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On February 13 we had elevated fine particle (PM2.5) readings across much of Indiana. The following describes what led to these elevated concentrations.
Occasionally during the winter months, there are periods of time where higher than normal PM2.5 events occur. These are generally wide-spread, regional events. They are driven by a combination of complex meteorological issues and natural and man-made emissions. These emissions may be derived from activities such as point source fuel combustion and automobile exhaust to nitrogen released from the soil. In the winter, the primary component of PM2.5 is nitrates. Wintertime nitrate-driven events typically occur during periods of inversions in the lower levels of the atmosphere.
Typically, when we discuss an inversion, we are talking about a temperature inversion. This is where air temperature at higher altitudes increases, which is opposite of normal, where the warmer air is closest to the ground. During an inversion, warm, less dense air will move over the top of cooler, denser air. Normally this occurs as warm fronts move in. When this happens, convection, or vertical mixing of air, can become stagnant up to a certain altitude. This will happen frequently at night during the summer but can happen as a more extreme situation in the winter. Often times, there is a distinctive boundary layer that forms between the warm and cold air masses. The mixing height extends to the boundary layer. There is no mixing of air between the two air masses during an inversion event, as the top of the boundary layer acts as a cap.
During wintertime inversions, a fresh snow cover can create a stronger temperature inversion at night by cooling and decreasing the height of the layer of the atmosphere closer to the surface. When the warmer air mass moves over the colder air, the ice and snow enhances the difference in the temperature between the two air masses. The combination of snow-cooled air at the surface and warm air aloft stabilizes the inversion overnight and can cause the layer of cold air close to the ground to “shrink” down. When it shrinks, there is less air in the bottom layer in which the pollution can mix. This is why PM2.5 concentrations may greatly increase overnight.
As an example, think about mixing Kool-Aid in water. If you mix one packet of Kool-Aid in a large pitcher, you will get normal Kool-Aid. If you mix the same packet of Kool-Aid in a small glass, you will have a much more concentrated form of Kool-Aid. There is not any more Kool-Aid in either container, but because the mixing volume is smaller, it is more concentrated in the cup. The same thing happens with PM2.5. When there is an inversion, the amount of air available to mix the PM2.5 is less, so the concentrations are higher, even though there may be no additional emissions. On a sunny day, the surface warming will break down the inversion and increase the mixing height, decreasing PM2.5 concentrations later in the day. This is also often accompanied by stronger winds which further increases dispersion of pollutants.
Snowfall and melting can further complicate issues in the winter. Snow will sometimes form around nitrogen particles in the air. When the snow falls, it will clean the nitrogen out of the air and bring it to the ground. However, when the snow melts, some of the snow sublimates (evaporates), which will then release the nitrogen back into the air. The amount of nitrogen released is not well documented by science at this point, but there is a lot of evidence that it occurs. As a result, the nitrogen can react in the air to form nitrates that could form PM2.5.
The atmospheric science surrounding these winter time events is complex and sometimes hard to predict. When they occur, however, it is not unusual to see PM2.5 concentrations rise. If the air mass is very stable, then an inversion can last for days and cause very high concentrations of PM2.5 to be observed.
Thanks to Ken Ritter, Mark Derf, Mark Neyman, Brian Wolff and Scott Deloney for putting this information together.