Introduction to the chemistry of atmospheric pollutants themed issue

Atmospheric chemistry is a challenging and interdisciplinary subject that spans both indoor and outdoor air environments. The role of the Anthropocene in changing climate, landscape, and atmospheric composition raises urgent concerns that previously-understood atmospheric chemical processes (in both the gas and condensed phases) are changing as well, and thus, affecting our ability to predict how future atmospheric chemistry on Earth affects public health, air quality, and its associated feedback on the climate system.

The various scientific approaches for studying atmospheric chemistry were succinctly distilled by Abbatt et al.,¹ who used the metaphor of a three-legged stool: these legs represent controlled laboratory experiments, ambient air observations and modelling studies. The balance between each leg and their individual sturdiness contributes to our overall understanding of atmospheric chemistry. With these principles in mind, we are pleased to present this themed issue that covers recent advances in the chemistry of atmospheric pollutants. The papers are broad-ranging and include photochemical transformations in the gas-phase, properties and reactivity of airborne aerosols, gas–particle interactions, biogeochemistry, and chemical tracers, each covering one or more of the metaphorical legs.

Five papers in this themed issue explore aspects of the chemistry in the condensed-phase that impact atmospheric chemistry: El Syed and Hennigan (https://doi.org/10.1039/D2EM00115B) present wintertime particulate water-soluble organic carbon measurements that demonstrate the importance of multiphase chemistry in its formation. They observed water-soluble organic carbon formation in residential biomass burning-impacted environments and – despite the typically low wintertime relative humidity values – make a case for the importance of aerosol liquid water in these reactions; in their contribution, Malek et al. (https://doi.org/10.1039/D2EM00163B) highlight the importance of nitrogen-containing organic carbon, a prominent contributor to organic aerosol with a wide range of primary and secondary emission sources. They developed a mechanistic understanding of the water uptake ability of organic aerosol containing aromatic nitrogen compounds using laboratory experiments. Their findings add to the growing knowledge relating the functional groups of organic constituents to the physicochemical properties of organic aerosol, specifically the water uptake mechanism and changes to particle morphology; likewise Yang et al. (https://doi.org/10.1039/D2EM00004K) provided insights into functional groups within organic aerosols. They investigated phenolic precursors to brown carbon; specifically, studying how the aqueous-phase acidity impacts the nitrate-mediated photooxidation of four phenols that are known to be emitted from biomass burning and fossil fuel combustion. Their findings demonstrate that pH influences reactivity, but is highly dependent upon the initial phenolic compound; Maben and Ziemann (https://doi.org/10.1039/D2EM00124A) advance our understanding of oligomer-forming accretion reactions that contribute to secondary organic aerosol formation. They fill major knowledge gaps by exploring the kinetics for numerous accretion reactions under a range of aerosol-relevant conditions, and find that accretion reactions between hydroperoxides and aldehydes that form peroxyhemiacetal oligomers are most favourable (even in the presence of water and no acid catalyst) in SOA mimics.; Schneider et al. (https://doi.org/10.1039/D2EM00111J) present experiments characterizing the release of gas-phase iodine-containing compounds from the heterogeneous ozonolysis of iodide under a range of conditions. Notably, they conducted experiments with live phytoplankton cultures to produce biologically relevant marine organic species in order to further study iodine–organic reactions. Their findings suggest that the inconsistency of their experimental results with a multilayer kinetic model highlights the need for improved understanding of halide oxidation chemistry.

