RESUMO
Organic molecular composition of fine aerosols in the free troposphere is poorly understood. Here, PM2.5 (particles with aerodynamic diametersâ¯≤â¯2.5⯵m) samples were collected at the summit of Mt. Emei (3080â¯m a.s.l.) in the Southwestern China on a daytime and nighttime basis during summer 2016 (June-July). The samples were analyzed by solvent-extraction followed by derivatization and gas chromatography/mass spectrometry (GC/MS). Four classes of organic compounds, i.e. n-alkanes, fatty acids, saccharides and lignin/resin acids were measured quantitatively. Fatty acids were found to be the most abundant species with an average concentration of 401⯱â¯419â¯ngâ¯m-3 (range 25.7-1490â¯ngâ¯m-3) in the daytime, similar to the average concentration at night (399⯱â¯447â¯ngâ¯m-3, 19.6-1970â¯ngâ¯m-3). However, the concentrations of biomass burning tracers (e.g., levoglucosan), primary biological aerosol tracers (e.g., mannitol and arabitol) and low molecular weight n-alkanes derived from fossil fuel combustion in daytime samples were obviously higher than those in nighttime samples. The results suggest that valley breezes transported a large number of aerosols and their precursors from the ground surface to the summit of Mt. Emei in the daytime. Estimated with tracer-based methods, the contributions of biogenic primary sources (plant debris, fungal spore, and biomass burning) to organic carbon was in the range of 3.28-83.5% (22.0⯱â¯17.5%) in the daytime and 3.45-37.4% (10.9⯱â¯8.97%) at night. As the largest contributor, biomass burning was an important anthropogenic/natural source of aerosol particles in the free troposphere over Mt. Emei. CAPSULE: Valley/mountain breeze is an important constraint to the temporal variations in organic aerosols over Mt. Emei.