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Air Pollution Climate Change

Convective clouds: from aerosol filter to smoking machine

Picture of Ran Korber
By Ran Korber on January, 4 2015

We can say that there are two kinds of air pollutants:

1) Gas phase pollutants

2) Solid phase pollutants.

Some of the solid phase pollutants (for example, soot, black carbon and dust) can interact with water vapor and they serve as Cloud Condensation Nuclei (CCN), that means that these particles won't follow the air movement in the presence of clouds. Enter your address to check outdoor air quality levels below.

A recent research from Nir BenMoshe lab's at Princeton University that was just presented in the last AGU meeting in San Fransisco, suggests that this interaction is very important for the CCN distribution because otherwise one can assume that convective clouds (the clouds that form big storms), which sometimes have updrafts of more than 60mph, will carry all the ground CCN aloft.

So yes, beneath of convective storm there will be much less CCN because of the updraft, but not all the CCN will go to the upper levels of the clouds and will be transported with the strong winds up there. The nucleation mechanism that convert CCN to droplets will remove some of the CCNs, some of these droplets will later on become rain drops and wash the CCN to the ground.

Because we know that in free weather (without clouds) the CCN concentration is becoming lower above a height of a mile, we can guess that in convective clouds there will be more CCN than in free air (because of the transport off CCN with the updraft).
But this is true only in polluted areas. In clean areas, because there are only small number of CCN, the precipitation rate and the condensation rate will be low and therefor the relative humidity can reach very high values, as we go up the relative humidity tend to go up because the lower temperatures, but if there are many droplets some of the humidity will condensate on them, so, if we in "clean" clouds the relative humidity goes up, then more CCN will nucleate above cloud base and the number of CCN can be lower than it was before the cloud was formed.

The attached figure show the CCN concentration in polluted cloud (left) and clean cloud (right) done by a computer simulation, look how the concentration inside the clouds are different than outside the clouds.

Air quality Cloud Condensation Nuclei (CCN) CCN concentration in polluted cloud (left) and clean cloud (right) done by a computer simulation, look how the concentration inside the clouds are different than outside the clouds.

 

It seems that the clouds have positive feedback to air pollution, convective clouds in clean environment will clean it even more, and convective clouds in polluted environment will pollute it even more.

About the author: Nir Benmoshe is a Post-Doc fellow at Princeton University. Benmoshe graduated the Hebrew University in Jerusalem (B.Sc. in Physics and Ph.D. in atmospheric sciences) and currently working on cloud-aerosol interaction. In both cloud and global scale.

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