CLOUD is an experiment at CERN. We put the atmosphere in a 
large chamber, a very special chamber 
and we can recreate the conditions exactly in the atmosphere
 anywhere in the world. 
The processes we’re looking at, namely aerosols, which form 
the seeds for clouds, may be affected by cosmic rays, 
so what CERN provides is a beautiful adjustable source of 
cosmic rays from the CERN PS.
We can turn the intensity upon the PS and see if we are 
making more cloud-forming particles in our CLOUD Chamber.
It is a cloud chamber, not a conventional cloud chamber as 
we’re used to in particle physics; 
we can actually pressurize the chamber and do a pressure 
reduction in the chamber
exactly as what would happen with a parcel of air in the 
atmosphere:
as it rises, the pressure goes down; the parcel expands, it 
cools down and eventually a cloud forms,
when it reaches 100% humidity.
We can recreate that in CLOUD and we do that. Occasionally 
we dedicate a complete run, like the last run that we did, 
where we make clouds inside the CLOUD chamber, but 80% or 
more of our work has been to look at the formation of 
aerosols so far and the growth of these aerosols, which are 
the seeds for cloud droplets.
Now we’ve been studying anthropogenic pollution in cities 
and we found a new mechanism that wasn’t known before,
which we think is very important for forming winter smog 
episodes in polluted open environments.
 What it is, is two vapors, nitric acid and ammonia, these 
are quite light vapors, so called volatile and they are 
 continuously exchanging in the atmosphere. Previously it 
was thought they had just a passive role, 
adding a bit of mass to smog, but not really driving the 
processes. 
We found they’re actually key players and they’re helping 
new particles form in highly polluted environments. 
So when there’s a winter smog episode, we’re seeing new 
particles continuously forming 
and these are making the cloud thicker and more opaque.
it was not understood how these particles could form, 
because as soon as a small particle forms
in a very polluted environment , it will collide with 
pre-existing particles and basically be removed
from creating new particles. This new mechanism is allowing 
them to grow
there are two very important vapors, one of which has been 
controlled previously.
Nitric acid is produced by the so-called NOX and the 
emission of that is controlled from vehicles. 
But the other one, ammonia, is not controlled and ammonia is
 just as important. And in fact, it is growing in cities 
because all the latest catalytic converters on gasoline and 
diesel vehicles are actually creating ammonia 
in the catalytic converter and throwing it out in the 
atmosphere and it is not controlled. 
It will need to be controlled in the future.
Those ammonia mixtures have to be very very carefully 
optimized so that we don’t emit an excess of ammonia.
This result we’re just publishing now is a very important 
one for human health and urban pollution 
It's highlighted the importance of controlling ammonia 
inside urban environments. 
it is very rewarding to work on something that has direct 
interest and importance for society. 
I think the skills we’ve learned in particle physics about 
cooperating and pulling resources 
have been incredibly important in the success of CLOUD
It’s the CERN model conveyed to another discipline, the CERN
 culture, and its working extremely well.
