Systematic reduction of complex tropospheric chemical mechanisms, Part I: sensitivity and time-scale analyses

[摘要] Explicit mechanisms describing the complex degradation pathwaysof atmospheric volatile organic compounds (VOCs) are important, since they allow the study of thecontribution of individual VOCS to secondary pollutant formation.They are computationally expensive to solve however, since theycontain large numbers of species and a wide range of time-scales causingstiffness in the resulting equation systems.This paper and the following companion paper describe the application of systematic and automated methods for reducing suchcomplex mechanisms, whilst maintaining the accuracy of the model with respect to importantspecies and features. The methods are demonstrated via application to version2 of the Leeds Master Chemical Mechanism. The methods of Jacobian analysis andoverall rate sensitivity analysis proved to be efficient and capable ofremoving the majority of redundant reactions and species in the scheme across a wide range ofconditions relevant to the polluted troposphere. The application of principal component analysis of the rate sensitivitymatrix was computationally expensive due to its use of thedecomposition of very large matrices, and did not produce significant reduction over and above theother sensitivity methods.The use of the quasi-steady state approximation (QSSA) proved to be an extremely successfulmethod of removing the fast time-scales within the system, as demonstrated by a local perturbation analysisat each stage of reduction.QSSA species were automatically selected via the calculation of instantaneousQSSA errors based on user-selected tolerances.The application of the QSSA led to the removal of a large number of alkoxy radicalsand excited Criegee bi-radicals via reaction lumping. The resulting reduced mechanism was shown toreproduce the concentration profiles of the important species selected from the full mechanism over a wide range ofconditions, including those outside of which the reduced mechanism was generated. As a result of a reduction in thenumber of species in the scheme of a factor of 2, and a reduction in stiffness, the computational timerequired for simulations was reduced by a factor of 4 when compared to the full scheme.

[学科分类] 大气科学