Particles in the atmosphere

Small particles suspended in the atmosphere (often known as aerosol; see Glossary) affect its energy balance because they both absorb radiation from the Sun and scatter it back to space. We can easily see the effect of this on a bright day in the summer with a light wind when downwind of an industrial area. Although no cloud appears to be present, the Sun appears hazy. We call it 'industrial haze'. Under these conditions a significant proportion of the sunlight incident at the top of the atmosphere is being lost as it is scattered back and out of the atmosphere by the millions of small particles (typically between 0.001 and 0.01 mm in diameter) in the haze. The effect of particles can also be seen often when flying over or near industrial or densely populated areas for instance in Asia when although no cloud is present, it is too hazy to see the ground.18

Atmospheric particles come from a variety of sources. They arise partially from natural causes; they are blown off the land surface, especially in desert areas, they result from forest fires and they come from sea spray. From time to time large quantities of particles are injected into the upper atmosphere from volcanoes - the Pinatubo volcano which erupted in 1991 provides a good example (see Chapter 5). Some particles are also formed in the atmosphere itself, for instance sulphate particles from the sulphur-containing gases emitted from volcanoes. Other particles arise from human activities. Over the past ten years a large number of observations especially from satellite-borne instruments have provided much needed information about the aerosol distribution from both natural and anthropogenic sources in both space and time (Figure 3.7a).

The most important of the aerosols from anthropogenic sources are sulphate particles that are formed as a result of chemical action on sulphur dioxide, a gas that is produced in large quantities by power stations and other industries in which coal and oil (both of which contain sulphur in varying quantities) are burnt. Because these particles remain in the atmosphere only for about five days on average, their effect is mainly confined to regions near the sources of the particles, i.e. the major industrial regions of the northern hemisphere (Figure 3.7b). Sulphate particles scatter sunlight and provide a negative forcing, globally averaged estimated as -0.4 ± 0.2 W m-2. Over limited regions of the northern hemisphere the radiative effect of these particles is comparable in size, although opposite in effect, to that of human-generated greenhouse gases up to the present time. Figure 3.8 illustrates a model estimate of the substantial effect on global atmospheric temperature of removing all sulphate aerosol in the year 2000.

An important factor that will influence the future concentrations of sulphate particles is 'acid rain' pollution, caused mainly by the sulphur dioxide emissions. This leads to the degradation of forests and fish stocks in lakes especially in regions downwind of major industrial areas. Serious efforts are therefore under way, especially in Europe and North America, to curb these emissions to a substantial degree. Although the amount of sulphur-rich coal being burnt elsewhere in the world, for instance in Asia, is increasing rapidly, the damaging

Total aerosol optical depth

Total aerosol optical depth

Figure 3.7 Distribution of atmospheric aerosols. (a) Total aerosol optical depth at a mid-visible wavelength (for definition see Glossary) due to natural plus anthropogenic aerosols determined from observations by the satellite instrument MODIS, averaged from August to October 2001. Also indicated are the locations of aerosol lidar network sites (red circles). (b) The amount of sulphate (SO4) aerosol in the atmosphere in mg[SO4] m- 2 from human activities, 'background' non-explosive volcanoes and natural di-methyl sulphate (DMS) from ocean plankton, averaged over the decade of the 1990s calculated by the Hadley Centre model HadGEMI.

effects of sulphur pollution are such that controls on sulphur emissions are being extended to these regions also. For the globe as a whole therefore, sulphur emissions are likely to rise much less rapidly than emissions of carbon dioxide. In fact, they are likely to fall during the twenty-first century to below their 2000 value (Figure 6.1) thus removing part of the offset they are currently providing against the increase in radiative forcing from greenhouse gases.

The radiative forcing from particles can be positive or negative depending on the nature of the particles. For instance, soot particles (also called black carbon) from fossil fuel burning absorb sunlight and possess a positive forcing globally averaged estimated as 0.2 ± 0.15 W m-2. Other smaller anthropogenic contributions to aerosol radiative forcing come from biomass burning (e.g. the burning of forests), organic carbon particles from fossil fuel and nitrate and mineral dust particles. Because of the interactions that occur between particles from different sources and with clouds it is not adequate to add simply estimated radiative forcing from different particles to find the total forcing. That is why in Figure 3.11 estimates are given of the total radiative forcing from aerosol particles together with the associated uncertainty.

So far for aerosol we have been describing direct radiative forcing. There is a further way by which particles in the atmosphere could influence the climate; that is through their effect on cloud formation that is described as indirect radiative forcing. The mechanism of indirect forcing that is best understood arises from the influence of the number of particles and their size on cloud radiative properties (Figure 3.9). If particles are present in large numbers when clouds are forming, the resulting cloud consists of a large number of smaller drops - smaller than would otherwise be the case - similar to what happens as polluted fogs form in cities. Such a cloud will be more highly reflecting to sunlight than one consisting of larger particles, thus further increasing the energy loss resulting from the presence of the particles. Further the droplet size and number influence the precipitation efficiency, the lifetime of clouds and hence the geographic extent of cloudiness. Figure 3.10 is an illustration of the effect as it applies in the wakes of ships where clouds form possessing drop sizes much smaller than those pertaining to other clouds in the vicinity. There is now substantial observational evidence for these mechanisms but the processes

Year

Figure 3.8 A model calculation of the effect on global mean surface air temperature of removing all sulphate aerosols in the year 2000 (red line) compared with maintaining the global burden of sulphate aerosols at the 2000 level for the twenty-first century (blue line).

