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In 2014, Pha Din (1466 m a.s.l.) was established as a Global Atmosphere Watch (GAW) regional station for aerosol and trace gas measurements in northwestern Vietnam. This study presents a five-year climatology of aerosol optical properties derived from nephelometer and aethalometer measurements and a comparison with ground-based remote sensing measurements at the nearby AERONET station of Son La. The annual variations of the aerosol measurements at Pha Din are clearly dominated by annually recurring periods with high biomass burning activity in northern Southeast Asia (February-May). During these periods, a large majority of air masses arriving at Pha Din originate to the southwest (northern Thailand, Laos and Myanmar). Both the meteorological conditions and the aerosol optical properties are very similar between individual periods (increased temperature: > 20 degrees C; moderate ambient relative humidity: 60-70%; decreased single scattering albedo: 0.8-0.9; increased absorption Angstrom exponent: 1.6-2.0; and scattering Angstrom exponent: >>1). Prior to the biomass burning season (October-January), the meteorological conditions at Pha Din are influenced by the SE Asian monsoon, leading to the frequent transport of air masses from SW China with moderate aerosol loadings. The lowest pollution levels are observed from June to September, which represents the wet season.
