Primary versus Secondary Organic Carbon

Contributions of the primary and secondary components of aerosol OC have been difficult to quantify. Lack of a direct chemical analysis method for the identification of either of these OC components has led researchers to employ several indirect methods. These methods include use of tracer compounds for either the primary or the secondary OC (Larson et 

FIGURE 13.7 Diurnal variation of organic nitrate size distributions in Southern California. Organic nitrates are secondary organic aerosol compounds. Reprinted from Atmos. Environ., 25A, Mylonas et al., 2855-2861, Copyright 1991, with kind permission from Elsevier Science, Ltd., The Boulevard, Langford Lane, Kidlington 0X5 GB1, UK. 

Elemental carbon has often been used as a tracer of primary OC (Chu and Macias, 1981 Wolff et al., 1983 Novakov, 1984 Gray et al., 1986 Huntzicker et al., 1986 Turpin et al., 

The underlying hypothesis is that because EC and primary OC often have the same sources, there is a representative ratio of OC/EC for the primary aerosol. If the measured ambient OC/EC ratio exceeds this expected value, then the additional OC can be considered to be secondary in origin. A weakness of that approach is that OC/EC emission rates vary by source and therefore the primary ratio will be influenced by meteorology, diurnal and seasonal fluctuations in emissions, and local sources. 

Particulate organic carbon comprises a large number of compounds having significant variations in volatility as a result, a number of these compounds can be present in both the gas and particulate phases. The ability of such semivolatile compounds to coexist in both phases complicates the distinction between primary and secondary OC. Strictly speaking, secondary OC starts its atmospheric life in the gas phase as a VOC, undergoes one or more chemical transformations in the gas phase to a less volatile compound, and finally transfers to the particulate phase by condensation or nucleation. Therefore the term 

FIGURE 14.6 Annual mean OC concentration distribution in the United States based on samples from remote stations (a) pgm-3 (b) percent of total aerosol mass. The influence of the urban centers has been eliminated to a large extent and the results depict the regional distributions (U.S. EPA 1996). 

Primary OC is emitted mainly by combustion or combustion-related sources (e.g., resuspension of combustion particles) and is accompanied by EC, but there is generally also a noncombustion component mainly from biogenic sources  

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Mediterranean Basin. Additional studies of the finer particle fractions and chemical characterisation will elucidate the sources and impact of particle pollution in the area. Particularly there is a lack of data on carbonaceous aerosol that organic and elemental carbon observations could help to distinguish the contribution of secondary versus primary sources. 

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