Seasonal air concentration variability, gas–particle partitioning, precipitation scavenging, and air–water equilibrium of organophosphate esters in southern Canada

In response to increasing production and application volumes, organophosphate esters (OPEs) have emerged as pervasively detected contaminants in various environmental media, with concentrations often exceeding those of traditional organic contaminants. Despite the recognition of the atmosphere's important role in dispersing OPEs and a substantial number of studies quantifying OPEs in air, investigations into atmospheric phase distribution processes are rare. Using measurements of OPEs in the atmospheric gas and particle phase, in precipitation, and in surface water collected in southern Canada, we explored the seasonal concentration variability, gas–particle partitioning behaviour, precipitation scavenging, and air–water equilibrium status of OPEs. Whereas consistent seasonal trends were not observed for OPE concentrations in precipitation or atmospheric particles, gas phase concentrations of several OPEs were elevated during the summer in suburban Toronto and at two remote sites on Canada's eastern and western coast. Apparent enthalpies of air–surface exchange fell mainly within or slightly above the range of air–water and air–octanol enthalpies of exchange, indicating the influence of local air–surface exchange processes and/or seasonally variable source strength. While many OPEs were present with a notable fraction in both the gas and particle phase, no clear relationship with compound volatility was apparent, although there was a tendency for higher particle-bound fractions at a lower temperature. High precipitation scavenging ratios for OPEs measured at the two coastal sites are consistent with low air–water partitioning ratios and the association with particles. Although beset by large uncertainties, air–water equilibrium calculations suggest net deposition of gaseous OPEs from the atmosphere to the Salish Sea and the St. Lawrence River and St. Lawrence Estuary. The measured seasonal concentration variability is likely less a reflection of temperature-driven air–surface exchange and instead indicates that more OPEs enter or are formed in the atmosphere in summer. More research is needed to better understand the atmospheric gas–particle partitioning behaviour of the OPEs and how it may be influenced by transformation reactions.