Three papers in this themed issue report on how indoor chemistry could affect the outdoor atmosphere: in Angelucci et al. (https://doi.org/10.1039/D2EM00411A) the first outdoor atmospheric measurements of chloramines are reported. They demonstrate that these measurements are likely related to indoor disinfection chemistry and describe the potential impact of chloramines on atmospheric oxidation; the remaining two studies describe experimental simulations of indoor cooking. In Pothier et al., (https://doi.org/10.1039/D2EM00250G) the chemical composition and volatility of organic aerosols resulting from cooking activities are reported. They use these findings to construct a model to describe how organic aerosol from this emission source, dilutes from indoor to outdoor environments. Takhar et al. (https://doi.org/10.1039/D1EM00532D) conducted a study on food cooking emissions to identify their contribution to aldehyde budgets. They apply their findings to present a mechanistic framework including reaction rates that could allow better prediction of emissions of organic compounds , having implications for ozone and particulate matter formation.

Interestingly, there are several call–answer companion studies in the themed issue. Al-Abadleh et al. (https://doi.org/10.1039/D2EM00176D) share a perspective article on iron in the atmosphere. Here they discuss multiphase photochemical reactions of Fe-containing minerals and present several different avenues for further research including the reactivity of Fe and organics in aerosol particles. Perhaps an answer to this call is the work by West et al. (https://doi.org/10.1039/D1EM00503K) who report on the laboratory investigation of Fe(III) secondary organic aerosol proxies undergoing photolysis to produce gas-phase oxygenated volatile organic compounds. Specifically they determine the chemical composition of Fe(III) citrate photocatalytic components and associated colloidal products. Particularly noteworthy was their use of ultra high resolution Orbitrap mass spectrometry and zwitterionic hydrophilic interaction liquid chromatography to identify highly polar organometallic Fe complexes. Given that such investigations are dependent on labour-intensive data mining spectra, West et al. also employed photodiode array chromatograms in the second dimension to support preliminary identification of 31 components in the reaction system and products. The products are then used to shed light on a comprehensive reaction mechanism for Fe(III)-citrate photochemistry. While these experiments present a multiphase mechanism, the detection of colloidal products here warrants their investigation in the atmosphere.

Further call–answer examples can be found in the cluster of papers on per- and polyfluoroalkyl substances (PFAS), a research area that was previously the subject of another ESPI themed issue.² The review article by Faust (https://doi.org/10.1039/D2EM00002D) on PFAS on atmospheric aerosol particles kicks off the topic. Here, Faust summarizes detection of PFAS in remote and continental locations, presenting the prevailing mechanisms for their atmospheric transport and deposition. Faust emphasizes the need for further study of PFAS on particulate matter, bulk seawater, the sea surface microlayer and the gas-phase as well as more diverse global sampling. As an answer to this, Tao et al. (https://doi.org/10.1039/D2EM00261B) also highlight the limited availability in reliable atmospheric PFAS measurements and the limited understanding of gas-particle partitioning. Here, they present a model that incorporates partitioning among air, aerosol liquid water, particulate water-insoluble organic matter, physicochemical properties and chemical speciation under a range of atmospheric conditions. Further to this call to action is the study by Bowers et al. (https://doi.org/10.1039/D2EM00275B) who introduce a novel method for gas- and aerosol-phase PFAS analysis using iodide chemical ionization mass spectrometry. Finally, one study in the collection explores the impact of particulate matter on human health. Root and Chorover (https://doi.org/10.1039/D2EM00182A) present uptake measurements of arsenic and lead from mine tailings particles into simulated biological fluids. They demonstrate that chemical speciation was a major driver of bioavailability of the metals.

We hope that you enjoy this collection of papers focused on the chemistry of atmospheric pollutants. We are grateful to all authors who contributed their work to this themed issue and to the reviewers, associate editors, editorial board, and staff who aided in bringing this collection together.

References

1. J. Abbatt, et al., New Directions: Fundamentals of atmospheric chemistry: Keeping a three-legged stool balanced, Atmos. Environ., 2014, 84, 390–391.
2. L. Ahrens, J. P. Benskin, I. T. Cousins, M. Crimi and C. P. Higgins, Themed issues on per- and polyfluoroalkyl substances, Environ. Sci.: Process. Impacts, 2019, 21, 1797–1802.

---