In 2014, Pha Din (1466 m asl) has been established as a Global Atmosphere Watch (GAW) regional station for aerosol and trace gas measurements in North Western 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 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 to May). During these periods, air masses at Pha Din to a large majority arrive from South West (Northern Thailand, Laos and Myanmar). Both the meteorological conditions and the aerosol optical properties are very similar during the individual high biomass burning periods (increased temperature: > 20 deg C, moderate ambient relative humidity: 60–70%, decreased single scattering albedo: 0.8–0.9, increased absorption Angstrom exponent: 1.6–2.0 and a scattering Angstrom exponent significantly larger than 1). Prior to the biomass burning season (October to January), the meteorological conditions at Pha Din are influenced by the SE Asian monsoon, leading to frequent air mass transport from SW China with moderate aerosol loadings. The lowest pollution levels are measured from June to September, which time period represents the wet season.