Insights into aerosol chemical composition and optical properties at Lulin Atmospheric Background Station (2862 m asl) during two contrasting seasons.

Continental outflows from peninsular Southeast Asia and East Asia dominate the widespread dispersal of air pollutants over subtropical western North Pacific during spring and autumn, respectively. This study analyses the chemical composition and optical properties of PM aerosols during autumn and spring at a representative high-altitude site, viz., Lulin Atmospheric Background Station (23.47°N, 120.87°E; 2862 m a.s.l.), Taiwan. PM mass was reconstructed and the contributions of major chemical components were also delineated. Aerosol scattering (σ) and absorption (σ) coefficients were regressed on mass densities of major chemical components by assuming external mixing between them, and the site-specific mass scattering efficiency (MSE) and mass absorption efficiency (MAE) of individual components for dry conditions were determined. NHNO exhibited the highest MSE among all components during both seasons (8.40 and 12.58 m g at 550 nm in autumn and spring, respectively). (NH)SO and organic matter (OM) accounted for the highest σ during autumn (51%) and spring (50%), respectively. Mean MAE (mean contribution to σ) of elemental carbon (EC) at 550 nm was 2.51 m g (36%) and 7.30 m g (61%) in autumn and spring, respectively. Likewise, the mean MAE (mean contribution to σ) of organic carbon (OC) at 550 nm was 0.84 m g (64%) and 0.83 m g (39%) in autumn and spring, respectively. However, a classification matrix, based on scattering Ångström exponent, absorption Ångström exponent, and single scattering albedo (ω), demonstrated that the composite absorbing aerosols were EC-dominated (with weak absorption; ω = 0.91-0.95) in autumn and a combination of EC-dominated and EC/OC mixture (with moderate absorption; ω = 0.85-0.92) in spring. This study demonstrates a strong link between chemical composition and optical properties of aerosol and provides essential information for model simulations to assess the imbalance in regional radiation budget with better accuracy over the western North Pacific.