Seeding Ice Clouds with Wildfire Emissions

Cloud formation is a complex process that varies depending on temperature and atmospheric dynamics. Ice-containing clouds (e.g., cumulonimbus, cirrus), which dominate continental precipitation, often depend on floating particulate matter to help kick off the crystallization process in the troposphere. Such ice nucleating particles (INPs) range from bacteria and bits of organic matter to mineral fragments and can influence cloud radiative properties and precipitation.

Because wildfires generate enormous amounts of particulate matter, they may considerably influence local cloud dynamics. Barry et al. sampled INPs from smoke plumes during the 2018 wildfire season in the western United States, the first measurements made at heights where smoke particles can directly affect cloud formation. Their results show that overall, INPs increase in quantity by up to 2 orders of magnitude in smoke plumes compared with background air. However, the specific types of particles and the exact degree to which they increased depended heavily on the conditions of a given fire (e.g., location and vegetation burned) and even how hot the fire was. For all samples, though, the INPs were dominated by organic material.

Electron microscopy also revealed that tiny spherical tar balls accounted for nearly a quarter of total INPs in certain conditions. The overall contribution of these tar balls to wildfire-derived INPs likely also depends on fuel and fire type and is “an open question,” according to the researchers.

Wildfires are predicted to become more common with ongoing climate change, so understanding interplays between fires and the broader climate will become increasingly vital, the authors say. The new results confirm that wildfires generate abundant INPs in the troposphere, potentially modifying cloud formation and precipitation. But, the researchers note, more modeling and sampling studies are needed to understand in detail how fuel and combustion conditions contribute to regional INP concentrations and thus the range of ways that wildfires can affect the climate system at large. (Journal of Geophysical Research: Atmospheres, https://doi.org/10.1029/2020JD033752, 2021)

—David Shultz, Science Writer