Low-molecular weight and oligomeric components in secondary organic aerosol from the photooxidation of p-xylene

A laboratory study was performed to investigate the composition of secondary organic aerosol (SOA) products from photooxidation of the aromatic hydrocarbon p-xylene. The experiments were conducted by irradiating p-xylene/CH3ONO/NO/air mixtures in a home-made smog chamber. The aerosol time-of-flight mass spectrometer (ATOFMS) was used to measure the size and the chemical composition of individual secondary organic aerosol particles in real-time. According to a large number of single aerosol diameters and mass spectra, the size distribution and chemical composition of SOA were determined statistically. Experimental results showed that aerosol created by p-xylene photooxidation is predominantly in the form of fine particles, which have diameters less than 2.5 mu m (i.e. PM2.5), and aromatic aldehyde, unsaturated dicarbonys, hydroxyl dicarbonys, and organic acid are major product components in the SOA after 2 hours photooxidation. After aging for more than 8 hours, about 10% of the particle mass consists of oligomers with a molecular mass up to 600 daltons. The possible reaction mechanisms leading to these products are also proposed.