Relative humidity , organic aerosol

On the average, the requirements for application of the statistical technique to filter data were met. Analysis of the 254 measured particle size distributions in 1979 indicates that the fine aerosol volume distribution preserved its shape. The measured sulfur mass distribution followed that of the total submicron volume. By difference, it was assumed that the organics did the same. The low relative humidity at China Lake minimized the formation of aqueous solutions due to water condensation on the particles. Therefore, it is expected that the statistical technique can be used with some success with the China Lake filter data. 

Care must be taken in extrapolating the results of laboratory studies to the lower concentrations and generally higher relative humidities (RH) found in ambient air. For example, Izumi et al. (1988) showed that the organic aerosol yield for the cyclohexene-03 reaction decreased in a nonlinear fashion as the initial reactant concentrations decreased from 5 ppm in addition, the concentration of condensation nuclei increased steeply with RH above 30%. This may be at least in part due to the effects of gas-particle partitioning on the measured aerosol yields discussed in more detail below. 

Chemical oxidation reactions and radical-induced hydrophobic-to-hydrophilic aging processes tend to increase the water solubility of OAs and, therefore, are thought to enhance the activity of atmospheric OAs as cloud condensation nuclei (CCN). As discussed by Gysel et al. (2004), at 75-90% of relative humidity (RH) the inorganic fraction dominates the water uptake (59-80%). Nevertheless, under the same conditions of RH, between 20% and 40% of total particulate water is associated with water-soluble organic matter. More data concerning the multiphase aerosol and cloud processes, as well as the chemical reactivity of carbonaceous aerosol components, have been compiled in the reviews of Jacobson et al. (2000), Kanakidou et al. (2005), and Poschl (2005) (and references therein). 

Similarly, the relative humidity has a strong influence on the chemical composition of the secondary organic aerosol formed in the atmosphere by the reaction of ozone with 1-tetradecene <2000EST2116> thermal desorption particle beam mass spectrometric determinations found that the main products are a-hydroxytridecyl hydroperoxide and a peroxy-hemiaceta 1. 

Generating Aerosols by Dark Phase Ozone Reaction. Reactants, various hydrocarbons and O3, were brought together in low ppm or tenths of a ppm concentration in 150-liter Teflon bags. The organic reactants used were a-pinene, cyclohexene, 1,5-hexadiene, cyclohexane, and 2-hexene. In all but one series of experiments, relative humidity was virtually zero (dew point <—48°C). In experiments using cyclohexene and O3, the water vapor pressure was varied relative humidities of -. 0%, — 50%, and 95-100% were used at 21°C. a-Pinene and other hydrocarbons alone in clean air in Teflon bags do not deteriorate detectably in 24 hours ozone deteriorates less than 5% in 18 hours. 

The primary purpose of the Sterilamp tube in air-conditioning systems has been to destroy microorganisms (22). Tests on the effect of 1 to 2 p.p.m. by voliune of ozone on E. coli sprayed into an air duct revealed that the organisms were not destroyed. This would confirm the data of Elford and Van den Ende ( ) that ozone is a poor disinfectant of air at low relative humidity. At high relative humidity these authors found that as low as 0.04 p.p.m. by volume destroyed bacteria dispersed in an aerosol. This would also agree with the results reported here, that organisms on surfaces and seeded on Petri plates can be destroyed by minute amounts of ozone. 

Water is a major component of the accumulation mode aerosol in amounts that depend on the relative humidity. The uptake of water is driven by the strongly hygroscopic nature of the secondary aerosol components, especially the ainmoniuin sulfates and nitrate. The water content depends in a complex way on both the inorganic and organic components. The resulting aerosol phase solutions are likely to be highly concentrated compared with fog droplets, for example. 

Deviations from the theory outlined above may occur if the organic solution is not ideal. In this case the mole fraction x, should be replaced by the product y,jc where y is the activity coefficient of the component i in the given solution. Saxena et al. (1995) have shown that a series of organic aerosol compounds can interact also with the aqueous phase in atmospheric particles. Therefore, for high relative humidities, the secondary organic aerosol compounds may exist in three phases—namely, the gas, the organic aerosol material, and the aqueous phase. 

Tobias H. J., Docherty, K. S., Beving, D. E., and Ziemann, P. J. (2000) Effect of relative humidity on the chemical composition of secondary organic aerosol formed from reactions of 1 -tetradecene and 03, Environ. Sci. Technol. 34, 2116-2125. 

Fig. 12 Relative oxidation rates by OH radicals of condensed-phase cholestane vs gas-phase m-xylene in different organic-aerosol matrices, all of which include a high fraction of motor oil. Cholestane oxidation is independent of OA concentration or the presence of a substantial SOA coating consisting of up to half of the total particle mass. However, high relative humidity slows cholestane oxidation by an order of magnitude. This suggests that a thin film of water on oil can significantly retard cholestane oxidation, perhaps by excluding the cholestane from the particle surface the SOA, on the other hand, does not coat the particle surface but rather mixes with the oil and thus does not impede cholestane oxidation...

Thornton JA, Braban CF, Abbatt JPD (2003) N205 hydrolysis on sub-micron organic aerosols the effect of relative humidity, particle phase, and particle size. Phys Chem Chem Phys 5 4593 603... 

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