Themed issue on aquatic photochemistry

I am delighted to present to you the Themed Issue on Aquatic Photochemistry in Environmental Science: Processes & Impacts, for which I served as the Guest Editor. This issue contains 20 invited contributions that represent both the excitement and breadth of current aquatic photochemical research. The 16 research articles and 4 critical reviews in this issue cover a diverse range of topics and sub-disciplines within environmental science, showcasing photochemistry's centrality as a driver in natural systems. At the same time, the contributions nicely highlight ES:P&I's clear focus on environmental processes, a fact underscored by its name change from Journal of Environmental Monitoring just over one year ago.

The four critical reviews contained in this issue reflect two major themes found in the research articles: organic matter-sensitized formation of photochemically produced reactive species (PPRIs) and the photochemistry of anthropogenic compounds. Charlie Sharpless and Neil Blough (DOI: 10.1039/c3em00573a) take up the first topic in their review on the possible involvement of charge transfer states in the photochemistry of chromophoric dissolved organic matter (CDOM). Reviews by Christy Remucal, Weihua Song and Charles Wong discuss different aspects of the photochemistry of organic environmental contaminants. Remucal (DOI: 10.1039/c3em00549f) reviews the role of indirect photochemistry and PPRIs in the photochemical fate of pesticides. Yan and Song as well as Wong and co-workers have contributed critical reviews on pharmaceutically active compounds (PhACs). While Wong and co-workers (DOI: 10.1039/c3em00615h) made their focus defining the outline of what are well studied and poorly studied compounds in this class, Yan and Song (DOI: 10.1039/c3em00502j) have focused on what is known about the chemical mechanisms of PhACs.

Davide Vione along with Marco Bodrato (DOI: 10.1039/c3em00541k) have synthesized what is known about direct and indirect photochemistry in a different manner, by producing a photochemical model and accompanying software package called APEX (Aqueous Photochemistry of Environmentally occurring Xenobiotics). I am very happy that they have chosen this issue to launch APEX, which promises to be a valuable tool for modeling and predicting the photochemistry of aquatic systems using simple input values.

Another personal highlight of this issue is the inclusion of a set of papers relevant to atmospheric photochemistry. Photochemistry in a water matrix does not only happen in lakes, rivers and oceans. Liquid and frozen water are important atmospheric matrices where photochemistry is most definitely important. Faye McNeill and her postdoc Greg Drozd (DOI: 10.1039/c3em00579h) have the distinction of presenting the only article in this issue with no photochemical reactions, but rather have focused one step earlier – on the factors that lead to the formation of light-absorbing compounds in aerosols. The Anastasio and Klán groups, in separate papers, examine ice photochemistry and show that frozen water does not mean slow. Cort Anastasio along with postdoc Jon Bower (DOI: 10.1039/c3em00565h) report that reactions with singlet oxygen are in fact enhanced in ice, while Petr Klán and his co-workers (DOI: 10.1039/c3em00665d) found the remarkable result that phototransformations of aromatic hydrocarbons on ice are actually faster at lower temperatures.

Coming back to the theme of relating relatively simple inputs into model predictions, Bill Arnold (DOI: 10.1039/c3em00479a) has contributed an article exploring the use of computational chemistry to predict the rates of PPRI-mediated oxidation of nitrogen-containing compounds. Bill Miller and graduate student Leanne Powers (DOI: 10.1039/c3em00617d) leave the planet altogether, using satellite ocean color observations to model global hydrogen peroxide photoproduction. Along similar lines, Dave Kieber and co-workers (DOI: 10.1039/c4em00036f) examined the wavelength and temperature dependence of hydrogen peroxide photoproduction, which should allow even better predictions from remote sensing data.

The theme of measurements of PPRI production, particularly hydroxyl radical, continues in three other contributions in this issue. Ken Mopper and co-workers (DOI: 10.1039/c3em00587a) examined the production of hydroxyl radical in natural waters from the Eastern United States over extended irradiation times. From a collaborative study between Rose Cory's and my group, led by postdoctoral researcher Sarah Page (DOI: 10.1039/c3em00596h), come results on the photochemical formation of hydroxyl from organic matter-rich waters on the North Slope of Alaska. Bill Cooper and his co-workers (DOI: 10.1039/c3em00591g) examined the photochemical production of PPRIs along an estuarine gradient in the Southeastern United States.

While many articles examined the production of PPRI from organic matter, there are two papers that examined the reaction of organic matter with PPRI. Krista Wigginton and her co-workers (DOI: 10.1039/c3em00663h) demonstrate a new method for quantifying the rate constant between hydroxyl radical and natural organic matter. Rose Cory and her co-workers (DOI: 10.1039/c3em00597f) look at the photochemistry of dissolved black carbon, a poorly studied, but important class of dissolved organic carbon.

The final set of papers all deal with the photodegradation of anthropogenic compounds. Daniel Mártire, Norman García and co-workers (DOI: 10.1039/c3em00576c) examined the indirect photochemistry of 2,6-dibromo-4-cyanophenol (bromoxynil), with the added twist that they explored the particulate phase photochemistry of bromoxynil by studying it covalently attached to nanoparticulate silica. Claire Richard and her co-workers (DOI: 10.1039/c3em00537b) report on the photochemistry of 1,3-dicyanotetrachlorobenzene (chlorothalonil), identifying the products and demonstrating that chlorothalonil functions as a good singlet oxygen sensitizer. Fabio Temussi and co-workers (DOI: 10.1039/c3em00400g) have contributed a study on two structurally similar carbamates with very different direct photochemistry. Finally, Thomas Borch and co-workers (DOI: 10.1039/c3em00581j) have reported on the pH dependent photochemistry of lamotrigine, an antiepileptic drug that is being found in wastewater-impacted waters.

What is clear from this collection of research articles and critical reviews is that the field of aquatic photochemistry is diverse and strong. Scientists are meeting challenges on several fronts: from the field to the laboratory to the silicon computer chip; from lakes and rivers to atmospheric aerosols; and, from anthropogenic chemicals to natural organic matter. The future of aquatic photochemistry appears bright indeed. I hope that you enjoy reading these studies as much as I enjoyed bringing them together as guest editor.

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