High daytime abundance of primary organic aerosols over Mt. Emei, Southwest China in summer.

Organic molecular composition of fine aerosols in the free troposphere is poorly understood. Here, PM2.5 (particles with aerodynamic diameters ≤ 2.5 µm) samples were collected at the summit of Mt. Emei (3080 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€¯419 ng m-3 (range 25.7-1490 ng m-3) in the daytime, similar to the average concentration at night (399 ±â€¯447 ng m-3, 19.6-1970 ng 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.