the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Opinion: Challenges and needs of tropospheric chemical mechanism development
Abstract. Chemical mechanisms form the core of atmospheric models to describe degradation pathways of pollutants and ultimately inform air quality and climate policy makers and other stakeholders. The accuracy of chemical mechanisms relies on the quality of their input data, which originate from experimental (laboratory, field, chamber) and theoretical (quantum chemistry, theoretical kinetics, machine learning) studies. The development of robust mechanisms requires rigorous and transparent procedures for data collection, mechanism construction and evaluation, and creation of reduced or operationally defined mechanisms. Developments in analytical techniques have led to a large number of identified chemical species in the atmospheric multiphase system which have proved invaluable for our understanding of atmospheric chemistry. At the same time, advances in software and machine learning tools have enabled automated mechanism generation. We discuss strategies for mechanism development, applying empirical or mechanistic approaches. We show the general workflows, how either approach can lead to robust mechanisms and that the two approaches complement each other to result in reliable predictions. Current challenges are discussed related to global change, including shifts in emission scenarios that result in new chemical regimes (e.g. low NO scenarios, wildfires, mega/gigacities) and require the development of new or expanded gas- and aqueous-phase mechanisms. In addition, new mechanisms should be developed to also target oxidation capacity, and aerosol chemistry impacting climate, human and ecosystem health.
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Status: open (until 13 Aug 2024)
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CC1: 'Comment on egusphere-2024-1316', Mike Jenkin, 09 Jul 2024
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A chemical mechanism is one of a number of a key components of science and policy models applied to air quality and climate issues - and it is an insurmountable task for an individual modeller to be able to appraise the vast wealth of published information to assemble that mechanism. It is therefore essential that sustainable methods and activities are in place to allow mechanisms to be constructed and freely available to the atmospheric chemistry community, and that such activities are well supported. This opinion paper provides a detailed and comprehensive overview and discussion of the many challenges and requirements for sustainable development of tropospheric chemical mechanisms, authored by a set of experts with considerable expertise and experience in a number of fields relevant to this topic. It thus provides an important benchmark reference and summary document of those methods and activities, which can help to guide and inform future mechanism development strategies.
My previous work in this field largely focused on gas phase mechanism development for organic compounds, mainly through contributions to the Master Chemical Mechanism (MCM). Since that mechanism was first constructed (manually) in the mid 1990s, there have been progressive and considerable developments in understanding of VOC degradation that have required increasing detail to be evaluated and represented in tropospheric chemical mechanisms. I therefore fully concur with the requirements for reliable (and ideally more extensive) evaluated data (sections 3.1 and 4.1); the development of SARs (section 3.2) and generation protocols (section 3.3); and the need to apply automated methods (section 3.4) to allow that detail to be included easily, for mechanisms to be updated regularly and efficiently, and to allow sensitivity tests to be carried out.
The developments in understanding have also highlighted the important requirement for systematic and justifiable methods of mechanism reduction, which are now more important than ever. Accordingly, this is covered in section 3.5, and I agree that this is one of the major challenges for the construction of practical mechanisms that are fully traceable to the fundamental detail derived from laboratory, chamber and theoretical studies. Some particular developments in understanding that have caused a potential explosion of detail in explicit mechanisms, and therefore an increased need for mechanism reduction strategies, are:
- "Autoxidation" mechanisms leading to rapid formation of highly-oxidised organic molecules (HOMs). These mechanisms can involve any number of (1,n) H-shift or ring-closure RO2 isomerisation reactions (sometimes reversible), the rate coefficients for which depend on the precise structure of the species and cannot be represented generically - and which can lead to orders of magnitude increases in mechanism size.
- The formation of large, involatile ROOR' (or other accretion) products from RO2 + R'O2 (permutation) reactions. These cannot be represented easily with the pseudo-unimolecular "peroxy radical pool" parameterisation which is so essential to restrict mechanism size in some mechanisms, including GECKO-A and the MCM.
- New information on the formation and reactions of Criegee intermediates (CIs) formed from O3 + alkene reactions. For example, this includes the very rapid reactions of all stabilised CIs with all organic acids in the mechanism, and the degradation of the hydroperoxyester products (i.e. essentially another class of permutation reaction with large association products).
These mechanistic features are all highlighted in this paper. In view of this, the particular impact of these (and other) features on mechanism size could possibly be given a little more emphasis, along with the resultant related challenges for mechanism reduction.
Minor comments
Page 3, line 73: For consistency with elsewhere, “geckoa” should read “GECKO-A”.
Page 7, Figure 3: For consistency with the section title, “Protocols of mechanism generation” should read “Protocols for mechanism generation”. There is also an “off” that should be an “of” in the auto generation box.
Page 19, lines 525-527: I do not fully understand how elevated O3 and suppressed NOx in megacities might invalidate prevailing understanding of RO2 chemistry. Perhaps this could be explained (note also that the reader is referred to “section 5.2” here, which is the same section). However, I do agree with the general point that changes in the ranges of ambient conditions need to be borne in mind.
Citation: https://doi.org/10.5194/egusphere-2024-1316-CC1
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