I went to grad school to study how aerosols affect climate. I enjoyed atmospheric physics a lot during my undergraduate degree and I wanted to do research that could help solve big problems being faced by humanity. Climate change is one of those problems, and we need to reduce the uncertainty in our predictions of how climate will change to make it easier for individuals and policy makers to plan for the future. The effects of aerosols and clouds are some of the least well understood parts of the climate picture, so this seemed like an area where I could make a useful contribution.
I joined a group working on the CLOUD (Cosmics Leaving OUtdoor Droplets) experiment, a large chamber based at CERN, in which we simulated different atmospheric conditions to study how aerosol particles (tiny solid or liquid drops suspended in the air) form and grow. These aerosols can become the seeds on which clouds form, and cloud reflect sunlight, which cools the planet. So more aerosols leads to more clouds leads to cooler climate.
When I joined the CLOUD experiment they had just finished looking at the most simple chemical system for making aerosol particles – sulphuric acid and water. Sulfuric acid in the atmosphere is mainly made from the sulfur dioxide emitted by burning fossil fuels and biomass, but some comes from natural sources like volcanic eruptions and certain gases emitted from the oceans. It was found that this process didn’t really occur in many places in our atmosphere (Kirkby et al. 2011).
Then we started adding in extra gases found in the atmosphere that are also involved in making aerosols. We found that ammonia and amines (mostly produced by animal farming) make aerosols form and grow many times faster than just sulphuric acid and water (Kirkby et al. 2011). When we put our experimental results in global climate models we saw that this process was responsible for a lot of the particles formed in our atmosphere (Dunne et al. 2016).
The we added an organic molecule, alpha-pinene, which is emitted by trees (and very similar to lots of other organic molecules emitted by plants generally) to sulphuric acid and water, and saw that it made the particles form and grow faster (Riccobono et al. 2012, Tröstl et al, 2016), and could explain some of the particles formed in the atmosphere. We were also surprised to see that the organic molecule was able to make particles without any sulphuric acid (Kirkby et al. 2016). This was very interesting because we had previously thought that sulphuric acid was always necessary to make aerosols.
Since we need aerosols to make clouds, and to most sulphuric acid comes from human activities, we thought that the pre-industrial atmosphere was much less cloudy than the present-day atmosphere. This new, natural way of making aerosols, means that actually the pre-industrial atmosphere was more cloudy than we used to think (Gordon et al. 2016). So this result changes how we model the pre-industrial atmosphere and this affects how much we predict temperatures are going to change in the future. By including this new way of making atmospheric aerosols in models, we predicted that global temperatures are actually changing a little slower than we thought. It’s a very tiny correction, so it doesn’t change the big picture of climate change and global warming, but it’s important for the atmospheric science community because we need our temperature predictions to be incredibly precise so that we can predict changes long into the future. Results like these make those predictions more precise.
Having seen how important organics were for making aerosols in the chamber, we wanted to check that it was really happening in the atmosphere. We took our instruments up to the top of a mountain, where we could measure air high enough up that it wasn’t being strongly influenced by stuff happening locally on the ground. Here we saw organics taking part in forming and growing aerosol particles in the air, just like we had in the chamber (Bianchi et al. 2016).
So my PhD was all about discovering different ways the aerosol particles can be formed in the atmosphere, and how these affect climate. This was done as part of a big collaboration, and, as you will see from the papers I’ve linked, each discovery is the work of many scientists working together.
