Implementation of a high-resolution Source-Oriented WRF/Chem model at the Port of Oakland

Abstract

A version of the Source-Oriented WRF/Chem (SOWC) model with 250 m spatial resolution (SOWC-HR) was developed and implemented to perform high resolution simulations over the community of Oakland, California, during March 2010. A multiscale set of nested domains was used to predict contributions to airborne particulate elemental carbon (EC) concentrations from ships, trains, and on-road diesel trucks. The final domain at 250 m resolution used Large Eddy Simulation (LES) to predict turbulent mixing at scales where traditional first order closure models are not valid. Results of the high resolution simulation with the nested LES (HR case) and without the nested LES (non-HR case) were compared to speciated particulate matter (PM) measurements and source contributions calculated using Positive Matrix Factorization (PMF). The PMF results showed that on-road diesel traffic was a major EC contributor, a result consistent with previous studies for Oakland. The average EC concentration predicted at the site by the SOWC-HR model was 0.42 μg m−3, with source contributions of 0.22 μg m−3 from on-road diesel, 0.05 μg m−3 from ship fuel combustion, 0.08 μg m−3 from trains, and 0.09 μg m−3 from other sources. Both simulation cases predicted similar total EC concentrations and source contributions at the sampling sites, but more substantial differences were predicted at other locations in the study region. The HR case predicted higher average and maximum hourly EC contributions from all sources compared the non-HR case. The greatest relative increase of maximum hourly EC was seen in the on-road diesel source, which increased by nearly a factor of 2 (3.74 μg m−3 to 6.69 μg m−3) when spatial resolution was increased from 1 km to 250 m. The SOWC-HR model predicted greater population-weighted EC concentrations from all sources when compared to the SOWC model without HR. The increase in period-averaged EC exposure from each source ranged from +1% to +17%, while the increase in maximum hourly EC exposure from each source ranged from +9% to +32%. This evaluation shows that resolving neighborhood scales through the representation of local mixing phenomena can significantly impact pollutant concentration predictions, especially when examining extreme exposures in a densely populated area with many sources and complex terrain.