PAH concentrations simulated with the AURAMS-PAH chemical transport model over Canada and the USA

Abstract

Abstract. The offline Eulerian AURAMS (A Unified Regional Air quality Modelling System) chemical transport model was adapted to simulate airborne concentrations of seven PAHs (polycyclic aromatic hydrocarbons): phenanthrene, anthracene, fluoranthene, pyrene, benz[a]anthracene, chrysene + triphenylene, and benzo[a]pyrene. The model was then run for the year 2002 with hourly output on a grid covering southern Canada and the continental USA with 42 km horizontal grid spacing. Model predictions were compared to ~5000 24 h-average PAH measurements from 45 sites, most of which were located in urban or industrial areas. Eight of the measurement sites also provided data on particle/gas partitioning which had been modelled using two alternative schemes. This is the first known regional modelling study for PAHs over a North American domain and the first modelling study at any scale to compare alternative particle/gas partitioning schemes against paired field measurements. The goal of the study was to provide output concentration maps of use to assessing human inhalation exposure to PAHs in ambient air. Annual average modelled total (gas + particle) concentrations were statistically indistinguishable from measured values for fluoranthene, pyrene and benz[a]anthracene whereas the model underestimated concentrations of phenanthrene, anthracene and chrysene + triphenylene. Significance for benzo[a]pyrene performance was close to the statistical threshold and depended on the particle/gas partitioning scheme employed. On a day-to-day basis, the model simulated total PAH concentrations to the correct order of magnitude the majority of the time. The model showed seasonal differences in prediction quality for volatile species which suggests that a missing emission source such as air–surface exchange should be included in future versions. Model performance differed substantially between measurement locations and the limited available evidence suggests that the model's spatial resolution was too coarse to capture the distribution of concentrations in densely populated areas. A more detailed analysis of the factors influencing modelled particle/gas partitioning is warranted based on the findings in this study.

GEM-MACH-PAH (rev2488): a new high-resolution chemical transport model for North American PAHs and benzene

“…Since PAHs have very small concentrations relative to criteria air contaminants, as in Galarneau et al (2014), we assume they do not have a significant effect on oxidant concentrations (O 3 and OH). Thus, the PAHs in GEM-MACH-PAH make use of the outcomes of the model's gas and aqueous-phase chemistry in a diagnostic fashion for PAH oxidation.…”

“…AURAMS-PAH included two parametrizations to calculate gas/particle partitioning: Junge-Pankow, JP (Junge, 1977;Pankow, 1987) and DE (Dachs and Eisenreich,150 2000), both of which, when applied for partitioning in AURAMS-PAH, assigned too much PAH mass to the gas phase. The two schemes resulted in surprisingly similar gas-particle partitioning (Galarneau et al, 2014). We carried out post-processing and analysis on the AURAMS-PAH model output from both schemes as well as the observations of gas and particle PAHs from the Galarneau et al (2014) study, in order to determine which scheme to proceed with in GEM-MACH-155 PAH, and how it could be improved.…”

“…The two schemes resulted in surprisingly similar gas-particle partitioning (Galarneau et al, 2014). We carried out post-processing and analysis on the AURAMS-PAH model output from both schemes as well as the observations of gas and particle PAHs from the Galarneau et al (2014) study, in order to determine which scheme to proceed with in GEM-MACH-155 PAH, and how it could be improved.…”

“…Our modifications to GEM-MACH include adding benzene and seven gas-phase and 14 particlephase (7 species × 2 size bins) PAHs to the species arrays, and adding the gas-particle partitioning 130 subroutine described in Galarneau et al (2014), but with updated partitioning parameters (see Section 2.2.1). Since PAHs have very small concentrations relative to criteria air contaminants, as in Galarneau et al (2014), we assume they do not have a significant effect on oxidant concentrations (O 3 and OH).…”

“…Some of these processes and/or their controlling parameters were updated relative to Galarneau et al (2014) and are described in the subsections below.…”