Cloud number concentration

FIGURE 14-44 Effect of ship emissions on (a) cloud number concentration, N, (b) effective cloud droplet radius, / cM, (c) cloud liquid water content, LWC, and (d, e) down- and upwelling radiation at (d) 744 nm and (e) 2.2 /im (adapted from King el al., 1993). 

Clouds, fogs, and rain, however, have much greater liquid water contents and thus have the potential for contributing more to atmospheric aqueous-phase oxidations. Clouds typically have liquid water contents of the order of 1 g m-3, with droplet diameters of the order of 5-50 yxm the number concentration and size distribution depend on the type of cloud. Fogs, on the... 

Effect of aerosol particles on cloud drop number concentrations and size distributions Clouds and fogs are characterized by their droplet size distribution as well as their liquid water content. Fog droplets typically have radii in the range from a few /an to 30-40 /an and liquid water contents in the range of 0.05-0.1 g m" Clouds generally have droplet radii from 5 /an up to 100 /im, with typical liquid water contents of 0.05-2.5 gin"5 (e.g., see Stephens, 1978, 1979). For a description of cloud types, mechanisms of formation, and characteristics, see Wallace and Hobbs (1977), Pruppacher (1986), Cotton and Anthes (1989), Heyms-field (1993), and Pruppacher and Klett (1997). 

Gillani, Leaitch, and co-workers (1995) carried out a detailed study of the fraction of accumulation mode particles (diameters from 0.17 to 2.07 /Am) that led to cloud droplet formation in continental stratiform clouds near Syracuse, New York. When the air mass was relatively clean, essentially all of the particles were activated to form cloud droplets in the cloud interior and the number concentration of cloud droplets increased linearly with the particle concentration. However, when the air mass was more polluted, the fraction of particles that were activated in the cloud interior was significantly smaller than one. This is illustrated by Fig. 14.40, which shows the variation of this fraction (F) as a function of the total particle concentration, Nun. In the most polluted air masses (as measured by large values of Nun), the fraction of particles activated was 0.28 + 0.08, whereas in the least polluted, it was as high as 0.96 + 0.05. The reason for this is likely that in the more polluted air masses, the higher number of particles provided a larger sink for water vapor, decreasing the extent of supersaturation. 

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). 

A number of field studies have quantitatively examined the relationship between the CCN number concentration and/or cloud droplet concentration and the mass concentration of non-sea salt sulfate (nss). Figure 14.45 shows one summary of some of these studies in the form of a log-log plot (Van Dingenen et al., 1995). Given the variety of measured parameters and wide range of conditions encompassed by these data (CCN... 

Similarly, Martin and co-workers (1994) measured aerosol particles in the size range from 0.05 to 1.5 /rm below the base of stratocumulus clouds, along with cloud droplet number concentrations in maritime and in continental air masses. Figure 14.46 shows the relationship between cloud droplet number concentration and the aerosol particle concentration for a set of flights carried out in the vicinity of the British Isles and in the South Atlantic (Martin et al., 1994). There is an almost linear relationship between the two for maritime air masses. Given that the cutoff for particle measurements was 0.05 /xm, these concentrations may have been underestimated, so that the slope of the line for maritime air masses can be taken as unity. That is, essentially all of the maritime particles at the cloud base could act as CCN under the range of supersaturations in these studies. 

FIGURE 14.46 Average cloud droplet number concentration as a function of subcloud aerosol particle concentration (0.05-1.5 fj.m) in marine ( ) and continental ( ) air masses (adapted from Martin et al., 1994). 

Number concentrations of ice crystals in cirrus clouds have also been observed to increase with aerosol particle concentrations (with diameters >0.018 /tm) and, in particular, with the concentration of light-absorbing materials in the ice crystals (Strom and Ohlsson, 1998). 

