Atmospheric Aerosols and Their Importance

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Raghava R. Kommalapati1 and Kalliat T. Valsaraj2

'Department of Civil and Environmental Engineering, P.O. Box 519, Mail Stop 2510, Prairie View A&M University, Prairie View, TX 77446 2Cain Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA 70803

Introduction

Apart from trace gases, the atmosphere contains a variety of liquids and solids that exist as dispersed phases in the air. They are collectively called aerosols. An aerosol is considered a two phase system consisting of solid or liquid particles and the gas (air) they are suspended in. Aerosols result from both natural and anthropogenic sources. Examples are dust particles generated by wind erosion of surface soils, agricultural activities, sea salt and wave breaking over oceans. Other sources include generation via chemical reactions in the atmosphere. For example, sulfate aerosols are generated by oxidation of sulfur dioxide in atmospheric moisture, particles are generated in automobile exhaust and incomplete combustion of fossil fuels in power plants. The aerosols have a direct radiative forcing effect on climate because they scatter and absorb solar and infrared radiation in the atmosphere. Aerosols also alter warm, ice and mixed-phase cloud formation processes by increasing droplet number, concentrations and ice particle concentrations. They decrease the precipitation efficiency of warm clouds and thereby cause an indirect radiative forcing associated with these changes in cloud properties. Aerosols have most likely made a significant negative contribution to the overall radiative forcing (/). Although the net effect is cooling, there is also evidence that black carbon in aerosols heats up the atmospheric layer in which they reside (2). Aerosols also impact the health of biota and has other biological effects (e.g., nutrient availability). Bioaerosols appear to have effects as cloud condensation nuclei (CCN) in some regions of the world (3). An important characteristic of aerosols

© 2009 American Chemical Society

is that they have varying atmospheric lifetimes. Aerosols generally have sizes ranging from 2 nm to hundreds of micrometers (4,5). Their shapes are also variable, however, an aerodynamic equivalent diameter is a useful means of representing their size. Concentration of aerosols is typically expressed in mass of particles per unit volume of the mixture (mg/m3 or ^g/m3) or number of particles per unit volume (#/m3) though the mass concentration is more commonly used in standards and measurements. Typical mass concentrations and particle sizes are given in Table 1 and particle sizes of various aerosol particles are listed in Table 2.

Table 1. Mass concentrations and particle sizes of aerosols (6)

Area

Concentration / ßg.m'3

Diameter / /jm

Urban

> 100

0.03

Rural

30-50

0.07

Marine

>10

0.16

Table 2. Properties of atmospheric aerosols (7,8)

Nature of Droplet

Size/¡im

Surface area/(m2/m3)

Liquid water content / (m3/m3 of air

Typical atmospheric lifetime

Aerosols

10"2-10

lxlO*3

10"11- 10"10

4-7 days

Fog droplets

1-10

8X10-4

5x10"8-5x10"7

3 hours

Cloud Drops

10-102

2x10"*

10"7 -10"6

7 hours

Raindrops

102 - 103

5x10~4

10 7 -10"6

3-15 minutes

Snowflakes

103- 105

3xl0"1

15-50 minutes

The Intergovernmental Panel for Climate Change, IPCC in 2007 (1) reported significant progress over its 2001 assessment with respect to aerosol sources. The following are some of the aerosol sources along with a brief description as provided in the latest assessment report:

• Soil dust: Soil dust is a major contributor to aerosol loading and optical thickness, particularly in tropical and sub-tropical regions. Dust source regions are mainly deserts, dry lake beds, and semi-arid desert fringes, but also areas in drier regions where vegetation has been reduced or soil surfaces have been disturbed by human activities.

• Sea salt: Sea salt aerosols are generated by various physical processes, especially the bursting of entrained air bubbles during whitecap formation. This type of aerosol may be the dominant contributor to both light scattering and cloud nuclei in those regions near the marine atmosphere.

• Industrial dust, primary anthropogenic aerosols: Transportation, coal combustion, cement manufacturing, metallurgy, and waste incineration are among the industrial and technical activities that produce primary aerosol particles.

• Carbonaceous aerosols (organic and black carbon): Carbonaceous compounds make up a large but highly variable fraction of the atmospheric aerosol. Organics are the largest single component of biomass burning aerosols. The main sources for carbonaceous aerosols are biomass and fossil fuel burning, and the atmospheric oxidation of biogenic and anthropogenic volatile organic compounds (VOC).

