Ice-nucleating particle concentrations of the past: insights from a 600-year-old Greenland ice core

Schrod, Jann; Kleinhenz, Dominik; Hörhold, Maria; Erhardt, Tobias; Richter, Sarah; Wilhelms, Frank; Fischer, Hubertus; Ebert, Martin; Twarloh, Birthe; Della Lunga, Damiano; Jensen, Camilla M.; Curtius, Joachim; Bingemer, Heinz G. (2020). Ice-nucleating particle concentrations of the past: insights from a 600-year-old Greenland ice core. Atmospheric chemistry and physics, 20(21), pp. 12459-12482. European Geosciences Union 10.5194/acp-20-12459-2020

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Ice-nucleating particles (INPs) affect the microphysics
in cloud and precipitation processes. Hence, they
modulate the radiative properties of clouds. However, atmospheric
INP concentrations of the past are basically unknown.
Here, we present INP measurements from an ice core
in Greenland, which dates back to the year 1370. In total 135
samples were analyzed with the FRIDGE droplet freezing assay
in the temperature range from -14 to -35 °C. The sampling
frequency was set to 1 in 10 years from 1370 to 1960.
From 1960 to 1990 the frequency was increased to one sample
per year. Additionally, a few special events were probed,
including volcanic episodes. The typical time coverage of a
sample was on the order of a few months. Historical atmospheric
INP concentrations were estimated with a conversion
factor, which depends on the snow accumulation rate of the
ice core, particle dry deposition velocity, and wet scavenging
ratio. Typical atmospheric INP concentrations were on the
order of 0.1 L-1 at -25 °C. The INP variability was found to
be about 1–2 orders of magnitude. Yet, the short-term variability
from samples over a seasonal cycle was considerably
lower. INP concentrations were significantly correlated to
some chemical tracers derived from continuous-flow analysis
(CFA) and ion chromatography (IC) over a broad range
of nucleation temperatures. The highest correlation coefficients
were found for the particle concentration (spherical
diameter dp>1.2 μm). The correlation is higher for a time
period of seasonal samples, where INP concentrations follow
a clear annual pattern, highlighting the importance of the
annual dust input in Greenland from East Asian deserts during
spring. Scanning electron microscopy (SEM) analysis of
selected samples found mineral dust to be the dominant particle
fraction, verifying their significance as INPs. Overall, the
concentrations compare reasonably well to present-day INP
concentrations, albeit they are on the lower side. However,
we found that the INP concentration at medium supercooled
temperatures differed before and after 1960. Average INP
concentrations at -23, -24, -25, -26, and -28 °C were
significantly higher (and more variable) in the modern-day
period, which could indicate a potential anthropogenic impact,
e.g., from land-use change.

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