Why We Can't Just Ignore Secondary Organic Aerosols

Often when air quality is studied, if a substantial amount of organic aerosols is measured, it is attributed to nature. Organic aerosols are a natural emission from vegetation, and so in public policy it is considered a constant. However, secondary organic aerosols (SOAs) aren't completely natural--they may have an anthropogenic part. This point is exactly highlighted by Carlton et. al in their study of SOAs, using a model to understand what portion of SOAs have anthropogenic parts. These SOAs can then theoretically be controlled, if their anthropogenic parts didn't exist in the atmosphere. The following diagram shows the various ways SOAs can form, and the degree to which they can be controlled.

We can see from the map above that it isn't simple how SOAs are formed, and that there are clear pathways that end up in SOAs. But this chart doesn't show the actual relative amounts of SOAs that are controllable, which was the aim of the study. The results are summarized below:

The graph above is the average concentration (top) and fraction (bottom) of biogenic SOAs from controllable emissions. The model was for August to September. The bottom figure shows that in general about up to 50% of SOAs are controllable. That is a very substantial amount and suggests that we can actually control these species. As it turns out, a lot of species like SOx, NOx, NH3 etc. end up forming SOAs by combining with other primary organic aerosols. Thus, without the emissions of the aforementioned species, there can be a great decrease in SOA concentration.

A good question to ask is--why do we care if SOAs are in the atmosphere? Well SOAs can be harmful to humans. While there is still need for more research on  SOAs, it is relatively accepted that SOAs can cause health problems. Thus, if we do not start looking at how humans are contributing to their presence in the atmosphere, true safe air quality cannot be achieved.