Exploring the Relationship Between Particle and Gas Phase Chlorine in Continental Winter

Abstract Road salt is widely used in many North American and European urban wintertime environments, yet its impacts on tropospheric reactive chlorine species are still uncertain. We present ambient size‐resolved particle‐phase chloride (Cl − ) and hydrogen chloride (HCl) measurements that were made in continental urban winter (Toronto, ON) concurrent with several applications of road salt. We also show long‐term monitoring of Cl − and Na + (road salt ions) mass loadings from local National Air Pollution Surveillance (NAPS) stations in Toronto. Long‐term levels of road salt ions were higher by a factor of 3.7 ± 1.5 and 2.9 ± 0.9, respectively, during winter months (December–March) in comparison to summer months (May–September). Cl − mass loadings from stations as well as size‐resolved measurements rival those observed in coastal/marine environments. Cl − mass fractions were 32 ± 9% and 8 ± 3% of the coarse and fine modes, respectively. Although Cl − levels were comparable to those in coastal locations, HCl mixing ratios were lower, ranging from <4 to 896 parts per trillion by volume (pptv). Fresh application of road salt was not associated with elevated HCl. A thermodynamic model (E‐AIM IV) was used to assess the contribution of HCl partitioning to observed mixing ratios using measured levels of key particulate and gas phase species. Simulations showed that HCl partitioning was a minor contributor to the observed HCl levels under low‐temperature winter conditions. Particles in both the coarse and fine mode were Cl − deficient, providing evidence for repartitioning of Cl − from the coarse to the fine mode.