# Evaluation of Different Chemical Mechanisms on O3 and PM2.5 Predictions in Alberta, Canada
## 3 Results and Discussion
### Fine Particulate (PM\({}_{2.5}\))
#### 3.3.3 Discussion  
All three mechanisms under-predicted PM\({}_{2.5}\) concentrations. Of the three, RACM2 performed slightly better, except at Edson where it was slightly worse. The Secondary Organic Aerosol Model (SORGAM) used in this study was originally developed on the basis of the absorption properties of organic aerosol, to better model the formation of low-volatility products from reactive organic gases via oxidation by OH, NO\({}_{3}\), or O\({}_{3}\), as well as their subsequent gas/particle partitioning [52]. This method was then extended to include the available species in the RACM chemistry, by creating eight classes representing anthropogenic (four--two aromatic, one higher alkane, one higher alkene) and biogenic (four--two for \(\alpha\)-pinene, two for limonene) gas/particle equilibria [52]. Of particular note was the complexity of describing the non-linear relationship between the primary and secondary aerosols, as well as the strong dependence on temperature for driving the equilibria, i.e., lower temperatures favored particle formation. The present study employed more up-to-date chemical mechanisms with improved reaction rates, rate constants, and products. As an example, RACM2 has been updated to incorporate better chemistry for aromatics, isoprene, alcohols (in remote areas), and acetone (in the upper atmosphere) [12]. The enhanced production of SOA-Anth and sulfate (Figure 8) by RACM2, over the other mechanisms for all seasons, suggests both the organic and inorganic anthropogenic particulate is best modelled by this mechanism. SOA-Bio and NH\({}_{4}\) production is similar between the mechanisms, with SAPRC07 slightly better at producing SOA-Bio, though this difference is probably not significant. The most striking change in the PM\({}_{2.5}\) formation is the change from being a SO\({}_{4}\)/SAO-Bio-dominated region in the summer to a NO\({}_{3}\)-dominated region in the winter (Figure 9). Clearly the PM\({}_{2.5}\) chemistry has changed during these periods. [PERSON] et al. [53] noted the sensitivity of SOA formation on the ratio of the molar concentration of NH\({}_{4}\) to SO\({}_{4}\), with SO\({}_{4}\) being preferentially formed in NH\({}_{4}\)-poor regions, i.e., where [NH\({}_{4}\)]/[SO\({}_{4}\)] molar <1.5. [PERSON] et al. [54], in a similar study in NH\({}_{4}\)-rich agricultural region, noted the molar ratio of NO\({}_{3}\) to SO\({}_{4}\) would indicate whether the SOA was being produced by mobile sources or non-mobile sources, with a low ratio being characteristic of non-mobile emissions and a high ratio being characteristic of mobile emissions (13:1--gasoline; 8:1 diesel). Table 4 presents a summary of relevant information from this study for comparison. During the summer, the region is NH\({}_{4}\)-poor (NH\({}_{4}\)/SO\({}_{4}\) < 1.5), which should drive SOA towards high SO\({}_{4}\) content. This is indeed the case (Figure 8). During the winter the  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Species**} & \multicolumn{2}{c}{**Summer**} & \multicolumn{2}{c}{**Winter**} \\  & **[\(\mu\)g/m\({}^{3}\)]** & **[\(\mu\)mol/m\({}^{3}\)]** & **[\(\mu\)g/m\({}^{3}\)]** & **[\(\mu\)mol/m\({}^{3}\)]** \\ \hline NO\({}_{3}\) & 2.00 \(\times\) 10\({}^{-2}\) & 3.20 \(\times\) 10\({}^{-4}\) & 9.00 \(\times\) 10\({}^{-1}\) & 1.45 \(\times\) 10\({}^{-2}\) \\ SO\({}_{4}\) & 6.50 region is relatively NH\({}_{4}\)-rich, and it is colder (which should favor NO\({}_{3}\) formation), which is observed, albeit with an appreciable amount of SO\({}_{4}\) still present. The NO\({}_{3}\) component may be attributable to local mobile sources (preferably diesel over gasoline), while the persistent SO\({}_{4}\) contribution may be a blend of mobile sources and non-mobile sources, such as the oil and gas activity in the area. Comparing the spatial distribution of PM\({}_{2.5}\) for winter and summer (Figure 2) also provides some indication of the nature of SOA production in the region.