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Aerosol Impacts on Climate and Biogeochemistry EG36CH03-Mahowald-NEW ARI 29 October 2011 14:11 Aerosol Impacts on Climate and Biogeochemistry Natalie Mahowald,1 Daniel S. Ward,1 Silvia Kloster,3 Mark G. Flanner,4 Colette L. Heald,5 Nicholas G. Heavens,1 Peter G. Hess,2 Jean-Francois Lamarque,6 and Patrick Y. Chuang7 1Department of Earth and Atmospheric Sciences, 2Department of Biological and Environmental Engineering, Cornell University, Ithaca, New York 14853; email: [email protected] 3Land in the Earth System, Max Planck Institute for Meteorology, 20146 Hamburg, Germany 4Department of Atmospheric, Ocean, and Space Sciences, University of Michigan, Ann Arbor, Michigan 48105 5Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado 80523 6Atmospheric Chemistry and Climate and Global Dynamics Divisions, National Center for Atmospheric Research, Boulder, Colorado 80307 7Department of Earth and Planetary Sciences, University of California, Santa Cruz, California 95064 Annu. Rev. Environ. Resour. 2011. 36:45–74 Keywords The Annual Review of Environment and Resources aerosols, climate change is online at environ.annualreviews.org by 68.62.18.60 on 11/05/11. For personal use only. This article’s doi: Abstract 10.1146/annurev-environ-042009-094507 Aerosols are suspensions of solid and/or liquid particles in the atmo- Copyright c 2011 by Annual Reviews. sphere and modify atmospheric radiative fluxes and chemistry. Aerosols All rights reserved move mass from one part of the earth system to other parts of the 1543-5938/11/1121-0045$20.00 Annu. Rev. Environ. Resourc. 2011.36:45-74. Downloaded from www.annualreviews.org earth system, thereby modifying biogeochemistry and the snow sur- face albedo. This paper reviews our understanding of the impacts of aerosols on climate through direct radiative changes, aerosol-cloud in- teractions (indirect effects), atmospheric chemistry, snow albedo, and land and ocean biogeochemistry. Aerosols play an important role in the preindustrial (natural) climate system and have been perturbed sub- stantially over the anthropocene, often directly by human activity. The most important impacts of aerosols, in terms of climate forcing, are from the direct and indirect effects, with large uncertainties. Similarly large impacts of aerosols on land and ocean biogeochemistry have been estimated, but these have larger uncertainties. 45 EG36CH03-Mahowald-NEW ARI 29 October 2011 14:11 uncertainties remain. Human activities, such as Contents energy production, industrial production, and land-use management, have radically altered INTRODUCTION.................. 46 the emission of many aerosols over the past Aerosol Characteristics . 47 160 years (Figure 1) (1, 2). Most anthro- Aerosol Transport and Lifetime . 47 pogenic aerosols tend to cool the climate (10), IMPACTS OF AEROSOLS . 49 thus they potentially hid the twentieth-century Direct Radiative Forcing . 49 warming that occurred as a result of increas- Cloud-Aerosol Interactions: ing greenhouse gases (2). Because of this, IndirectEffects................. 52 the magnitude of anthropocene changes in Aerosol Impacts on Atmospheric aerosol climate forcing in the current climate Gases.......................... 54 is very important for understanding climate Snow-Albedo Interactions . 54 sensitivity and for projecting future climate Land Biogeochemistry Impacts . 55 (11). Ocean Biogeochemistry Impacts . 56 In the first section of this paper, we in- ClimateEffects.................... 57 troduce aerosols, their properties, and their NET EFFECT OF AEROSOLS transport and removal processes. In the sec- ON CLIMATE AND ond section, we present the main mechanisms BIOGEOCHEMISTRY........... 57 through which both natural and anthropogenic Aerosols from Land Sources . 57 aerosols interact with climate and biogeochem- Aerosols from Ocean Sources . 60 istry. This is in contrast to previous reviews, Lightning......................... 61 which focused either on natural or anthro- Volcanoes......................... 61 pogenic aerosols and rarely considered aerosol Secondary Organic Aerosols . 61 impacts on biogeochemistry (12–16). We then Other Anthropogenic Aerosols . 62 look at different sources of aerosols from land SUMMARY/CONCLUSIONS . 63 and oceans and assess their natural contribu- tion to climate and biogeochemistry as well as how humans have changed, and are likely to change, their impacts on climate and biogeo- Supplemental Material INTRODUCTION chemistry. Many of these mechanisms operate Aerosols are solid or liquid particles suspended on long timescales and are poorly understood. by 68.62.18.60 on 11/05/11. For personal use only. in the atmosphere. They represent a small Here, we highlight the uncertainties. Impor- Aerosols: liquids or fraction of atmospheric mass but have a tant impacts on land and ocean ecosystems from solids suspended in the disproportionately large impact on climate aerosols are reviewed and highlighted in the atmosphere and biogeochemistry. These particles modify sections on biogeochemistry. Climate sensitivity: atmospheric radiation in both the short- and To compare the effects of aerosols on cli- Annu. Rev. Environ. Resourc. 