Tropospheric New Particle Formation and the Role of Ions
Jan Kazil
0
1
2
R. Giles Harrison
0
1
2
Edward R. Lovejoy
0
1
2
0
E.R. Lovejoy NOAA Earth System Research Laboratory, Chemical Sciences Division
, 325 Broadway, R/CSD2, Boulder,
CO 80305-3337, USA
1
R.G. Harrison Department of Meteorology, University of Reading
, Earley Gate, PO Box 243, Reading, RG6 6BB,
UK
2
J. Kazil ( ) Max Planck Institute for Meteorology
, Bundesstr. 53, 20146 Hamburg,
Germany
Aerosol particles play an important role in the Earth's troposphere and in the climate system: They scatter and absorb solar radiation, facilitate chemical processes, and serve as condensation nuclei for the formation of clouds. Tropospheric aerosol particles are emitted from surface sources or form in situ from the gas phase. Formation from the gas phase requires concentrations of aerosol precursor molecules aggregating to form molecular clusters able to grow faster than they evaporate. This process is called nucleation. Gas phase ions can reduce the concentration of aerosol precursor molecules required for nucleation, as they greatly stabilize molecular clusters with respect to evaporation. Therefore, ions are a potential source of aerosol particles. Since atmospheric ionization carries the signal of the decadal solar cycle due to the modulation of the galactic cosmic ray intensity by solar activity, a possible connection between the solar cycle, galactic cosmic rays, aerosols, and clouds has been a long-standing focus of interest. In this paper, we provide an overview of theoretical, modeling, laboratory, and field work on the role and relevance of ions for the formation of tropospheric aerosol particles, and on subsequent effects on clouds, and discuss briefly related research needs.
1 Introduction
The continuous formation of charged particles in the atmosphere has been recognized since
the late nineteenth century. These particles were identified as ions following studies on
radioactivity and electrical conduction in gases (see Aplin et al. 2008). In early laboratory
experiments, Wilson (1897, 1899) reported ion and particle formation in air and other gases
in the presence of ultraviolet radiation and radioactive sources, and was the first to suggest
that ions may be involved in atmospheric particle formation, and to realize the importance
this could have for clouds (Galison 1997). Particle formation from the gas phase requires the
emergence of molecular clusters of a size at which the condensation of further molecules
is more likely than their evaporation, a process referred to as nucleation. Molecular
clusters forming around ions are more stable compared to their neutral counterparts; ions can
therefore trigger nucleation in conditions where neutral molecules would not. In the
twentieth century, laboratory studies have confirmed Wilsons initial experiments (e.g. Bricard
et al. 1968; Vohra et al. 1969; Raes and Janssens 1985), but research on atmospheric
particle formation from ions has intensified only in the dawn of the twenty-first century with
the heightened awareness of the relevance of new particle formation from the gas phase,
which can significantly increase concentrations of aerosol particles and cloud
condensation nuclei in the troposphere (Kulmala et al. 2004a), and thereby establishes a
connection between atmospheric chemical composition, aerosols, clouds, and climate. Interest for
the topic was also fueled by the possibility that the solar cycle modulation of atmospheric
ionization through galactic cosmic rays could give rise to a similar modulation in aerosol
and cloud droplet concentrations, and provide a mechanism to explain reported
correlations between the decadal solar cycle and tropospheric observables, such as cloud cover
(Svensmark and Friis-Christensen 1997; Marsh and Svensmark 2000; Carslaw et al. 2002;
Harrison and Carslaw 2003).
2 Tropospheric Aerosol
Tropospheric aerosol particles are either emitted from the surface of the Earth, or form
from the gas phase. Transport of aerosol particles from the stratosphere represents only a
minor contribution to tropospheric aerosol. Major surface sources of aerosol particles
include the oceans, which emit sea-salt particles, arid and semi-arid regions, where wind lifts
mineral dust, and volcanoes, which inject particulate sulfate and ash into the atmosphere,
while carbonaceous aerosols are produced by wildfires and by the combustion of fossil
and biomass fuels. The formation of new aerosol particles from the gas phase requires
sufficiently high concentrations of gas phase molecules with low saturation vapor pressure:
for example, sulfuric acid which has a very low saturation vapor pressure in atmospheric
conditions has been frequently reported as component of freshly formed aerosol, and
appears to drive new particle formation in clean areas, such as over oceans (Clarke 1992;
Brock et al. 1995). Over continents and, in particular, within the continental boundary
layer, recently nucleated aerosol particles may contain, in addition to sulf (...truncated)