Contrasting Source Apportionment Approaches with Zero Out Testing


I would like to draw your attention to the outcomes of our recent CMAQ/ISAM model runs (Case 1) and CMAQ runs (Case 2) conducted with emissions set to zero in a particular region (referred to as ‘Region A’ here). This region is masked in the CMAQ/ISAM model.

Upon analyzing the data, I observed a significant difference in the ozone contribution attributed to anthropogenic emissions from this region. In particular, the Source Apportionment analysis based on Case1 indicated that the ozone contribution from anthropogenic sources in Region A is approximately double that of Case2, please refer to the table attached at the end for further details. This finding suggests that the elimination of anthropogenic emissions in Case2 has a profound impact on the atmospheric chemistry, resulting in varying levels of ozone contributions. The ozone contribution exhibits a nonlinear response to emissions in different regions, and the complex interplay between emissions and atmospheric chemistry. For example, the impact of a simultaneous reduction of two sources differs from the sum of their individual impacts. In addition, for higher emission reductions, impacts are not suited because of their non-additivity and lack of robustness [reference].

This raises questions about the validity of contrasting the source apportionment approach with zero out testing or CMAQ with the brute force method.

Please let me know your thoughts and any additional suggestions you may have.


This is an active area of research and these tools are meant to provide additional and complementary information to air quality modelers. As you certainly realize, interpretation of results and comparison between different techniques depends of many factors including some that are very specific to the application. As such, I can only recommend that you become very familiar with your simulation episode and also with the tools themselves and make your own conclusions.