Particle number concentration, aerosols

Particle Number Concentration and Size Distribution. The development of aerosol science to its present state has been directly tied to the available instrumentation. The introduction of the Aitken condensation nuclei counter in the late 1800s marks the beginning of aerosol science by the ability to measure number concentrations (4). Theoretical descriptions of the change in the number concentration by coagulation quickly followed. Particle size distribution measurements became possible when the cascade impactor was developed, and its development allowed the validation of predictions that could not previously be tested. The cascade impactor was originally introduced by May (5, 6), and a wide variety of impactors have since been used. Operated at atmospheric pressure and with jets fabricated by conventional machining, most impactors can only classify particles larger... 

Rodriguez S, Cuevas E (2007) The contributions of minimum primary emissions and new particle number formation enhancements to the particle number concentration in urban air. J Aerosol Sci 38 1207-1219... 

The submicron particle number size distribution controls many of the main climate effects of submicron aerosol populations. The data from harmonized particle number size distribution measurements from European field monitoring stations are presented and discussed. The results give a comprehensive overview of the European near surface aerosol particle number concentrations and number size distributions between 30 and 500 nm of dry particle diameter. Spatial and temporal distributions of aerosols in the particle sizes most important for climate applications are presented. Annual, weekly, and diurnal cycles of the aerosol number concentrations are shown and discussed. Emphasis is placed on the usability of results within the aerosol modeling community and several key points of model-measurement comparison of submicron aerosol particles are discussed along with typical concentration levels around European background. 

As particle number size distributions can be complex, and the instruments used generate large amount of size distribution data, which can be hard to effectively describe, a common method is to calculate integrated particle number concentrations for specific aerosol particle diameter ranges, depending on which part of the particle number size spectrum is needed for the application. In this chapter, three different ranges are used (Fig. la) ... 

Number concentrations are dominated by submicron particles, whereas the mass concentrations are strongly influenced by particle concentrations in 0.1-10 pm diameter range [13]. Similarly, the variability of the number-based measurements is strongly dominated by variability in smaller diameter ranges, whereas the variability of mass-based properties, such as PM10, are dominated by variability in the accumulation mode (usually around 500 nm of mass mean diameter) and in the coarse mode. This means the variabilities of these properties are not necessarily similar in shorter timescales, due to sensitivity of variance from very different air masses and thus aerosol types. This is demonstrated in Fig. lb, where the variance of the each size class of particle number concentrations between 3 and 1,000 nm is shown for SMEAR II station in Hyytiala, Finland. The variance has similarities to the particle number size distribution (Fig. la), but there are also significant differences, especially on smaller particles sizes. Even though in the median particle number size distribution the nucleation mode is visible only weakly, it is a major contributor to submicron particle number concentration variability. 

Figure 9 (adapted from [18]) shows some of the typical correlations between particle number concentrations between 30 and 100 nm (here referred to as Aitken mode, although a more rigorous derivation would require actual modal fitting) and concentrations between 100 and 500 nm ( accumulation mode ). The idea of this kind of plot is to show the possible correlation between the two aerosol modes, to indentify some of the main particle number size distribution types, and whether the particle number concentrations in both modes increase in the same rate. 

The submicron aerosol populations in the European background air are variable from location to location. The concentrations and variability of aerosol distributions do, however, show similarities over large geographical areas (Fig. 11). The particle number concentrations are generally lower in more northern and higher mountain locations, naturally as they are generally located farther from the emission areas. 

Paatero P, Aalto P, Picciotto S, Bellander T, Castano G, Cattani G, Cyrys J, Koster M (2005) Estimating time series of aerosol particle number concentrations in the five HEAPSS cities on the basis of measured air pollution and meteorological variables. Atmos Environ 39 2261-2273... 

A comparison between the rates of formation and dissociation of doublets as well as the calculation of the critical particle size are presented in the next section. The subsequent section discusses the implications of the above two mechanisms of aerosol growth on the calculation of the Brownian coagulation coefficient from the evolution of particle number concentration. 

