Surface aerosols

The two fundamental theories for calculating the attachment coefficient, 3, are the diffusion theory for large particles and the kinetic theory for small particles. The diffusion theory predicts an attachment coefficient proportional to the diameter of the aerosol particle whereas the kinetic theory predicts an attachment coefficient proportional to the aerosol surface area. The theory... 

The model results were compared with the HOx concentrations measured by the FAGE (Fluorescence Assay by Gas Expansion) technique during four days of clean Southern Ocean marine boundary layer (MBL) air. The models overestimated OH concentrations by about 10% on two days and about 20% on the other two days. HO2 concentrations were measured during two of these days and the models overestimated the measured concentrations by about 40%. Better agreement with measured HO2 was observed by using data from several MBL aerosol measurements to estimate the aerosol surface area and by increasing the HO2 uptake coefficient to unity. This reduced the modelled HO2 overestimate by 40%, with little effect on OH, because of the poor HO2 to OH conversion at the low ambient NOx concentrations. 

Heterogeneous uptake on surfaces has also been documented for various free radicals (DeMore et al., 1994). Table 3 shows values of the gas/surface reaction probabilities (y) of the species assumed to undergo loss to aerosol surface in the model. Only the species where a reaction probability has been measured at a reasonable boundary layer temperature (i.e. >273 K) and on a suitable surface for the marine boundary layer (NaCl(s) or liquid water) have been included. Unless stated otherwise, values for uptake onto NaCl(s), the most likely aerosol surface in the MBL (Gras and Ayers, 1983), have been used. Where reaction probabilities are unavailable mass accommodation coefficients (a) have been used instead. The experimental values of the reaction probability are expected to be smaller than or equal to the mass accommodation coefficients because a is just the probability that a molecule is taken up on the particle surface, while y takes into account the uptake, the gas phase diffusion and the reaction with other species in the particle (Ravishankara, 1997). 

Aerosol surface area is likely to be variable even within a remote marine air mass. Previous MBL aerosol studies describe changes in aerosol concentration and composition due to entrainment from the free troposphere (Bates et al., 1998, 2001 Covert et al., 1998). Raes et al. (1997) found an observable link between vertical transport patterns and aerosol variability in the MBL specifically in the Aitken mode (<0.2/u.m). Hence entrainment of aerosol from the free troposphere appears to occur frequently, even in remote MBL air masses. In addition, aerosols have the capacity to travel great distances in the free troposphere, before being entrained into the MBL. 

Reactive aerosol surface area (RASA) data were not available for SOAPEX-2 so a constant value of 1.0x 10-7 cm-1,... 

Notholt et al. (1992) and Andres-Hernandez et al. (1996) measured HONO, NO, N02, and aerosol surface areas at both urban and nonurban locations. They observed that at Ispra, Italy, HONO concentrations tended to correlate with N02, NO, and aerosol surface areas. Such studies support the formation of HONO from heterogeneous reactions of N02 at the surfaces of aerosol particles, fogs, buildings, and the ground. 

Junkermann, W., and T. Ibusuki, FTIR Spectroscopic Measurements of Surface Bound Products of Nitrogen Oxides on Aerosol Surfaces—Implications for Heterogeneous HN02 Production, Atmos. Enriron., 26A, 3099-3103 (1992). 

Notholt, J., J. Hjorth, and F. Raes, Formation of HN02 on Aerosol Surfaces during Foggy Periods in the Presence of NO and N02, Atmos. Em iron., 26A, 211-217 (1992). 

For example, the rate constant for dissociation of hydrated S02, kl2, is 3.4 X 10r s l so that the half-life for dissociation of the hydrated S02 is only 0.2 yu,s. Similarly, the second ionization, reaction (13), occurs on time scales of less than a millisecond (Schwartz and Freiberg, 1981). Thus, regardless of which of the three species, S02 H20, HSO, or SO3-, is the actual reactant in any particular oxidation, the equilibria will be reestablished relatively rapidly under laboratory conditions, and likely under atmospheric conditions as well. The latter is complicated by such factors as the size of the droplet, the efficiency with which gaseous S02 striking a droplet surface is absorbed, the chemical nature of the aerosol surface, and so on for example, the presence of an organic surface film on the droplet could hinder the absorption of S02 from the gas phase. 

As we have seen, a great deal is known about emission sources and strengths, ambient levels, and mutagenic/carcinogenic properties of the particle-phase PAHs in airborne POM. However, because of the tremendous physical and chemical complexity of the aerosol surfaces on which photolysis, photooxidations, and gas-particle interactions take place in real polluted ambient air, much less is known about the structures, yields, and absolute rates and mechanisms of formation of PAH and PAC reaction products, especially for the more polar PACs. This is one area in which there exists a major gap in our knowledge of their atmospheric chemistry and toxicology. 

