Aerosols number size distribution

Keywords Aerosol number concentration, Aerosol number size distribution, Atmospheric aerosols, CCN... 

Spatial Differences and Similarities of Aerosol Number Size Distributions. 305... 

Fig. 1 (a) Typical clean Northern European median number size distribution (measured at SMEAR II station in Hyytiala, Finland). The approximate modal locations and size ranges of different integral properties of the aerosol number size distribution used are shown (b) variance of number concentration as a function of particle diameter (c) variance of (computed) volume concentration in the same station (adapted from Asmi (2012), [11])... 

Fig. 2 Stations used in measurements of aerosol number size distributions. Black symbols are EUSAAR stations, white GUAN (MPZ was in both networks). Triangles denote high-altitude mountain stations (over 1,000 m from mean sea level). Figure adapted from figure published in [18], which also has more details on the locations and types of the stations...

One of the stations, Zeppelin (ZEP), is located far north of the European mainland, on the Svalbard archipelago, 78°N. This far northern position creates many environmental drivers for the aerosol size distribution, almost never seen at the more southern stations. As the station is located north of the northern polar circle, the station is good part of the year in complete daylight (midnight sun), and in complete darkness (polar night). Although the data quality was not always optimal, some indication of the aerosol number size distributions can be made. 

One of the main uses of comparable size distribution dataset is model-measurement comparison. Without comparable data, the air quality and climate models do not have a way to reliably validate their results. In model-to-measurement comparison, many application-based conditions must be taken into account, and especially for aerosol number size distribution, some of the key points are [18] ... 

Covert DS, Wiedensohler A, Aalto P, Heintzenberg J, McMuny PH, Leek C (1996) Aerosol number size distributions from 3 to 500-nm diameter in the Arctic marine boimdary layer during suimner and autunm. Tellus B-Chem Phys Meteorol 48 197-212... 

Very early on, Aitken (1923) showed that most particles in the atmosphere are smaller than 0.1 pm diameter and that their concentrations vary from some hundreds per cm over the ocean to millions per cm in urban areas. Junge (1955,1963,1972) measured the atmospheric aerosol number size distribution and concentration in urban and non-urban areas as functions of altitude and site. He established the standard form for plotting size distribution data log of AN/ADp versus logD, where N = number and Dp = particle diameter. He observed that this plot was a straight line that could be described by the equation AN/ADp = AD, where A and k were constants. He also noted that in the range from 0.1 to 10.0 pm particle diameter, k was approximately equal to 4.0. This distribution mode was widely known as the Junge distribution or the power law distribution. 

For example, as seen in Fig. 14.41, aerosol particle number size distributions in the clean marine boundary layer outside of clouds are often observed to have a bimodal distribution. The larger mode above 0.1 /xm... 

Thimmaiah D, Hovorka J, Hopke PK (2009) Source apportionment of winter submicron Prague aerosols from combined particle number size distribution and gaseous composition data. Aerosol Air Qual Res 9 209-236... 

Rodriguez S, Van Dingenen R, Putaud JP, Dell Acqua A, Pey J, Querol X, Alastuey A, Chenery S, Kin-Fai H, Harrison RM, Tardivo R, Scamato B, Gianelle V (2007) A study on the relationship between mass concentration, chemistry and number size distribution of urban fine aerosols in Milan, Barcelona and London. Atmos Chem Phys 7 2217-2232... 

Birmili W, Weinhold K, Nordmann S et al (2009) Atmospheric aerosol measurements in the German ultrafine aerosol network (GUAN). Part 1. Soot and particle number size distributions. Gefahrst Reinhalt Luft 69 137-145... 

Abstract The aerosol particle number size distribution is a key component in aerosol indirect climate effects, and is also a key factor on potential nanoparticle health effects. This chapter will give background on particle number size distributions, their monitoring and on potential climate and health effects of submicron aerosol particles. The main interest is on the current variability and concentration levels in European background air. 

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. 

Measuring the Aerosol Particle Number Size Distributions. 302... 

