Particles indirect effects

Thus the atmospheric component of the planet s radiation budget is strongly modulated by the indirect effects of oceanic gas and particle exchange. As will be... 

In addition, aerosol particles have indirect effects. The most important of these is their effect on cloud properties, since clouds obviously also have major effects on climate. In addition, since heterogeneous chemistry can occur on aerosol particles (see Chapter 5), it is possible that such chemistry can alter the concentrations of other contributors to the climate system, such as the greenhouse gases. One example is the formation of N20 from reactions of HONO on the surface of aerosol particles (see Chapter 7.C). 

Based on such correlations, it is reasonable to assume that the Twomey proposal is applicable, i.e., that anthropogenic emissions of S02 and other species that form particles in the atmosphere may contribute to CCN and hence have indirect effects on climate. 

The following sections focus on the potential indirect effects of aerosol particles due to anthropogenic contributions, which, unlike the natural emissions, are expected to provide a contribution that changes with time. 

There are two questions with respect to potential indirect effects of aerosol particles on properties of clouds (1) What are the sources of new particles (2) How do these new particles grow to sufficient size (> 50 nm) to act as CCN ... 

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... 

In short, it is becoming clear that although the focus to date has been mainly on sulfate, the effects of other components, including both natural and anthropogenic species, need to be taken into account in both the direct and indirect effects of particles on global climate. 

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. 

Necessarily for any number of particles more than two, eqn. (211) cannot be solved exactly, even if v° = 0 and U = 0. When there are more than two particles, the motion of one particle, say j, causes both k and / to move. Now because k and / are perturbed by j, then the perturbation to the motion of k is felt by /. The motion of j affects / directly and also indirectly through k. These indirect effects are not usually very important, especially in chemical kinetics, because the particles most likely to react are those which are closest together. Under such circumstances, the direct effect is stronger than the transmitted and reflected components. These effects have been considered by Adelman [481], Freed and Muthukumar [482] and Allison et al. [483]. Adelman draws an interesting parallel between the screening of hydrodynamic repulsion and the electrolyte screening of a coulomb interaction [481]. 

The indirect climatic impact of aerosol at the ABL is determined by numerous interactions between aerosol and the dynamics of the microphysical and optical properties of clouds. The input to the atmosphere of anthropogenic aerosol particles functioning as CCN favors an increase in cloud droplet number density. As mentioned above, the related increase in the optical thickness and albedo of clouds, with their constant water content, was called the first indirect effect , which characterizes the climatic impact of aerosol. 

The impact of secondary aerosols on indirect radiative forcing is the most variable and is the least understood [3]. The reasons why the indirect effect of secondary aerosols is so difficult to describe is that it depends upon [1] (1) a series of molecular-microphysical processes that connect aerosol nucleation to cloud condensation nuclei to cloud drops and then ultimately to cloud albedo and (2) complex cloud-scale dynamics on scales of 100-1000 km involve a consistent matching of multiple spatial and time scales and are extremely difficult to parameterize and incorporate in climate models. Nucleation changes aerosol particle concentrations that cause changes in cloud droplet concentrations, which in turn, alter cloud albedo. Thus, macro-scale cloud properties that influence indirect forcing result from both micro-scale and large-scale dynamics. To date, the micro-scale chemical physics has not received the appropriate attention. 

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. 

Self-Feedback Effect Photochemistry Effect Smudge-Pot Effect Daytime Stability Effect Particle Effect through Surface Albedo Particle Effect through Large-Scale Meteorology Indirect Effect Semi-direct Effect... 

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. 

Indirect effects of aerosols on climate arise from the fact that the particles act as nuclei on which cloud droplets form. In regions distant from land, the number density of SOj particles is an important determinant of the extent and type of clouds. By contrast, over land there are generally plenty of particles for cloud formation from wind-blown soil dust and other sources. Since clouds reflect solar radiation back to space, the potential link to climate is clear. The effect is likely to be most sensitive over the oceans far from land and for snow-covered regions like Antarctica, where land sources of particles have least effect. In such areas a major source of aerosols is the DMS route to SO - particles (Fig. 7.23). Thus, marine phytoplankton are not only the major source of atmospheric acidity but also the main source of cloud condensation nuclei (CCN) and so play an important role in determining cloudiness and hence climate. 

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