# Evaluation of Different Chemical Mechanisms on O3 and PM2.5 Predictions in Alberta, Canada
## 3 Results and Discussion
### O\({}_{3}\)  
#### 3.2.1 Diurnal Variation  
Since O\({}_{3}\) is a photochemical pollutant, its diurnal variation is important to understand. Figure 4 illustrates the diurnal plot for the Edson, Grande Prairie, and Beaverlodge air monitoring stations. The station diurnal plots include O\({}_{3}\) collected at the respective stations in addition to the predictions by the three mechanisms. The diurnal variation of O\({}_{3}\) follows a standard pattern over the course of the day by increasing after sunrise, reaching a maximum at roomtime, and subsiding towards the evening. The pattern is more prominent in the summer than in the winter, as O\({}_{3}\) production is elevated due to higher temperatures and actinic flux [51]. All three mechanisms share many of the same pathways and reaction rates for the inorganic formation and destruction of O\({}_{3}\). The organic pathways for the formation and destruction of O\({}_{3}\) are more complicated to understand, given the differences in definition or clumping of the organic compounds involved, but the similarity of the predicted O\({}_{3}\) levels suggests the organization of the organic compounds is essentially the same. In addition, the reaction rates for the formation via the many available organic pathways are lower, in general, than the inorganic pathways. One notable exception to this is the formation of O\({}_{3}\) from some ketones with the hydroxyl and peroxide ions in the CB6R3 model, which shows an elevated O\({}_{3}\) formation rate that is comparable to the inorganic pathways. There are two qualitative comparisons we can make between the different mechanisms and O\({}_{3}\) observations that may shed light on the quality of the mechanisms: the first is the general fit by the mechanisms to the diurnal O\({}_{3}\) pattern, and the second is the hourly dispersion of O\({}_{3}\) values within each mechanism and within the observations. In general, during the summer, all three mechanisms produce reasonable peak values of O\({}_{3}\) at all three sites. However, the nighttime values as well as the dawn and dusk values of predicted O\({}_{3}\) for all three mechanisms are generally lower than observations. One notable exception is at the Edson site, where the predicted dawn values are higher than observed. In the winter, all three mechanisms tend to under-predict O\({}_{3}\), though there is some variability in the degree of this from site to site and for the time when it occurs. In a previous study undertaken in Alberta [10], it was noted that the variation in KZMIN (Table 1: KZMIN = 0.1 m\({}^{2}\)/s urban, 2.0 m\({}^{2}\)/s rural) allowed better fitting of mean predicted primary species and O\({}_{3}\) concentrations to observations. Too high of a KZMIN resulted in an over-prediction of O\({}_{3}\) at night, and too low of a KZMIN resulted in an over-prediction of the primary species. The selected KZMIN values may not represent the optimal values for diffusion in the Planetary Boundary Level (PBL) and may account for some of the discrepancy between the predicted and observed O\({}_{3}\) values. Additional local effects, i.e., winter pile burning or wood heating, may account for the some of the intermittent variability seen during the winter [10]. The dispersion of predicted hourly values may provide some insight into the performance of the different mechanisms. In all cases, it should be noted that the hourly variability for the three mechanisms is similar. This is not surprising, given the common photochemical reactions and rates the mechanisms are built on. During the summer, the night, dusk, and dawn variability in the mechanisms and the observations are similar. During daytime, there is less variability in the predicted O\({}_{3}\) values than the observed values, which may indicate there are some additional pathways for the creation or destruction of O\({}_{3}\). Perhaps the added ketone pathways included in the CB6R3 mechanism are pointing at the need to further identify and include similar pathways, or perhaps the reaction rates are not quite right.