Description: Dry deposition to the surface is one of the main removal pathways of tropospheric ozone (O₃). We quantified for the first time the impact of O₃ deposition to the Arctic sea ice on the planetary boundary layer (PBL) O₃ concentration and budget using year-round flux and concentration observations from the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) campaign and simulations with a single-column atmospheric chemistry and meteorological model (SCM). Based on eddy-covariance O₃ surface flux observations, we find a median surface resistance on the order of 20,000 s m¯¹, resulting in a dry deposition velocity of approximately 0.005 cm s¯¹. This surface resistance is up to an order of magnitude larger than traditionally used values in many atmospheric chemistry and transport models. The SCM is able to accurately represent the yearly cycle, with maxima above 40 ppb in the winter and minima around 15 ppb at the end of summer. However, the observed springtime ozone depletion events are not captured by the SCM. In winter, the modelled PBL O₃ budget is governed by dry deposition at the surface mostly compensated by downward turbulent transport of O₃ towards the surface. Advection, which is accounted for implicitly by nudging to reanalysis data, poses a substantial, mostly negative, contribution to the simulated PBL O₃ budget in summer. During episodes with low wind speed (〈5 m s¯¹) and shallow PBL (〈50 m), the 7-day mean dry deposition removal rate can reach up to 1.0 ppb h¯¹. Our study highlights the importance of an accurate description of dry deposition to Arctic sea ice in models to quantify the current and future O₃ sink in the Arctic, impacting the tropospheric O₃ budget, which has been modified in the last century largely due to anthropogenic activities.

Description: The MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) expedition was the largest Arctic field campaign ever conducted. MOSAiC offered the unique opportunity to monitor and characterize aerosols and clouds with high vertical resolution up to 30 km height at latitudes from 80 to 90 N over an entire year (October 2019 to September 2020). Without a clear knowledge of the complex aerosol layering, vertical structures, and dominant aerosol types and their impact on cloud formation, a full understanding of the meteorological processes in the Arctic, and thus advanced climate change research, is impossible. Widespread ground-based in situ observations in the Arctic are insufficient to provide these required aerosol and cloud data. In this article, a summary of our MOSAiC observations of tropospheric aerosol profiles with a state-of-the-art multiwavelength polarization Raman lidar aboard the icebreaker Polarstern is presented. Particle optical properties, i.e., light-extinction profiles and aerosol optical thickness (AOT), and estimates of cloud-relevant aerosol properties such as the number concentration of cloud condensation nuclei (CCN) and ice-nucleating particles (INPs) are discussed, separately for the lowest part of the troposphere (atmospheric boundary layer, ABL), within the lower free troposphere (around 2000 m height), and at the cirrus level close to the tropopause. In situ observations of the particle number concentration and INPs aboard Polarstern are included in the study. A strong decrease in the aerosol amount with height in winter and moderate vertical variations in summer were observed in terms of the particle extinction coefficient. The 532 nm light-extinction values dropped from 〉50 Mm-1 close to the surface to 〈5 Mm-1 at 4-6 km height in the winter months. Lofted, aged wildfire smoke layers caused a re-increase in the aerosol concentration towards the tropopause. In summer (June to August 2020), much lower particle extinction coefficients, frequently as low as 1-5 Mm-1, were observed in the ABL. Aerosol removal, controlled by in-cloud and below-cloud scavenging processes (widely suppressed in winter and very efficient in summer) in the lowermost 1-2 km of the atmosphere, seems to be the main reason for the strong differences between winter and summer aerosol conditions. A complete annual cycle of the AOT in the central Arctic could be measured. This is a valuable addition to the summertime observations with the sun photometers of the Arctic Aerosol Robotic Network (AERONET). In line with the pronounced annual cycle in the aerosol optical properties, typical CCN number concentrations (0.2 % supersaturation level) ranged from 50-500 cm-3 in winter to 10-100 cm-3 in summer in the ABL. In the lower free troposphere (at 2000 m), however, the CCN level was roughly constant throughout the year, with values mostly from 30 to 100 cm-3. A strong contrast between winter and summer was also given in terms of ABL INPs which control ice production in low-level clouds. While soil dust (from surrounding continents) is probably the main INP type during the autumn, winter, and spring months, local sea spray aerosol (with a biogenic aerosol component) seems to dominate the ice nucleation in the ABL during the summer months (June-August). The strong winter vs. summer contrast in the INP number concentration by roughly 2-3 orders of magnitude in the lower troposphere is, however, mainly caused by the strong cloud temperature contrast. A unique event of the MOSAiC expedition was the occurrence of a long-lasting wildfire smoke layer in the upper troposphere and lower stratosphere. Our observations suggest that the smoke particles frequently triggered cirrus formation close to the tropopause from October 2019 to May 2020.