1950

2000

2050

Year

Figure 3.8 A model calculation of the effect on global mean surface air temperature of removing all sulphate aerosols in the year 2000 (red line) compared with maintaining the global burden of sulphate aerosols at the 2000 level for the twenty-first century (blue line).

1950

2000

2050

Figure 3.9 Schematic illustrating the cloud albedo and lifetime indirect effect on radiative forcing. Larger numbers of smaller particles in polluted clouds lead to more reflection of solar radiation from the cloud top, less radiation at the surface, less precipitation and a longer cloud lifetime.

Figure 3.9 Schematic illustrating the cloud albedo and lifetime indirect effect on radiative forcing. Larger numbers of smaller particles in polluted clouds lead to more reflection of solar radiation from the cloud top, less radiation at the surface, less precipitation and a longer cloud lifetime.

higher albedo

Smaller cloud \ particles i —less precipitation higher albedo

Smaller cloud \ particles i —less precipitation

Higher optical depth

-less radiation at surface

Higher optical depth

-less radiation at surface

Cloud droplet effective radius ( jjm)

Figure 3.10 Cloud droplet radii for ship track clouds and background water clouds in the same region showing the smaller droplet sizes in the polluted ship track clouds. (Data from MODIS instrument on NASA's Aqua satellite.)

involved are not easy to model and will vary a great deal with the particular situation. Substantial uncertainty therefore remains in estimates of their magnitude as shown in Figure 3.1119. To refine these estimates, more studies are required especially through making careful measurements on suitable clouds.

RF terms

RF values (W m-2)

Spatial scale

LOSU

greenh

Long-lived ouse gases

r

1

i

i

H

1.66 [1.49 to 1.83]

Global

High

N2O

arbo

s

0.48 [0.43 to 0.53] 0.16 [0.14 to 0.18]

Global

High

CH4 I- H Haloc

Ozone

Stra

tospheric I-1

h—1 Tropospheric

-0.05 [-0.15 to 0.05] 0.35 [0.25 to 0.65]

Continental to global

Med

Stratospheric water vapour from CH4

jj1

0.07 [0.02 to 0.12]

Global

Low

Surface albedo

La

id use 1— —

_ j Black carbon on snow

-0.2 [-0.4 to 0.0] 0.1 [0.0 to 0.2]

Local to continental

Med -Low

Total aerosol

" Direct effect

1-'-1

-0.5 [-0.9 to -0.1]

Continental to global

Med -Low

Cloud albedo effect

1-

—--1

-0.7 [-1.8 to -0.3]

Continental to global

Low

Linear contrails

!

0.01 [0.003 to 0.03]

Continental

Low

Solar irradiance

-i 1 H

0.12 [0.06 to 0.30]

Global

Low

Total net anthropogenic

-1-

-1-

-1-1-1-

-

-

1.6 [0.6 to 2.4]

Radiation forcing (W m -2)

Radiation forcing (W m -2)

Figure 3.11 Global, annual mean radiative forcings (W m-2) due to a number of agents for the period from pre-industrial (1750) to 2005. The size of the rectangular bar denotes a best estimate value; the horizontal lines indicate estimates of the uncertainty (90% confidence) ranges. To each forcing an indication is given of the geographical extent (spatial scale) and a 'level of scientific understanding' (LOSU) index is accorded. This latter represents a judgement about the reliability of the forcing estimate involving factors such as the assumptions necessary to evaluate the forcing, the degree of knowledge of the mechanisms determining the forcing and the uncertainties surrounding the quantitative estimate of the forcing.

The estimates for the radiative effects of particles as in Figure 3.11 can be compared with the global average radiative forcing to date due to the increase in greenhouse gases of about 2.6 W m-2. Comparing global average forcings, however, is not the whole story. Although the effects of particles on the global climate are well indicated by using globally averaged forcing estimates, for their effects on regional climate, information about their regional distribution (Figure 3.7) has also to be included (see Chapter 6).

A particular effect on cloudiness arises from aircraft flying in the upper troposphere which influence high cloud cover through their emissions of water vapour and of particles that can act as nuclei on which condensation can occur. As we shall see in Chapter 5 (page 111) high cloud provides a blanketing effect on the Earth's surface similar to that of greenhouse gases and therefore leads to positive radiative forcing. Extensive formation of contrails in the upper troposphere by aircraft frequently occurs; an estimate of radiative forcing from this cause is included in Figure 3.11. Persistent contrails also tend to lead to increased overall cloudiness in the region where the contrails have formed. This is called aviation induced cloudiness and is difficult to quantify. Because of these effects of aircraft and also the effect of increased ozone (reduced by methane reduction) mentioned on page 57, the overall greenhouse effect of aircraft has been estimated as the equivalent of two or possibly up to four times the effect of their carbon dioxide emissions.20

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