Elsevier

Atmospheric Environment

Volume 34, Issue 25, 26 July 2000, Pages 4215-4240
Atmospheric Environment

Formation and cycling of aerosols in the global troposphere

https://doi.org/10.1016/S1352-2310(00)00239-9Get rights and content

Abstract

Aerosols are formed, evolve, and are eventually removed within the general circulation of the atmosphere. The characteristic time of many of the microphysical aerosol processes is days up to several weeks, hence longer than the residence time of the aerosol within a typical atmospheric compartment (e.g. the marine boundary layer, the free troposphere, etc.). Hence, to understand aerosol properties, one cannot confine the discussion to such compartments, but one needs to view aerosol microphysical phenomena within the context of atmospheric dynamics that connects those compartments. This paper attempts to present an integrated microphysical and dynamical picture of the global tropospheric aerosol system. It does so by reviewing the microphysical processes and those elements of the general circulation that determine the size distribution and chemical composition of the aerosol, and by implementing both types of processes in a diagnostic model, in a 3-D global Chemical Transport Model, and in a General Circulation Model. Initial results are presented regarding the formation, transformation, and cycling of aerosols within 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,

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