Our first paper on the science from the ATom mission came out in Nature today! You can read the full manuscript here, or a shorter press release about it here. In this post I’ll tell you a little about what we discovered and how it came about.
On ATom we flew these amazing flight paths from the arctic to the antarctic down the middle of the Pacific and up the middle of the Atlantic. As we flew we fairly-constantly ascended and descended between just a few hundred meters above the ocean, and about 12 km altitude. When we reached the tropics we kept noticing that, as we got to high altitude we saw lots and lots of very small particles. Because of their small size, we knew that these particles had come from a process called gas-to-particle-conversion or new particle formation, whereby gases in the atmosphere make small liquid or solid particles.
We weren’t entirely surprised to see this signature of new particle formation high up in the tropics, as other scientists had noticed this in different locations over the Pacific before. However, it was very interesting for us to see how consistently this appeared every time we were high altitude over the Pacific. A week or so later, when we hit the tropics again, this time traveling North up the Atlantic, exactly the same thing happened, and our signal for small particles would go very high at the high altitudes. Each of the four ATom deployments (the same route but each flown in a different season) showed the same pattern.
Back on the ground we started delving into the data to see what we could learn about these small particles forming high up in the tropics. We found that the gases that form these particles seem to come from deep convective clouds. These clouds loft air from just above the ocean to high altitudes very quickly.
We were interested in the fate of these particles. They are so small when they form that they don’t really have any effect on climate because they’re too small to scatter the sunlight or to act as seeds for cloud droplets. In the tropics, once air exists those deep convective clouds, it tends to descend slowly back towards the earth’s surface. We saw evidence for the particles growing in size in this descending air (this happens when more gases condense onto the surface of the particles, or when two small particles collide and stick together making one larger particle). In-fact they grew so large that by the time the air got down to the lowest layers above the ocean, the particles were big enough to act as seeds for cloud droplets to form on (we call these cloud-condensation nuclei).
The number of seed-particles present in a cloud determines the number of droplets that make up the cloud. Generally, if there are more droplets in a cloud, the cloud is brighter and reflects more sunlight. The amount of sunlight reflected by clouds is an important part of what we call the earth’s energy balance – that is the balance between the amount energy entering our earth-atmosphere system from the sun and the amount of energy being radiated back to space. Knowing the pieces that make up the energy balance is crucial for modeling and predicting climate.
Some colleagues of ours who work with climate models looked at what their models were seeing in this tropical region and we compared it to our observations. Together, we saw that, in general, the models were not seeing as many of the larger particles that influence the clouds (the ones that had grown from the tiny particles formed up high) as we had observed. This means that their clouds reflected less sunlight than our observations suggest they should. We found a number of factors causing this, most noticeably a mathematical issue to do with how particles are removed by clouds in the model being slightly different from how it’s happening in reality.
Improving the way climate models represent these particles is just one small correction in a very large and complex system of modeling and predicting climate. It is this process of observing the atmosphere, understanding what those observations are telling us, and then learning how to represent this in our models that enables us to predict future climate and understand how our earth-atmosphere system responds to changes.
