the surface can become depleted under limited vertical mixing. Here, we demonstrate novel deployment of low cost small sensors measuring PM2.5, gases (CO, NO, NO2, O3) and meteorological parameters (P, T, RH) to characterize Arctic atmospheric composition and properties, including mapping vertical distributions.

Our three-week pre-ALPACA (Alaskan Layered Pollution and Chemical Analysis) intensive field-campaign took place in downtown Fairbanks in Nov-Dec 2019. Small sensor temperature-dependencies were characterized by instrument cross-comparisons and correction-algorithms developed. Sensors were then deployed near-ground, on the roof of a 19 m building, and on a vertical pulley system set-up along the side of the building for vertical profiling. The small sensors show a strong capability to capture temporal variations in PM2.5, CO, NO and NO₂ and O₃, across a wide temperature range: surface gas and particle abundances became elevated during a cold-polluted period (temperatures as low as -30 C) and again became elevated during a subsequent warm-polluted period (temperatures around -3 C). Vertical profiling during the warm-polluted period identified strong temperature inversions associated with near-surface layers of high PM2.5 and CO that are distinct from an overlying clean, warm, humid air-mass. During the cold-polluted period, temperature inversions were present but less strong, there was little vertical structure in composition, and PM2.5 was often greater at 20m than at the surface. This finding contrasts with a full winter-season analysis that shows cold surface temperatures typically associated with strong inversions and PM highest at the surface. We invoke plume-rise modelling to show how buoyant plumes from local emissions (e.g. home-heating) can reach heights of about 10-20 m, allowing polluted emissions to rise and accumulate at altitude unless inversions are sufficiently strong to constrain the plume-rise. Causes of the temperature inversions include radiative cooling and advection of overlying warm-air. Our study highlights how small sensor measurements and vertical profiling can help elucidate the coupled processes of atmospheric chemistry, physics, dynamics and emissions that lead to surface air pollution episodes at high latitudes.

This study forms part of the Alaskan Layered Pollution and Chemical Analysis (ALPACA) project https://alpaca.community.uaf.edu/. We are grateful for technical support from Alaska-DEC, LPC2E, UAF, SEOSS, Alphasense and SouthCoastScience.

How to cite: Roberts, T., Cesler-Maloney, M., Simpson, W., Mao, J., Barret, B., Bekki, S., Temine-Roussel, B., d'Anna, B., Maillard, J., Ravetta, F., Raut, J.-C., Woods, A., Ioannidis, E., and Law, K.: In-situ characterization of layered pollution in the wintertime Arctic atmosphere by small sensors, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15351, https://doi.org/10.5194/egusphere-egu21-15351, 2021.

The wintertime Arctic is influenced by air pollution transported from mid-latitudes, leading to formation of Arctic Haze, as well as local emissions such as combustion for heating and power production in very cold winter conditions. This contributes to severe air pollution episodes, with enhanced aerosol concentrations, inter-dispersed with cleaner periods. However, the formation of secondary aerosol particles (sulphate, organics, nitrate) in cold/dark wintertime Arctic conditions, which could contribute to these pollution episodes, is poorly understood.

In this study, which contributes to the Air Pollution in the Arctic: Climate, Environment and Societies - Alaskan Layered Pollution and Arctic Chemical Analysis (PACES-ALPACA) initiative, the Weather Research Forecasting Model with chemistry (WRF-Chem) is used to investigate wintertime pollution over central Alaska focusing on the Fairbanks region, during the pre-ALPACA campaign in winter 2019-2020. Fairbanks is the most polluted city in the United States during wintertime, due to high local emissions and the occurrence of strong surface temperature inversions trapping pollutants near the surface.

Firstly, different WRF meteorological and surface schemes were tested over Alaska with a particular focus on improving simulations of the wintertime boundary layer structure including temperature inversions. An optimal WRF set-up, with increased vertical resolution below 2km, was selected based on evaluation against available data.

Secondly, a quasi-hemispheric WRF-Chem simulation, using the improved WRF setup, was used to assess large-scale synoptic conditions and to evaluate background aerosols originating from remote anthropogenic and natural sources affecting central Alaska during the campaign. The model was run with Evaluating the Climate and Air Quality Impacts of Short-Lived Pollutants (ECLIPSE) v6b anthropogenic emissions and improved sea-spray aerosol emissions. Discrepancies in modelled aerosols compared available data are being investigated (e.g. missing dark formation mechanisms, treatment of removal processes).

Thirdly, fine resolution simulations, using high resolution emissions (e.g. 2019 CAMS inventory), including local point sources, over the Fairbanks region, were used to investigate chemical and dynamical processes influencing aerosols under different meteorological conditions observed during the field campaign including a cold stable episode and a period with possible mixing of air masses from aloft. The model was evaluated against available aerosol, oxidant (ozone) and aerosol precursor data from surface monitoring sites and collected during the pre-campaign, including vertical profile data collected in the lowest 20m. The sensitivity of modelled aerosols to meteorological factors, such as relative humidity, temperature gradients and vertical mixing under winter conditions are investigated.

How to cite: Ioannidis, E., Law, K. S., Raut, J.-C., Onishi, T., Marelle, L., Roberts, T. J., Barret, B., D'Anna, B., Temine-Roussel, B., Mölders, N., Mao, J., and Simpson, W. R.: Wintertime Arctic Air Pollution over central Alaska: pre-ALPACA campaign, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15477, https://doi.org/10.5194/egusphere-egu21-15477, 2021.