Year‐Round In Situ Measurements of Arctic Low‐Level Clouds: Microphysical Properties and Their Relationships With Aerosols

Abstract

Two years of continuous in situ measurements of Arctic low‐level clouds have been made at the Mount Zeppelin Observatory (78°56′N, 11°53′E), in Ny‐Ålesund, Spitsbergen. The monthly median value of the cloud particle number concentration (Nc) showed a clear seasonal variation: Its maximum appeared in May–July (65 ± 8 cm−3), and it remained low between October and March (8 ± 7 cm−3). At temperatures warmer than 0 °C, a clear correlation was found between the hourly Nc values and the number concentrations of aerosols with dry diameters larger than 70 nm (N70), which are proxies for cloud condensation nuclei (CCN). When clouds were detected at temperatures colder than 0 °C, some of the data followed the summertime Nc to N70 relationship, while other data showed systematically lower Nc values. The lidar‐derived depolarization ratios suggested that the former (CCN‐controlled) and latter (CCN‐uncontrolled) data generally corresponded to clouds consisting of supercooled water droplets and those containing ice particles, respectively. The CCN‐controlled data persistently appeared throughout the year at Zeppelin. The aerosol‐cloud interaction index (ACI = dlnNc/(3dlnN70)) for the CCN‐controlled data showed high sensitivities to aerosols both in the summer (clean air) and winter–spring (Arctic haze) seasons (0.22 ± 0.03 and 0.25 ± 0.02, respectively). The air parcel model calculations generally reproduced these values. The threshold diameters of aerosol activation (Dact), which account for the Nc of the CCN‐controlled data, were as low as 30–50 nm when N70 was less than 30 cm−3, suggesting that new particle formation can affect Arctic cloud microphysics.

Frequent new particle formation over the high Arctic pack ice by enhanced iodine emissions

“…Low-level clouds in the Arctic are generally characterized by an SS of 0.3% which would not be sufficient to activate 20-40 nm particles 50 . However, it has been indirectly shown that droplets often form on particles smaller than 50 nm 50,51 . We provide here direct evidence that particles in the 20-40 nm size range activate as CCN in Arctic fog when the concentration of larger aerosols is low enough; this suggests that iodine NPF may be a relevant CCN source in the region.…”

High Sensitivity of Arctic Black Carbon Radiative Effects to Subgrid Vertical Velocity in Aerosol Activation

“…We used BC mass concentrations observed at two surface sites, Barrow (71.3°N, 156.6°W) and Ny‐Ålesund (78.9°N, 11.9°E), in the Arctic (Sinha et al, 2017) and during some aircraft campaigns over East Asia, Pacific, and Arctic (Kondo et al, 2011; Matsui, Kondo, et al, 2011; Oshima et al, 2012; Schwarz et al, 2010). We also used observation‐based SSmax values estimated at midlatitude (Japan) (Moteki et al, 2019) and in the Arctic (Koike et al, 2019; Leaitch et al, 2016) to evaluate our model simulations (supporting information Text S4).…”

The importance of Aitken mode aerosol particles for cloud sustenance in the summertime high Arctic – a simulation study supported by observational data

“…The ability of an aerosol particle to act as a CCN depends upon multiple factors, such as its size and chemical composition (Köhler, 1936), surface tension (e.g. Ovadnevaite et al, 2017) and the ambient relative humidity (e.g.…”

“…Rastak et al, 2017). A larger maximum supersaturation within an air parcel allows smaller and less hygroscopic particles, potentially, to act as CCN (Köhler, 1936;Petters and Kreidenweis, 2007). On the other hand, the maximum supersaturation is also dependent on the relative abundance of particles, particularly the number of water soluble accumulation or coarse mode particles as they easily act as CCN and subsequently take up water when they grow (e.g.…”