Unintentional persistent organic pollutants (UPOPs) are generated from combustion and industrial processes and require continuous monitoring due to their toxicity and persistence. Hexachlorobutadiene (HCBD) has recently been listed as a POP, whereas chlorostyrenes have rarely been reported in ambient air, except for octachlorostyrene (OCS). In this study, ambient air concentrations of chlorostyrenes and HCBD were investigated, and their analytical methods including HCB and PeCB were established. Particulate and gaseous samples were collected using a high-volume air sampler and analyzed using high-resolution mass spectrometry (HRMS). Monochlorostyrenes were not detected in most samples, whereas OCS concentrations were comparable with those reported in previous studies. HCBD concentrations were slightly lower than reported values. The feasibility of passive air sampling for spatially extensive monitoring in industrial areas was also evaluated, indicating that a 2–3 month sampling period may provide sufficient analytical mass to exceed the HRMS detection limits for quantitative analysis. The limits of quantification for GC/MS were 2–6 times higher than those of HRMS. Comparable quantification results between the two instruments were observed at higher concentration levels; however, OCS quantification by GC/MS was limited at low concentration levels. These findings demonstrate that GC/MS has limitations for the analysis of UPOPs (e.g., OCS) at low concentration levels, and confirm the suitability of passive air sampling as a practical approach for spatially extensive ambient monitoring.