Heterogeneous uptake of gaseous hydrogen peroxide on mineral dust

Molecular characterization of diverse quinone analogs for discrimination of aerosol-bound persistent pyrolytic and photolytic radicals

Aerosol-bound organic radicals, including environmentally persistent free radicals (EPFRs), are key components that affect climate, air quality, and human health. While putative structures have been proposed, the molecular characteristics of EPFRs remain unknown. Here, we report a surrogate method to characterize EPFRs in real ambient samples using mass spectrometry. The method identifies chemically relevant oxygenated polycyclic aromatic hydrocarbons (OxPAH) that interconvert with oxygen-centered EPFR (OC-EPFR). We found OxPAH compounds most relevant to OC-EPFRs are structurally rich and diverse quinones, whose diversity is strongly associated with OC-EPFR levels. Both atmospheric oxidation and combustion contributed to OC-EPFR formation. Redundancy analysis and photochemical aging model show pyrolytic sources generated more oxidized OC-EPFRs than photolytic sources. Our study reveals the detailed molecular characteristics of OC-EPFRs and shows that oxidation states can be used to identify the origins of OC-EPFRs, offering a way to track the development and evolution of aerosol particles in the environment.

Persistent Uptake of H2O2 onto Ambient PM2.5 via Dark-Fenton Chemistry

Particulate matter (PM) and gaseous hydrogen peroxide (H₂O₂) interact ubiquitously to influence atmospheric oxidizing capacity. However, quantitative information on H₂O₂ loss and its fate on urban aerosols remain unclear. This study investigated the kinetics of heterogeneous reactions of H₂O₂ on PM_2.5 and explored how these processes are affected by various experimental conditions (i.e., relative humidity, temperature, and H₂O₂ concentration). We observed a persistent uptake of H₂O₂ by PM_2.5 (with the uptake coefficients (γ) of 10^-4-10^-3) exacerbated by aerosol liquid water and temperature, confirming the critical role of water-assisted chemical decomposition during the uptake process. A positive correlation between the γ values and the ratio of dissolved iron concentration to H₂O₂ concentration suggests that Fenton catalytic decomposition may be an important pathway for H₂O₂ conversion on PM_2.5 under dark conditions. Furthermore, on the basis of kinetic data gained, the parameterization of H₂O₂ uptake on PM_2.5 was developed and was applied into a box model. The good agreement between simulated and measured H₂O₂ uncovered the significant role that heterogeneous uptake plays in the sink of H₂O₂ in the atmosphere. These findings suggest that the composition-dependent particle reactivity toward H₂O₂ should be considered in atmospheric models for elucidating the environmental and health effects of H₂O₂ uptake by ambient aerosols.

Shining New Light on the Kinetics of Water Uptake by Organic Aerosol Particles

The uptake of water vapor by various organic aerosols is important in a number of applications ranging from medical delivery of pharmaceutical aerosols to cloud formation in the atmosphere. The coefficient that describes the probability that the impinging gas-phase molecule sticks to the surface of interest is called the mass accommodation coefficient, α_M. Despite the importance of this coefficient for the description of water uptake kinetics, accurate values are still lacking for many systems. In this Feature Article, we present various experimental techniques that have been evoked in the literature to study the interfacial transport of water and discuss the corresponding strengths and limitations. This includes our recently developed technique called photothermal single-particle spectroscopy (PSPS). The PSPS technique allows for a retrieval of α_M values from three independent, yet simultaneous measurements operating close to equilibrium, providing a robust assessment of interfacial mass transport. We review the currently available data for α_M for water on various organics and discuss the few studies that address the temperature and relative humidity dependence of α_M for water on organics. The knowledge of the latter, for example, is crucial to assess the water uptake kinetics of organic aerosols in the Earth's atmosphere. Finally, we argue that PSPS might also be a viable method to better restrict the α_M value for water on liquid water.