Molecular and Elemental Tracers-Based Receptor Modeling of PM2.5 in Suburban Hong Kong With Hourly Time-Scale Air Quality Considerations

Abstract

Continuing control efforts in recent decade have notably brought down fine-particulate-matter (PM2.5) pollution to compliance with the annual average air quality objective in Hong Kong. However, short-term (several hours to few days) PM2.5 pollution sporadically occurred in winter and has become a major focus of attention, calling research in characterizing sources and their variations at hourly timescale. This study reports hourly measurements of PM2.5 and its chemical speciation, including molecular and elemental tracers in suburban Hong Kong in 2020 winter. Hourly PM2.5 was 20.3 ± 7.33 μg m−3, dominated by organics and sulfate. We deployed tracer-based source apportionment using positive matrix factorization, which resolved six secondary and seven primary sources. Sulfate-rich and nitrate-rich factors were the leading PM2.5 contributors. Variation in source contributions under different air masses revealed the Pearl River Delta region and coastal areas to the north are main origins. Elevation in secondary sources was the driving factor in causing high PM2.5 polluted hours. During Chinese New Year holiday, firework emissions partially offset the reductions in anthropogenic sources, even leading to an increased PM2.5. Hour-by-hour examination of most polluted and firework event hours enhanced understanding on dynamic variations of haze and firework burning evolution. The mass concentration of firework-derived PM2.5 was notably lower under “prohibition” than “restriction” control scenario (avg. 3.5 vs. 8.4 μg m−3). Our results highlight the effectiveness of tracer-based source apportionment in revealing hourly PM2.5 source variations and the necessity of regional joint management to lessen the severity of episodic PM2.5 pollution and health risks.