Unveiling the origins of Northern Thailand’s haze: comprehensive chemical characterization and source apportionment of PM2.5 using targeted molecular markers

Annual haze in Northern Thailand has become increasingly severe, impacting health and the environment. However, the sources of the haze remain poorly quantified due to limited observational data on aerosol molecular tracers. This study comprehensively investigates chemical composition of PM_2.5, including both inorganic and organic compounds throughout haze and post-haze periods in 2019 at a rural site of Northern Thailand. Average PM_2.5 concentrations during haze and post-haze period were 87 ± 36 and 21 ± 11 µg/m³, respectively. Organic matter was the dominant contributor in PM_2.5 mass, followed by water soluble inorganic ions and mineral dust. Molecular markers, including levoglucosan, dehydroabietic acid, and 4-nitrocatechol, and ions (Cl^-, and K⁺), were used to characterize low haze (PM_2.5 < 100 µg/m³) and episodic haze (PM_2.5 > 100 µg/m³). Low haze is associated with local aerosols from agricultural waste burning, while episodic haze is linked to aged aerosols from mixed agricultural waste, softwood, and hardwood burning. Source apportionment incorporating these molecular markers in receptor modelling (Positive matrix factorization), identified three distinct biomass burning sources: mixed, local, and aged biomass burnings, contributing 31, 19 and 13 % of PM_2.5 during haze period. During post-haze period, contributions shifted, with local biomass burning (32 %) comparable to secondary sulfate (34 %) and mixed dust and traffic sources (26 %). These findings demonstrate that both regional and local sources contribute to severe haze, highlighting the need for integrated policies for cross-border cooperation as well as stricter regulations to reduce biomass burning in Northern Thailand and Southeast Asia.