Relative humidity dictates how smoke pollutants transform after dark, as water vapor shifts nighttime oxidation pathways of biomass-burning compounds like catechol from nitration toward oxidative ring fragmentation, according to research published on October 28, 2026, by Phys.org.
Water Vapor Redirects Nighttime Catechol Oxidation Pathways
When the sun sets on biomass-burning emissions, chemical transformations do not halt. Nitrate radicals take over as primary atmospheric oxidants. Researchers Md. Sohel Rana and Marcelo I. Guzman at the University of Kentucky published findings in Environmental Science & Technology examining how catechol reacts with these nitrate radicals as air conditions transition from dry to humid.
Under dry conditions, nitrate-radical oxidation favors nitration and oligomerization. Previous work by the same researchers in ACS Earth and Space Chemistry identified nitrocatechols, dinitrocatechols, oligomeric compounds, and higher-generation oxidation products during dry runs. These dry-air products came paired with strong near-ultraviolet absorption linked directly to brown-carbon chromophores.
Water changes that balance entirely. As relative humidity increases, the chemistry shifts away from nitration and oligomerization. Instead, it drives toward oxidative ring fragmentation, producing highly oxygenated products such as dicarboxylic acids.
Water does not merely dilute the reacting material. It actively alters which chemical pathways dominate.
Tracking Molecular Shifts Through Spectroscopy and Mass Spectrometry
To uncover these competing pathways, Rana and Guzman deployed Fourier transform infrared spectroscopy, UV-visible spectroscopy, and complementary mass-spectrometric measurements. Tracking parent compounds and reaction products across varying humidity levels allowed them to build a detailed molecular picture.
The resulting data shows that phenolic compounds released in smoke maintain active chemistry long after daylight fades. Nitrate radicals supply a continuous route for nighttime transformation, but humidity dictates the final molecular composition.
Smoke plumes routinely pass through environments featuring wildly varying relative humidities during transport. Consequently, otherwise identical biomass-burning organic compounds follow distinct nighttime chemical trajectories depending on ambient conditions.
Implications for Atmospheric Aerosols and Light Absorption
These divergent pathways influence more than just molecular structure. They alter the wavelength-dependent light absorption of aged organic aerosol.
Nitroaromatic and oligomeric products generated under dry environments contribute directly to light-absorbing brown carbon. Conversely, the highly oxygenated products and dicarboxylic acids favored by high humidity alter aerosol optical properties differently.
Relative humidity functions as an active governor of chemical evolution during nighttime aging. Resolving how water changes these competing molecular pathways provides essential mechanistic data, connecting controlled laboratory measurements directly with the evolving composition of smoke plumes in the atmosphere.