• Primary biogenic aerosols: Primary biogenic aerosol consists of plant debris (cuticular waxes, leaf fragments, etc.), humic matter, and microbial particles (bacteria, fungi, viruses, algae, pollen, spores, etc.).

• Sulphate aerosols: Sulphate aerosols are produced by chemical reactions in the atmosphere from gaseous precursors (with the exception of sea salt sulphate and gypsum dust particles).

• Nitrate aerosols: Aerosol nitrate is closely tied to the relative abundances of ammonium and sulphate.

• Aerosols from volcanoes: Two components of volcanic emissions are of most significance for aerosols: primary dust and gaseous sulphur.

Many aerosol species (e.g., sulphates, secondary organics) are not directly emitted, but are formed in the atmosphere from gaseous precursors and aerosol species often combine to form mixed particles with optical properties and atmospheric lifetimes different from those of their components. Also clouds affect aerosols in a complex way by scavenging aerosols, by adding mass through liquid phase chemistry, and through the formation of new aerosol particles in and near clouds. The secondary aerosols which are formed as a result of atmospheric interactions also play a significant role in the atmosphere.

IPCC's fourth assessment report focus included a significant discussion on the radiative forcing due to atmospheric aerosols. Aerosols interact both directly and indirectly with the Earth's radiation budget and climate. As a direct effect, the aerosols scatter sunlight directly back into space. As an indirect effect, aerosols in the lower atmosphere can modify the size of cloud particles, changing how the clouds reflect and absorb sunlight, thereby affecting the Earth's energy budget. The indirect effects thus modify the radiative properties, amount and life time of the clouds. These direct and indirect effects of aerosols on radiative forcing are depicted schematically in Figure 1. Aerosols also can act as sites where chemical reactions take place (heterogeneous chemistry).

Aerosols also contribute to atmospheric pollution, effecting human health and visibility. For example, aerosols are implicated in mega cities pollution effects on human health. They also affect global climate in several ways. The various effects manifest themselves through the following specific interactions between aerosols and gases in the atmosphere (6, 7):

• The condensed phase (aerosols - solid or liquid) either absorb or adsorb materials. This can lead to potential transformations of compounds and change the overall characteristics of the condensed phases. Thus, they act as catalysts for reactions in the atmosphere via heterogeneous processes.

• Solid aerosol particles can hygroscopically grow and act as solvent media for reactions such as conversion of gaseous materials to secondary organic aerosols.

• Aerosols that adsorb or absorb pollutants can mediate the wet and dry deposition of pollutants to the earth's surface and water bodies.

• Aerosols are capable of scattering light and can change the overall visibility in the lower troposphere. This can also affect the temperature and overall circulation patterns in the lower atmosphere.

• In the upper atmosphere, the presence of aerosols can significantly decrease the temperature, thus contributing to the overall greenhouse effect and global climate change.

The additional reflection caused by aerosol pollution is expected to have an effect on the climate comparable in magnitude to that of increasing concentrations of atmospheric greenhouse gases. The effect of the aerosols, however, will be opposite to the effect of the increasing atmospheric trace gases - cooling instead of warming the atmosphere. The warming effect of the greenhouse gases is expected to take place everywhere, but the cooling effect of the pollution aerosols will be somewhat regional, e.g., near and downwind of industrial areas. No one knows what the outcome will be of atmospheric warming in some regions and cooling in others. Climate models are still too primitive to provide reliable insight into the possible outcome (9).

Whereas, the effects in the lower atmosphere are better understood, there is a real lack of understanding and uncertainty with respect to the effects in the upper atmosphere. IPCC also identified that there are significant gaps in the literature on aerosol radiative forcing (Figure 2). Notice the large uncertainty in the predicted radiative forcing by aerosols and the low scientific understanding of the aerosol radiative forcing.

While there are few books available on atmospheric aerosols, they typically tend to focus on specific aspects, for example, health effects of aerosols (JO) and

Un perturbed cloud

Driuja tncraaaad cloud h*ight Increased cloud suppression. [Pineus & Baker. 1934) lifetime

Increased LWC (Afcrecht, 1989)

Scattering & absorption of radiation

\jDkwcflhc* J

Un perturbed cloud

C/ovd albado affect/ t"/ndlf»cfa<fectf \ Twomf a/fecf I

Driuja tncraaaad cloud h*ight Increased cloud suppression. [Pineus & Baker. 1934) lifetime

Increased LWC (Afcrecht, 1989)

Ckwd UHttm* a/fecf 2" indirect effect/ Aibr+cht e/fecf

Haating causaa cloud bum-off (Ackarman at 2000)

Sfnt-difct effect

Figure 1. Schematic diagram showing the various radiative mechanisms associated with cloud effects in relation to aerosols (from Climate Change 2007, IPCC Assessment - with permission from Cambridge University Press, NY)

properties and formation aspects of aerosols (11-13) and aerosol chemistry (1416). The present book is an attempt to address some of the gaps in the literature particularly in the area of characterization and chemistry of atmospheric aerosols and modeling. The authors of these chapters are emerging leaders in this field and their contributions presented in this book are reviews of their work along with recent advances in the field.

