Deptartment of Environmental Science and Engineering
Oregon Graduate Center
Although carbonaceous species comprise a large fraction of urban aerosol, the contributions of primary and secondary sources to carbonaceous aerosol are not well understood. The purpose of this research was to establish a method of identifying primary and secondary organic aerosol through the use of time-resolved organic and elemental carbon measurements. Because of the need to obtain improved time-resolution, low detection limits, and minimal influence from sampling artifacts, an in situ carbon analyzer was developed, characterized, and compared with other methods. The instrument combines the sampling function of a conventional filter sampler with the analytical function of a thermal-optical carbon analyzer. Field experiments were conducted in Los Angeles to investigate volatilization and adsorption sampling artifacts. Some ambient filter samples were also analyzed for specific organic compounds using direct thermal desorption/gas chromatography/mass spectroscopy. The in situ carbon analyzer measured particulate organic and elemental carbon in the Los Angeles Basin during the Carbonaceous Species Methods Comparison Study in 1986 and the Southern California Air Quality Study in 1987. Organic and elemental carbon concentrations showed strong diurnal variations. Peak concentrations occurred during the daylight hours in the summer and at night in the fall. The maximum concentrations observed in the winter were two to three times higher than the summer maxima. On several summer days in 1987 the diurnal profiles of organic and elemental carbon were quite similar. Good correlations, comparable to those observed during the fall, were observed between organic and elemental carbon, suggesting that the organic aerosol on those days was principally primary. Comparison of the diurnal profiles of organic carbon with those of elemental carbon and ozone provided evidence for considerable secondary formation of organic aerosol during three sampling periods in 1987: July 11 - 13, July 25 - 29, and August 27 - 31. At the height of the August 27 - 31 episode secondary formation accounted for roughly 50% to 70% of the organic aerosol at 1700 hours (PDT).
Turpin, Barbara J., "Secondary formation of organic aerosol investigation of the diurnal variations of organic and elemental carbon" (1989). Scholar Archive. 170.