Secondary aerosol carbon

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). 

Our rationale for this analysis procedure is that during periods of increased concentration of secondary sulfate aerosol there might also be an increase in secondary carbon aerosol. The frequency distribution for all samples is shown in Figure 7a. 

The carbon to lead ratio in St. Louis on four winter days is plotted as a function of time of day in Figure 10. Far less variation can be seen in comparison to the LA data plotted in Figure 2 although an increase in the C/Pb ratio in St. Louis is seen at midday on two days. The C/Pb ratio is also much lower in the St. Louis data. This supports the previous conclusion that primary carbon is dominant in St. Louis in the winter sampling period. It also subbests that more secondary carbon aerosol is present in LA than in St. Louis. 

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. 

Air-poUutant effects on neural and sensory functions in humans vary widely. Odorous pollutants cause only minor annoyance yet, if persistent, they can lead to irritation, emotional upset, anorexia, and mental depression. Carbon monoxide can cause death secondary to the depression of the respiratory centers of the central nervous system. Short of death, repeated and prolonged exposure to carbon monoxide can alter sensory protection, temporal perception, and higher mental functions. Lipid-soluble aerosols can enter the body and be absorbed in the lipids of the central nervous system. Once there, their effects may persist long after the initial contact has been removed. Examples of agents of long-term chronic effects are organic phosphate pesticides and aerosols carrying the metals lead, mercury, and cadmium. 

Primary alkyl chlorides are fairly stable to fluorine displacement. When fluorinated, 1-chloropropane is converted to 1-chloroheptafluoropropane and 1-chloto-2-methylbutane produces 39% l-chlorononafluoro-2-methylbutane and 19% perfluoro-2-methylbutane. Secondary and tertiary alkyl chlorides can undergo 1,2-chlorine shifts to afford perfluonnated primary alkyl chlorides 2-Chloro-2-methylpropane gives l-chlorononafluoro-2-methylpropane, and three products are obtained by the fluorination of 3-chloropentane [7] (equation 1). Aerosol fluorina-tion of dichloromethane produces dichlorodifluoromethane which is isolated in 98% purity [4 (equation 2). If the molecule contains only carbon and halogens, the picture is different. Molecular beam analysis has shown that the reaction of fluorine with carbon tetrachlonde, lodotrichloromethane, or bromotrichloromethane proceeds first by abstraction of halogen to form a trichloromethyl radical [5]... 

The extent of gas-to-aerosol conversion of secondary pollutants can be estimated by measuring gas particle distribution factors for carbon, nitrogen, and sulfur species. For example, /c = P/ P + G), where P = particulate organic carbon ng/m as carbon) and G = gas-phase... 

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... 

Rosen, Hansen, Dod and Novakov found a high correlation between optical absorptivity and the particulate carbon loading in 24-h samples from several California cities ( 5). Elemental carbon, a primary pollutant which is directly related to the absorptivity, was found to be a large fraction of the carbonaceous aerosol. They were able to place a low limit on the amount of secondary organic aerosol produced in correlation with ozone. 

In this paper we present results which reconcile the widely different results just discussed ranging from a carbon aerosol dominated by secondary organic material on the one hand to a carbon aerosol composed largely of primary carbon compounds on the other. We have employed an approach which uses lead or elemental carbon as a tracer for primary emissions and combines several analysis techniques to reexamine the published ACHEX data. We also present a new data set from St. Louis which is analyzed in a similar manner to contrast the aerosol in a midwestern city with that on the California coast. 

This was calculated in the same manner as that described by Cass, Boone and Macias (9). The carbon to lead ratio determined from this inventory can be examined for several limiting cases ranging from the output of an automobile using leaded fuel (C/Pb /2) to aerosol in the well mixed LA air basin with significant secondary conversion of carbon (C/Pb>7). This approach assumes, of course, that lead is present only as a primary pollutant. For highway traffic composed of vehicles and fuel types in the same proportions as for the entire urban area, the ratio of aerosol carbon to lead was calculated to be about 4. 

Turpin, B. J., J. J. Huntzicker, S. M. Larson, and G. R. Cass, Los Angeles Summer Midday Particulate Carbon Primary and Secondary Aerosol, Environ. Sci. Technoi, 25, 1788-1793 (1991). 

Biogenic particles which comprise primary (fungal spores, bacteria, viruses, plant debris) and secondary organic aerosol (SOA) from biogenic non-methane VOCs are part of the commonly measured organic carbon fraction. Model results [51] indicate a... 

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. 

Carbonaceous aerosols in the atmosphere are complex in nature and are found in both coarse particles (> about 2.5 pm) and fine particles (< about 2.5 pm). Sources of carbon-containing particles are varied and include resuspended soil particles, pollen, plant waxes, etc. in the coarse fraction, and soot particles, sorbed organics including PAHs, and secondary aerosols resulting from... 

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