Aerosol surface area

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. 

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. 

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). 

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. 

This qualitative explanation for the oscillatory behavior of the particle number density is supported by theory. Assume the system can be modeled as a continuous steady state stirred tank reactor (CSTR) that is, reactants enter and products leave from a perfectly mixed tank with composition equal to that of the products. The set of equations derived in the previous section applies, but a new term must be subtracted from the right-hand side in each case to account for the loss of particles from the CSTR by the flow process. Equation (10.49) for the change in aerosol surface area with time becomes... 

Asgrowth continues, the aerosol surface area becomes sufficiently large to accommodate the products of gas-to-particle conversion. The saturation ratio decreases, leading to a reduction in the particle formation rate. The decay in the number concentration for r > 80 min in Fig. 11.4 is probably due to coagulation calculations for free molecule aerosols support this hypothesis. 

Figure 5. Distributions of the aerosol surface area versus particle size, for ABC in various types of AM, in the lower a) and upper b and c) troposphere.

The clouds observed in the lower troposphere when the inflow of arctic air (BT IV) dominates in the region are on average characterized by the same values of the optical parameters as the clouds in the previously reviewed case. However, the spectra of the distribution of the aerosol surface area versus particle size are noticeably shifted to the range of smaller particles (radiuses r=0.1-0.2 //m), and are described by a broader logarithmically normal function (curves 3 and 4). The observation can be explained by the offset of aerosol containing a great amount of loes dust and products of anthropogenic pollution composed of outbursts from the industrial plants of Kazakhstan. 

In case of the pollution import by cool tropical air flows (BT If), die distribution of aerosol surface area versus particle size in the cloud generallly follows curves 3 and 4, but the dominating mode of the size spectrum occurs at the radius of i=0.15 pm. The cloud composition is likely enriched with a saline component of a desert aerosol (r<0.4 pm) and by... 

In the longer-term, and with contemporary loads of chlorine in the stratosphere, the presence of volcanic aerosols causes a reduction in the ozone abundance resulting from the large increase in aerosol surface area available for heterogeneous reactions (Hofmann and Solomon, 1989 Brasseur and Granier, 1992 Michelangeli et al., 1992 Pitari and Rizzi, 1993 Tie et al., 1994). These effects will be further discussed in Chapter 6. 

Figure 5.65. Median meridional distribution of the sulfate aerosol surface area density (/im2cm-3) for the period 1985-1994, based on observations by the SAGE II satellite. From Thomason et al, 1997.

Primary sources of sulfur to the stratosphere are carbonyl sulfide (Crutzen, 1976 Chin and Davis, 1995) and explosive volcanic eruptions that inject SO2 gas directly into the stratosphere (e.g., McCormick et al., 1995) which subsequently forms liquid sulfate aerosols (see Section 5.7.1). Observations of PSC extinction show that the major eruptions of El Chichon in 1981 and Pinatubo in 1991 led to large increases in particle surface areas in polar regions (e.g., McCormick et al., 1995 Deshler et al., 1992 Thomason et al., 1997 Hofmann et al., 1992 1997). Hofmann and Oltmans (1993) showed that enhanced aerosol surface areas due to Pinatubo and Hudson (a South American... 

Fig. 2.1 Scheme of an atmospheric aerosol surface area distribution showing the three modes, the main source of mass for each mode, the principal processes involved in inserting mass into each mode, and the principal removal mechanisms. (From Whitby 1978.)... 

FIGURE 5.24 Variation of stratospheric aerosol surface area with altitude as inferred from satellite measurements. Maximum in surface area concentration occurs at about 18-20km. [Reprinted from McElroy et al. (1992) with kind permission from Elsevier Science Ltd., The Boulevard, Langford Lane, Kidlington OX5 1GB, UK.]... 

Figure 5.24 shows a distribution of stratospheric aerosol surface area as a function of altitude from 18 to 30 km inferred from satellite measurements. The surface area units used in Figure 5.24 are cm2 cm 3, and typical values of the stratospheric surface area at, say, 18 km altitude are about 0.8 x 10-8 cm2 cm-3. This is equivalent to 0.8pm2 cm 3. As a useful rule of thumb, stratospheric aerosol surface area in the lower stratosphere ranges between 0.5 and 1.0 pm2 cm-3. 

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