# Evaluation of Different Chemical Mechanisms on O3 and PM2.5 Predictions in Alberta, Canada
## 1 Introduction
Additional improvements to the model included updated rate constants, updated photolysis rates, updated chlorine chemistry involving aldehydes and ketones, and, perhaps most importantly for this discussion, improved treatment of proxy radicals under low NOx conditions leading to better estimates of the formation of hydroperoxides and Secondary Organic Aerosols. The version of SAPRC07 (saprc07_ae6_aq) employed in this work comprised 97 explicit species and 64 lumped species, which allowed the modelling of 732 reactions. The CB model was originally developed in the 1970s [16], with the most commonly used version being the CB-IV model [14]. The CB models use a blend of explicit species (e.g., ethene, isoprene, formaldehyde) with the remaining organic species represented by a lumped-structure model made up of a simple sum of the more basic carbon bond types making up the species. In some cases, the implied reactivity may be moderated by assigning some of the carbon atoms to a non-reactive group. The lumped-structure approach allows the simple addition of new organic species but may make interpretation of the predicted concentrations complicated. Significant changes were incorporated into the CB05 model [15] including updated kinetic reaction and photolysis rates, additional inorganic pathways, and improved organic chemistry related to alkanes, alkenes (particularly isoprene), and oxygenated products and intermediaries [23]. Alkyl nitrates in the model are more reactive, liberating more NO\({}_{2}\), while simultaneously allowing for greater deposition of the soluble alkyl nitrates. The net results of these improvements in CB05 are the higher predicted concentrations of O\({}_{3}\) during the summer and winter compared to CB-IV. The current CB version is CB6R3 [24], which was developed from CB6R2 to better represent wintertime high-ozone events in the Rocky Mountains, where modelled O\({}_{3}\) was significantly lower (-one third) than the observed O\({}_{3}\), by accounting for temperature and pressure effects on alkyl nitrate formation not accounted for in earlier CB models [25]. In particular, the reduced rate of formation of alkyl nitrates at lower temperatures enhanced the formation of O\({}_{3}\), given the additional availability of nitric oxide (NO) and alkanes to produce O\({}_{3}\). The prediction of O\({}_{3}\) did improve using CB6R3, albeit modestly, but elevated NOz (total nitrogen containing species--NO\({}_{X}\)) species, particularly nitric acid, in both summer and winter, suggests additional research on reaction rates and/or removal processes is warranted [26]. For this work, we used cb6r3_ae6_aq, which comprised 60 explicit species and 68 lumped species, allowing the modelling of 339 possible reactions. The three chemical mechanisms discussed above all share the concept of reaction rates and products; however, as previously stated, they differ in terms of rate constants, photolysis (due to pressure and temperature changes), and the treatment of organic and inorganic chemistry. Several studies [9; 27; 28; 29; 30; 31; 32; 33] have compared these mechanisms and found considerable variations in the model predictions, particularly for O\({}_{3}\). [PERSON] et al. [34] investigated uncertainties in O\({}_{3}\) formation caused by SAPRC07, CB05, RADM2, and SAPR99 and found that differences in reaction rate constants and lumped volatile organic compounds may be the cause of the differences in O\({}_{3}\) production. [PERSON] et al. [35] compared the RADM series of ACTMs to the RACM series and noted the greatest daily maximum O\({}_{3}\) values predicted for RACM2 used updated photolysis rates, particularly more accurately predicted HCHO and NO\({}_{2}\) photolysis rates, which led to enhanced OH radical production. [PERSON] et al. [36] compared the CB6 series (CB6r1, r2, r3) under urban conditions and noted the sensitivity of these models to temperature and reduced O\({}_{3}\) production, due to the enhanced formation of organic nitrates.