Particles number concentration

EPM has been developed to simulate as a function of time all the phases, species, and the detailed )tinetic mechanism of the previous section. The structure of EPM consists of material balances, the particle number concentration balance, an energy balance, and the calculation of important secondary variables. 

Rate of Reactions. The rates of reaction in the aqueous and polymer phases were calculated using the appropriate kinetic constants according to the kinetic mechanisms described above, radical and molecular concentrations, and the particle number concentration. 

Prindle and Ray (ZB.) have recently analyzed the same styrene data using a hybrid model consisting of the micellar nucleation mechanism above the CMC and of the homogeneous nucleation and coagulation mechanism below the CMC. Their simulations show a much steeper rise in the particle number concentration precisely at the CMC than predicted by EPM. Their hybrid model does not appear to predict that the particle concentration levels off at high surfactant concentrations. 

The same applies to crystallizers, in which particle sizes and particle number concentrations not only depend on nucleation and growth from supersaturated mother liquid, but are also affected by shear-dominated agglomeration and by secondary nucleation as a result of particle particle and particle-impeller collisions. Some of the subprocesses involved may be limited to specific and different parts of the vessel e.g., nucleation may be restricted to a flame-like region around the outlet of a feed pipe (Van Leeuwen, 1998). In addition, in... 

Hollander (2002) and Hollander et al. (2001 a,b, 2003) studied agglomeration in a stirred vessel by adding a single transport equation for the particle number concentration mQ (actually, the first moment of the particle size distribution)... 

Fig. 3. Effect of particle number concentration on flocculation rate.

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... 

The increase of PM in air is one of the main indicators showing the presence of LRT smoke. The increase in PM due to LRT smoke is more pronounced in PM or PM2 5, and most of the mass is usually located in fine particles. However, elevated levels of coarse particles have also been attributed to the LRT smoke [19]. The episodes in April to May and August 2006 can clearly be seen in the PM] data from Helsinki for the period of March 2006 to February 2007 (Fig. 2). Also particle number concentration increases with the smoke plume but usually less than the PM mass. In spring... 

Pey J, Querol X, Alastuey A, Rodriguez S, Putaud JP, Van Dingenen R (2009) Source apportionment of urban fine and ultra-fine particle number concentration in a Western Mediterranean city. Atmos Environ 43 4407 (415... 

Reche C, Querol X, Alastuey A, Viana M, Pey J, Moreno T, Rodriguez S, Gonzaliz Y, Femandez-Camacho R, de La Rosa J, Dall Osto M, Pret ASH, Hueglin C, Harrison RM, Quincey P (2011) New considerations for PM, Black Carbon and particle number concentration for air quality monitoring across different European cities. Atmos Chem Phys 11 6207-6227... 

Schneider J, Kirchner U, Borrmann S, Vogt R, Scheer V (2008) In situ measurements of particle number concentration, chemically resolved size distributions and black carbon content of traffic-related emissions on German motorways, rural roads and in city traffic. Atmos Environ 42 4257-4268... 

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... 

Gomiscek B, Hauck H, StopperS, Preining O (2004) Spatial and temporal variations of PM, PM2.5, PM10 and particle number concentration during the AUPHEP—project. Atmos Environ 38 3917-3934... 

The link between exposure to air pollutants and adverse health effects is well established, but the causal biological mechanisms are not clear and this is especially the case for particulate matter health effects. Airborne particulate matter is extremely variable in chemical composition, size and morphology all parameters of possible health relevance. This and the different health endpoints affected by exposure to ambient PM make the situation very complex. It may well be that more than one particle characteristic is needed to effectively describe the harmful outcomes of exposure. Possible parameters under discussion are particle number concentration, which is dominated by particles below 100 nm in size the so-called ultrafines [33], particle surface area concentration, which is dominated by particles around 200-800 nm in diameter [34, 35], black carbon or black smoke [36], or the reactivity of particles with respect to redox reactions, or their potential to form radical oxidative species (ROS) [37]. These and some other alternative particulate indicators are currently discussed [38] and investigated in several large European and US studies such as ESCAPE and Transphorm2. 

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. 

Also, the particle number concentration histograms can have multiple concentration modes. These can be interpreted as different air masses or emission periods affecting the number and mass concentrations measured at the stations. Such behavior is often seen in locations with time-dependent influence of either very clean air (resulting in a log-normal mode of smaller concentrations) or very polluted air (resulting in a log-normal mode of higher concentrations). Similar multi-modal histograms can also be detected in some cases at stations with high seasonality, where each mode corresponds to a seasonal concentration distribution. 

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. 

Environment Typical particle number concentration between 30 and 100 nm (cm-3) Particle number concentration between 100 and 500 nm (cm-3) Spatial variability... 

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. 

We focus in this chapter on particles from ambient origin. We first illustrate differences in outdoor and personal exposure using data on real-time particle number concentrations (PNC) from a recent study in Augsburg, Germany. We then present a model of indoor PM concentrations, illustrating the factors that affect indoor air quality. We summarize empirical studies that have assessed indoor-outdoor relationships for particle mass, particle number, and specific components of particulate matter. The focus is on European studies, but we included key studies from outside Europe as well. We conclude by comparing the strength of indoor-outdoor relationships of various particle fractions and components. 

Cyrys J, Pitz M et al (2004) Relationship between indoor and outdoor levels of fine particle mass, particle number concentrations and black smoke under different ventilation conditions. J Expo Anal Environ Epidemiol 144 275-283... 

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