christina j williamson

One of the things I’m most interested in in the the data we’re collecting on the ATom mission is new particle formation in the atmosphere. This is a process, also called nucleation, is where molecules in the gas phase come together under the right conditions to form liquid or solid particles. It’s been observed all over the place, from polluted cities, to very clean arctic conditions, to above forests and oceans, pretty much everywhere we’ve looked.

Compared to all the particles out there that come from primary sources (meaning they were solid or liquid particles before being lofted into the atmosphere) like sea spray, desert dust and particles from biomass burning, these newly formed particles are very small and so have a less direct effect on radiation and clouds. So why am I so interested in them?

We only started looking into atmospheric new particle formation as a scientific community relatively recently as its importance for climate and air quality has come to light. It is quite a difficult phenomenon to measure and characterize. For these reasons, there is still much that we don’t understand about it and this makes it hard to put it into our climate models correctly. The effect itself is quite small, but because it’s one of the least-well-understood parts of our models, it punches above its weight in the amount of uncertainty it accounts for.

Under the right conditions, newly formed particles can grow to sizes large enough to scatter sunlight or act as cloud condensation nuclei (the seeds needed for a cloud to form). These conditions involve having enough condensible vapors present to grow the particle and, crucially, having not too many larger particles around to eat up the little ones before they have a chance to grow. If you imagine a lot of different sized particles bobbing around in a parcel of air, some of them collide, and sometimes, when they collide, they stick together and become one particle. We call this process coagulation.

Now, in general, we think there are fewer particles in the air today than there used to be in the pre-industrial era. That’s because a lot of the particles we put in the atmosphere today come from man-made pollution, as do a lot of the gases that condense onto them and make them grow. So, newly formed particles in the atmosphere may have played a bigger role back then than they do today because they had a better chance of growing before colliding with something that was just going to eat them up.

It’s this combination of needing to reduce the uncertainty the aerosol particles cause in our climate models, along with the idea that their effect on climate may have been larger in pre-industrial times than it is today that makes understanding atmospheric new particle formation so compelling to me. That and pure curiosity to seek out exactly what is causing this rather mysterious phenomenon.