Description: Near-surface mercury and ozone depletion events occur in the lowest part of the atmosphere during Arctic spring. Mercury depletion is the first step in a process that transforms long-lived elemental mercury to more reactive forms within the Arctic that are deposited to the cryosphere, ocean, and other surfaces, which can ultimately get integrated into the Arctic food web. Depletion of both mercury and ozone occur due to the presence of reactive halogen radicals that are released from snow, ice, and aerosols. In this work, we added a detailed description of the Arctic atmospheric mercury cycle to our recently published version of the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem 4.3.3) that includes Arctic bromine and chlorine chemistry and activation/recycling on snow and aerosols. The major advantage of our modelling approach is the online calculation of bromine concentrations and emission/recycling that is required to simulate the hourly and daily variability of Arctic mercury depletion. We used this model to study coupling between reactive cycling of mercury, ozone, and bromine during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) spring season in 2020 and evaluated results compared to land-based, ship-based, and remote sensing observations. The model predicts that elemental mercury oxidation is driven largely by bromine chemistry and that particulate mercury is the major form of oxidized mercury. The model predicts that the majority (74%) of oxidized mercury deposited to land-based snow is re-emitted to the atmosphere as gaseous elemental mercury, while a minor fraction (4%) of oxidized mercury that is deposited to sea ice is re-emitted during spring. Our work demonstrates that hourly differences in bromine/ozone chemistry in the atmosphere must be considered to capture the springtime Arctic mercury cycle, including its integration into the cryosphere and ocean.

Description: The rapid melt of snow and sea ice during the Arctic summer provides a significant source of low-salinity meltwater to the surface ocean on the local scale. The accumulation of this meltwater on, under, and around sea ice floes can result in relatively thin meltwater layers in the upper ocean. Due to the small-scale nature of these upper-ocean features, typically on the order of 1 m thick or less, they are rarely detected by standard methods, but are nevertheless pervasive and critically important in Arctic summer. Observations during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition in summer 2020 focused on the evolution of such layers and made significant advancements in understanding their role in the coupled Arctic system. Here we provide a review of thin meltwater layers in the Arctic, with emphasis on the new findings from MOSAiC. Both prior and recent observational datasets indicate an intermittent yet longlasting (weeks to months) meltwater layer in the upper ocean on the order of 0.1 m to 1.0 m in thickness, with a large spatial range. The presence of meltwater layers impacts the physical system by reducing bottom ice melt and allowing new ice formation via false bottom growth. Collectively, the meltwater layer and false bottoms reduce atmosphere-ocean exchanges of momentum, energy, and material.The impacts on the coupled Arctic system are far-reaching, including acting as a barrier for nutrient and gas exchange and impacting ecosystem diversity and productivity.

Description: Atmospheric gaseous elemental mercury (GEM) concentrations in the Arctic exhibit a clear summertime maximum, while the origin of this peak is still a matter of debate in the community. Based on summertime observations during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition and a modeling approach, we further investigate the sources of atmospheric Hg in the central Arctic. Simulations with a generalized additive model (GAM) show that long-range transport of anthropogenic and terrestrial Hg from lower latitudes is a minor contribution (~2%), and more than 50% of the explained GEM variability is caused by oceanic evasion. A potential source contribution function (PSCF) analysis further shows that oceanic evasion is not significant throughout the ice-covered central Arctic Ocean but mainly occurs in the Marginal Ice Zone (MIZ) due to the specific environmental conditions in that region. Our results suggest that this regional process could be the leading contributor to the observed summertime GEM maximum. In the context of rapid Arctic warming and the observed increase in width of the MIZ, oceanic Hg evasion may become more significant and strengthen the role of the central Arctic Ocean as a summertime source of atmospheric Hg.

Description: This dataset contains HClO3 and HClO4 (30min-averaged) data, measured with a nitrate-chemical ionization atmospheric pressure interface time-of-flight mass spectrometry (CI-APi-TOF), during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition from 15 January 2020 to 30 June 2020. These measurements were performed in the Swiss container on board of Research Vessel Polarstern during the MOSAiC campaign. The data columns include the Time in Coordinated Universal Time (UTC), the latitude and longitude of Research Vessel Polarstern, the MOSAiC event label, and the unit of the data has been indicated in each data column.

