Indirect Effect of Aerosols on Climate

The indirect effect of aerosols on climate, which at present contributes a major uncertainty in understanding anthropogenic perturbations on climate, is a very active area of research. For some typical model treatments of this indirect effect and how it interacts with those due to other, simultaneous, perturbations, see, for example, Jones et al. (1994), Hansen et al. (1997a—d), C. C. Chuang et al. (1997), Lohmann and Feichter (1997), and Pan et al. (1998). 

Brenguier J.-L. Pawlowska H. and Schuller L. (2003). Cloud microphysical and radiative properties for parameterization and satellite monitoring of the indirect effect of aerosol on climate. J. Geophys. Res., 108(D15), CMP6/1-CMP6/14. 

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. 

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... 

Brenguier et al. (2003) discussed the results of eight series of aircraft measurements of the microphysical characteristics of marine stratocumulus clouds in a broad range of observation conditions (different physico-chemical properties of aerosol, number density values in the interval 50 cm 3-25 cm-3, etc.)- The unique complex of synchronous observations of the microphysical and radiative characteristics of cloud cover obtained can be used to assess the indirect impact of aerosol on clouds and climate based on analysis of the ratio between the cloud optical thickness and effective radius of cloud droplets. Correlation between these values is usually negative, but in a heavily polluted atmosphere it can be positive. From the observational data obtained during ACE-2, the polluted systems of clouds turned out to be somewhat drier and therefore thinner, resulting in the positive correlation between the indirect impact of aerosol on climate and the effective radius of droplets. 

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. 

The first indirect climatic effect of aerosol (the Twomey effect) is based on the assumption that with a constant equivalent liquid water content (LWC) of clouds an increase in atmospheric aerosol number density (and, hence, concentration of CCN)... 

The main difficulty in understanding the causes of climate change is connected with the impossibility of considering climatic feedbacks sufficiently reliably. Primarily, this refers to cloud-radiation feedback, direct and indirect (by the effect on radiation properties of clouds) impacts of atmospheric aerosols on climate, and the impact of the atmosphere-ocean interaction on climate formation. 

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. 

It is necessary to highlight the effects of aerosols and other chemical species on meteorological parameters, which have many different pathways (direct, semi-direct, and indirect effects, etc.) and they have to be prioritised and considered in on-line coupled modelling systems. Sensitivity studies are needed to understand the relative importance of different feedback mechanisms for different species and conditions relevant to air quality and climate interactions. A concerted action to mobilise and coordinate research in this area is needed. 

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. 

The atmospheric aerosol has profound effects on the nature of the air environment. Effects on human health have led to the establishment of ambient air qualtiy standards by the United States and other industrial nations. The optical properties of the aerosol alTect local and regional visibility and Earth s radiation balance, hence global climate. There is evidence that reactions that take place on the surface of the stratospheric aerosol play a major role in the destruction of the stratospheric ozone layer. Particularly complex (and poorly understood) arc the indirect effects of the aerosol serving as condensation nuclei for the formation of clouds which in turn affect the radiation balance. For an extensive review of the properties of the atmospheric aerosol and its effects, especially health related, the reader is referred to the document prepared by the U.S. EPA (1996) for use in setting the ambient air quality standard for particulate matter. Atmospheric aerosol properties and dynamics are reviewed in detail by Seinfeld and Pandis (1998). 

Indirect climate effects of aerosols are more complex and more difficult to assess than direct effects because they depend on a chain of phenomena that connect aerosol levels to concentrations of cloud condensation nuclei, cloud condensation nuclei concentrations to cloud droplet number concentrations (and size), and these, in turn, to cloud albedo and cloud lifetime. Changes in the number concentration of aerosols are observed to cause variations in the population and sizes of cloud droplets, which are expected to cause... 

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). 

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. 

Giorgi F. Bi X. and Qian Y. (2003). Indirect and direct effects of anthropogenic sulfate on the climate of East Asia as simulated with a regional coupled climate-chemistry/aerosol model. Climatic Change, 58, 345-376. 

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