You always knew that the exhaust from a car’s tailpipe or the smoke from a smokestack was bad for you and your surrounding environment. But have you ever wondered what happens to it after it is emitted and you don’t see it anymore? In less than a few minutes, it mixes with the air to become invisible, which makes it very hard to track. To track air pollution, scientists have built computer models that tell us how pollution from a polluting source (say, a power plant) is transported and transformed in our atmosphere and more importantly, who will be affected and to what extent. One could argue that it doesn’t take a computer model to tell us that you are breathing bad air if you live besides a highway but you definitely need one if you want to know if a power plant hundreds of miles away is responsible for your next asthma attack.
Models are only as good as our current understanding of physics and chemistry. Although we know a lot about how much our polluting sources emit (cars, trucks, ships, power plants, refineries), we know little to nothing about their transformation, particularly when it comes to organic fine particles. Organic is a fancy word for anything that has carbon and hydrogen as its primary ingredients and fine particles are tiny invisible pollutant particles that are few thousand times smaller than the period at the end of this sentence. Transformation in the atmosphere is very important for organic particles; it probably endures the same fate as any organic matter left exposed to the elements. Think of the apple slice that turns brown when left untouched.
In the work that we write about in the journal Atmospheric Chemistry and Physics, we make a smart computer model even smarter by incorporating two recent discoveries about the sources and transformation of organic particles. The first addition we made was to add a source of fine particles that we had completely missed earlier (oops!) while the second addition was to incorporate the transformation that I mentioned earlier (appleturningbrown); until a few years ago we knew that the apple turned brown but we were not sure how it exactly happened. Although Wwe had made the model smarter, we had to test if that was true; this is usually done by comparing predictions from the model to real-world measurements. We found that our revised model predicted the amount of organic particle mass reasonably well at over 200 different locations in the US over the summer months. In the winter, our model predicted too little, meaning that our models could be missing a polluting source that is only present in the winter (may be residential wood heaters). We also showed, based on our model, that most of the organic particles in our atmosphere had a lot of oxygen in them, which agrees with our observations; the previous generation of models told us that there wasn’t a lot of oxygen.
Overall, we make a modest improvement over the previous generation of models in predicting organic particles in the atmosphere across the globe. This is an important contribution since organic particles account for about half of the fine particle mass in the atmosphere, which are believed to result in a million deaths a year globally. Our model could help inform the next generation of regulations and policies that will reduce organic fine particles and thereby clean our air.