Investigation of Anthropogenic Monoterpenes in Canadian Cities

Volatile organic compounds from traffic-related emissions, such as benzene, toluene, ethylbenzene, and xylenes (BTEX), have decreased in Canada due to mitigation measures. As a result, other volatile organic compounds (VOCs), such as those from volatile chemical products (VCPs), which include the unregulated monoterpene limonene, have become one of the more important components of urban VOCs. To understand the extent to which anthropogenic sources contribute to monoterpene concentrations and the impact on atmospheric reactivities within major Canadian urban areas, we examine historical monoterpene concentrations (limonene, α-pinene, and β-pinene) provided by the Environment and Climate Change Canada air monitoring database with respect to their temporal and spatial trends and relative contributions to total OH reactivity. Ratios of limonene to monoterpene serve as proxies for comparing monoterpene distributions among forested, residential, and industrial sites. Stations with higher levels of human influence show limonene to α-pinene and limonene to β-pinene ratios that are 3.2 and 2.7 times greater, respectively, compared with ratios observed in forested sites. In evaluating the relationship between monoterpene abundance and population density, the Spearman’s rank correlation coefficient was found to be the highest for limonene with a value 0.51. In the case of α-pinene, the coefficient was 0.26, while there was no statistically significant correlation detected for β-pinene. In larger cities such as Toronto, the monoterpene concentrations are higher than those in proximal forested areas, especially during the winter months. While monoterpenes only constitute 5% of the total OH reactivity of monitored VOCs, their relative contribution has remained consistent over the last two decades, emphasizing the imminent need to shift efforts to monitoring and regulating limonene and many other VCP compounds.