Decoupled direct 3D sensitivity analysis for particulate matter (DDM-3D/PM)

The decoupled direct method (DDM) and DDM-3D have been implemented in air quality models in order to efficiently compute sensitivities. Initial implementation of DDM/DDM-3D in models was confined only to gas-phase species as the treatment of sensitivities in the dynamics of secondary aerosol formation is more complex. Here, it is extended to calculate particulate matter sensitivities. DDM-3D/particulate matter (PM) results compare well spatially and temporally with the traditional brute-force approach, particularly for species responses to emissions of their “parent” precursor (e.g., sulfate to SO2 emissions.) Correlations of more indirect relationships between aerosols and gaseous emissions (e.g., nitrate to SO2 emissions) are worse, but these sensitivities are usually small. DDM-3D/PM appears to work better than the brute-force approach in some cases due to numerical noise and other factors, as identified from the application on a southeastern US domain for a summer episode. DDM-3D/PM is also computationally efficient. While CPU usage was found to scale linearly with the number of sensitivity parameters of interest (for a given domain size), it was significantly less than using the brute-force approach.