Improving the computational efficiency of a source-oriented chemical mechanism for the simultaneous source apportionment of ozone and secondary particulate pollutants

Abstract

Source-oriented chemical mechanisms enable direct source apportionment of air pollutants by explicitly representing precursor emissions and their reaction products in atmospheric models. These mechanisms use source-tagged species to track emissions and their evolution. However, scalability was previously limited by the large number of reactions required for interactions between two tagged species, such as the NOx-NOx or volatile organic compound (VOC)-NOx reactions. This study improves computational efficiency and scalability with a new method that tracks the total concentration of tagged species, reducing the n2 second-order reactions for n sources into 2n pseudo-first-order reactions. The overall production and removal rate of individual species remain unchanged in the new approach. The number of reactions and number of model species increase linearly with the number of source types, thus greatly improving the computational efficiency. In addition, a source-oriented Euler backward iterative (EBI) solver was implemented to replace the Gear solver used in previous applications of the source-oriented mechanism. The source-oriented EBI solver has been assessed by comparing predicted results with those of the Gear solver. Good agreement between those two methods has been achieved, as the results from the EBI scheme are linearly correlated to Gear and the average of absolute relative error is below 5 %. In the timing assessment, the proposed EBI scheme can effectively reduce the total chemistry time by 73 %-90 % for grids with different resolutions, which leads to a reduction in total simulation time by 46 %-74 %. The proposed source-oriented scheme is efficient enough for practical long-term source apportionment applications on nested domains.