Not only do CCN affect the number of cloud droplets formed, but they also affect the size distribution of these droplets. This also affects cloud albedo and its sensitivity to changes in the number concentration (see Eqs. (JJ) and (KK)). Figure 14.47, for example, shows the size distribution for cloud droplets measured in urban and nonurban air around Denver, Colorado (Al-kezweeny et al., 1993). The median volume diameter was 14 jj,m for the urban air cloud, and this was only 50% of that of the much larger droplets in the... 

Leaitch, W. R G. A. Isaac, J. W. Strapp, C. M. Banic, and H. A. Wiebe, The Relationship between Cloud Droplet Number Concentrations and Anthropogenic Pollution Observations and Climatic Implications, J. Geophys. Res., 97, 2463-2474 (1992). 

Van Dingenen, R., F. Raes, and N. R. Jensen, Evidence for Anthropogenic Impact on Number Concentration and Sulfate Content of Cloud-Processed Aerosol Particles over the North Atlantic, J. Geophys. Res., 100, 21057-21067 (1995). 

A third commonly used method for determining cloud liquid water content is integration of the droplet size spectrum as measured by a PMS FSSP probe. Estimates of cloud liquid water content using this technique are subject to large errors due to uncertainties in determining the number concentrations of droplets in the largest size ranges. 

Similar to N50, we also show the number concentrations between 100 and 500 nm in diameter (Ni00), representing CCNs for lower updraft velocity clouds, and a rough estimate of the number concentration in accumulation mode as the Hoppel minimum between the Aitken and the accumulation modes is often around 100 nm of diameter. 

Notably, the instruments used in the European Supersites for Atmospheric Aerosol Research (EUSAAR)/Aerosols, Clouds, and Trace gases Research Infrastructure Network (ACTRIS) and German Ultrafine Aerosol Network (GUAN) measurements used in this chapter are from intercalibrated measurements, where the abilities of the instruments were determined in common intercalibration workshops [15]. Overall, the instruments agree well on particle sizes between 20 and 200 nm, with the differences above 200 nm still relatively minor for number concentrations. In smallest particles sizes the instrument deviation is large, and for this reason we only consider particles larger than 30 nm in diameter in this chapter. 

Even though there is no significant change in the CCN number concentrations between days of the week, this is not yet the complete picture of potential aerosol-cloud weekend effects. There are many other potential aerosol processes, such as semi-direct effect, which could have significant effect on the local meteorology. Also, the CCN number concentrations have a weak weekday variation within the cities, which could then lead to meteorological weekend effect directly above the urban environment. However, as the urban or semi-urban areas cover spatially quite small area of the Europe (around 5%) [27], the weekend effect is then of much more a local effect, if it exists at all. 

Although these reactions result in an increased respirable aerosol mass, little is known about human health effects associated with these particular condensed reaction products. Tamas et al. (2006) observed a correlation between the number concentration of SOAs, from the limonene-ozone reaction in an office, and sensory load reported by 20 human subjects. Additionally, a screening study indicates that perfume wearers will be subjected to a personal reactive cloud of reaction products, such as fine aerosols (Corsi et al., 2007). 

The impact of sea salt aerosol determines the decrease in droplet number concentration in marine clouds (CDNC) by 20%-60%, with a maximum decrease in the NH Roaring Forties (40%-70%) and NH mid-latitudes (20%-40%), characterized by the high concentration of sea salt aerosol. Some increase in CDNC due to sea salt aerosol was also observed in the equatorial band. The impact of changes in aerosol content and cloud cover on global climate will be estimated in future. 

CCN). Changes in the concentrations of CCN may alter the cloud droplet concentration, the droplet surface reflectivity, the radiative properties of clouds (cloud albedo) (2), and hence, the earth s climate (8-101. This mechanism has been proposed for the remote atmosphere, where the radiative properties of clouds are theoretically predicted to be extremely sensitive to the number of CCN present (ID). Additionally, these sulfate particles enhance the acidity of precipitation due to the formation of sulfuric acid after cloud water dissolution (11). The importance of sulfate aerosol particles to both radiative climate and rainwater acidity illustrates the need to document the sources of sulfur to the remote atmosphere. 

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. 

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