Examining single-source secondary impacts estimated from brute-force, decoupled direct method, and advanced plume treatment approaches.

In regulatory assessments, there is a need for reliable estimates of the impacts of precursor emissions from individual sources on secondary PM 2.5 (particulate matter with aerodynamic diameter less than 2.5 microns) and ozone. Three potential methods for estimating these impacts using Eulerian grid photochemical models are the brute-force (B–F) method, the decoupled direct method (DDM), and advanced plume treatment (APT). Here, we systematically inter-compare and assess the B–F, DDM, and APT approaches using hypothetical sources in a consistent modeling platform for a wide range of source conditions (i.e., emissions amount and composition, location within two California air basins, and stack parameters). The impacts of NOx and VOC sources on ozone and SO 2 sources on PM 2.5 sulfate calculated by these methods are in general agreement. The agreement is evident in the similar magnitudes, spatial patterns, and strong correlations among the impacts. This result, along with previous model evaluations based on similar Eulerian grid modeling, builds confidence in the reliability of the impact estimates. Disagreement among methods is evident in calculations of PM 2.5 nitrate impacts associated with NH 3 and NOx sources. Numerical instabilities in DDM sensitivity calculations compromise the nitrate impact estimates from that approach. The B–F and APT methods, which use brute-force differencing to identify impacts, are affected by numerical artifacts to a lesser degree than (H)DDM, with the artifacts being more prominent for APT than B–F. Overall, our results indicate that the (H)DDM, B–F, and APT approaches are viable for use in estimating single-source impacts for ozone and secondary PM 2.5 sulfate, while the B–F method appears to be the most reliable for estimating nitrate impacts. There is a need for additional field study measurements to better constrain model estimates of single-source secondary impacts. [ABSTRACT FROM AUTHOR]