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Vol. 34 No. 5
September-October 2012

The Project Place | Information about new, current, and complete IUPAC projects and related initiatives.
See also www.iupac.org/projects

Chemical Kinetic Data Evaluation for Atmospheric Chemistry and the IUPAC Kinetics Database

It is well established that emissions from human activities are leading to important changes in the composition of the global atmosphere. The impact of emissions on air quality on local and regional scales has been recognized for decades. The importance of intercontinental and global transport of air pollutants is now becoming apparent. More recently, it has become clear that anthropogenic emissions of greenhouse gases and aerosols are responsible for radiative forcing of climate change. To plan effective policy and actions to address these issues and plan a path to a more sustainable future, it is critical that decision makers have the best available scientific understanding upon which to base their decisions.

Computational models are used extensively to understand the impacts of emissions on air quality on local, regional, and global scales and to understand the effects of human activities on the global climate system. An accurate representation of atmospheric chemistry is of critical importance in these models. In past models, the focus was on gas-phase chemistry. Modern models include the important effects of heterogeneous reactions and the formation and fate of atmospheric aerosols. The IUPAC Task Group on Chemical Kinetic Data Evaluation for Atmospheric Chemistry is engaged in a project to provide evaluated kinetic data available to the scientific community in an online database for use in the next generation of atmospheric models. This project is furthering the IUPAC strategic goals of (i) serving as a scientific, international, non-governmental body that objectively addresses global issues involving the chemical sciences and (ii) facilitating the development of effective channels of communication in the international chemistry community.

The Task Group is continuing to update and expand the evaluations, which have been published in a series of 10 peer-reviewed articles in J. Phys. Chem. Ref. Data, and five recent articles in Atm. Chem. Phys. The updated evaluations are available at <www.iupac-kinetic.ch.cam.ac.uk>, the website of the Centre for Atmospheric Science in the Department of Chemistry, University of Cambridge, UK. The group is updating data sheets and providing recommendations for rate constants for selected gas phase and heterogeneous processes for which new information has become available. In 2012, the panel will compile and submit for publication recommended data for heterogeneous reactions on liquid water and sulfuric acid surfaces and homogeneous reactions of halogenated organic compounds. These data are needed as inputs for the next generation of global and regional atmospheric models.

There is growing interest within the scientific community in the formation, fates, and environmental impacts of atmospheric aerosols. The chemistry and physics associated with atmospheric aerosols is very complex. With support from IUPAC, the project has put great effort into collecting and evaluating kinetic data for atmospherically relevant reactions and processes involving aerosols and has added several hundred data sheets containing evaluated data for heterogeneous reactions (e.g., reactions on ice surfaces, soot, mineral dust, sulfuric acid aerosol) over the past couple of years. This data facilitates more accurate representations of important heterogeneous processes in atmospheric models of climate and air pollution.

We take the completion of the migration of the kinetic and thermodynamic IUPAC database on atmospheric chemistry to its website as an opportunity to highlight several enhancements and capabilities recently implemented by the webmasters Glenn Carver and Hannah Barjat of Cambridge University. The website has been active since 1999. For details on the history of the website see Jan-Feb 2007 CI, p. 15.

The database is divided into summary tables for families of reactions, data sheets for individual reactions and supplementary information providing guides to the gas-phase and the heterogeneous data sheets and thermodynamic data used in the database. At the top of the home page there is a link to the five volumes published in Atmospheric Chemistry and Physics which are “snapshots” of the database at time of publication. Publication in Atmospheric Chemistry and Physics provides permanent documentation of the state of the database on a periodic basis in an easy to access and familiar format.

The summary tables provide kinetic data on (a) gas phase Ox, NOx, HOx, SOx, organic compounds, inorganic and organic halogens; (b) heterogeneous reactions on ice substrates; (c) heterogeneous reactions on mineral dust substrates. Previous editions of the summary tables are available to facilitate the tracking of changes in the recommended kinetic information with time as new data or new aspects in evaluation become implemented.

The data sheets proper are brief summaries of available experimental data with notes giving details of the experimental procedures and are organized in four broad groups, namely (i) gas phase reactions; (ii) photolysis reactions; (iii) heterogeneous reactions on solid substrates; (iv) heterogeneous reactions on liquid substrates. It is notable that the evaluation of heterogeneous reactions has been entirely refurbished from previous evaluations and also goes beyond what other evaluations have provided. It is based on an advanced framework of kinetic parameters to describe heterogeneous and multiphase processes in more detail. Particularly useful is the recently implemented search capability based on SMILES strings and IUPAC’s InChI unique identifier of chemical compounds. It is planned to install an additional search capability based on functional group recognition to make it even more user-friendly given the more than 1000 data sheets now in the database. For each thermal reaction, a preferred value of the rate coefficient at 298K is given together with its temperature dependence, if available. The selection of the preferred value is discussed in some detail and estimates of the accuracies of the rate coefficients and temperature coefficients have been given for each reaction depending on the number of studies and their quality. For each photochemical reaction the data sheets list the preferred values of the photoabsorption cross sections and the quantum yields of the photochemical reactions together with comments on how these were selected.

The supplementary information at the bottom of the home page includes a brief guide to the data sheets explaining key definitions of kinetic parameters; a glossary of terms; an updated introduction to the heterogeneous data sheets; and a referenced list of thermodynamic data of relevant atoms, free radicals, and closed-shell molecules and the order of reactions within a family.

The advantages of the IUPAC kinetic database for atmospheric chemistry may be summarized as follows:

  • extensive database accessible free of charge
  • critical evaluation with justifying documentation as opposed to compilation of literature data
  • frequent updates
  • hard copy “snapshots” of the state of the database at a given point in time for easier traceability of recommendation changes

For more information contact Tim Wallington <twalling@ford.com>.

www.iupac.org/project/2011-049-1-100


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