Aerosol heterogeneous chemistry

Fig. 1 Schematic of surface-active organic material in a deliquesced aerosol particle. Surface organics can potentially inhibit the uptake of gas-phase species to the particle, enhance ice nncle-ation, and depress particle surface tension, with important implications for aerosol heterogeneous chemistry and cloud formation...

In the following sections we review laboratory, field, and modeling studies of the influence of surfactant films on aerosol heterogeneous chemistry. 

Early studies of the influence of interfacial organic films on gas uptake to aqueous films and aerosols were reviewed previously by Donaldson and Vaida [131]. Donaldson and Valsaraj also recently reviewed the adsorption of VOCs at the air-aqueous interface and their reaction with atmospheric oxidants [132]. For an overview of techniques and principles of laboratory studies of aerosol heterogeneous chemistry the reader is referred to the recent review article of Kolb et al. [133]. 

While ample evidence from the laboratory suggests that surfactant films significantly influence aerosol heterogeneous chemistry, no direct evidence of this effect for ambient aerosols exists at this time. In part this is due to the challenges of characterizing aerosol morphology in situ. 

It is clear that surface-active organics in atmospheric aerosols can significantly impact aerosol heterogeneous chemistry, cloud formation, and freezing. Most of these effects stem from the tendency of surface-active molecules to partition to the... 

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. 

Atmospheric particulates (sea salt, carbonaceous soot, and sulfuric acid aerosols) are known to provide a condensed phase for conq>lex heterogeneous chemistry to occur. Although the presence of atmospheric particulates are known to alter trace gas concentrations, details of the specific chemical mechanisms for condensed phase chemistry have not been identified. 

Atmospheric aerosols have a direct impact on earth s radiation balance, fog formation and cloud physics, and visibility degradation as well as human health effect[l]. Both natural and anthropogenic sources contribute to the formation of ambient aerosol, which are composed mostly of sulfates, nitrates and ammoniums in either pure or mixed forms[2]. These inorganic salt aerosols are hygroscopic by nature and exhibit the properties of deliquescence and efflorescence in humid air. That is, relative humidity(RH) history and chemical composition determine whether atmospheric aerosols are liquid or solid. Aerosol physical state affects climate and environmental phenomena such as radiative transfer, visibility, and heterogeneous chemistry. Here we present a mathematical model that considers the relative humidity history and chemical composition dependence of deliquescence and efflorescence for describing the dynamic and transport behavior of ambient aerosols[3]. 

Stelson, A. W and J. H. Seinfeld, Chemical Mass Accounting of Uban Aerosol, Environ. Sci. Technol., 15, 671-679(1981). Stickel, R. E., J. M. Nicovich, S. Wang, Z. Zhao, and P. H. Wine, Kinetic and Mechanistic Study of the Reaction of Atomic Chlorine with Dimethyl Sulfide, J. Phys. Chem., 96, 9875-9883 (1992). Swartz, E J. Boniface, I. Tchertkov, O. V. Rattigan, D. V. Robinson, P. Davidovits, D. R. Worsnop, J. T. Jayne, and C. E. Kolb, Horizontal Bubble Train Apparatus for Heterogeneous Chemistry Studies Uptake of Gas-Phase Formaldehyde, Environ. Sci. Technol, 31, 2634-2641 (1997). 

Zhang, R., M.-T. Leu, and L. F. Keyser, Heterogeneous Chemistry of HONO on Liquid Sulfuric Acid A New Mechanism of Chlorine Activation on Stratospheric Sulfate Aerosols, . /. Phys. Chem., 100, 339-345 (1996). 

There are several reasons for the dramatic ozone destruction (see Fig. 2.17) low temperatures may have prolonged the presence of polar stratospheric clouds, which play a key role in the ozone destruction, the polar vortex was very stable, there were increased sulfate aerosols from the 1991 Mount Pinatubo volcanic eruption, which also contribute to heterogeneous chemistry, and chlorine levels had continued to increase. These issues are treated in more detail shortly. 

These aerosols play a major role in stratospheric chemistry by directly providing surfaces for heterogeneous chemistry (discussed in more detail later) as well as serving as nuclei for polar stratospheric cloud formation. Figure 12.21 schematically shows the processes believed to be involved in PSC formation. The thermodynamic stability of the various possible forms of PSCs at stratospherically relevant temperatures and the transitions between them are discussed in detail by Koop et al. (1997a). 

The finding that the heterogeneous chemistry that occurs on polar stratospheric clouds also occurs in and on liquid solutions in the form of liquid aerosol particles and droplets in the atmosphere provided a key link in understanding the effects of volcanic eruptions on stratospheric ozone in both the polar regions and midlatitudes. As discussed herein, the liquid particles formed from volcanic emissions are typically 60-80 wt% H2S04-H20, and hence the chemistry discussed in the previous section can also occur in these particles (Hofmann and Solomon, 1989). We discuss briefly in this section the contribution of volcanic emissions to the chemistry of the stratosphere and to ozone depletion on a global scale. For a brief review of this area, see McCormick et al. (1995). 

Hofmann, D. J S. J. Oltmans, W. D. Komhyr, J. M. Harris, J. A. Lathrop, A. O. Langford, T. Deshler, B. J. Johnson, A. Torres, and W. A. Matthews, Ozone Loss in the Lower Stratosphere over the United States in 1992-1993 Evidence for Heterogeneous Chemistry on the Pinatubo Aerosol, Geophys. Res. Lett, 21, 65-68 (1994b). 

In addition, aerosol particles have indirect effects. The most important of these is their effect on cloud properties, since clouds obviously also have major effects on climate. In addition, since heterogeneous chemistry can occur on aerosol particles (see Chapter 5), it is possible that such chemistry can alter the concentrations of other contributors to the climate system, such as the greenhouse gases. One example is the formation of N20 from reactions of HONO on the surface of aerosol particles (see Chapter 7.C). 

The last years attention has been also paid on the importance of heterogeneous reactions in the troposphere. On the basis of the extremely limited laboratory studies and numerous assumptions it has been calculated that scavenging of compounds such as N03, N205 and HOBr followed by reactions in clouds or/and onto aerosols can be crucial for the oxidant levels in the troposphere [24 - 27]. However, contrary to the gas phase tropospheric chemistry, it does not exist a reference scheme for heterogeneous chemistry nor any consistent compilation of recommended relevant kinetic data. Finally, although CTMs have nowadays more or less sophisticated parameterizations of heterogeneous chemistry,... 

Heterogeneous chemistry involves a number of processes that combine the overall rale of transport and chemical conversion between the gas and condensed phases. These processes include (a) gas diffusion to the surface of the aerosol, (b) accommodation ("sticking") at the surface, (c) diffusion within the condensed phase, (d) chemical reaction in the condensed phase, and (e) diffusion of the resultant products to the surface and evaporation from the interface. 

Arnold, F Buhrke, T., and Qiu, S. (1990) Evidence for stratospheric ozone-depleting heterogeneous chemistry on volcanic aerosols from El Chicon, Nature 348,49-50. 

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