The CERN/CLOUD results are surprisingly interesting…

The long-awaited first paper from the CERN/CLOUD project has just been published in Nature. The paper, by Kirkby et al, describes changes in aerosol nucleation as a function of increasing sulphates, ammonia and ionisation in the CERN-based ‘CLOUD’ chamber. Perhaps surprisingly, the key innovation in this experimental set up is not the presence of the controllable ionisation source (from the Proton Synchrotron accelerator), but rather the state-of-the-art instrumentation of the chamber that has allowed them to see in unprecedented detail what is going on in the aerosol nucleation process (this is according to a couple of aerosol people I’ve spoken about this with).

This paper is actually remarkably free of the over-the-top spin that has accompanied previous papers, and that bodes very well for making actual scientific progress on this topic.

The paper first confirms some results that are well known: aerosol nucleation increases enormously when you add H2SO4 into the air (the biggest contributor to human aerosol impacts via the oxidation of our emissions of SO2), it increases further when you add ammonia (NH3), and it increases even more when you increase ionisation levels from neutral, to ambient ground levels, and to upper atmospheric levels (as long as you are below what is called the ‘ion-pair’ limit). However, the most intriguing result is that despite going to a lot of trouble to make sure the chamber was ultra-free of contaminants, the researchers found that within most of the aerosols that formed, there were traces of organic nitrogen compounds that must have been present in almost undetectably low concentrations. The other intriguing finding is that aerosol nucleation rates in the chamber don’t match (by a an order of magnitude or more) actual formation rates seen in real world near-surface atmospheric layers at realistic temperatures (only in unrealistically cold conditions do rates come close). The authors speculate (quite convincingly) that this is precisely because they didn’t have enough volatile organic compounds (which are ubiquitous in the real world) to help get the nucleation started. This result will surely inspire some of their next experiments. All-in-all this is a treasure trove of results (and potential future results) for people tasked with trying to model or understand aerosol processes in the atmosphere.


Annotated Fig 5 from Kirkby et al

Figure 1: Annotated version of fig 5 in Kirkby et al. Small dots are in situ observations, lines are other lab data. Colours for the CLOUD results are coded with respect to temperature. Going from open to filled symbols denote increasing NH3. All results are for ambient CR ionisation (changes in CR only make a difference below the ion-pair limit).


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