The rates of formation and dissociation of a doublet consisting of equal-size aerosol particles, in air at 1 atm and 298 K, have been calculated for a Hamaker constant of 1CT12 erg from Eqs. [6] and [7]-[12]. The rate of formation of a doublet rf is plotted in Fig. 1 as a function of 2 for different values of 4>p. The rate of formation of a doublet is found to decrease with increasing particle size at constant (j>p because of the predominant effect of the decrease in the particle number concentration (Eq. [2]). The rate of formation of a doublet does not show as strong a dependence... 

Let us consider a system of monodispersed aerosol particles. Because of the collisions between the particles, they will coagulate to form doublets. The rate of formation of a doublet depends, of course, on the particle radius, the particle density, the viscosity and temperature of the suspending medium, the Hamaker constant, and the particle number concentration. If the rate of formation of a doublet is itnuch less than its rate of dissociation, the doublets are unstable. These unstable doublets attain... 

For nonspherical particles, Muller (1928) postulated that since the diffusion equation applicable to aerosol problems is the same (except for definition of terms) as the general equation for electric fields (Laplace s equation), there should be analogs among the electrostatic terms for various properties of coagulation. For example, the potential should be analogous to particle number concentration, and field strength to particle agglomeration rate. Zebel (1966) pointed out that... 

Atmospheric particles influence the Earth climate indirectly by affecting cloud properties and precipitation [1,2], The indirect effect of aerosols on climate is currently a major source of uncertainties in the assessment of climate changes. New particle formation is an important source of atmospheric aerosols [3]. While the contribution of secondary particles to total mass of the particulate matter is insignificant, they usually dominate the particle number concentration of atmospheric aerosols and cloud condensation nuclei (CCN) [4]. Another important detail is that high concentrations of ultrafine particles associated with traffic observed on and near roadways [5-7] lead, according to a number of recent medical studies [8-11] to adverse health effects. 

Both the number concentrations and sizes of aerosol particles directly affect many of their properties and effects. For example, the ability of particles to serve as nuclei for cloud droplet formation depends on their composition as a function of size, although their effectiveness in any given situation depends also on the number of particles present. Knowledge of these aerosol properties is required to evaluate the indirect effects (Section 4.04.7.3) of aerosol particles on climate, i.e., the effect of aerosol particles on cloud reflectivity and persistence. Therefore much attention has been and continues to be focused on determining particle number concentrations and size distributions. 

Atmospheric particles have spherical equivalent diameters (Dp) ranging from 1 nm to 100 pm. Plots of particle number concentration (as well as surface area and volume) as a function of particle size usually show that an atmospheric aerosol is composed of three or more modes, as illustrated in Figure 1. By convention, particles are classified into three approximate categories according to their size Aitken (or transient) nuclei mode (Dp <0.1 pm), accumulation mode (0.1 < Dp < 2.5 pm), and coarse mode (Dp > 2.5 pm) (Seinfeld and Pandis 1998). Particles smaller than 2.5 pm are generally classified as fine. The terms PM2.5 and PMio refer to particulate matter with aerodynamic equivalent diameters under 2.5 and 10 pm, respectively. These terms are often used to describe the total mass of particles with diameters smaller than the cutoff size. 

Figure 2. Trimodal stmcture of the submicron particle number size distribution observed at a boreal forest in Hyytia la, Finland on June 17, 1996, 08 09-08 19. The total particle number concentration of the submicron aerosol is 1011 particles cm. From Ma kela et al. (1997). Used by permission of the American Geophysical Union.

As shown in Figure 1, within an atmospheric aerosol the smallest particles usually dominate the total number of particles, while the accumulation and coarse modes often determine the total surface area and volume (i.e., mass), respectively. For example. Figure 3 shows results from a study in Atlanta where nanoparticles (Dp = 3-10 nm) and nano- and ultrafme particles (Dp = 10-100 nm) contributed approximately 30 and 60%, respectively, to the total particle number concentration (Dp < 2 pm). However, in terms of particle mass, the accumulation mode particles were dominant, and nanoparticles with Dp < 10 nm contributed insignificantly. 