The climate effects of atmospheric aerosol particles are a matter of continuous interest in the research community. The aerosol-climate effects are divided into two groups The direct effect represents the ability of the particle population to absorb and scatter short-wave radiation - directly affecting the radiation balance. These direct effects depend primarily on the aerosol optical properties and particle number size distribution, as the particle size significantly affects the scattering efficiency of... 

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. 

GUAN is a network of multiple German institutes with an interest in submicron aerosol properties, which was established in 2008 [17]. The methodologies of particle number size distribution measurements and data handling procedures in both GUAN and EUSAAR networks are very similar, and the size distribution measurement results are comparable between the two networks. The EUSAAR measurements were available (with some station-to-station variability) for the year 2008-2009 and the GUAN measurements were mostly from 2009. The locations of the stations are shown in Fig. 2. 

Levels and Variability of Aerosol Number Concentrations 3.1 General Properties of Number Size Distributions... 

Combining the physical aerosol measurements from a high number of European background stations shows that there are clear similarities between particle number size distributions and concentration levels measured at different locations over wide geographical regions. These similarities are connected to similar emissions, particle loss processes, and meteorological patterns. The main aim of this section is just to provide key factors of each station categorization, more details and complete analysis of individual stations are available in [18] and references therein. 

Overall, the results suggest that the particle number size distributions in Central Europe are very similar over very large region, and even though the mean concentrations somewhat vary from station to station, the background air in Europe is homogenous from the aerosol point of view. 

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. 

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.

Number size distribution of aerosol particles under urban conditions according to Whitby (1978). N number of particles tln diameter of particles FT total volume concentration r. correlation coefficient between power law given in the figure and experimental data. (By courtesy of Atmospheric Environment)... 

Aerosol scattering, absorption, and extinction coefficients are functions of the particle size, the complex refractive index of the particles m, and the wavelength X of the incident light. The extinction coefficient for a monodisperse ensemble of particles was given in terms of the dimensionless extinction efficiency by (15.27). With a population of differentsized particles of identical refractive index m with a number size distribution function of n(Dp), the extinction coefficient is given by1... 

The CCN behavior of ambient particles can be measured by drawing an air sample into an instrument in which the particles are subjected to a known supersaturation, a so-called CCN counter (Nenes et al. 2001). If the size distribution and chemical composition of the ambient particles are simultaneously measured, then the measured CCN behavior can be compared to that predicted by Kohler theory on the basis of their size and composition. Such a comparison can be termed a CCN closure, that is, an assessment of the extent to which measured CCN activation can be predicted theoretically [see, for example, VanReken et al. (2003), Ghan et al. (2006), and Rissman et al. (2006)]. The next level of evaluation is an aerosol-cloud drop closure, in which a cloud parcel model, which predicts cloud drop concentration using observed ambient aerosol concentration, size distribution, cloud updraft velocity, and thermodynamic state, is evaluated against direct airborne measurements of cloud droplet number concentration as a function of altitude above cloud base. The predicted activation behavior can also be evaluated by independent measurements by a CCN instrument on board the aircraft. Such an aerosol-cloud drop closure was carried out by Conant et al. (2004) for warm cumulus clouds in Florida. 

Wehner B, Birmili W, Gnauk T, Wiedensohler A (2002) Particle number size distribution in a street canyon and their transformation into the urban-air background measurements and a simple model study. Atmos Environ 36 2215-2223 Weingartner E, Keller C, Stahel WA, Burtscher H, Baltensperger U (1997) Aerosol emission in a road tunnel. Atmos Environ 31 451-462... 

It is out of the scope of this book to describe the AP mechanics, i. e. microphysics and dynamics (Friedlander 1977, Hinds 1882, Kouimtzis and Samara 1995, Harrison and van Grieken 1998, Meszaros 1999, Spumy 1999, 2000, Baron and Willeke 2001). Here, we only summarize the important topic of atmospheric aerosol size distribution (Jaenicke 1999). Fig. 4.15 shows that the size range covers several orders of magnitude. Therefore, the common logarithm of the radius is useful to describe the different distribution functions dN r)ld gr = f( gr) or dN r)ldr =/(Igr)/2.302 r. N r) cumulative number size distribution (or the integral of radii) having dimension cm , r radius of particle ... 

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