Qinchu Fan, Xiaohong Yao, Leiming Zhang
Fine particulate matter (PM2.5) remains a major air-quality and public-health concern despite increasingly stringent emission regulations. While PM2.5 pollution has been extensively studied in urban and industrial regions, severe episodes in remote communities remain poorly characterized. Using observations from the Canadian National Air Pollution Surveillance (NAPS) network, we identified severe PM2.5 pollution episodes in two remote mountain communities in British Columbia, Canada. These episodes were associated primarily with residential space-heating emissions during winter and episodic wildfire smoke impacts during summer. Wintertime daily PM2.5 exceedance frequencies varied substantially among years, reaching seasonal maxima of approximately 30%. Winter exceedances exhibited recurrent nocturnal episodes of rapid PM2.5 accumulation under low wind speeds, occurring at consistent times of day but with highly variable hourly growth rates. This behavior is consistent with a dominant contribution from primary emissions, particularly condensable particulate matter (CPM) formed during the early evolution of residential-combustion plumes under subzero conditions. Regression analyses suggested that CPM contributions may increase by as much as ∼100 μg m⁻3 as ambient temperature decreased from 0 °C to -30 °C. Paired PM2.5 and PM10 measurements further revealed substantial evaporation of semi-volatile CPM during plume transport, implying that wintertime PM2.5 variability was governed largely by the accumulation and partial evaporation of primary particulate matter. In the absence of concurrent PM2.5 chemical composition measurements at the two study sites, chemical composition data from other NAPS sites were used to provide supplementary evidence for the analysis. The findings obtained from this study highlight the importance of improving CPM quantification under subzero conditions for effective air-quality management in cold-climate communities.