Simulation of Neighborhood-Scale Air Quality with two-way coupled
WRF-CMAQ over Southern Lake Michigan-Chicago Region
Abstract
The southern Lake Michigan region of the United States, home to Chicago,
Milwaukee, and other densely populated Midwestern cities, frequently
experiences high pollutant episodes with unevenly distributed exposure
and health burdens. Using the two-way coupled Weather Research Forecast
and Community Multiscale Air Quality Model (WRF-CMAQ), we investigate
criteria pollutants over a southern Lake Michigan domain using 1.3 and 4
km resolution hindcast simulations. We assess WRF-CMAQ’s performance
using data from the National Climate Data Center and EPA Air Quality
System. Our 1.3 km simulation slightly improves on the 4 km simulation’s
meteorological and chemical performance while also resolving key details
in areas of high exposure and impact, i.e., urban environments. At 1.3
km, we find that most air quality-relevant meteorological components of
WRF-CMAQ perform at or above community benchmarks. WRF-CMAQ’s chemical
performance also largely meets community standards, with substantial
nuance depending on the performance metric and component assessed. For
example, hourly simulated NO2 and O3 are highly correlated with
observations (r > 0.6) while PM2.5 is less so (r = 0.4).
Similarly, hourly simulated NO2 and PM2.5 have low biases
(<10%), whereas O3 biases are larger (<30%).
Simulated spatial pollutant patterns show distinct urban-rural
footprints, with urban NO2 and PM2.5 20-60% higher than rural, and
urban O3 6% lower. We use our 1.3 km simulations to resolve
high-pollution areas within individual urban neighborhoods and
characterize changes in O3 regimes across tight spatial gradients. Our
findings demonstrate both the benefits and limitations of
high-resolution simulations, particularly over urban settings.