Spontaneous chemistry on aerosol surface: Interfacial redox chemistry plays an important role in the formation of gas molecules and aerosol particles. However, the characterization of such heterogeneous processes is challenging, and they are often omitted in chemical kinetics models. Using ambient-pressure x-ray photoelectron spectroscopy combined with molecular dynamics simulations, Kong et al . discovered spontaneous redox chemistry promoted by the surface at the first stages of the solvation process on a typical inorganic aerosol surface of ammonium sulfate (see the Perspective by Ruiz-Lopez). Several unexpected species have been identified as being possible products of a sulfate-reducing ammonium oxidation reaction, and this may help to resolve some of the enduring conundrums of atmospheric chemistry. The present results could also be useful for the development of wastewater treatments and other industrial technologies. —YS

Modern small-scale biomass burners have been recognized as an important renewable energy source because of the economic and environmental advantages of biomass over fossil fuels. However, the characteristics of their gas and particulate emissions remain incompletely understood, and there is substantial uncertainty concerning their health and climate impacts. Here, we present online measurements conducted during the operation of a residential wood-burning boiler. The measured parameters include gas and particle concentrations, optical absorption and chemical characteristics of gases and particles. Positive matrix factorization was performed to analyze data from a high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS) equipped with a filter inlet for gases and aerosols (FIGAERO). Six factors were identified and interpreted. Three factors were related to the chemical composition of the fuel representing lignin pyrolysis products, cellulose/hemicellulose pyrolysis products, and nitrogen-containing organics, while three factor were related to the physical characteristics of the emitted compounds: volatile compounds, semi-volatile compounds, and filter-derived compounds. An ordinal analysis was performed based on the factor fractions to identify the most influential masses in each factor, and by deconvoluting high-resolution mass spectra fingerprint molecules for each factor were identified. Results from the factor analysis were linked to the optical properties of the emissions, and lignin and cellulose/hemicellulose pyrolysis products appeared to be the most important sources of brown carbon under the tested burning conditions. It is concluded that the emissions from the complex combustion process can be described by a limited set of physically meaningful factors, which will help to rationalize subsequent transformation and tracing of emissions in the atmosphere and associated impacts on health and climate.