Eighteen-year trends of local and non-local impacts to ambient PM₁₀ in Hong Kong based on chemical speciation and source apportionment

Along with rapid economic development in China in the past twenty years, air pollution has deteriorated significantly and then gradually improved afterwards. Emission changes in response to the economic development and control measures and their impacts to ambient pollution levels are thus of great scientific and policy-making implications. By using the eighteen-year PM₁₀ speciation dataset that have been continuously collected at multiple stations over Hong Kong since 1998, we demonstrated a novel approach to identify long-term trends of local and non-local contributions to ambient PM₁₀ in Hong Kong based on Positive Matrix Factorization source apportionment and backward trajectory cluster analysis. It is found that vehicle exhaust contribution had decreased from 9.9 μg·m⁻³ in 1998 to 3.4 μg·m⁻³ in 2015, corresponding to a 66% decrease. However, residual oil contribution increased threefold, which indicated ship emission needs to be primarily addressed in air pollution management in Hong Kong. The mostly non-local sources of secondary sulfate, industry, and coal / biomass burning all showed inverse V-shape trends during the eighteen years, indicating the dual roles of economic development and emission control in shaping the ambient pollution levels. Local contributions dominated for vehicle exhaust and aged sea salt, while non-local ones dominated for secondary sulfate, secondary nitrate, industry, and coal / biomass burning. Although with small fluctuations, non-local contributions consistently accounted for 63–70% of total PM₁₀ during the eighteen years, similar as previous estimates using a different approach. Such a dominant non-local contribution highlights the importance of regional collaboration to reduce PM levels in Hong Kong.