View over Greenland from the NASA DC8 during ATom, 2018
In 2020 I was looking through some of the aerosol data I collected on the Atmospheric Tomography Mission (ATom) (https://csl.noaa.gov/projects/atom/) and noticed something weird. In a layer of the atmosphere called the lowermost stratosphere, we always saw a lot more small particles in the Northern Hemisphere than in the Southern Hemisphere. I hadn’t been looking out for this, it just jumped out at me from the data when I was looking for something different, but it was there, and it was such a big contrast I couldn’t help but investigate further.
ATom was a set of research flights we did between 2016 and 2018 on the NASA DC8 aircraft. We flew almost pole-to-pole over the middle of the Pacific and Atlantic Oceans, and as we flew we constantly ascended and descended between just a few 100 m above sea level and about 13 km altitude.
We generally expect to see more aerosols in the northern hemisphere in the troposphere, the lower part of the atmosphere, because it is directly affected by emissions from urban areas and industry, which are heavily concentrated in the northern hemisphere. The stratosphere, the layer above the troposphere, is generally less affected by these sources. On ATom flights, we got into the lowermost stratosphere at mid and high latitudes in both hemispheres, once we were flying above around 10 km altitude.
When I investigated the data, the extra particles we saw in the northern hemisphere lowermost stratosphere were very small, mostly between about 3 and 20 nm in diameter. Because of their small size, these particles are most often produced by a process called new particle formation, or aerosol nucleation, which is when, under the right conditions, specific gases in the atmosphere convert to liquid or solid droplets. We had data to show that one of the gases that is most often responsible for new particle formation, sulphur dioxide, which produces sulphuric acid, was present in much higher concentrations in the lowermost stratosphere in the northern hemisphere than in the southern hemisphere. We also conducted some modeling and thermodynamic calculations to test if it was possible for these particles to be forming where we observed then – and it turns out it was!
The next question of course was where was the sulphur dioxide in this part of the stratosphere coming from, and why was there so much more of it in the north? We looked at the spatial and seasonal pattern in our data (on ATom we took data in all four seasons, and made lots of vertical profiles) and had to conclude that both the small particles, and the gases that could be forming them, were not being transported up from the troposphere, nor down from higher up in the stratosphere. This meant that there had to be a source within the lowermost stratosphere itself. With the stratosphere being so far removed from the surface of the earth, the list of possible sources was helpfully pretty short.
Monsoon cycles can transport material very quickly from the earth’s surface to the stratosphere, but these have a strong seasonal pattern which did no match our data. Volcanoes and a type of convective cloud that forms over large fires, called Pyrocumulus, can also deposit particles and gases into the lowermost stratosphere, but our tracers of these phenomena and record of emissions during the ATom flights did not match our aerosol observations
Aircraft emit both sulphur dioxide. Scientists compile and update inventories of all different sorts of emissions, including aircraft sulfur dioxide, so I was able to use one of these to look at where in the atmosphere this is emitted and how much of it. Current flight paths are heavily concentrated in the northern hemisphere, and cruise altitude of most commercial aircraft is high enough to be in the lowermost stratosphere at mid and high latitudes. I ran some calculations and compared locations of our data with the emissions, and the sulphur dioxide from aircraft emissions closely matched both the locations and amounts we observed.
We cannot say definitively that aircraft are causing this phenomenon, we don’t have the “smoking gun” – something like an established tracer of aircraft emissions correlating well with the relevant gases and particles. But our data and investigations so far show that aircraft emissions may well be responsible for the hemispheric imbalance in particle concentrations. And that’s interesting.
Aerosols in the stratosphere are important for a number of reasons. Firstly, aerosols can provide surfaces for chemical reactions to take place on, including reactions with ozone. The stratosphere is home to the ozone layer, which protects the earth’s surface from UV radiation. Aerosols in the stratosphere also interact with solar radiation, affecting the earth’s energy balance and therefore climate. The particles we focused on in this study are themselves so small that their own impact on these phenomena is almost negligibly small. However, they have the potential to grow to larger sizes and this could have more of an impact. That is something we need to investigate further. Aircraft emissions are set to increase in coming years, and increased rocket launches will add to those, so this is something we need to be aware of.
We recently published a research paper on this which is available for anyone to read in the journal Atmospheric Chemistry and Physics There have also been a number of popular science articles on this work including from Climate Wire and NOAA.