Pollution’s effect on the environment is perhaps the hottest of today’s hot-button issues. One long disputed issue: do aerosol (airborne) particles increase or decrease rainfall? The answer, according to findings by an international team of scientists, headed by Prof. Daniel Rosenfeld of the Institute of Earth Sciences at the Hebrew University of Jerusalem, is that both are true – depending on local environmental conditions.

“Both camps are right”, said Prof. Meinrat O. Andreae, director of the Max Planck Institute for Chemistry in Germany, a co-author of the publication. “But you have to consider how many aerosol particles there are.” Added Rosenfeld, “The amount of aerosols is the critical factor controlling how the energy is distributed in the atmosphere.”

In an article entitled, “Flood or Drought: How Do Aerosols Affect Precipitation?” appearing in the Sept 5 issue of the journal Science, the team proposed a conceptual model that explains the apparent dichotomy, following the energy flow through the atmosphere and the ways it is influenced by aerosol particles. This model allows the development of more exact predictions of how air pollution affects weather, water resources and future climates.

A major source of uncertainty

The role of man-made aerosols has been a major source of uncertainty in understanding of the climate system, including the question of global warming.

“With these results we can finally improve our understanding of aerosol effects on precipitation and climate since the direct contradiction of the different aerosol effects has seriously hindered us from giving more accurate predictions for the future of our climate, and especially for the availability of water,” stated Andreae.

Aerosols serve as cloud condensation nuclei (CCN) and thus have a substantial effect on cloud properties and the initiation of precipitation.

Clouds, and therefore precipitation, come about when moist, warm air rises from ground level and water condenses or freezes on the aerosols aloft.

Large concentrations of human-made aerosols have been reported to both decrease and increase rainfall as a result of their radiative and CCN activities. At one extreme, pristine tropical clouds with low CCN concentrations “rain out” too quickly to mature into long-lived clouds. At the other extreme, heavily polluted clouds evaporate much of their water before precipitation can occur, if they can form at all given the reduced surface heating resulting from the aerosol haze layer.

On one hand, aerosols can act like a sunscreen reducing the amount of sun energy reaching the ground. Accordingly, less water evaporates and the air at ground level stays cooler and drier, with less of a tendency to rise and form clouds.

Gathering points for air humidity

On the other hand, aerosols can act as gathering points for air humidity, so called condensation nuclei. Water condenses on these tiny particles, releasing energy in the process. This is the same energy that was earlier used to evaporate the water from the earth’s surface. The released heat warms the air parcel so that it can rise further, taking the cloud droplets with it.

But if there is a surplus of these gathering points, the droplets never reach the critical mass needed to fall to earth as rain – there is not enough water to share between all the aerosol particles. Also, with a rising number of droplets their overall surface increases, which increases the amount of sunlight reflected back to space and thus cooling and drying the earth.

The study results showed the following: With rising pollution, the amount of precipitation at first rises, reaches a maximum and finally falls off sharply at very high aerosol concentrations. The practical result is that in relatively clean air, adding aerosols up to the amount that releases the maximum of available energy increases precipitation.

Beyond that point, increasing the aerosol load even further lessens precipitation. Therefore, in areas with high atmospheric aerosol content, due to natural or manmade conditions, the continuation or even aggravation of those conditions can lead to lower than normal rainfall or even drought.

“These results have great significance for countries like Israel where rainfall is scarce and can be easily affected by over-production of aerosols,” stated Rosenfeld. “Our study should act as a red light to all of those responsible for controlling the amounts of pollution we release into the atmosphere.”

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