The smallest air pollution particles can increase the intensity of the largest storm clouds

Until now it has been known that aerosol particles need to be larger than one tenth of a micron (100 nm) in order to be able to change the properties of the clouds and rain they create. The particles do this by serving as condensate nuclei for cloud droplets at its base. Therefore, contaminated clouds are composed of smaller droplets that do not easily blend into raindrops, making them more "target hard". Due to their small size, their large numerical concentrations can be generated by emitting relatively small amounts of pollutants at a weight, for example, diesel engines emit very high concentrations of such small particles.

Areas relatively clean of human influences in tropical areas, such as the Amazon, contain small concentrations of particles both large and small. Cloud and aerosol measurements in the Amazon documented the properties of the clouds in the clean air and in the wind down from Manaus - a city of more than two million people that is isolated in the center of the Amazon. It was found that the intensity of the winds rising in the clouds downwind from Manaus was more than doubled precisely when the concentration of particles smaller than 50 nm was highest. Our cloud simulations showed the mechanism of action of the small particles. For comparison, about a decade ago we documented the mechanism of action of particles larger than 100 nm. These particles form multiple and small cloud droplets at the base of the cloud. The cloud droplets are too small to coalesce efficiently into raindrops, so most of the cloud water rises with the vertical winds to heights where they freeze into ice particles. Freezing is accompanied by heat release that increases the intensity of the clouds. As a result the clouds create thunderstorms and hail instead of milder rain with no lightning.

In contrast, if the air has a small concentration of particles that can form cloud droplets at its base, the cloud is formed with a small concentration of large droplets that rush to coalesce with each other and fall as raindrops. As a result the total surface area of ​​the droplets in the cloud is small. The surface area limits the rate of condensation of vapors in cloud water and the relative humidity inside the air rising in the cloud rises well above 100%. Relatively high humidity can cause even the smallest particles to form thickening nuclei of cloud droplets. Therefore, when we add such particles to the cloud, usually as a result of air pollution, the additional droplets that will form in the cloud will cause additional condensation of water resulting in additional cloud water, increased condensate heat in much larger amounts And its precipitation.

This mechanism works mainly in clouds in the tropics and regions of summer rains like in the southeastern United States and the monsoon in India and Southeast Asia.

According to this mechanism, much smaller amounts of air pollution than have been estimated so far are enough to cause very significant changes in rains in large areas of the world. In addition to the fact that the rains are more powerful - but not necessarily more numerous - this could have implications for the global energy scales that are driving the global climate system in an unprecedented way so far.

Links to articles:

•    From the Grapevine, Is man-made pollution making storms more severe?  
•    Eureka, Amazon rainforest provides a unique natural lab to study effects of aerosols
•    Drovers, Tiny Particles Have Outsize Impact on Precipitation
•    Green Car progress, PNNL-led international study finds ultrafine aerosols have outsize impact on storm clouds, precipitation
•    Xihhua, Tiniest emission particles may lead to big storms: study
•    Israel 21C, New multinational research reveals that even small amounts of manmade aerosol particles can wreak havoc.  
•    Jerusalem Online, New US-Israeli study shows how tiny man-emitted particles affect weather, crops