Humidity can change how smoke pollutants transform after dark

The way smoke pollutants transform after sunset can be influenced significantly by the amount of water vapor in the air. A recent study conducted by Md. Sohel Rana and Marcelo I. Guzman at the University of Kentucky reveals that relative humidity plays a crucial role in determining the chemical reactions that occur during the nighttime oxidation of catechol, a phenolic compound found in biomass burning emissions.

Published in Environmental Science & Technology, the research paper titled "Relative Humidity Modulates Nitration and Ring Fragmentation during Heterogeneous Oxidation of Catechol by Nitrate Radicals" investigates how catechol reacts with nitrate radicals (NO₃), key nighttime atmospheric oxidants, as humidity levels vary from dry to increasingly humid conditions.

The study, which is part of a larger research effort focusing on how nighttime chemistry transforms phenolic compounds associated with biomass-burning emissions, highlights the importance of relative humidity as a controlling factor in this nighttime chemical transformation. Under dry conditions, nitrate-radical oxidation tends to favor nitration and the formation of oligomerization products.

Previous research by Rana and Guzman, published in ACS Earth and Space Chemistry, identified various oxidation products such as nitrocatechols, dinitrocatechols, oligomers, and higher-generation oxidation compounds, all of which exhibited strong near-ultraviolet absorption due to brown-carbon chromophores. However, as humidity levels increase, the chemical reactions undergo a notable shift.

The new study demonstrates a transition from nitration and oligomerization-dominated chemistry to oxidative ring fragmentation and the production of highly oxygenated products, including dicarboxylic acids. This shift indicates that the addition of water does not merely dilute the reacting materials but actively alters the pathways through which catechol undergoes nighttime oxidation.

This difference is significant because the chemical products resulting from these distinct pathways have different chemical structures and properties. Nitroaromatic and oligomeric compounds formed under dry conditions can contribute to the formation of light-absorbing brown carbon, linking nighttime chemistry with the optical properties of atmospheric particles.

The researchers utilized advanced analytical techniques, including Fourier transform infrared and UV-visible spectroscopy alongside mass-spectrometric measurements, to track the transformation of catechol and its oxidation products across different humidity levels. By comparing the parent compound and its products under varying humidity conditions, the team was able to construct a detailed molecular picture of how the chemistry evolves as water content in the air increases.

The findings contribute to a growing body of research on the atmospheric chemistry of biomass burning, emphasizing that phenolic compounds released in smoke continue to react even after sunset. Under nighttime conditions, nitrate-radical chemistry provides an additional route for the transformation of these compounds. The study's results underscore the potential atmospheric implications of humidity on the behavior of biomass-burning organic compounds.

As biomass-burning smoke plumes move through different atmospheric environments with varying relative humidities, the same compounds may follow different chemical pathways depending on the specific humidity conditions they encounter. This finding has important implications for understanding the molecular composition and wavelength-dependent light absorption characteristics of aged organic aerosols.

By moving beyond treating relative humidity as merely a background atmospheric variable, the study emphasizes its active role in governing the chemical evolution of biomass-burning compounds during nighttime aging. The research not only advances our understanding of nighttime oxidation processes but also provides mechanistic insights that can help connect laboratory findings with the evolving composition and optical properties of biomass-burning aerosols observed in the atmosphere.