What Do My Measurements Mean?

By Bonne Ford

So, the AMOD sampler is set up in your backyard, and you’ve logged on to the website and started seeing your data. What does the data tell you?

Remember, the instrument is taking different air-quality measurements. It measures small particles (“particulate matter” [PM]) down here at the surface where you breathe and it measures the particles in the column of the atmosphere above you (this value is often called “aerosol optical depth” [AOD]). When these particle values go up, it means that air quality has gone down. These two values will change throughout the day, and the values in your backyard might differ  from those in a backyard just a few blocks away. There are a lot of different sources of particles in the air, both natural (like dust or sea salt) and from human activities (like from vehicle or power plant emissions).

There are many reasons why air quality varies in time and place. For example, we often see higher values during rush hour, because more cars are on the road (emission sources). So, your backyard air quality might be slightly worse than your neighbors’ if you live closer to a busy street. You could also see a spike in particle mass if someone drives an old pickup by your house, you use a gas-powered lawn mower,  or if you like to grill in your backyard.    The weather can also affect air quality. Rain can wash out particles. Wind can stir up particles. Cold, calm nights can allow particles to just hang around and build up.  

Sometimes the two measurements (AOD and PM2.5) will show similar changes, and sometimes one value will change more than the other. This difference is mostly because particle concentrations can change with height. While there are a lot of emission sources at the surface, particles can also come from higher up (for example, volcanoes, big forest fires, or smoke stacks). Particles can also be lifted up into the air by the wind. If particles are higher up in the air, you might see your AOD reading go up while your PM2.5 reading stayed the same. If particles are all near the surface, you might see more change in PM2.5 and not much change in AOD.