Nighttime Smoke from Fires Continues to Evolve as Humidity Determines Whether It Turns Brown or Breaks Up

Particles released by wildfires do not stop reacting after sunset. A study from the University of Kentucky, published on October 24, 2026, shows that ambient humidity steers the nocturnal chemistry of a key smoke compound, catechol. This finding matters for estimating the impact of plumes on climate and air quality.

Catechol, a phenol released by biomass burning, reacts after sunset with the nitrate radical

The combustion of biomass emits numerous phenolic compounds, among which the catechol, a recognized precursor of secondary organic aerosols. By day, sunlight drives most of the atmospheric oxidation. However, at night, another actor takes over, the nitrate radical, noted NO3.

This radical forms when ozone reacts with nitrogen dioxide, two gases abundant in polluted plumes. By day, sunlight almost immediately destroys it, making it an exemplary nighttime oxidant. Thus, smoke that travels at night continues to transform, far from the fire that produced it.

Md. Sohel Rana and Marcelo Guzman exposed thin films of catechol to relative humidity ranging from 0 to 90 %

The two chemists published their work in Environmental Science & Technology. Their setup involved depositing thin layers of catechol, then exposing them to nitrate radicals generated from ozone and nitrogen dioxide. Additionally, the relative humidity varied from completely dry air to 90 %.

To identify the products, the team combined Fourier-transform infrared spectroscopy, UV–visible spectroscopy and several mass spectrometry techniques, including one coupled to ion chromatography. Consequently, the researchers could track both the formed molecules and their capacity to absorb light.

In dry air, nitrocatechols form brown carbon, while humid air favors dicarboxylic acids

Under dry conditions, oxidation promotes nitration and the assembly of molecules. It yields mono-, di- and trinitrocatechols, accompanied by dimers and trimers of catechol. These products absorb strongly between 300 and 500 nanometers, a signature of brown carbon that traps part of the solar radiation.

As humidity increases, the nitration capacity wanes. At the same time, the aromatic ring opens and yields dicarboxylic acids, including cis,cis-muconic acid, 2,3-dihydroxymalytic acid and oxalic acid. Thus, the chemistry shifts from a functionalization logic to a fragmentation logic.

This chemical shift could modify how models estimate the climate effect of wildfire plumes

Brown carbon locally warms the atmosphere by absorbing light, unlike many lighter aerosols. Consequently, smoke traversing a humid air mass could lose some of its light-absorbing molecules in favor of highly oxidized compounds. The authors emphasize that the water associated with aerosols governs the chemical and optical aging of these particles.

However, these measurements stem from laboratory experiments on a single model compound. They will need to be tested against real plumes, where hundreds of molecules react together. Yet, with the rise of megafires, incorporating nocturnal humidity into air-quality models now appears to be a logical step.