Figure 11. Particle size distributions (top panel) and particle number concentrations (bottom panel) at Hyytia la, Finland as a function of time of day (Julian day 263.5 = noon on September 19, 1996). Note the burst of nanoparticle nucleation occurring near noon and its subsequent growth. From Clement et al. (2001). Used by pemussion of the editor of The Journal of Aerosol Science.

At the above optimal concentration, the rate of coagulation of the aerosol happens to be rather small. According to Smoluchowski s equation, the particle number concentration decreases like... 

Figure 10.11 Scaling of particle number concentration in a turbulent jet. (a) Aerosol number density measured at 20 nozzle diameters on the jet centerline. (Qld = 0.375 cm. Re = 4700 Arf = 0.235 cm. Re = 4700 (+) d = 0.235 cm. Re = 7100. For a given value of. rp- N increased with d and ko. (b) Data of (a) replotted as Nun/d versus xq the data collapse to a single curve. The data demonstrate the effects of varying both nozzle diameter and velocity.

The condensable material which includes molecules, clusters, and stable nuclei may be efficiently scavenged by the existing aerosol. In this case, there is little new particle formation the particle number concentration remains constant or decreases by mixing. Deposition to particles in the accumulation mode (0.1 < dp < 2.5 jam) is diffusion-controlled and can be calculated from an expression of the form (Chapter 10)... 

Figure 6- Measured time profiles of pressure, gas temperature, relative humidity with respect to ice, back-scattered laser light intensity, as well as ice particle number concentration for two expansion cooling experiments with different flame soot aerosol samples from the CAST burner as seed aerosol (Mdhler et al, 2004b). See text for details.

FIGURE 8.1 Histogram of aerosol particle number concentrations versus the size range for the distribution of Table 8.1. The diameter range 0-0.2 pm for the same distribution is shown in the inset. 

The indirect effect of aerosols on climate is exemplified by the processes that link S02 emissions to cloud albedo. Sulfur dioxide is oxidized in gas and aqueous phases to aerosol sulfate. Although increased S02 emissions can be expected to lead to increased mass of sulfate aerosol, the relation between an increased mass of aerosol and the corresponding change of the number concentration of aerosol is not well established. Yet, it is the aerosol number concentration that is most closely related to the cloud drop number concentration. Aerosol mass is created by gas-to-particle conversion, which can occur by growth of... 

A direct relationship between CDNC and ambient aerosol mass is not necessarily to be expected. As we saw in Chapter 8, aerosol mass and number concentrations lie in different size regimes aerosol mass peaks at much larger particle size than particle number. Thus aerosol mass may not accurately reflect aerosol number, and it is the latter that determines the CCN concentration. In addition, the lifetimes of particles at the peak in the mass and number distributions differ the larger particles have a longer lifetime than do the smaller particles since the paths for removal of small particles are more efficient. Empirically,... 

Total deposition can also be detennined by measuring the mean particle number concentration in samples taken from inspired and expired aerosols over periods of several breaths and taking into account particle losses due to the sampling procedure. 

However, regardless of the technique used, the particle number concentration of the inspired aerosol must remain constant during the entire inspiration. In case the particle number concentration increases during inspiration, deposition will be different from that obtained for the case of a decreasing inspiratory particle number concentration. It must therefore be recognized that it is necessary to use only aerosols of uniform particle number concentration for the experimental determination of total deposition. 

FIGURE 9 Left distribution of particle number concentration versus aerodynamically equivalent radius for the rural continental aerosol. Right The corresponding size distributions for the concentrations of particle surface and volume. The size distribution of mass concentration is nearly equivalent to that of volume. [From Zellner, R., ed. (1999). Global Aspects of Atmospheric Chemistry, Steinkopff/Springer, Darmstadt, Germany.]... 

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