Wandering dust, the seed of rain – the international journey of an aerosol particle

Saudi Arabia; a storm in May. The column of air, water, and ice towers over the sand dunes. As the hail falls, it melts to form rain, cooling the air. As the rain continues to fall, most of it evaporates in the drier air nearer the ground, cooling the air still further. And as the cool dense air sinks, it spreads out rapidly as it approaches the surface. A few kilometres away, a Nubian ibex, a goat sporting impressive horns, feels the gust of air. Sand is blown along the surface of the desert. Some grains are lifted up, airborne for a few metres, before crashing back to the surface, knocking yet more sand and dust into the air. As the process continues, fuelled by the gusts of cool air, a wall of dust begins to form. The “haboob” dust storm makes its way across the arid landscape, coating plants, roads, and buildings in a layer of dust. As the haboob closes in on the Gulf of Oman, most of the larger grains of sand are deposited. But some of the finer dust particles, the size of bacteria, have been lifted up to a height of five kilometres and are blown across the Arabian Sea to Pakistan and northern India.

In northern India, a family sits around a wood-burning stove. Inside their home, the air is heavy with black carbon soot. Much of this smoke escapes into the air outside, adding to the black carbon aerosol released from a nearby coal-fired power station. The power station adds yet another chemical to the noxious airborne soup: sulphur dioxide. Over the next few days, much of this sulphur dioxide gas is oxidized to sulphuric acid, forming small liquid particles of sulphate aerosol. Some of the sulphate coats the particles of black carbon and the dust that has been blown across the Arabian Sea.

When the sun rises, the family observes that it is yet another hazy day, a common sight during the pre-monsoon season. The milky mixture of aerosol reduces the amount of sunlight reaching the ground. Much of the sunlight is scattered back to space by the sulphate and dust. However, some of the sunlight is absorbed by the dust and black carbon, warming the atmosphere, inhibiting the formation of clouds. But the inhibition of cloud formation is only temporary: the monsoon storms are on their way, and the atmospheric heating may even hasten their arrival.

Over the course of the next few days, energy builds up gradually in the warm moist air near the ground. Finally, one sunny afternoon, a parcel of air gains enough energy to rise upwards. As it rises and cools, water condenses on many of the sulphate particles, diluting the sulphate to form cloud droplets. As the water condenses, heat is released and the air parcel rises even further. After a few minutes, the top of the cloud is so high that the air temperature is below 0°C. But the cloud droplets do not freeze immediately. The water molecules do not have a surface to freeze onto. Even when the “supercooled” droplets have risen high enough to cool below -15°C, most of the droplets remain unfrozen. But some of the cloud droplets have grown on dust that has been coated by sulphate. And some of these dust particles act as the seed, or “ice nucleus”, on which ice can grow. As the air rises and cools even further, the ice crystals grow rapidly at the expense of the liquid droplets. Some of the ice crystals collide with other ice crystals and liquid droplets, accelerating their growth. When they grow large enough, their downward terminal velocity becomes faster than the upward movement of the air, and they fall through the cloud, collecting yet more water on the way, growing to form hailstones. By the time the hail reaches the ground, most of it has melted to form rain.

The family recognizes the sound of thunder. They feel a gust of cool air. The monsoon rains have finally come.

[Benjamin Grandey, June 2015]

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