Particulate matter of smaller than 2.5 μM in diameter (PM_2.5) consists of a large fraction of organic matter. However, their composition at molecular level and sources were not yet fully understood. This hinders the effort to understand aerosol’s net climate impacts, environment impacts and most importantly, its health effects. In this work, more than twenty organic acids and cholesterol in PM_2.5 were studied for their abundance, temporal variations and potential sources at three different characteristic sites: Yuen Long (YL, urban); Mong Kok (MK, urban roadside) and Lok Ma Chau (roadside) in Hong Kong. The target organic acids include secondary organic aerosol (SOA) tracers of five precursors, two fatty acids, and a number of dicarboxylic acids and their hydroxyl-substituted derivative...[ Read more ]

Particulate matter of smaller than 2.5 μM in diameter (PM_2.5) consists of a large fraction of organic matter. However, their composition at molecular level and sources were not yet fully understood. This hinders the effort to understand aerosol’s net climate impacts, environment impacts and most importantly, its health effects. In this work, more than twenty organic acids and cholesterol in PM_2.5 were studied for their abundance, temporal variations and potential sources at three different characteristic sites: Yuen Long (YL, urban); Mong Kok (MK, urban roadside) and Lok Ma Chau (roadside) in Hong Kong. The target organic acids include secondary organic aerosol (SOA) tracers of five precursors, two fatty acids, and a number of dicarboxylic acids and their hydroxyl-substituted derivatives.

Over 100 Filter-based samples were collected between 2015-2017 and analyzed by gas chromatography-mass spectrometry with MSTFA derivatization. A sample treatment procedure was develop to solve an issue of high contamination of palmitic acid and stearic acid in method blanks. The background contamination severely affected the quantification of these fatty acids, which, together with cholesterol, serve as effective tracers for cooking emissions. Plastic syringe and plastic syringe filter case were identified to be the major contamination source. Replacing the plasticware with glassware and stainless-steel material has minimized the contamination, allowing the reliable quantification of the two fatty acids in ambient PM_2.5 samples.

Correlation analysis between dicarboxylic acids and SOA tracers or oxidant (Ox) revealed their different origins. Smaller diacids such as glutaric acid and adipic acid correlated best with oxidant (Rp as high as 0.9) among other acids suggesting their dominant secondary formation in the atmosphere while larger diacids such as suberic acid and azelaic acid correlated well with biomass burning related compounds (Rp~0.7-0.8) implying they shared source commonality with biomass burning source. Hydroxy-diacids tended to correlate well with biogenic SOA tracers indicating their dominant biogenic origins (Rp as high as >0.9). Higher abundance of fatty acids in urban sites suggest anthropogenic input in those areas. The highest abundance of fatty acids and cholesterol observed at MK site was probably contributed from both vehicle and cooking emissions. The data sets have provided key molecular source marker concentrations for quantitative PM_2.5 source apportionment.