Mobility distribution measurements, aerosol

Details of the calibration, use, performance, and artifactual problems are given in a proceedings entitled Aerosol Measurement (Lundgren et al., 1979) this also shows data for the mobility distribution for monodis-perse aerosols. 

Figure 7.11 Coagulation of aerosol panicles much smaller than the mean free path. Size distributions measured with the electrical mobility analyzer (Husar. 1971).

The condensation particle counter battery measurements are complemented by aerosol particle size distribution measurements using a dual differential mobility particle sizer system covering a size range of 3-900 nm, and an aerodynamic particle sizer covering aerosol particle sizes between 0.7 and 20 pm. In addition, air ions are detected using a balanced scanning mobility analyser and an air ion spectrometer. During the period of measurements, several new particle formation (nucleation) events occur in tropospheric air. 

On-line aerosol measurements were made using a Thermo-Systems, Inc., Model 3030 Electrical Aerosol Size Analyzer (EAA). This instrument uses the electrical mobility of the particles to measure the size distribution in the 0.01 to 0.5 ym range. 

Although the SEMS represents a marked advance in the state of the art for measurement of aerosol size distribution, an important gap remains in current measurement technology, namely, the ability to make rapid, high-resolution measurements of the accumulation-mode aerosols on-line. The limitation of the DMA or SEMS for measurement of particles larger than 0.2 xm in diameter is the multiple charging that allows particles of two or more different sizes to contribute a given mobility fraction. Regardless of... 

Size-resolved chemical information is much more difficult to obtain. The many applications of the differential mobility analyzer in measuring properties of size-classified particles are important tools for the characterization of aerosol systems, but the approaches demonstrated to date yield limited data. Vapor pressures, surface tension, and optical absorption have been measured on mobility-classified aerosols. Direct measurements of the distribution of chemical composition with particle size are needed. Elemental... 

Weimer S, Mohr C, Richter R, Keller J, Mohr M, Prevot ASH, Baltensperger U (2009) Mobile measurements of aerosol number and volume size distributions in an Alpine valley influence of traffic versus wood burning. Atmos Environ 43 624—630... 

Kulkarni, P. and Wang, J. (2006) New fast integrated mobility spectrometer for real-time measurement of aerosol size distribution -1 concept and theory. Journal of Aerosol Science, 37,1303-25. 

The particle distribution graphs are done as commonly used in aerosol measurement the channel width, which represents particle diameter (dp) range, is plotted on a logarithmic scale against the total number concentration (TNC), that is calculated from the measured number of particles (dN) divided by the logarithm of the channel width (d og(dp)), where dp is the mobility diameter. 

To measure the distribution of particle mobilities, the applied potential between the rod and tube is systematically varietl and the particle concentrations are measured. The. size distribution of the inlet aerosol can be calculated from the distribution of particle mobilities using (6.5) ajid the Boltzmann distribution (Chapter 2) or an equivalent relationship. 

Husar( 1971) studied the coagulation of ultrahne particles produced by a propane torch aerosol in a 90-m polyethylene bag. The size distribution was measured as a function of time with an electrical mobility analyzer. The results of the experiments are shown in Fig. 7.11 in which the size distribution is plotted as a function of particle diameter and in Fig. 7.12 in which is shown as a function of t) both based on particle radius. Numerical calculations were carried out by a Monte Carlo method, and the results of the calculation are also shown in Fig. 7.12. The agreement between experi ment and the numerical calculations is quite satisfactory. 

Experiments on simultaneous coagulation and growth were made by Husar and Whitby (1973). A 90-m polyethylene bag was filled with laboratory air from which paniculate matter had been removed by filtration. Solar radiation penetrating the bag induced photochemical reactions among gaseous pollutants, probably SO2 and organics, but the chemical composition was not determined. The reactions led to the formation of condensable species and photochemical aerosols. Size distributions were measured in 20-min intervals using an electrical mobility analyzer. The results of one set of experiments for three different time,s are shown in Fig. 11.3. 

Studies of aerosol size distributions in power plant plumes clearly show that gas-Co-panicle conversion is an important source of submicron aerosol. Condensable material is formed primarily by the oxidation of SOi to sulfates and NOj, to nitrates, both usually present as ammonium salts. It is somewhat easier to analyze plume aerosol dynamics than urban aerosol behavior because (in selected cases) the plume aerosol originates from a single source and is sufficiently well-defined to follow many kilometers downwind. By using ground-based mobile laboratories and/or suitably instrumented aircraft, the aerosol and associated gasc.s can be measured. 

The smaller aerosol particles can be captured from the air for subsequent counting and size measurement by means of so-called thermal precipitators. In these instruments, metal wires are heated to produce a temperature gradient. Aerosol particles move away from the wire in the direction of a cold surface, since the impact of more energetic gas molecules from the heated side gives them a net motion in that direction. The particles captured are studied with an electron microscope. Another possible way to measure Aitken particles is by charging them electrically under well-defined conditions. The charged particles are passed through an electric field and are captured as a result of their electrical mobility (see equation [4.6]). Since size and electrical mobility are related, the size distribution of particles can be deduced. These devices are called electrical mobility analyzers. 

Haaf, W. (1980). Accurate measurements of aerosol size distribution II, construction of a new plate condensor electric mobility analyser and first results. J. Aerosol Sci. 11, 201-212. 

FIGURE 2. Mobility and size spectra of tropospheric aerosol ions. The wide bars mark the fraction concentrations theoretically estimated on the basis of the standard size distribution of tropospheric aerosol. The pin bars with head + and - mark average values of positive and negative aerosol ion fraction concentrations measured in a rural site every 5 min during 4 months. 

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