The western United States is getting hotter and drier, boosting concerns about wildfire emissions. Wildfire plumes consist of tiny particles known as aerosols, which are emitted when organic matter such as brush or trees burns. Once in the atmosphere, aerosols can impact sunlight absorption, weather, and public health.
To better understand these effects, researchers are interested in the composition, structure, and evolution of wildfire aerosol plumes. Until now, most knowledge of these plumes has come from satellites and other remote data. But in a new study, Mardi et al. illuminated the properties of plumes using data from aircraft that flew straight through them.
For the new study, the research team analyzed data collected aboard a research aircraft in coastal California during the summers of 2013 and 2016. In 81 out of 231 total sounding measurements, the airplane flew straight through aerosol plumes of several wildfires, including the Douglas Complex fires in southern Oregon and the costly Soberanes fire near Big Sur in California.
During each flight, the scientists captured atmospheric properties using a variety of instruments mounted on the airplane. These included a spectrometer to measure the amount and proportion of aerosol particles of different sizes and another spectrometer to analyze the same properties for cloud droplets as the plane passed through layers of cloud and smoke. The research team also studied aerosol data collected by two NASA aircraft.
The aircraft data revealed a variety of characteristics of the wildfire plumes. After rising vertically, most of the observed plumes began moving horizontally through the atmosphere in single layers, but some stratified into multiple horizontal layers, especially over the ocean. Most of the plume layers were above the boundary layer, a layer of the atmosphere that lies closest to Earth’s surface. This finding contrasts with studies from some other regions that suggested plume layers are found within the boundary layer in close proximity to the shore. Regional and case differences arise because of factors such as fuel type, fire intensity, and atmospheric conditions.
The researchers also found that wildfire plume layers contained a higher concentration of larger aerosol particles, and plume layers at lower altitudes above the boundary layer had higher levels of these larger aerosols. Clouds seemed to enhance the atmospheric heating effects of plume layers located above them.
These findings could help inform future research into wildfire smoke plumes and their effects. The researchers demonstrated this potential by comparing their observations with aerosol distribution patterns predicted by the Navy Aerosol Analysis and Prediction System (NAAPS), a computer model. They found that the model agreed better with their observations when they changed the height at which wildfire smoke begins horizontal movement from a default to an average height based on aircraft data. (Journal of Geophysical Research: Atmospheres, https://doi.org/10.1029/2018JD029134, 2018)
—Sarah Stanley, Freelance Writer