5.1 Introduction
Aerosols have a direct radiative forcing because they scatter and absorb solar
and infrared radiation in the atmosphere. Aerosols also alter the formation
and precipitation efficiency of liquid- water, ice and mixed-phase clouds, thereby
causing an indirect radiative forcing associated with these changes in cloud
properties.
The quantification of aerosol radiative forcing is more complex than the quantification
of radiative forcing by greenhouse gases because aerosol mass and particle number
concentrations are highly variable in space and time. This variability is largely
due to the much shorter atmospheric lifetime of aerosols compared with the important
greenhouse gases. Spatially and temporally resolved information on the atmospheric
burden and radiative properties of aerosols is needed to estimate radiative
forcing. Important parameters are size distribution, change in size with relative
humidity, complex refractive index, and solubility of aerosol particles. Estimating
radiative forcing also requires an ability to distinguish natural and anthropogenic
aerosols.
The quantification of indirect radiative forcing by aerosols is especially
difficult. In addition to the variability in aerosol concentrations, some quite
complicated aerosol influences on cloud processes must be accurately modelled.
The warm (liquid- water) cloud indirect forcing may be divided into two components.
The first indirect forcing is associated with the change in droplet concentration
caused by increases in aerosol cloud condensation nuclei. The second indirect
forcing is associated with the change in precipitation efficiency that results
from a change in droplet number concentration. Quantification of the latter
forcing necessitates understanding of a change in cloud liquid-water content
and cloud amount. In addition to warm clouds, ice clouds may also be affected
by aerosols.
5.1.1 Advances since the Second Assessment Report
Considerable progress in understanding the effects of aerosols on radiative
balances in the atmosphere has been made since the IPCC WGI Second Assessment
Report (IPCC, 1996) (hereafter SAR). A variety of field studies have taken place,
providing both process-level understanding and a descriptive understanding of
the aerosols in different regions. In addition, a variety of aerosol networks
and satellite analyses have provided observations of regional differences in
aerosol characteristics. Improved instrumentation is available for measurements
of the chemical composition of single particles.
Models of aerosols have significantly improved since the SAR. Because global
scale observations are not available for many aerosol properties, models are
essential for interpolating and extrapolating available data to the global scale.
Although there is a high degree of uncertainty associated with their use, models
are presently the only tools with which to study past or future aerosol distributions
and properties.
The very simplest models represent the global atmosphere as a single box in
steady state for which the burden can be derived if estimates of sources and
lifetimes are available. This approach was used in early assessments of the
climatic effect of aerosols (e.g., Charlson et al., 1987, 1992; Penner et al.,
1992; Andreae, 1995) since the information and modelling tools to provide a
spatially- and temporally-resolved analysis were not available at the time.
At the time of the SAR, three-dimensional models were only available for sulphate
aerosols and soot. Since then, three-dimensional aerosol models have been developed
for carbonaceous aerosols from biomass burning and from fossil fuels (Liousse
et al., 1996; Cooke and Wilson, 1996; Cooke et al., 1999), dust aerosols (Tegen
and Fung, 1994; Tegen et al., 1996), sea salt aerosol (Gong et al., 1998) and
nitrate and ammonia in aerosols (Adams et al., 1999, 2001; Penner et al., 1999a).
In this report, the focus is on a temporally and spatially resolved analysis
of the atmospheric concentrations of aerosols and their radiative properties.
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