Description: This dataset contains 30min-averaged data of HBr (normalized signal) and H2SO4, obtained from nitrate-chemical ionization atmospheric pressure interface time-of-flight mass spectrometry (CI-APi-TOF) for period of 22 February 2020 to 30 April 2020 during the MOSAiC campaign. These measurements were performed in the Swiss container on board of Research Vessel Polarstern during the MOSAiC campaign. The data columns include the Time in Coordinated Universal Time (UTC), the latitude and longitude of Research Vessel Polarstern, the MOSAiC event label, and the unit of the data has been indicated in each data column.

Description: This dataset contains hourly-averaged ozone dry air mole fractions measured during the year-long Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition from October 2019 to September 2020. This is a merged dataset that combines cross-evaluated measurements performed in the University of Colorado (CU), the Atmospheric Radiation Measurement (ARM) Program, and Swiss containers onboard Research Vessel Polarstern. The data columns include the Date and Time in Coordinated Universal Time (UTC), the latitude and longitude of the Research Vessel Polarstern, the ozone dry air mole fraction in nmol/mol, and the sampling location.

Description: This dataset contains hourly-averaged carbon monoxide dry air mole fractions measured during the year-long Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition from October 2019 to September 2020. This is a merged dataset that combines cross-evaluated measurements performed in the Atmospheric Radiation Measurement (ARM) Program and Swiss containers on the D-deck of Research Vessel Polarstern, along with data from discrete whole air samples collected for post-cruise analysis at the National Oceanic and Atmospheric Administration (NOAA) Global Monitoring Laboratory (GML). The data columns include the Date and Time in Coordinated Universal Time (UTC), the latitude and longitude of the Research Vessel Polarstern, the carbon monoxide dry air mole fraction in nmol/mol, and the sampling location.

Description: This dataset contains ambient concentrations of aerosol precursor vapors measured in the central Arctic during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. The timeseries includes a full year of sulfuric acid (SA), methanesulfonic acid (MSA), and iodic acid (IA) concentrations retrieved at a time resolution of 5 minutes between October 2019 and September 2020. The data were collected using a nitrate chemical ionization mass spectrometer (NO3-CIMS) as described by Jokinen et al. (2012). The instrument was located in the Swiss container, which was placed on the starboard side of Polarstern's bow on the D-deck during the campaign (Shupe et al., 2022). The concentration retrievals were obtained by integrating peaks from the high-resolution mass spectra for each compound of interest (either as a deprotonated ion or as its corresponding cluster with nitrate), normalizing the result with the sum of charger ions (NO3-, HNO3NO3-, (HNO3)2NO3-), and multiplying by the calibration factor (6×109 molec·cm-3) obtained from a dedicated calibration using SA. Since the instrument calibration was only performed using SA, the concentrations of MSA and IA are low limit estimations. SA was determined by peaks at mass to charge ratios (m/z) of 96.9601 Th (HSO4-) and 159.9557 Th (H2SO4NO3-), MSA was determined by m/z peaks at 94.9808 Th (CH3SO3-) and 157.9765 Th (CH3SO3HNO3-), and IA was determined by m/z peaks at 174.8898 Th (IO3-) and 237.8854 Th (HIO3NO3-). Zero measurements were performed periodically by placing a filter on the inlet of the instrument to determine the detection limit for each individual species. The detection limits were calculated as μ + 3 × σ, where µ is the average concentration and σ is the standard deviation, both of which were evaluated during filter measurements. The resulting detection limits are 8.8e4, 1.5e5, and 5.5e4 molec·cm-3 for SA, MSA, and IA, respectively. The dataset includes flags to specify the data that are below the detection limit. The influence of local pollution from the research vessel and other logistic activities was identified by applying a pollution detection algorithm (Beck et al., 2022) to particle number concentrations from a condensation particle counter (CPC3025, TSI) that was also located in the Swiss container. Periods that were potentially affected by primary pollution are flagged in the dataset. The columns in the data file include the date and time in Coordinated Universal Time (UTC); the concentration of SA, MSA, and IA in molec·cm-3; a detection limit flag for each individual species (1 = below detection limit); and a local pollution flag where the data may have influence from the vessel and logistical activities (1 = pollution was detected).