C VgH

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where H is the depth of water.

Fig. 2.26. Schematic of bubble bursting at the sea surface. The lower diagram shows a bubble rising through the water which has just reached the air-sea interface. Milliseconds later, in the top diagram, the bubble has burst, with a jet of water, caused by the pressure differences between the bubble and its environment, throwing water droplets into the air.

2.9.2 Breaking waves and marine aerosols

Waves break at sea, and on impact with the shore. Fig. 2.24 illustrates important facets of the wave breaking process. It shows large numbers of whitecaps, and spray thrown liberally into the air. Whitecaps consist of strongly oxygenated water; breaking waves are an important pathway for atmospheric gases to enter the sea. The spray shows that the breaking waves throw sea water into the air. Some of this is evaporated before falling back into the ocean; more, with its dissolved salts, is carried away by turbulence to contribute to the atmospheric collection of condensation nuclei.

A typical vertical profile of the concentration of dissolved oxygen is shown in Fig. 3.9. The surface maximum is accentuated by breaking waves injecting additional gas into the water. Regions of strong winds, and many breakers, will therefore have enhanced levels of dissolved oxygen in their surface waters. We shall return to the mechanisms of gas transfer across the air-sea interface in Chapter 3.

Many of the bubbles that are injected into the water as a wave breaks, however, rise back to the surface before their gases dissolve. As the bubble breaks through the surface its pressure, being higher than the atmosphere's, causes it to explode, with a jet of water erupting from the base of the bubble, as illustrated in Fig. 2.26. This jet breaks into a collection of droplets, some of which fall back into the water, but some of which are whisked away by the wind. The splashes resulting from the impact of the remnant jet droplets will also add further water droplets, and salt particles, to the air.

Bubbles would seem to be directly linked to wave breaking, and indeed experiments at low wind speeds, using sonar to trace bubble flocks, show discrete bubble events associated with breaking waves. This can be seen when travelling by aeroplane as a burst of white water appearing where, a second or two earlier, a wave has just broken. At higher wind speeds, sufficient wave breaking occurs for large areas to have considerable bubble concentrations to depths of 10 m or more. A background population of bubbles exists, however, even in the absence of breaking. This appears to be because of the longevity of some bubbles within the water. Addition of salt particles to the atmosphere will consequently occur continuously, although the process will be significantly enhanced during periods of strong winds.

Table 2.2. Sources for condensation nuclei over sea and land

Concentration (^gm 3)

Substance

Sources

Land

Ocean

SO42- Sulphate

Oxidation of SO2

1

0.1-1.0

Combustion of fuels, forest fires,

(10 in polluted air)

volcanoes, marine biology, sea

spray, airborne soil particles

NH4+ Ammonium

Combustion, decay of organic

0.1-1.0

0.01

matter in soils

(10 in polluted air)

NO3- Nitrate

Industrial processes, automobiles,

<1

0.1

sea spray

(1 in polluted air)

NaCl Sodium

Sea-salt particles, some soil

<1

1-10

chloride

particles

Organic carbon

Combustion, automobiles

1

0.1-1.0

The salt particles injected into the atmosphere through bubble bursting, splashes from breaking waves and water droplets torn by the wind from the sea surface provide the majority, by mass, of the condensation nuclei over maritime areas, and a significant proportion over land. Condensation nuclei are crucial to cloud formation and are considered next.

2.9.3 Condensation nuclei

In the previous references to cloud formation (§§1.7, 2.2.1) we have implied that clouds form due to condensation of atmospheric water vapour into droplets. This is correct, but if the condensation is unaided by some initiating mechanism then supersaturations of 400% can occur before the spontaneous appearance of droplets. The atmosphere, however, is never completely clean: there are always particles, or aerosols, present. These act as nuclei for condensation, allowing droplets to form with only very small supersaturations of less than 1%. Some nuclei are hygroscopic, which means that water molecules tend to stick to their surfaces at humidities below saturation, in some cases as much as 15% below. Anyone who has left a salt-cellar exposed to the air will know that salt strongly attracts atmospheric water vapour! Such particles are thus of major significance for aiding cloud formation and rainfall, particularly for air temperatures above freezing. Typical sources for cloud nuclei are shown in Table 2.2.

The formation of ice in clouds, characteristic of high clouds such as cirrus, or the upper reaches of vigorous cumulus clouds or the cumulonimbus associated with thunderstorms, is also dependent on aerosols. Drops of water of cloud droplet size - a few micrometres - will not spontaneously freeze until the temperature reaches -40°C. Ice condensation nuclei allow ice crystals to form at temperatures only a few degrees below 0°C. Clay particles are better nuclei than sea salt for ice crystal formation, and hence snow production. Silver iodide is another material that aids ice formation; it was used extensively in the 1950s and 1960s when cloud seeding experiments were performed in the (essentially unrealized) hope of aiding rainfall initiation.

In the maritime environment sea salt is the major aerosol by mass; over land it is important but other particles have similar, or greater, concentrations in the lower atmosphere.5 The fact that the oceans cover 70% of the surface area of the globe means that production of sea salt particles, as discussed in the previous section, is of considerable climatic impact. Alteration to marine tropospheric circulation, modifying the wave regime of the ocean, and therefore the distribution and quantity of wave breaking, may feedback on climate by ultimately influencing cloud formation, rainfall and the Earth's radiative properties. This will be pursued further in Chapters 3,4, 6 and 7.

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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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