Aerosol particles indirect effects

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). 

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... 

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. 

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 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. 

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. 

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... 

It has become clear only recently that the atmospheric sierosol plays an important role for the climate on earth. It is common to distinguish between direct and indirect effects of the aerosols on the climate. Aerosols effect directly the radiation balance of the earth due to scattering and absorption of electromagnetic radiation (radiative forcing). On the other hand they influence the physics and chemistry of the atmosphere as condensation nuclei for cloud droplets and their chemical reactions with atmospheric trace gases. Though these indirect aerosol effects are difficult to quantify, they are at least as important as the direct radiative forcing. An especially important and complex example for the indirect influence of aerosols on the chemistry and radiation balance of the earth is the role of stratospheric aerosol particles on the polar ozone depletion, which is discussed in more detail below. 

Aerosols, including those of biogenic origin, have important impacts on atmospheric radiation, both directly, and indirectly through the nucleation of atmospheric water. The impact of biogenic aerosols on radiation through direct and indirect effects depends on their physical and chemical properties. Table 5 shows the main chemical and physical properties of aerosols that influence their interaction with radiation, and with other atmospheric compounds, resulting in the formation of secondary particles, as well as their influence on cloud formation. Chemical... 

Besides the direct effects of heterogeneous reactions, equally important indirect effects of PSCs on gas phase chemistry were revealed. It was suggested that HNO3 condenses onto the aerosol particles allowing them to grow at temperatures above those at which pure ice crystals can exist ... 

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