Aerosol particles direct effect

The electrostatic force is directly proportional to the net charge of an aerosol particle. Therefore, effective charging of the particles is of great importance. Airborne particles are normally charged either due to their birth processes or due to charge transfer from gas ions to particles. The natural charging of particles is normally so weak that it has no practical importance for electrostatic air cleaning. 

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. 

Aerosol-Based Direct Fluorination. A technology that works on Hter and half-Hter quantities has been introduced (40—42). This new aerosol technique, which functions on principles similar to LaMar direct fluorination (Fig. 5), uses fine aerosol particle surfaces rather than copper filings to maintain a high surface area for direct fluorination. The aerosol direct fluorination technique has been shown to be effective for the synthesis of bicycHc perfluorocarbon such as perfluoroadamantane, perfluoroketones, perfluoroethers, and highly branched perfluorocarbons. 

Like the inhaled gases or vapors, soluble and insoluble aerosol particles can directly exert desirable and undesirable local effects at the site of deposition and/or systemic effects after solubilization, absorption, and metabolization. 

In short, the direct effects of aerosol particles in terms of backscattering solar radiation out to space and hence leading to cooling are reasonably well understood qualitatively and provided the aerosol composition, concentrations, and size distribution are known, their contribution can be treated quantitatively as well. However, major uncertainties exist in our knowledge of the physical and chemical properties, as well as the geographical and temporal variations, of aerosol particles and it is these uncertainties that primarily limit the ability to accurately quantify the direct effects at present. 

As we have seen in Chapter 9, there are a variety of dissolved solutes in atmospheric particles, which will lower the vapor pressure of droplets compared to that of pure water. As a result, there is great interest in the nature and fraction of water-soluble material in atmospheric particles and their size distribution (e.g., Eichel el al., 1996 Novakov and Corrigan, 1996 Hoffmann et al., 1997). This vapor pressure lowering effect, then, works in the opposite direction to the Kelvin effect, which increases the vapor pressure over the droplet. The two effects are combined in what are known as the Kohler curves, which describe whether an aerosol particle in the atmosphere will grow into a cloud droplet or not under various conditions. 

The first major link between the indirect effects of aerosol particles and climate is whether there has been an increase in particles and in CCN due to anthropogenic activities. As discussed in Chapter 2, anthropogenic emissions of particles and of gas-phase precursors to particles such as S02 have clearly increased since preindustrial times, and it is reasonable that CCN have also increased. Ice core data provide a record of some of the species that can act as CCN. Not surprisingly, sulfate and nitrate in the ice cores have increased substantially over the past century (Mayewski et al., 1986, 1990 Laj et al., 1992 Fischer et al., 1998). For example, Figure 14.43 shows the increases in sulfate and nitrate since preindustrial times in an ice core in central Greenland (Laj et al., 1992). Sulfate has increased by 300% and nitrate by 200%. This suggests that sulfate and nitrate CCN also increased, although not necessarily in direct proportion to the concentrations in the ice core measurements. 

As discussed in Section C.la, sea salt particles in the marine boundary layer have been shown to likely play a major role in backscattering of solar radiation (Murphy et al., 1998), i.e., to the direct effect of aerosol particles. However, they also contribute to the indirect effect involving cloud formation, since they can also act as CCN. Since such particles are a natural component of the marine atmosphere, their contribution will not play a role in climate change, unless their concentration were somehow to be changed by anthropogenic activities, e.g., through changes in wind speed over the... 

On the other hand, aerosol particles from anthropogenic activities tend to be concentrated over or near industrial regions in the continents. Because both the direct and indirect effects of particles are predominantly in terms of scattering solar radiation, their effects are expected primarily during the day. 

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... 

To study the chemistry of highly concentrated particles in bulk solution one must avoid mass transfer limitations and the effects of container surfaces. Both of these problems are eliminated by directly using aerosol particles. Two approaches have been used to study aerosol chemistry (1) aerosol reactors in which the evolution of a suspension of particles is followed, and (2) experiments in which the changes occurring in a single particle can be followed. 

Two-dimensional trajectories of a typical gas molecule and a typical aerosol particle can be compared in Fig. 9.2. The molecule shows sharp changes in direction, each change occurring when it strikes another molecule. As discussed in Chap. 3, the average distance between hits is defined as the mean free path of the molecule. For the particle, a hit by a single molecule does not appreciably affect its motion. Therefore, its path is not characterized by sharp changes in direction, but by smooth curves representing the combined effect of hits by many molecules. 

The role of aerosols in climate can be divided into two types direct and indirect. In the direct effect the particles absorb and scatter energy coming from the sun back to space. This tends to cool the atmosphere since solar radiation, which, in the absence of the aerosols, would warm the air, is now partially absorbed by the particles or reflected upwards out of the atmosphere. 

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