High-latitude dust as a potential source of ice-nucleating particles in the Arctic

Ice-nucleating particles (INPs) play a key role in the climate and weather systems on the Earth.
They are a small subset of atmospheric aerosol particles and are able to facilitate primary
ice nucleation in mixed-phase clouds above 235 K. Through this process, they can control
the occurence of ice crystals in mixed-phase clouds, which, in turn, determines their radiative
properties as well as their lifetime via the formation of precipitation. This mechanism is part of
aerosol-cloud interactions and results in an overall net negative radiative forcing, although it
is associated with significant uncertainties. Improving the representation of clouds in current
weather and climate models requires a better understanding of the types of INPs and their
spatiotemporal distribution in the atmosphere.
Different types of aerosol particles are able to act as INPs, which vary in their abundance and ice
nucleation ability with temperature. The INP concentration in the atmosphere typically follows
an exponential increase towards lower temperatures, with only about one in a million of ice-active
aerosol particles at about 268 K, but about one in a thousand ice-active aerosol particles at
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about 240 K. Mineral dust often dominates the INP concentration at lower temperatures, while
biogenic aerosol particles dominate the INP concentration at temperatures above around 258 K.
The climate is changing faster in the Arctic compared to the rest of the world, a phenomena
knows as Arctic amplification, which is not yet fully understood, partly due to missing knowledge
on the primary ice formation in clouds. This study aims at better capturing the concentration
and types of INPs in the Arctic and sub-Arctic region. Mineral dust in the Arctic was found
to be an important type of INP in the Arctic, as well as biogenic aerosol particles. One often
overlooked source of mineral dust is high-latitude dust, which can be defined as all mineral dust
that originates from dust sources located above 60◦ N. In this work, I conducted measurements
at Svalbard at 79◦ N, as well as in Northern Finland (68◦ N). Longterm studies were conducted
using the Portable Ice Nucleation Experiment to measure the INP concentration at different
nucleation temperatures at a high temporal resolution of minutes. Together, these measurements
were used to assess the impact of local emission of aerosol on the INP concentration as well
as the contribution from long-range transport. In addition, vertical measurements of the INP
concentration were conducted utilizing a lightweight filter-based aerosol sampler onboard of a
uncrewed aerial vehicle to assess the distribution of INPs in the sub-Arctic. This sampler was
developed and characterized during this work and offered a novel approach in measuring the
vertical distribution of INPs.
Additional modelling results from the FLEXible PARTicle dispersion model model were used to
investigate the source and age of mineral dust arriving at the high Arctic. To understand the
INP population, the INP concentration was normalized with the aerosol surface concentration to
obtain the ice nucleation active site density. During periods, where a low ice nucleation active site
density was observed, mineral dust was typically originating from sources in Russia. Higher ice
nucleation active site densities were observed for sources in Africa, indicating that high-latitude
dust might have a lower ice nucleation ability compared to other mineral dust. Furthermore, dust
that travelled through the atmosphere for longer time periods, generally was connected to an
increase in ice nucleation active site density, which might hint to possible aging effect enhancing
the ice nucleation ability. The reason for this behaviour remains unclear.
The vertical measurements using the newly developed uncrewed aerial vehicle based setup often
showed either similar INP concentrations between the ground and higher altitudes (<600 m
above ground level), or even higher INP concentrations at higher altitudes. Given that ground-
based measurements are more impacted by local emission, this finding suggests that either the
atmospheric layers are well-mixed and ground-based measurements are also representative for
the INP concentration at higher altitudes, or that the air masses arriving at elevated levels
contain air masses with aerosol particles that are more efficient INPs. The dataset is still too
small to draw a conclusive picture, but it is clear, that more vertical measurements of the INP
concentration are needed to understand this behaviour.