FRANKFURT. When winter smog takes over Asian
mega-cities, more particulate matter is measured in the streets than expected.
An international team, including researchers from Goethe University Frankfurt,
as well as the universities in Vienna and Innsbruck, has now discovered that
nitric acid and ammonia in particular contribute to the formation of additional
particulate matter. Nitric acid and ammonia arise in city centres predominantly
from car exhaust. Experiments show that the high local concentration of the vapours
in narrow and enclosed city streets accelerates the growth of tiny nanoparticles
into stabile aerosol particles (Nature, DOI 10.1038/s41586-020-2270-4).
In crowded urban centres, high
concentrations of particulate matter cause considerable health effects.
Especially in winter months, the situation in many Asian mega-cities is dramatic
when smog significantly reduces visibility and breathing becomes difficult.
Particulates, with a diameter of less than
2.5 micrometres, mostly form directly through combustion processes, for example
in cars or heaters. These are called primary particulates. Particulates also
form in the air as secondary particulates, when gases from organic substances,
sulphuric acid, nitric acid or ammonia, condense on tiny nanoparticles. These grow
into particles that make up a part of particulate matter.
Until now, how secondary particulates
could be newly formed in the narrow streets of mega-cities was a puzzle.
According to calculations, the tiny nanoparticles should accumulate on the abundantly
available larger particles rather than forming new particulates.
Scientists in the international research
project CLOUD have now recreated the conditions that prevail in the streets of
mega-cities in a climate chamber at the particle accelerator CERN in Geneva,
and reconstructed the formation of secondary particulates: in the narrow and
enclosed streets of a city, a local increase of pollutants occurs. The cause of
the irregular distribution of the pollutants is due in part to the high pollutant
emissions at the street level. Furthermore, it takes a while before the
street air mixes with the surrounding air. This leads to the two pollutants
ammonia and nitric acid being temporarily concentrated in the street air. As the
CLOUD experiments demonstrate, this high concentration creates conditions in
which the two pollutants can condense onto nanoparticles: ammonium nitrate
forms on condensation cores the size of only a few nanometres, causing these
particles to grow rapidly.
“We have observed that these nanoparticles
grow rapidly within just a few minutes. Some of them grow one hundred times
more quickly than we had previously ever seen with other pollutants, such as
sulphuric acid," explains climate researcher Professor Joachim Curtius from
Goethe University Frankfurt. “In crowded urban centres, the process we observed
therefore makes an important contribution to the formation of particulate
matter in winter smog – because this process only takes place at temperatures
below about 5 degrees Celsius." The aerosol physicist Paul Winkler from the
University of Vienna adds: “When conditions are warmer, the particles are too
volatile to contribute to growth."
The formation of aerosol particles from
ammonia and nitric acid probably takes place not only in cities and crowded
areas, but on occasion also in higher atmospheric altitudes. Ammonia, which is primarily
emitted from animal husbandry and other agriculture, arrives in the upper
troposphere from air parcels rising from close to the ground by deep convection,
and lightning creates nitric acid out of nitrogen in the air. “At the
prevailing low temperatures there, new ammonium nitrate particles are formed
which as condensation seeds play a role in cloud formation," explains ion physicist
Armin Hansel from the University of Innsbruck, pointing out the relevance of
the research findings for climate.
The experiment CLOUD (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 on the formation of clouds and
particulate matter. CLOUD is carried out by an international consortium
consisting of 21 institutions. The CLOUD measuring chamber was developed with
CERN know-how and achieves very precisely defined measuring conditions. CLOUD experiments
use a variety of different measuring instruments to characterise the physical
and chemical conditions of the atmosphere consisting of particles and gases. In
the CLOUD project, the team led by Joachim Curtius from the Institute for
Atmosphere and Environment at Goethe University Frankfurt develops and operates
two mass spectrometers to detect trace gases such as ammonia and sulphuric acid
even at the smallest concentrations as part of projects funded by the BMBF and
the EU. At the Faculty of Physics at the University of Vienna, the team led by
Paul Winkler is developing a new particle measuring device as part of an ERC
project. The device will enable the quantitative investigation of aerosol dynamics
specifically in the relevant size range of 1 to 10 nanometres. Armin Hansel
from the Institute for Ion Physics and Applied Physics at the University of Innsbruck
developed a new measuring procedure (PTR3-TOF-MS) to enable an even more
sensitive analysis of trace gases in the CLOUD experiment with his research
team as part of an FFG project.
Wang, M., Kong, W., et al. Rapid growth of
new atmospheric particles by nitric acid and ammonia condensation. Nature, DOI
information: Prof. Dr. Joachim Curtius, Institute for Atmosphere and Environment, Goethe University Frankfurt am Main, Tel: +49 69 798-40258, email: firstname.lastname@example.org
Prof. Dr. Armin Hansel, Institute for Ion Physics and Applied
Physics, University of Innsbruck, Tel.: +43 512 507 52640, email: email@example.com
Prof. Dr. Paul Winkler, Aerosol physics and Environmental Physics, Faculty for Physics, University of Vienna, Tel:
+43-1-4277-734 03, email: firstname.lastname@example.org