Aerosols, organic

Aerosol organics in the atmosphere could be reduced by control of emission of nitrogen oxides and tota/ hydrocarbons. However, the identification and control of a few specific aerosol precursors in gasoline and other sources might prove a more efficient approach. 

FIGURE 3-5 Hourly variatioiis of secondary aerosol organics, nitrates, sulfates, and ammonium as percent of total aerosol. Pasadena, Califixnia, July 25, 1973. Reprinted with permis from Grosjean and Friedlandn. ... 

As discussed earlier, the equilibrium constraint of low vapor pressure is particularly important for stability of the organic aerosols. Organic aerosol formation requires accumulation of condensable species in excess to their gas-phase saturation concentrations. In turn, examination of vapor-pressure for various oxygenated... 

A relation similar to Equation 3-9 has been found to account satisfactorily for organic aerosols formed from cyclopentene, cyclohexene, and 1,7-octadiene in smog chambers. The aerosol organic carbon concentration as a function of time was measured (Grosjean and Fried-lander, unpublished data) with an organic-carbon analyzer (Figure 3-22). The relation obtained was ... 

TABLE 3-12 Potential Aerosol Organic Precursors Identified in Gasolines and Automobile Exhaust... 

In short, the same types of aerosol organic products have been identified both in model systems and in polluted urban ambient air and can generally be rationalized based on the oxidation of known constituents of air. The measured yields of organics in the particles can depend on the amount of particle phase available for uptake of the organic if it is semivolatile and partitions between the gas and condensed phases. This partitioning, and its dependence on the amount of condensed phase available, may be at least in part responsible for discrepancies in the yields of secondary organic aerosol reported in a number of studies. 

Gordon, R. J., N. J. Trivedi, B. P. Singh, and E. C. Ellis, Characterization of Aerosol Organics by Diffuse Reflectance Fourier Transform Infrared Spectroscopy, Environ. Sci. Technol., 22, 672-677 (1988). 

Furthermore, little is known about the alkaline-extractable and insoluble fractions of aerosol organic matter. On average these fractions together can account for 7-20% of the total aerosol mass (Zappoli et al., 1999 Krivacsy et al., 2001b). Thus,... 

Figure 3. A comparison of the measured mass fiagmentation pattern and mass loading distribution of aerosol organics in different photochemical regimes as defined by the CO and O3 levels (HC, LO - high CO, low O3 and vice versa), (from Alfarra et al, 2004).

TABLE 14.4 Concentration of Aerosol Organic Carbon from Remote Marine Areas... 

TABLE 14.8 Compounds Identified in Primary Organic Aerosol Emissions from a Series of Sources and Ambient Aerosol Concentrations of Fine Aerosol Organic Compounds Measured in Pasadena, California, during 1982... 

This equation, first proposed by Odum et al. (1996) in a slightly different form, indicates that for low-organic-mass concentrations the aerosol yield will be directly proportional to the total aerosol organic mass concentration caer. For very nonvolatile products and/or for large organic mass concentrations, the overall aerosol yield will be independent of the organic mass concentrations and equal to a + a2. Finally, (14.43) suggests that the aerosol yields will be sensitive to temperature since c) and c depend exponentially on temperature. 

Assume extreme conditions (aerosol surface, aerosol liquid water content, aerosol organic carbon concentration) for these estimates and that the propionic acid vapor pressure is 0.005 atm. 

In ambient aerosols, organic compounds released in combustion processes will be incorporated into the particles through adsorption or condensation. As a result, organics are found predominantly in the accumulation mode of the aerosol, i.e. the respirable particles C<2yum) which have a large specific surface. Each aerosol particle should therefore be considered as a matrix of various shape and composition (e.g. fly-ash spherical particles built from metal oxides diesel exhaust conglomerates of carbon to which one or more surface layers of PAH are adsorbed. The chemical reactivity of" PAH will therefore be affected by two new parameters, depending on the particle matrix. 

Cheng JYW, Chan CK, Lee CT, Lau APS (2009) Carhon content of common airborne fungal species and fungal contribution to aerosol organic carbon in a subtropical city. Atmos Environ 43 2781... 

Condensed-phase chemistry is another possible source of aerosol surfactants. The oxidation of aerosol organic material yields functionalized products which may be surface-active [98]. Oxidative processing of water-soluble species in cloud or aerosol water may result in the formation of surface-active organic acids such as malonic and malic acids [99,100]. Surface-active HULIS or organosulfate species may also form in situ in aerosol or cloud water [84, 85, 101-104]. 

Fig. 2 Summary of N2O5 uptake suppression for coated particles and films. The ratio of the reactive uptake coefficient, YN205. with and without an organic surface coating, as a function of aerosol organic mass fraction top panel) and liactional surface coverage bottom panel)...

The laboratory studies discussed in this section highlight the importance of the solubility of aerosol organic material oti CCN activity. As stated previously, solubility affects the Raoult term in the Kohler equation (1). The solubility limits the amount of organic material that is incorporated into the particle and thus influences the potential for the formation of a surface film and/or micelles. Hence, solubility indirectly also impacts the surface tension of the particle, thereby influencing the CCN activity via the Kelvin effect. 

Finally, for a complete picture of the multiple roles of surface-active organic material in the chemistry and physics of aerosols, field experiments are needed which couple direct observations of aerosol heterogeneous chemistry [148], CCN, and ESf activity with studies of aerosol composition, surface tension, and particle morphology. New techniques which provide speciated ambient aerosol organic composition [297] or functional group information [298] are expected to yield additional insight. 

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