Implementing the decoupled direct method for sensitivity analysis in a particulate matter air quality model

The decoupled direct method (DDM) is an efficient and accurate way of performing sensitivity analysis to model inputs. As the impact of atmospheric particulate matter (PM) on human health and visibility became evident, the need to extend the DDM to PM sensitivity has grown. In this work, the DDM is implemented in the three PM modules employed in the Comprehensive Air-quality Model with extensions (CAMx): ISORROPIA for inorganic gas/aerosol partitioning, SOAP for secondary organic gas/aerosol partitioning, and RADM-AQ for aqueous-phase chemistry. The PM modules are complex and the DDM implementation is discussed in detail. Stand-alone tests are performed for each PM module in which first-order sensitivities computed bythe DDM are compared tothe traditional brute-force method (BFM). The DDM sensitivities are shown to be accurate and agree well with the BFM within the linear response range. The SOAP module showed nearly linear response for up to +/-30% changes in concentration inputs. The RADM-AQ module showed moderately nonlinear response in some tests but first-order sensitivities accounted for most of the response for input changes up to +/-20%. ISORROPIA shows greater deviation from linear response than the other PM modules and the near-linear range can be as restricted as +/-10% changes in concentration inputs. Nonlinearity in ISORROPIA results both from the equations that describe thermodynamic equilibrium and the computational approaches within ISORROPIA that are employed to boost efficiency.