Organization of the Book

The concept for the book came from the two symposia that we organized at the 2007 Fall National meeting of the American Chemical Society in Boston, MA. The symposia titled "Atmospheric Aerosol Processes" attracted a good audience and the need for a specialized book on the topic was pointed out by several attendees and thus the idea for the book emerged. The book is organized into three sections: Characterization, Chemistry and Modeling of Atmospheric Aerosols.

The characterization section of the book includes three chapters. The chapters include: The role of morphology on aerosol particle reactivity; The characteristics and the cytotoxic effects of particulate matter in the ambient air of the Chiang mai-Lamphun basin in Northern Thailand; and Toluene decomposition on water droplets in corona discharge. Professor Bear's research group presented experimental evidence on the role of morphology on the reactivity of aerosol particles. The ozonolysis of oleic acid particles which are released into the atmosphere is studied and the authors concluded from their work and the evidence from other studies that the particle morphology in addition to chemical composition must be considered when evaluating particle reactivity. The chapter on fine particle matter in the ambient air presented reviews of recent investigations on the effects of air borne pollutants on human heath and specifically the levels and distribution of particulate matter (< 10 jam and <2.5 ¿im) along with the mutagenicity and cytotoxicity of samples collected on filters in the Chiang Mai-Lamphun basin in Thailand. Professor Wu described the toluene (a model compound representing a long lived environmental and potentially carcinogenic pollutant) decomposition on water droplets in a corona discharge. The authors propose that these kinds of reactions will have many applications in air pollution control and in industrial manufacturing processes.

The chemistry portion of the book covers several interesting topics including secondary aerosols and the chapters include: Surface activity of perfluorinated compounds at the air-water interface; Atmospheric chemistry of urban surface films; Photochemistry of secondary organic aerosol formed from oxidation of monoterpenes; Effects of highly concentrated dry (NH4)2S04 seed

Radiative forcing of climate between 1750 and 2005

Radiatrve Forcing Terms__Cfcmate efficacy Spattal scab

Radiative forcing of climate between 1750 and 2005

Radiatrve Forcing Terms__Cfcmate efficacy Spattal scab

Scientific understanding

Figure 2. Anthropogenic and natural forcing of the climate for the year 2005 relative to 1750. Note the bars and vertical lines associated with aerosols, (from Climate Change 2007, IPCC Assessment - With permission from Cambride University Press, NY).

Scientific understanding

Figure 2. Anthropogenic and natural forcing of the climate for the year 2005 relative to 1750. Note the bars and vertical lines associated with aerosols, (from Climate Change 2007, IPCC Assessment - With permission from Cambride University Press, NY).

aerosols on ozone and secondary organic aerosol formation in aromatic hydrocarbons/NOx photooxidation Systems; and Adsorption and UV photo-oxidation of gas phase phenanthrene on atmospheric films. Professor Vaida and co-workers reported from experimental investigation that the perfluorinated compounds act as efficient surfactants on the surfaces of aqueous atmospheric aerosol particles and thus could potentially increase their atmospheric transport, distribution and deposition. Professor Donaldson and his group provided a summary of their on going research work on the atmospheric chemistry of large and stationary urban surface films coating buildings, roadways, etc. They hypothesize that chemistry of such films could be important in determining local oxidative strength and pollutant concentrations. The research group headed by Professor Nizkorodov in their contribution reported their work on secondary organic aerosols (SOA) which are formed from atmospheric oxidation of monoterpenes specifically the photochemical processes occurring inside these biogenic SOA particles. The photolysis of the SOA particles modifies the chemical composition of the SOA which in turn leads to emission of small volatile molecules back into the gas phase. This could explain the presence of a number of observed products in the atmosphere which are produced from the photolysis of organic peroxides and carbonyls. An international group consisting of researchers from China and Japan in their contribution presented experimental results on the effects of highly concentrated dry ammonia sulfate seed aerosols on ozone and SOA formation. The presence of highly concentrated dry aluminum sulfate aerosols had no effect on gas-phase reactions in aromatic hydrocarbon photooxidation systems, but enhanced SOA generation and SOA yield and the authors hypothesize that these results could be utilized in SOA formation modeling especially for air quality simulations involving particulate matter pollution. The last chapter in this section reports the experimental results on the adsorption and UV photooxidation of a gas phase semi volatile polycyclic aromatic hydrocarbon, phenanthrene on atmospheric water films. The interfacial partitioning of phenanthrene increased in the presence of surface active compounds . Moreover, the surface reaction of phenanthrene proceeded faster than the reaction in the bulk phase. The authors identified the different pathways of photooxidation for the surface reaction as well as reactions in the bulk water.

