Aerosols number concentration

FIGURE 14-41 Typical particle size number distributions for marine aerosols outside of clouds where the total aerosol number concentration was < 500 cm-3 (adapted from Anderson et al., 1994). 

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. 

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

Properties of Aerosol Number Concentrations and Size Distributions. 299... 

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

Table 1 Representational (geometric) mean aerosol number concentrations in European background environments (data from [18])...

Fig. II Overview of submicron aerosol number concentrations. The symbols indicate the typical concentrations, variability, and seasonality of N10o particles. Adapted from [18]...

Asmi A (2012) Weakness of the weekend effect in aerosol number concentrations. Atmos Environ 51 100-107. doi 10.1016/j.atmosenv.2012.01.060... 

Noc = total aerosol number concentration 4> — volumetric concentration... 

FIGURE 8.2 Aerosol number concentration normalized by the width of the size range versus size for the distribution of Table 8.1. The diameter range 0-0.1 im for the same distribution is shown in the inset. 

During the winter and early spring (February to April) the Arctic aerosol has been found to be influenced significantly by anthropogenic sources, and the phenomenon is commonly referred to as Arctic haze (Barrie 1986). During this period the aerosol number concentration increases to over 200 cm-3. The nucleation mode mean diameter is at 0.05 pm and the accumulation mode at 0.2 pm (Covert and Heintzenberg 1993)... 

The aerosol number concentration may increase or decrease exponentially with altitude and one suggestion of a form of the profile is (Jaenicke 1993)... 

The continuous distribution, introduced in Chapter 8, is usually a more useful concept in practice. The volume of the particle is often chosen for convenience as the independent variable for the continuous size distribution function n(v, t)( tirr3 cm-3), where v — is the volume of a particle with diameter Dp. Thus n(v, t)dv is defined as the number of particles per cubic centimeter having volumes in the range from v to v + dv. Note that the total aerosol number concentration Nt(t) (cm-3) is then given by... 

It is desired to estimate the relative contributions of the processes listed below to the rates of change of the aerosol number concentrations in each of the three size ranges ... 

Particles that can activate at a given supersaturation are defined as cloud condensation nuclei (CCN) for this supersaturation. In the cloud physics literature one often defines as condensation nuclei (CN) those particles that form droplets at supersaturations of >400% and therefore CN include all the available particles. One can therefore assume that the CN concentration is equal to the total aerosol number concentration. This CN definition should be contrasted with the CCN definition where supersaturations often well less than 2% are used. Therefore CCN represent the particles that can form cloud droplets under reasonable atmospheric supersaturations. We caution the reader that CCN concentrations always refer to a specific supersaturation, for example, CCN(1%) or CCN(0.5%) and one should be careful when comparing CCN concentrations measured or estimated at different supersaturations. 

If aerosol number concentrations are substantially increased as a result of anthropogenic emissions over that in the absence of such emissions, the number concentration of cloud droplets, which is governed by the number concentration of aerosol particles below cloud, may also be increased. An increased number concentration of cloud droplets leads, in turn, to an enhanced multiple scattering of light within clouds and to an increase in the optical depth and albedo of the cloud. The areal extent of the cloud and its lifetime may also increase. This is the essence of the indirect effect of aerosols on climate. A key measure of aerosol influences on cloud droplet number concentrations is the number concentration of cloud condensation nuclei (CCN). 

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... 

Primary particles (e.g., dust, pollens, plant waxes) and secondary oxidation products are the main components of remote continental aerosol (Deepak and Gali, 1991). Aerosol number concentrations average around 2000 to 10,000 cm and PM lo concentrations are... 

FIGURE 7.26 Representative vertical distribution of aerosol number concentration (Jaenicke, 1993). A range of concentrations is shown for marine and remote continental aerosols. 

Thus, at very short times, the deposition flux is that resulting from diffusion plus one-half that due to settling, whereas for long times the deposition flux becomes solely the settling flux. For particles of radii 0.1 /rm and 1 tm in air (at 1 atm, 298 K), Xds is about 80 seconds and 0.008 second, respectively, assuming a density of 1 g cm. For times longer than that. Brownian motion does not have any effect on the particle motion. The aerosol number concentration and removal flux are shown in Figures 8.9 and 8.10. The system reaches a... 

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