Ionic Composition of Particulate Matter across Canada Shows Evidence for Near-Ubiquitous Chloride Activation and Sulfate Neutralization.

Using publicly available data from the National Air Pollution Surveillance Program, water-soluble ion mole fractions in PM 2.5 and PM 2.5-10 were extracted for 2010, 2012, 2013, and 2015 at six locations across Canada. Fine particle ion content was dominated by ammonium sulfate, with ammonium to sulfate ratios largely approaching 2 and no seasonal or geographic trends. Sulfate and nitrate mole fractions were negatively correlated in the fine particles, consistent with their expected pathways of entry into PM 2.5 . Coarse particle composition varied depending on location and season. Coastal stations were dominated by sodium and chloride year-round while inland stations showed high amounts of these ions only during the colder months, where road salt is often applied. A negative correlation between nitrate and chloride mole fractions as well as a positive correlation between nitrate concentrations and "missing" chloride provides evidence for near-ubiquitous nitrate displacement of chloride. This displacement was strongly indicated in coastal sites and also evident inland, especially during cold months. Weak evidence for nitrogen dioxide as the source of nitrate was found for inland sites, but none was found for coastal sites, suggesting a nonlocal source for the particulate nitrate precursor.