A full year of aerosol size distribution data from the central Arctic under an extreme positive Arctic Oscillation: insights from the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition

Abstract

Abstract. The Arctic environment is rapidly changing due to accelerated warming in the region. The warming trend is driving a decline in sea ice extent, which thereby enhances feedback loops in the surface energy budget in the Arctic. Arctic aerosols play an important role in the radiative balance and hence the climate response in the region, yet direct observations of aerosols over the Arctic Ocean are limited. In this study, we investigate the annual cycle in the aerosol particle number size distribution (PNSD), particle number concentration (PNC), and black carbon (BC) mass concentration in the central Arctic during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. This is the first continuous, year-long data set of aerosol PNSD ever collected over the sea ice in the central Arctic Ocean. We use a k-means cluster analysis, FLEXPART simulations, and inverse modeling to evaluate seasonal patterns and the influence of different source regions on the Arctic aerosol population. Furthermore, we compare the aerosol observations to land-based sites across the Arctic, using both long-term measurements and observations during the year of the MOSAiC expedition (2019–2020), to investigate interannual variability and to give context to the aerosol characteristics from within the central Arctic. Our analysis identifies that, overall, the central Arctic exhibits typical seasonal patterns of aerosols, including anthropogenic influence from Arctic haze in winter and secondary aerosol processes in summer. The seasonal pattern corresponds to the global radiation, surface air temperature, and timing of sea ice melting/freezing, which drive changes in transport patterns and secondary aerosol processes. In winter, the Norilsk region in Russia/Siberia was the dominant source of Arctic haze signals in the PNSD and BC observations, which contributed to higher accumulation-mode PNC and BC mass concentrations in the central Arctic than at land-based observatories. We also show that the wintertime Arctic Oscillation (AO) phenomenon, which was reported to achieve a record-breaking positive phase during January–March 2020, explains the unusual timing and magnitude of Arctic haze across the Arctic region compared to longer-term observations. In summer, the aerosol PNCs of the nucleation and Aitken modes are enhanced; however, concentrations were notably lower in the central Arctic over the ice pack than at land-based sites further south. The analysis presented herein provides a current snapshot of Arctic aerosol processes in an environment that is characterized by rapid changes, which will be crucial for improving climate model predictions, understanding linkages between different environmental processes, and investigating the impacts of climate change in future Arctic aerosol studies.

Annual cycle of aerosol properties over the central Arctic during MOSAiC 2019–2020 — light-extinction, CCN, and INP levels from the boundary layer to the tropopause

“…In this section, we present our lidar results in terms of time series of CCN and INP concentration estimates at different height levels. We include the MOSAiC in situ observations of the particle number concentrations n 50,dry (Boyer et al, 2023) and of ice nucleating particles n INP (Creamean et al, 2022) In accordance with the observations of optical properties, strong differences in the CCN concentration between winter and summer are found at 250 m height in Fig. 10.…”

“…The in situ measured n 50,dry values (considering particles with diameters from 100-500 nm) were, on average, lower than the lidar estimates during the winter period (October-April). However, the monthly mean median particle number concentrations (not shown) were, as the lidar values, also clearly >100 cm −3 in the months of January, February, April and May (Boyer et al, 2023). The in situ observations also showed particle number concentrations <40 cm −3 in summer in terms of monthly median values.…”

“…To estimate the ice-nucleating potential of dust particles at the tropopause level, we applied the DIN INP parameterization of The number concentrations n 50,dry measured in situ (Boyer et al, 2023) consider particles with diameters from 100 to 500 nm and is also used as a proxy for the CCN concentration n CCN for S WAT = 1.002. CCN concentrations were also measured aboard Polarstern (Dada et al, 2022) and will be included in the discussions of the MOSAiC observations in Sect.…”

“…The in situ observations of n 50,dry were carefully checked and corrected for contamination by local pollution (exhaust plume of Polarstern and further aerosol sources on the near-by measurement field station on the pack ice) (Beck et al, 2022). About 40% of the measured data had to be removed (Boyer et al, 2023).…”

“…7 is the strong decrease of the particle extinction coefficient with height during the winter months (Arctic haze season) when aged anthropogenic aerosol, soil dust, and biomass burning smoke is transported into the Arctic from the surrounding continents (North America, Asia, Europe) (Stohl, 2006;Willis et al, 2018;Engelmann et al, 2021). Most of the pollution reaching Polarstern at lower heights in winter 2019-2020 originated from northern Asia (Creamean et al, 2022;Boyer et al, 2023). Arctic haze events observed on 4 February and 4 March 2020 were discussed in Engelmann et al (2021).…”