Faqiang Zhan, Knut Breivik, Frank Wania

Polychlorinated biphenyls (PCBs) have been identified as one of the main threats to the endangered St. Lawrence Estuary (SLE) beluga whale (Delphinapterus leucas) population. Atmospheric deposition is often considered one of the major sources of PCBs to the habitats of the whales. We use the Nested Exposure Model (NEM), a state-of-the-art multimedia fate model, to simulate the atmospheric delivery of PCBs to the SLE. NEM is able to simulate a user-defined region with high spatial resolution, while accounting for the influence of global and continental emissions. The use of nested domains greatly reduces the computational demands. In this study, a global simulation at resolution of 6° × 6° was followed by a regional simulation for North America at 1° × 1°, and finally a simulation of the target SLE region at 0.5° × 0.5°. We simulated the fate of seven highly abundant congeners (PCB-28, 52, 101, 118, 138, 153, 180) from 1930 to 2023. The model's ability to capture the spatial air concentration variability was evaluated by comparing results with measurements from a passive air sampler network deployed in the study regions from 2019 to 2022. NEM predicted air concentrations in the SLE region generally agree within a factor of three with the measurements, with even better performance for the lighter congeners, i.e., PCB-28 and 52. The simulated spatial patterns also agree well with observations by the passive air sampler network, e.g., predicting similarly elevated levels in the Montreal–Quebec City corridor. The simulated temporal trends are consistent with declining trend reported in the last few decades. NEM suggests that PCB levels in air in the SLE region have dropped over 95% since they peaked in the 1970s. The excellent agreement with observations allows us to use the model to derive information that is inaccessible by measurements. For example, the model suggests that Canadian emissions are on average responsible for half of the PCB-153 in the atmosphere of the SLE. This proportion is higher in cities such as Montreal and Quebec City where local sources are dominant. Emissions in the US and elsewhere in the world contribute almost equally to the other half. Domestic and international efforts are required to further reduce the input of PCBs to the SLE.