Aerosol concentration distribution

Fig. 7-12 Schematic of an atmospheric aerosol size distribution. This shows the three mass modes, the main sources of mass for each mode, and the principal processes involved in inserting mass into and removing mass from each mode (m = mass concentration. Dp = particle diameter). (Reproduced with permission from K. T. Whitby and G. M. Sverdrup (1983). California aerosols their physical and chemical characteristics. In "The Character and Origin of Smog Aerosols" (G. M. Hidy, P. K. Mueller, D. Grosjean, B. R. Appel, and J. J. Wesolowski, eds), p. 483, John Wiley, New York.)...

Theoretical calculations of unattached fractions of radon or thoron progeny involve four important parameters, namely, 1) the count median diameter of the aerosol, 2) the geometric standard deviation of the particle size distribution, 3) the aerosol concentration, and 4) the age of the air. All of these parameters have a significant effect on the theoretical calculation of the unattached fraction and should be reported with theoretical or experimental values of the unattached fraction. 

T e daily average aerosol concentration is 4060 x 10 particles per m with a fluctuation of + 50 percent based on the standard deviation of the mean. The mean particle size likewise shows large variation with a mean of 0.04 + 0.01 ym. There was some evidence in the aerosol analyzer data for major and minor modes in the size distribution as found by George et al. (1980, 1984). The mean diameter of the particles remained constant as seen in the graph. Both Rn-222 and PAEC (WL) values followed similar diurnal patterns. It is reasonable to expect higher WL levels during the morning due both to increase in radon and aerosol concentrations. The mean diurnal PAEC is 0.006 WL for this seven-day period in September. 

Together with the radon daughter measurements, nearly continuous measurements of the ventilation rate are performed by means of the release of N2O tracer gas and observation of its decay with an infrared spectrometer (Miran 101). Furthermore the aerosol concentration and size distribution are monitored every 20 to 30 min with an automated aerosol spectrometer (Raes et al.,1984). 

The deposition rate of the attached fraction, plotted in Figure 3, is calculated from the aerosol size distribution assuming diffusion and electrophoresis to be the most important deposition mechanisms (Raes et al.,1985a). The accuracy of the absolute values was checked by forming the aerosol mass balance after the generation of a high aerosol concentration.In Table II is compared the decay of the... 

Okamori K, Tanaka S, Hashimoto Y. 1991. Transport of soil particles an pollutants to the ocean and their concentration distribution in the marine atmosphere. A study of marine aerosols collected on board the antarctic observation ship "Shirase". Nippon Kagaku Kaishi 6 759-765. 

Figure 2. Normalized aerosol volume distribution, China Lake, CA (1979 average)—average of 254 measurements. The error bars are standard deviations. The distribution is normalized with respect to total aerosol volume concentration of particles less than 10 lOn in diameter.

It was found that the requirements were satisfied for application of the linear regression technique to species mass concentrations in a multicomponent aerosol. The results of 254 particle size distributions measured at China Lake in 1979 indicate that the normalized fine aerosol volume distribution remained approximately constant. The agreement between the calculated and measrued fine particle scattering coefficients was excellent. The measured aerosol sulfur mass distribution usually followed the total distribution for particles less than 1 ym. It was assumed that organic aerosol also followed the total submicron distribution. 

In a search for sources of alkaline materials in rural air and rain, we have sampled and performed multi-element analyses on ambient particulate matter and potential source materials. Ambient aerosols were sampled daily using single Nuclepore filters or Florida State University "streakers." Samples of soil and unpaved road materials were also collected and analyzed. The samples were analyzed by various multi-element methods, including ion-and proton-induced X-ray emission and X-ray fluorescence, as well as by atomic absorption spectrophotometry. Visual observations, as well as airborne elemental concentration distributions with wind direction and elemental abundances in aerosols and source materials, suggested that soil and road dust both contribute to airborne Ca. Factor analysis was able to identify only a "crustal" source, but a simple mass balance suggested that roads are the major source of Ca in rural central Illinois in summer. 

Based on the use of the NARCM regional model of climate and formation of the field of concentration and size distribution of aerosol, Munoz-Alpizar et al. (2003) calculated the transport, diffusion, and deposition of sulfate aerosol using an approximate model of the processes of sulfur oxidation that does not take the chemical processes in urban air into account. However, the 3-D evolution of microphysical and optical characteristics of aerosol was discussed in detail. The results of numerical modeling were compared with observational data near the surface and in the free troposphere carried out on March 2, 4, and 14, 1997. Analysis of the time series of observations at the airport in Mexico City revealed low values of visibility in the morning due to the small thickness of the ABL, and the subsequent improvement of visibility as ABL thickness increased. Estimates of visibility revealed its strong dependence on wind direction and aerosol size distribution. Calculations have shown that increased detail in size distribution presentation promotes a more reliable simulation of the coagulation processes and a more realistic size distribution characterized by the presence of the accumulation mode of aerosol with the size of particles 0.3 pm. In this case, the results of visibility calculations become more reliable, too. 

It is interesting to note that according to Eq. 12.36, the equilibrium charge distribution on aerosols is independent of both ion concentration and aerosol concentration. These factors are important, however, in establishing the length of time necessary for equilibrium conditions to develop. 

With an ion production rate of 104 ions/(cms s) and an aerosol concentration of 5 x 104 particles/cm3, equilibrium would be achieved in about 2 s. For atmospheric aerosols where the ion production rate may be only 10 ions/(cm3 s), even though aerosol concentrations of 5 x 104 particles/cm3 are not uncommon, it takes approximately 1700 s (or about 30 min) for equilibrium to be achieved. O Connor and Sharkey (1960) report that equilibrium conditions usually prevail in air coming from the ocean. Over an industrial city, however, measurements indicated that the equilibrium charge distribution is not attained (Nolan and Doherty, 1950). This difference is attributed to the shorter time span between the production of the aerosol over the city and its measurement. 

Example 19.3 Figure 19.2 shows the size distribution of a sneeze as given by Lidwell (1967) (10s droplets per sneeze). Assuming each of the droplets contains one or more viable microorganisms with a virus survival half-life of 2 min, determine the viable aerosol concentration as a function of time, using Eq. 19.1. 

The first stage of the consideration of the effect of aerosol on climate is the modeling of aerosol properties. The models (based on statistically reliable field-measurements data) are to parameterize such characteristics as complex refractive index of particles m = n — ki), their shape and size distribution, vertical profile of aerosol concentrations, as well as variability of these parameters in time and due to humidity. 

In summer, surface inversions over the ice cap are weaker, which stimulates aerosol mixing in the troposphere and its motion to the surface. A relative spreading of the tropopause favours the air transport from the lower stratosphere to the upper one. The transport takes place, apparently, near the boundaries of the continent, but it may also take place over its interiors. As a result, rather a homogeneous distribution of aerosol concentrations and composition over the Antarctic continent is observed [36]. 

Particle emissions resulting from natural processes should be roughly distributed according to the relative amounts of land and ocean areas in various latitudinal bands. Assuming this, natural amissions are estimated to be a function of latitude, based on the amount of land and water distributed in various latitude zones. When this is done for the northern hemisphere and compared with an estimated zonal distribution of pollutants based on Figure 3, the relative contribution of pollutant sources to atmospheric aerosol concentrations as a function of latitude is estimated. 

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