Sulfur 10-200% increase6 in surface area in sulfate particles Increased aerosol surface area, enhanced ozone depletion by CIO, decreased ozone depletion by NO,... 

FIGURE 12.31 Aerosol surface area, NO, NO,., and CIO as a function of altitude (adapted from Keim et al., 1996). 

FIGURE 12.34 Vortex-averaged total 03 (DU) from TOMS satellite data for October at 75°S compared to model predictions using the assumption of a constant aerosol surface area or the measured surface areas (adapted from Portmann et al., 1996). 

Keim et a.l. (1996) have shown that there is a significant reduction in NO and increase in NO. and CIO in a layer above the tropopause that has increased aerosol surface areas (Fig. 12.31). They attribute this to increased heterogeneous reactions of C10N02 on particles to form HN03 and active chlorine. 

If the aerosol surface area in the McKinney et al. studies in Problem 4 was 5 /xm2 cm 3, what would the lifetime be for BrONOz with respect to hydrolysis on particles Take a temperature of 200 K and assume a reaction probability of 0.8 (Table 12.8) and that the reaction is not diffusion-limited. Compare this to the lifetime with respect to photolysis calculated in Problem 4. 

Thomason, L. W L. R. Poole, and T. Deshler, A Global Climatology of Stratospheric Aerosol Surface Area Density Deduced from Stratospheric Aerosol and Gas Experiment II Measurements 1984-1994, J. Geophys. Res., 102, 8967-8976 (1997). 

The accumulation mode (0.1 < d ic < 1pm) particles included in this mode originate from coagulation of particles in the nucleation mode and from condensation of vapors onto existing particles. These particles usually accounts for a substantial part of the aerosol mass and for most of the aerosol surface area (Seinfeld and Pandis, 1998). 

So what is the fate of this dissolved iron in rainwater and the labile iron on aerosol surfaces upon deposition to seawater Based on the results of acid cycling... 

The processes by which ions are lost in the stratosphere and the troposphere are not completely understood due to a sparcity of laboratory data on ionic recombination. It is most likely that mutual neutralization of cluster ions [reaction (9)] will be the primary loss mechanism in the upper stratosphere, with the process of collision-enhanced (ternary) recombination becoming increasingly important at lower altitudes (Sect. 3.2.5). In the presence of aerosols (liquid or solid droplets), loss of both positive and negative cluster ions from the gas phase can occur by attachment to the aerosol surfaces 85 86) (see Sect. 4). 

Loss of ions occurs via the processes of mutual neutralization, ternary ionic recombination and attachment to aerosol surfaces, processes which urgently need further study in the laboratory. It is an interesting fact that the ion chemistry directly accelerates the loss of ionization from all regions of the atmospheric plasma. Atomic ions are converted into molecular ions, molecular ions into larger cluster ions which recombine more rapidly. The larger ions also act as nucleation sites for the formation of aerosols, thus involving a transition from the molecular to the liquid state. 

The night-time replenishment of ozone is caused by entrainment of ozone from the free troposphere into the boundary layer. The overnight loss of peroxide is due to deposition over the sea surface (and heterogeneous loss to the aerosol surface), as peroxide has a significant physical loss rate, in contrast to ozone which does not. Therefore, the daytime anti-correlation of ozone and peroxide is indicative of the net photochemical destruction of ozone. 

Figure 21 A schematic representation of the so-called Bromine explosion mechanism where effectively one BrO molecule is converted to two by oxidation of bromide from a suitable aerosol surface " ...

As noted in Sections 3.1 and 4.1, the atmospheric relevance of the two acids often extends beyond the issue of their surface dissociation in the conditions that we have considered here. Thus, for example, the widely occurring sulfate aerosols are concentrated sulfuric acid solutions and nitric acid trihydrate (NAT) is also clearly fairly concentrated. Elucidation of the acid ionization state of such aerosol surfaces—and thus the theoretical characterization of the mechanisms and rates of chemical reaction occurring on them—requires other tools to deal with the multiple proton transfer possibilities present in these systems. One such tool, a generalization of available reaction Monte Carlo methodologies [72] to treat such multiple proton transfers, is under development [73]. 

Nucleation of new particles occurs through the formation of low-vapor-pressure products of gas-phase reactions. The production of this material is generally accompanied by its condensation on existing aerosol particles. In some instances nucleation can be initiated where there is rapid production of new condensable material together with low concentration of aerosol particles and thus low existing aerosol surface area. This combination results in the supersaturation of the condensable vapor, which may build up sufficiently to overcome the free-energy barrier (critical supersaturation) associated with new particle production, thereby initiating nucleation. 

The distribution of aerosols (shown in Figure 2—per unit mass) can also be expressed in terms of the total number of particles, which places greater emphasis on the smaller particles and provides information about the nuclei mode and the process of accumulation. Aerosol concentrations are sometimes expressed in terms of aerosol surface area, which is closely related to visibility impacts. 

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