Investigation of the Community Multiscale air quality (CMAQ) model representation of the Climate Penalty Factor (CPF)

Abstract

Elevated levels of tropospheric ozone (O3) caused by anthropogenic emissions of NOx and VOCs have a negative impact on human health, crops, and ecosystems. Even if precursor emissions are reduced compared to current levels, predicted higher temperatures due to increased greenhouse gas emissions could impede resulting air quality benefits. The Climate Penalty Factor (CPF) has been devised to quantify the adverse impact of increasing temperatures on surface O3. We compute the CPF from surface observations at CASTNET (mostly rural) and AQS (mostly urban) sites and compare these values to CMAQ output for the contiguous U.S. and seven sub-regions for the O3 season (May 1 through September 30) for each year from 2002 through 2012. Here we show that CMAQ can in general reproduce with reasonable accuracy the CPF for the CONUS. In predominantly rural regions (CASTNET) observed CPFs are 1.94 ppb/°C while simulated CPFs are 2.11 ppb/°C. Observed CPFs are 2.00 ppb/°C in predominantly urban regions (AQS), lower than simulated CPFs of 2.39 ppb/°C. CMAQ reproduces CPFs at rural sites reasonably well but tends to somewhat overestimate CPFs at urban sites. Upon examination of each region, the CMAQ simulation for the Southeast shows a substantial overestimate (43%) of CPF for urban (AQS) sites. Conversely, the CMAQ simulations for the Northeast show a slight underestimate (8%) of the CPF at rural (CASTNET) sites. Both results for CPF (overestimated for urban sites and either underestimated or close to observed for rural sites) could be related to the tendency of CMAQ with CB05-TUCL (an excessively long lifetime of alkyl nitrates with respect to photolysis) to overestimate NOx or keep it too close to the urban source locations. Our analysis suggests that CMAQ may attribute a somewhat greater fraction of O3 production to an increase in temperature than observed. Finally, there is a ∼2% per year decrease in CPF throughout the CONUS during the 11-year period in both measurements and the CMAQ simulations, suggesting that reductions in NOx emissions can mitigate the impact of climate change on surface ozone.