Atmospheric fate and deposition of polyhalogenated carbazoles in urban environment

Polyhalogenated carbazoles (PHCZs) are of emerging environmental concern due to their dioxin-like toxicity and widespread distribution in the aquatic environment. However, knowledge of the occurrence, fate, and impacts of PHCZs in the atmospheric environment is limited. The present study investigated air concentrations and atmospheric deposition of 11 PHCZs in Toronto, Canada, during the winter of 2024. The event-based effects of atmospheric deposition on the mass load of PHCZs to urban streams were also evaluated to better understand the transport of PHCZs from urban environments to receiving watersheds. Results showed that 3-chloro-9H-carbazole (3-CCZ) and 3,6-dichloro-9H-carbazole (36-CCZ) were the predominant PHCZs in all matrices. Similar concentrations of 3-CCZ (0.020 ± 0.013 pg/m³) and 36-CCZ (0.030 ± 0.022 pg/m³) were found in the gas phase, while higher concentrations of 36-CCZ (0.086 ± 0.045 pg/m³) were observed in the particle phase compared to 3-CCZ (0.029 ± 0.022 pg/m³). Temperature positively affected the partitioning of 36-CCZ to the gas phase. Although these two PHCZs were frequently found in the air, the estimated Toxic Equivalent Quantity and Estimated Daily Intake suggested they were unlikely to pose risks to human health relevant to dioxin-like effects via outdoor inhalation. The atmospheric deposition of 3-CCZ and 36-CCZ was 45 ± 63 pg/m²/day and 89 ± 60 pg/m²/day, respectively. The scavenging of 3-CCZ increased at lower temperatures due to greater partitioning to precipitation. In addition, rain and snowmelt could increase the mass flux of these two PHCZs (approximately up to 3-fold) in urban streams and result in more PHCZs being delivered to aquatic environments.