Finally the modeling section of the book includes two very interesting chapters; Understanding climatic effects of aerosols: modeling radiative effects of aerosols; Environmental effects to residential New Orleans following hurricane katrina: indoor sediment, vapor-phase and aerosolized contaminants. Professor Jia and his research group presented a review of the elements of aerosol-climate interactions and the uncertainties underlying aerosol-climate modeling along with the climatic implications of radiative forcing. The last chapter deals with the aerosolized contaminants in the indoor environment of residential homes in New Orleans following hurricane Katrina. The authors report that indoor pollutants are found to be more highly concentrated than contaminants found outdoors during the flood events thus making it more harmful for the residents, first responders and recovery personnel during the aftermath of Hurricane Katrina.

This book, thus brings together the varied aspects of atmospheric aerosol characterization, chemistry and modeling. It represents a unique collection of articles that will, hopefully, bring further insight into the important area of atmospheric aerosols and their importance. Further research is certainly warranted in this important area, as was indicated during the Symposium that formed the basis for this book.

References

1. IPCC, 2007, Forster et al., "Changes in Atmospheric Constituents and in Radiative Forcing", In: Climate change 2007: The Physical Science Basis, contributions of Working Group 1 to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, UK, New York, NY.

2. Ramanathan, V., Ramana, M., Roberts, G., Kim, D., Corrigan, C., Cheng, C., Winkler, D., Nature, 2007, 448, 575-578.

3. Christner, B., Morris, C, Foreman, C., Cai, R., Sands, D., Science, 2008, 319, 1214.

4. Ghan J., Schwartz, S.E., 2007, Aerosol properties and processes - A path from field and laboratory measurements to global climate models, Bull. Amer. Meteor. Soc. 1059-1083, doi: 10.1175/BAMS-88-7-1059.

5. Hinds W.C., 1999, Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles, 2nd ed, Wiley, New York.

6. Warneck, P., 2000, Chemistry of the Natural Atmosphere, 2nd edition, Academic Press, San Diego, CA.

7. Seinfeld, J.H., Pandis, S.N., 2006, Atmospheric Chemistry and Physics -From Air Pollution to Climate Change, 2nd edition, John Wiley & Sons, Inc., New York, NY.

8. Valsaraj, K.T., 2000, Elements of Environmental Engineering -Thermodyanmics and Kinetics, 2nd Edition, CRC Press, Boca Raton, FL.

9. NASA, 2008, "Atmospheric Aerosols", NASA Online Facts, http://oea.larc.nasa.gov/PAIS/Aerosols.html

10. Ruzer, LS., and Harley NH., 2005, Aerosols Handbook: Measurement, Dosimetry and Heath Effects, CRC Press, Boca Raton, FL.

11. Kondratyev, KY., Ivley, LS., Krapivon, VF., Varostos, CA., 2005, Atmospheric aerosol properties: Formation, processes and impacts, Springer Verlag, New York.

12. Spurny, K.R. (editor), 2000, Aerosol Chemical Processes in the Environment, Lewis Publishers, Boca Raton, FL.

13. Baumgartel, H., Grunbein, W., Hensel, F., (editors), 1999, Global Aspects of Atmospheric Chemistry, Springer,, New York, NY.

14. Meszaros, E., 1999, Fundamental of Amospheric Aerosol Chemistry, Akademiai Kiado, Budapest, Hungary.

15. Gelenscer, A. 2004, Carbonaceous Aerosols, Springer, Dordrecht, The Netherlands.

16. Colbeck, I. (editor), Environmental Chemistry of Aerosols, Blackwell Publishing Ltd., Ames, IA.

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    How aerosols are important?
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