When sea ice melts and the water surface increases,
more iodine-containing vapours rise from the sea. Scientists from the
international research network CLOUD have now discovered that aerosol particles
form rapidly from such iodine vapours, which can serve as condensation nuclei
for cloud formation. The CLOUD researchers, among them atmospheric scientists
from the Goethe University Frankfurt, fear a mutual intensification of sea ice
melt and cloud formation, which could accelerate the warming of the Arctic and
Antarctic.
FRANKFURT. More
than two thirds of the earth is covered by clouds. Depending on whether they
float high or low, how large their water and ice content is, how thick they are
or over which region of the Earth they form, it gets warmer or cooler
underneath them. Due to human influence, there are most likely more cooling
effects from clouds today than in pre-industrial times, but how clouds
contribute to climate change is not yet well understood. Researchers currently
believe that low clouds over the Arctic and Antarctic, for example, contribute
to the warming of these regions by blocking the direct radiation of long-wave
heat from the Earth's surface.
All clouds are formed by aerosols,
suspended particles in the air, to which water vapour attaches. Such suspended
particles or aerosols naturally consist of dusts, salt crystals or molecules
released by plants. Human activities cause above all soot particles to be
released into the atmosphere, but also sulphuric acid and ammonia molecules,
which can cluster and form new aerosol particles in the atmosphere. Model
calculations show that more than half of the cloud droplets are formed from
aerosol particles that have formed in the atmosphere. For the formation of
clouds, it is not decisive what the aerosol particles are made of; what matters
most is their size: Aerosol particles only become condensation nuclei for cloud
droplets from a diameter of about 70 nanometres and up.
In the atmosphere over the sea, aerosols
released by humans play a much smaller role in the formation of low clouds than
over land. Besides salt crystals originating from sea spray, aerosol particles over
the sea mainly originate from certain sulphur compounds (dimethyl sufide) that
are released from phytoplankton and react to form sulphuric acid, for example. At
least, that is what previous research concluded.
Scientists from the CLOUD consortium have
now studied the formation of aerosol particles from iodine-containing vapours.
The slightly pungent smell of iodine is part of the aroma of the sea air you
breathe when walking along the North Sea. Every litre of seawater contains 0.05
milligrams of iodine, and when it enters the atmosphere, iodic acid or iodous
acid is formed through sunlight and ozone. The scientists simulated atmospheric
conditions in mid-latitudes and arctic regions in the CLOUD experimental chamber
at the CERN particle accelerator centre in Geneva, including cosmic rays
simulated by an elementary particle beam.
Their findings: aerosol particle formation
by iodic acid takes place very rapidly, much more rapidly than the particle
formation of sulphuric acid and ammonia under comparable conditions. Ions
produced by cosmic rays further promote particle formation. For the
transformation of the molecular iodine into the iodine-containing acids, not
even UV radiation and only a little daylight are necessary. In this way, very
large aerosol quantities can be formed very quickly.
Atmospheric researcher Prof. Joachim
Curtius from Goethe University explains: "Iodine aerosols can form faster
than almost all other aerosol types we know. If ions produced by cosmic rays
are added, each collision leads to the growth of the molecular clusters."
Curtius added that this is particularly important because global iodine
emissions on Earth have already tripled over the past 70 years. "A vicious
circle may have been set in motion here: The pack ice thaws, which increases
the water surface area and more iodine enters the atmosphere. This leads to
more aerosol particles, which form clouds that further warm the poles. The
mechanism we found can now become part of climate models, because iodine may
play a dominant role in aerosol formation, especially in the polar regions, and
this could improve climate model predictions for these regions."
The CLOUD
experiment (Cosmics Leaving OUtdoor Droplets) at CERN studies how new aerosol
particles are formed in the atmosphere out of precursor gases and continue to
grow into condensation seeds. CLOUD thereby provides fundamental understanding
of the formation of clouds and particulate matter. CLOUD is carried out by an
international consortium consisting of 21 institutes. The CLOUD measurement
chamber was developed with CERN know-how and is one of the cleanest experimental
rooms in the world. CLOUD measurement campaigns use a variety of different
measuring instruments to characterise the physical and chemical state of the
particles and gases that make up the atmosphere. The team led by Joachim
Curtius from the Institute for Atmosphere and Environment at Goethe University
Frankfurt developes and operates two mass spectrometers
in the CLOUD project to detect trace gases such as iodic acid and iodous acid
even in the smallest concentrations.
Publication:
Xu-Cheng He, Yee Jun Tham, Lubna Dada,
Mingyi Wang, Henning Finkenzeller, Dominik Stolzenburg, Siddharth Iyer, Mario
Simon, Andreas Kürten, et. al. Role of
iodine oxoacids in atmospheric aerosol nucleation, Science 05 Feb 2021: Vol. 371, Issue 6529, pp. 589-595, https://doi.org/10.1126/science.abe0298
Further
information:
Prof.
Joachim Curtius
Institute for Atmosphere and Environment
Goethe University Frankfurt am Main
Tel:
+49 69 798-40258
curtius@iau.uni-frankfurt.de
Dr. Andreas Kürten
Institute for Atmosphere and Environment
Goethe University Frankfurt am Main
Tel: +49 (69) 798-40256
kuerten@iau.uni-frankfurt.de
