Urban aerosol, carbonaceous

The relative contribution of primary and secondary carbon to urban aerosol is discussed in this paper. Some data from the ACKEX study in Los Angeles have been reexamined using new values for the carbon and lead emissions. Data on total carbon, elemental carbon and lead in fine particle samples collected in St. Louis are presented. Lead and elemental carbon have been shown to be useful tracers of primary carbonaceous aerosol. It is concluded that secondary carbon is most likely to be a significant portion of the urban carbonaceous aerosol in the summer and in the middle of the day. Secondary carbon can best be measured with short time resolution sampling (At 6h). 

Particulate carbon in the atmosphere exists predominantly in three forms elemental carbon (soot) with attached hydrocarbons organic compounds and carbonates. Carbonaceous urban fine particles are composed mainly of elemental and organic carbon. These particles can be emitted into the air directly in the particulate state or condense rapidly after Introduction into the atmosphere from an emission source (primary aerosol). Alternatively, they can be formed in the atmosphere by chemical reactions involving gaseous pollutant precursors (secondary aerosol). The rates of formation of secondary carbonaceous aerosol and the details of the formation mechanisms are not well understood. However, an even more fundamental controversy exists regarding... 

In this paper we have shown that there is no simple answer to the question posed in the title of this paper. Primary carbon particles dominate the carbonaceous aerosol under certain conditions while substantial secondary carbon may be present at other times. However, the importance of secondary carbon contributions is much less obvious when 24-h samples are examined. With shorter time averaged samples (e.g. 6-h or less) the increase in secondary carbon formation can be more easily detected. Secondary carbon appears to be more important in the summer rather than winter, in the afternoon father than the early morning, and in LA rather than St. Louis. It should be noted that these conditions of increased secondary carbon aerosol formation are also more favorable conditions for photochemical reactions. Our detailed emission inventory Indicates that much more primary carbon exists in the urban aerosol than was thought previously. This is in agreement with the data. Our analysis shows that even on the very smoggy days in the ACHEX study there were times when primary carbon dominated the carbonaceous aerosol. 

The ratio of elemental (EC) to organic carbon (OC) has been measured in many studies and is commonly found to be less than one. For example, Shah et al. (1986) measured EC and OC in particles from both urban and rural areas across the United States and found the urban average concentrations to be 6.6 /xg m 3 for OC and 3.8 gg m 3 for EC, i.e., a ratio of EC OC of 0.6. Similarly, in the Los Angeles area, EC typically represents about a third of the carbonaceous component of particles (Rogge et al., 1993d). For rural areas, the concentrations were about a third of those in the urban areas, but the ratio was about the same, at 0.5, consistent with ratios of 0.4-0.5 measured in the Ohio River Valley by Huntzicker et al. (1986). Novakov et al. (1997) measured carbonaceous aerosol off the east coast of the United States at altitudes from 0.2 to 3.0 km and report that the ratio of EC OC is 0.1. 

Kadowaki, S., Characterization of Carbonaceous Aerosols in the Nagoya Urban Area. 2. Behavior and Origin of Particulate n-Alkanes, Environ. Sci. Technol., 28, 129-135 (1994). 

Shah, J.J., R.L. Johnson, E.K. Heyerdahl, and J.J. Huntzicker. 1986. Carbonaceous aerosol at urban and rural sites in the United States. /. Air Pollut. Cont. Assoc. 36 254-257. 

Ambient aerosols, particularly those with diameters less than 3pm, are a serious pollution problem. Carbonaceous material is a major component of the fine particle concentration (10 and has undergone extensive study in the past few years (, ) in large part because of the concern that these particles play an Important role in urban haze and community health. 

Chu, L. C., and Macias, E. S. (1981) Carbonaceous urban aerosol Primary or secondary in Atmospheric Aerosol Source/Air Quality Relationships, E. S. Macias and P. K. Hopke, eds., American Chemical Society, Washington, DC, pp. 251-268. 

Katrinak, K. A., Rez, P, and Buseck, P. R., (1992) Structural variations in individual carbonaceous particles from an urban aerosol. Environ. Sci. TechnoL, 26, 1967-1976. 

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