Wildfire smoke does not simply disappear into the air: when it encounters rain, some of its material can return to the ground with the water.
A U.S. team studied this phenomenon on a large scale by combining satellite observations of smoke with rainfall measurements. More than 250 stations from the National Atmospheric Deposition Program provided data on the chemical composition of precipitation. The researchers studied three very different years—2014, 2020, and 2022—across several regions of the United States and Canada.
Wildfire in Europe.
Source: European Wildfires 2025 / ArcGIS StoryMaps
To understand the process, a fire releases gases and tiny particles from vegetation and other burned materials into the atmosphere. When rain passes through a smoky area, the water can capture some of these substances and carry them toward the ground. Scientists call this wet deposition. The dissolved elements then become directly accessible to plants, algae, and other organisms.
The researchers compared weeks that included days with both smoke and rain with other periods. The measured deposition increased for all nine types of ions studied. The sharpest increases involved orthophosphate, a form of phosphorus, at 46%, followed by potassium at 41% and ammonium at 36%. These values nevertheless varied greatly by region.
The phenomenon can have a substantial impact in a year with heavy smoke. In 2022, the stations experienced an average of 21 days combining smoke and rain, compared with four in 2014 and six in 2020. That year, these episodes accounted for approximately 20% to 30% of the annual rain deposition of several measured substances, including ammonium, phosphate, and potassium.
An influx of nutrients can promote growth in an environment that lacks them. Elsewhere, it can instead alter the existing balance. In a lake or waterway already rich in nutrients, an additional influx can in particular promote excessive algae growth.
One might also think that rainfall alone is enough to indicate what each fire transports. Yet the study does not make it possible to attribute the measured deposition to a specific fire.
The geographic differences show why the source of the smoke matters. The type of vegetation burned, fire conditions, the distance traveled, and transformations of the smoke in the atmosphere can change what ultimately reaches the ground. The results therefore mainly indicate that a plume can carry nutrients far beyond the area directly affected by the flames.
The next step will be to trace plumes back to the fires that produced them. The researchers want to determine whether a wildfire, another type of vegetation fire, or different combustion conditions leave distinct signatures in the rain. This will make it possible to better link each measured deposit to its source.