Secondary formation of organic aerosol in the Los Angeles basin: A descriptive analysis of organic and elemental carbon concentrations

An in situ carbon analyzer measured particulate organic and elemental carbon with 2 h time resolution during the Southern California Air Quality Study in 1987. Organic and elemental carbon concentrations showed strong diurnal variations. Peak concentrations occured during the daylight hours in the summer and at night in the fall. The maximum concentrations observed in the fall (maximum total carbon = 88 μmCm−3) were two to three times higher than the summer maxima (maximum total carbon = 36 μgCm−3). Two periods were examined in detail: 25–31 August and 17–19 November. Between 25 and 31 August the correlation (R2) between organic and elemental carbon decreased from 89% to 0% and then increased to 28%. 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 this period. Three other periods of high secondary formation were identified: 22–28 June, 11–13 July and 25–29 July.

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