# Evaluation of Different Chemical Mechanisms on O3 and PM2.5 Predictions in Alberta, Canada
## 1 Introduction  
Air pollution remain one of the most serious threats to human health and ecosystems, despite efforts to control anthropogenic emissions. Ground-level ozone (O3) and particulate matter (PM2.5) are two pollutants of most concern, given their adverse effect on human health and ecosystems [1; 2; 3; 4; 5; 6; 7]. However, O3 and a significant portion of PM2.5 (secondary PM2.5) are secondary pollutants formed by the complex synthesis of several precursor gases, such as volatile organic compounds (VOCs), oxides of nitrogen (NOX), and sulfur dioxide (SO2), in the atmosphere with an evolution and impact that may be difficult to predict [8]. Furthermore, given the limited availability of suitable temporal and spatial measurements of pollutants, atmospheric chemical transport models (ACTMs) are essential for estimating secondary pollutant concentrations. However, ACTMs are typically developed to understand place-based (urban/rural, high elevation) problems using updated chemical kinetics and photolytic rates, additional chemical species, and/or additional chemical pathways [9]. Furthermore, adjustments to the mechanical component of an ACTM, e.g., the minimum eddy diffusivity coefficient as a function of land use [10], if any, must be considered when evaluating an ACTMs performance. From an air-quality-management perspective, selection of the wrong ACTM may lead to incorrect conclusions regardingthe influence of precursors on the formation of secondary pollutants and, subsequently, a poor management response. Changes to the vertical diffusivity, mixing height, etc., [11] may also introduce an ad hoc bias in the model, which may be difficult to interpret. Hence, combining suitable temporal and spatial monitoring data, when available, serves to both guide in the selection of the most appropriate ACTM as well as better inform the choice of effective mitigation strategies. Many chemical mechanisms have been developed for ACTMs to address the issues associated with urban/rural O\({}_{3}\) formation. Three of the most widely used mechanisms are the Regional Atmospheric Chemistry Mechanism 2 (RACM2) model [12], the State Air Pollution Research Center (SAPRC07) model [13], and the Carbon Bond 6 (CB6) model [14; 15; 16]. The RACM2 model is built on earlier ACTMs, in particular the Regional Acid Deposition Model (RADM) [17] and its extension (RADM2) [18], which had been developed to understand the formation of atmospheric acids, the formation of organic and inorganic peroxides, and aqueous chemistry involving the oxidation of SO\({}_{2}\) in urban/polluted and rural/less polluted areas, respectively. Improved reaction rates, updated reaction schemes and product yields for a more extensive suite of inorganic and organic compounds continued with the development of the RACM [19] and RACM2 [12]. Of note were improvements to explicitly include organic compounds, i.e., alkanes, alkenes, carbonyls, and aromatics, believed to be significant reactants in a relatively clean (low NOx) atmosphere. While expanding the chemistry consideration of computational constraints was made by lumping species of similar reactivity together, the version of RACM2 [20] employed here comprised 54 explicit species and 80 lumped species, making available 582 reactions in total. SAPRC07 [21] is likely the most comprehensive and adaptable photochemical model considered here, allowing for specific representation of up to 780 types of VOCs [13], when needed for particular reactivity assessments of O\({}_{3}\) formation. However, for computational purposes, it also makes use of a lumped-chemical model developed from the earlier SAPRC99 [22], which uses a numerical parameter to weight the contribution from individual reactions.