2011.36:45-74. Downloaded from www.annualreviews.org the change in longwave as well as alter cloud properties mate and biogeochemistry, we estimate their equilibrium surface (Figure 1) (1). Variations in radiation and impacts on radiative forcing, based primarily temperature in response to a doubling clouds, in turn, impact climate (2). While on literature results, but when not available, of carbon dioxide in the atmosphere, particles take part with we make simple estimates and include these in Radiative forcing: photochemical reactions (3) and impact public tables and figures. Because the literature does the change in the health (4). When aerosols fall onto the surface, not usually provide information on the rela- radiation budget (both they darken snow albedo (5) and modify tive importance of various aerosols on differ- short and longwave) both land (6, 7) and ocean biogeochemistry ent impacts, we use a recent model simulation owing to the addition (Figure 1) (8, 9). Because of aerosols’ im- to calculate relative importance and changes of a constituent, either natural or portance to climate and biogeochemistry, as since preindustrial times (17) (more details in anthropogenic well as impacts on human health, they have the Supplement; follow the Supplemental been studied for many years; yet significant Material link from the Annual Reviews home 46 Mahowald et al. EG36CH03-Mahowald-NEW ARI 29 October 2011 14:11 page at http://www.annualreviews.org). We in the next section (Figures 2 and 3). In do not discuss the potentially important impact addition, aerosols can interact with gas phase of aerosols on health. chemistry through the following processes: Lifetime: time that an (a) equilibrium partitioning between the gas aerosol particle resides and particle phase (e.g., nitrate uptake onto in the atmosphere Aerosol Characteristics aerosols), (b) irreversible partitioning from the (e.g., if the lifetime is Aerosols vary widely in their impact on cli- gas to the particle phase (e.g., formation of 10 days, 1/e of the mate and biogeochemistry because of sub- ammonium sulfate), (c) aerosol liquid phase aerosol is left after 10 days) stantial variability in their size, composition, reactions of partitioned gases [e.g., organic and location in time and space. Here, we de- polymerization and secondary organic aerosol Primary aerosols: aerosols entrained into scribe in more detail some of the attributes (SOA) formation], and (d ) aerosol surface the atmosphere as of aerosols. The impact of aerosols on var- phase reactions for particles in the gas phase solids or liquids ious atmospheric and biochemical processes (18, 19). Chemical reactions within aerosols Secondary aerosols: are determined by their number, size, vol- can change their properties substantially. For aerosols formed in the ume, and composition (Figure 2). One of example, the ability of an aerosol to take up wa- atmosphere by the most important attributes of aerosols for ter (hygroscopicity) can be modified by adding chemical reaction or both impacts and their atmospheric lifetime is organic compounds (20). More discussion of condensation of gases the aerosol size (Figure 2). Aerosols vary in these reactions are described in sections below. SOA: secondary size from a diameter of 1 nm up to 100 μm Finally, the aerosol spatial and temporal organic aerosol (Figure 2). Particles with diameters less than variability is important for climate impacts. 1 μm are fine aerosols and are divided into Two identical aerosols, one close to the ground the Aitken mode (diameter <0.1 μm) and the and the other at high altitude, have differing cli- accumulation mode (diameter >0.1 μmand mate impacts and lifetimes. Thus, an important <1.0 μm), and particles greater than 1 μm com- property of aerosols is their location in space prise the coarse aerosols. Coarse aerosols are and time (e.g., Figure 3). The impact of an usually primary aerosols, meaning they were aerosol on the atmosphere is restricted to its directly emitted and are often entrained into generally brief time of actual residence in the at- the atmosphere by the wind [e.g., desert dust mosphere, but once deposited on the surface, an or sea-salt particles (18)]. In contrast, many an- aerosol’s effects can endure much longer than thropogenic aerosols occur in the fine mode. A its coherence as a particle (Figure 2). substantial portion of fine aerosols are not
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