Effect of aerosols on atmosphere (Rainfall)

Q. "Aerosols- whether they have a warming effect or a cooling effect in the atmosphere? In some cases, they've been linked with warming effect (Eg- Rainfall increase, Glacial melting, Black carbon etc) whereas in other cases they're seen as source of cooling by changing the properties of cloud, thereby increasing the reflectivity.

Please clarify between both phenomena and climatological effects of Aerosols."

http://m.earthobservatory.nasa.gov/Features/Aerosols/page3.php

"Black carbon....“semi-direct effect,” turns clouds into a smoky haze that suppresses precipitation..."

it says higher evaporation will follow due to black carbon (soot) absorbign more heat. I'd expect more rainfall on account of cloud formation. But, it gives a twist and says "Black carbon turns clouds into a smoky haze that suppresses precipitation".

How do the droplets become big?

Drops of water become big by condensing or fusing with each other.

Some catalytic items like salt encourage condensation and that results in bigger droplets

The more the number of droplets the greater the chance of fusion

If black soot results in evaporation of some of the droplets, the cloud turns into haze

So cloud formation doesn't automatically mean more chances of precipitation, right?

Global warming - higher evaporation from earth surface

Black soot - higher evaporation of the cloud itself

Rain is phase change for water. From gaseous state to liquid.Change of phase is controlled by many factors like temperature pressure etc. One important component is the purity of the matter itself.

Like if the water is impure it boils at above 100 degrees.

We can add an 'impurity' to play with the temperature at which the phase changes.

In case of the carbon soot it lowers the temperature at which water vapour solidfies to form water droplets. That means you need much lower temperature to make it water and fall down as rain. Else the cloud won't become water and it will instead be vapour only!!!

Precipitation will be less with more impurity in the cloud

Except water vapour, a green house gas, every other GHG would be considered a pollutant

That it's because more evaporation and cloud

All need not affect the phase change!

Indeed only those who disturb the molecular structure do that: eg carbon!! It's about solutions. Aerosol is a solution. Other may form mixture and not solution!! So all GHG need not increase the phase transition temperature of vapour to water!! Water will boil at 100 leaving sand behind!!! Then try boiling sugar and water. It will go above 100!

More rainfall can come by: more clouds and by more precipitation of clouds. All GHG lead to more clouds as they increase temperature of the globe. But all such clouds need not ppt due to the carbon soot pushing the ppt temp up.

So cloud formation doesn't automatically mean more chances of precipitation, right?

Correct

But more cloud more ppt, unless such carbon plays spoil sport

Yes, meaning carbon soot will add impurity thereby maintaining the gaseous state of cloud at higher temperatures? Thus avoiding precipitation.

Consider this:

If you imagine good pedagogy equivalent to GHG, then better pedagogy will lead to more intellectuals.

But intellectuals will lecture others only they are alone and presence of girlfriend increases their effort required to share their knowledge to students.

Now, carbon soot is the gf.

So good pedagogy will lead to more intellectuals, but all med to share knowledge by lecturing if they have girlfriends/boyfriends.

You know why carbon increaes the precipitation temp of cloud than any other aerosol

If you read the mechanism of phase transfer and effect of heat transfer involved, you will know why carbon increases the ppt temperature!

It need not be. It depends on many things. Heat capacity, molecular structure, humidity, concentration etc

As on day scientists know that carbon does this.

And salt and sulphate don't do.

The known GHGs, while all of them increase the evaporation rate thus cloud formation in general, carbon reduces precipitation due to its property of increasing the precipitation temperature.

"Due to its high heat capacity".