Tropospheric ozone

In 2010 the Gas Kinetics Discussion Group of the Royal Society of Chemistry held its 21st International Symposium on Gas Kinetics in Leuven, Belgium. At this meeting, we held an extended session of presentations with extensive discussion on tropospheric ozone, and in this issue of PCCP, five of six papers arising from the meeting are published. It is difficult to overestimate the importance of ozone in the chemistry of Earth's atmosphere, and its study has dominated the research of many atmospheric chemists for decades. In the troposphere, ozone is an important secondary pollutant, its photolysis leads to the formation of the hydroxyl radical, the major daytime oxidant in atmospheric chemistry, and it is also involved in the generation of the nitrate radical, the major night-time oxidant in atmospheric chemistry. Furthermore, ozone is an initiator of oxidation itself, reacting readily with unsaturated volatile organic compounds (VOCs). These reactions are not fully understood by any means, but they are known to generate hydroxyl radicals and—for larger unsaturated compounds such as the terpenes—secondary organic aerosol (SOA). Ozone thus has a major impact on the oxidising capacity of the atmosphere, and has a significant impact on climate.

In this collection of papers, the focus is on the ozonolysis of unsaturated VOCs, and there are studies that cover kinetics, mechanism, radical and final product yields and SOA generation in these reactions. The collection kicks off with Donahue et al.'s ‘Adventures in ozoneland’ Perspective (DOI: 10.1039/c0cp02643c) which describes a 15 year pursuit of the Stabilised Criegee Intermediate (SCI). Although the existence of the Criegee Intermediate has been widely accepted since Criegee's pioneering work in the 1940s and 1950s, finding direct physical evidence for its existence in the gas phase has proved elusive. In this article, indirect evidence from the study of the pressure dependence of reaction products is used to narrow down the conditions under which we may expect to find the SCI. The observations indicate a complex, multiple-well reaction system and emphasise the importance of the role that fundamental physical chemistry plays in understanding chemical processes that occur in the atmosphere. Of course, an understanding of reaction mechanism starts with a good knowledge of reaction rate constants; Percival and co-workers (DOI: 10.1039/c0cp02643c) have examined the kinetics of the ozonolysis of a number of large (C₈–C₁₄) 1-alkenes at high temperature. Their study indicates that previously observed trends in the reactivity of these alkenes towards ozone arise as a result of unexpected heterogeneous processes, and have thus resolved some observations that were difficult to explain.

Alam et al. (DOI: 10.1039/c0p02342f) focus on total radical yields of the ethene–ozone reaction in a study carried out at the EUPHORE chamber in Valencia. This group provides a fairly comprehensive picture of ethene ozonlysis, accounting for the loss of excited CI to form hydroxyl and hydroperoxyl radicals (both measured directly) and SCI (measured indirectly).

Finally, there are two papers concerning the formation of SOA. Beck et al. (DOI: 10.1039/c0cp02379e) describe a very careful study of the gas-phase ozonolysis of the sesquiterpene α-humulene, using proton-transfer mass spectrometry to tentatively identify 37 products in the aerosol phase and five in the gas phase, and are also able to provide a broad overview of the reaction mechanism. Wolf et al. (DOI: 10.1039/c0cp02499f) describe the pressure dependence of SOA formation of a number of cyclic alkenes; they show that aerosol formation is suppressed at low pressures and are able to link this observation to the chemical mechanisms leading to SOA formation.

Much of the driving force for studying ozonolysis comes from the need to understand atmospheric chemistry. However, the techniques used in its investigation are those of physical chemistry; it is appropriate therefore that this collection of papers should appear in PCCP, and I would very much like to than the staff of PCCP for their assistance in bringing the collection together. I would also very much like to thank the authors for their excellent contributions.

Finally, the next International Symposium on Gas Kinetics will be held in Boulder, Colorado in June, 2012, and I fully expect that there will be another excellent crop of presentations on this important and fascinating area of research.

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