This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: What happens to smoke particles after sunset can depend strongly on something as familiar as the amount of water in the air. Sohel Rana and Marcelo I.
Guzman at the University of Kentucky shows that relative humidity can redirect the chemical pathways involved in the nighttime oxidation of catechol, a phenolic compound associated with biomass burning. Published in Environmental Science & Technology, the study, "Relative Humidity Modulates Nitration and Ring Fragmentation during Heterogeneous Oxidation of Catechol by Nitrate Radicals," examines how catechol reacts with nitrate radicals (NO₃), important nighttime atmospheric oxidants, as conditions change from dry to increasingly humid air. The article is part of a broader research effort examining how nighttime chemistry transforms phenolic compounds associated with biomass-burning emissions.
The findings point to relative humidity as an important control on the direction that this nighttime chemistry takes. Under dry conditions, nitrate-radical oxidation favors nitration and oligomerization. Related work by Rana and Guzman in ACS Earth and Space Chemistry identified nitrocatechols, dinitrocatechols, oligomeric compounds and higher-generation oxidation products under dry conditions, accompanied by strong near-ultraviolet absorption associated with brown-carbon chromophores.
As relative humidity increases, however, the balance of reactions changes. The new study shows a transition away from chemistry dominated by nitration and oligomerization and toward oxidative ring fragmentation and highly oxygenated products, including dicarboxylic acids. In other words, adding water does not simply dilute or accompany the reacting material.
It changes which chemical pathways become important as catechol undergoes nighttime oxidation. That distinction matters because the two pathways produce chemically different organic material. Nitroaromatic and oligomeric products generated under dry conditions can contribute to light-absorbing brown carbon, connecting nighttime chemistry with the optical properties of atmospheric particles.
Previous work from the same research program demonstrated that heterogeneous nitrate-radical oxidation of catechol provides an efficient nighttime route to nitrocatechols, oligomers and brown carbon. The effect of humidity is also emerging in studies of other biomass-burning phenolics. To uncover the competing pathways, Rana and Guzman combined Fourier transform infrared and UV-visible spectroscopy with complementary mass-spectrometric measurements.
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