Formation and cycling of aerosols in the global troposphere
Introduction
Particles in the atmosphere arise from natural sources, such as wind-borne dust, sea spray, and volcanoes, and from anthropogenic activities, such as combustion of fuels (Table 1). Emitted directly as particles (primary aerosol) or formed in the atmosphere by gas-to-particle conversion processes (secondary aerosol), atmospheric aerosols range in size from a few nanometers (nm) to tens of micrometers (μm) in diameter. Once airborne, particles evolve in size and composition through condensation of vapour species or by evaporation, by coagulating with other particles, by chemical reaction, or by activation in the presence of supersaturated water vapour to become cloud and fog droplets. Particles smaller than 1 μm diameter generally have atmospheric concentrations in the range from 10 to 10,000s per cm3; those exceeding 1 μm diameter typically exhibit concentrations less than 10 cm−3.
There is evidence that anthropogenic particles, at concentrations typical of urban airsheds, directly affect human health. Biomass burning, especially in the tropics, leads to significant perturbations to tropospheric aerosol loadings in that region, perhaps accompanied by alterations of cloud behaviour. Aircraft exhaust particles in the upper atmosphere are a source of ice and cloud nuclei. Atmospheric particles provide surfaces for heterogeneous chemical reactions that may influence gas-phase chemistry in the troposphere. It is not possible to survey each of these aspects in a review of modest length; consequently, we focus here on aerosol processes in the global atmosphere, the dynamics that shape the size and composition of the global aerosol.
The first measurements of the aerosol number concentration in the atmosphere were performed by Aitken (1888) who used an expansion chamber to make water vapour condense on the particles and make them grow to visible droplets. Aitken proclaimed that “without aerosols there would be no clouds and no precipitation”. The water vapour supersaturation (=relative humidity (%)−100) created in the Aitken counter reached 300%, enough to activate any particle. In the atmosphere, however, supersaturations of at most 2% are reached (Pruppacher and Klett, 1980), and Köhler (1936) showed that at such low supersaturations only those particles will activate that are sufficiently hygroscopic, i.e. particles that contain sufficient amount of soluble material to reduce the equilibrium water vapour pressure above the solution droplet. Hence, aerosol chemical and physical properties do control cloud droplet formation, and accordingly cloud microphysical properties, precipitation potential and optical properties. There are now many observations that this is effectively the case (Boers et al., 1994; Cerveny and Balling, 1998; Rosenfeld, 1999; Pawlowska and Brenguier, 2000; Johnson et al., 2000; Chuang et al., 2000).
Aerosols are important players in the hydrological cycle and climate system. It is therefore necessary to understand their cycling in the atmosphere, and to be able to predict their characteristics. Within the context of global climate change, aerosol studies have focused either on descriptions of global sources and spatial distributions of aerosols, neglecting the microphysical aspects, or they have focused on the microphysics of their formation and evolution, without placing these processes in the context of atmospheric large-scale circulation. In this paper we will review progress achieved by the two approaches, and we will attempt to synthesise a combined microphysical and dynamical picture of the global tropospheric aerosol system. We will also review observations of some key aerosol characteristics in a number of environments, which have been helpful to constrain our understanding of aerosols. In the model studies, presented at the end of the paper, we draw particularly from the global sulphur cycle because much has been learned recently about this cycle, and it serves as an excellent vehicle to discuss the effect of global circulation on aerosol properties and behaviour.
Section snippets
Processes
Fig. 1 depicts generally the microphysical processes that influence the size distribution and chemical composition of the atmospheric aerosol, highlighting the large range of sizes that are involved in the formation and evolution of aerosol particles. Traditionally, atmospheric aerosols have been divided into two size classes: coarse (Dp>1 μm) and fine (Dp<1 μm), reflecting the two major formation mechanisms: primary and secondary. Both populations strongly overlap, however, in the 0.1–1 μm
Size distributions
Measurements of the atmospheric aerosol size distributions were essential in identifying the various processes involved in the formation and evolution of the atmospheric aerosol (Whitby, 1978; Hoppel et al., 1986, Hoppel et al., 1990). Jaenicke (1988) has reviewed such measurements up till the early 1980s and made a climatology of aerosol size distributions. Fig. 2 shows a similar climatology of number distributions and corresponding volume distributions as a function of particle diameter,
Modelling the clean marine boundary layer
The existence of significantly different size distributions and chemical compositions in various environments (see Section 3) has led the aerosol community to think in terms of atmospheric compartments, such as the marine boundary layer, the continental boundary layer, the free troposphere. Of those, the MBL has been studied extensively, because of its dominant role in the climate system (Charlson et al., 1987) and because of it simplicity relative to others.
The aerosol in the marine boundary
Tropospheric general circulation
Tropospheric general circulation is characterised by rapid, localized upward motion due to convection (in the tropics) or slantwise ascent along frontal surfaces (in the mid-latitudes), which is compensated by relatively slow and large-scale subsidence in the sub-tropical and polar regions. Horizontal transport in the lower and upper troposphere connects areas of upward and downward transport, in what are supposed to be toroidal circulation patterns. Long-term averages of both the meridional
Aerosol microphysics in the context of the general circulation
A straightforward way to link microphysics and the general circulation and treat fully the issues discussed above is to implement the descriptions of the processes depicted in Fig. 1 in a general circulation model or global CTM, which captures the transport patterns depicted in Fig. 3. However, in order to accurately treat aerosol dynamic processes such as nucleation, coagulation, and condensation, the aerosol size distribution between 1 nm and 1 μm should be described with a high resolution in
Summary and outlook
During the past decade enormous progress has been made in the understanding of the life cycle of aerosols in the global atmosphere. In the previous sections we argued that even a basic understanding of aerosols at a global scale requires the understanding and integration of both microphysical and large-scale dynamics processes. This is primarily because the time scales of aerosol evolution are in many cases longer than the residence time in particular atmospheric compartments. Furthermore,
References (133)
- et al.
cis-Pinic acid, a possible precursor for organic aerosol formation from ozonolysis of alpha-pinene
Atmospheric Environment
(1998) - et al.
An analysis of various nucleation mechanisms for sulphate particles in the stratosphere
Journal of Aerosol Sccience
(1982) - et al.
Heteromolecular nucleation in the sulphuric acid-water system
Atmospheric Environment
(1989) - et al.
A study of processes governing the maintenance of aerosols in the marine boundary layer
Journal of Aerosol Science
(1999) - et al.
The self-preserving particle size distribution for Brownian coagulation in the free-molecular regime
Journal of Colloid and Interface Science
(1972) - et al.
Time scales to achieve atmospheric gas-aerosol equilibrium for volatile species
Atmospheric Environment
(1996) - et al.
The role of ion-induced aerosol formation in the lower atmosphere
Journal of Aerosol Science
(1986) - et al.
Modelling the dynamics of H2SO4–H2O aerosols with AERO2model description, uncertainty analysis and experimental validation
Journal of Aerosol Science
(1992) - et al.
Global concentrations of tropospheric sulphate, nitrate, and ammonium simulated in a general circulation model
Journal of Geophysical Research
(1999) On the number of dust particles in the atmosphere
Nature
(1888)