Slide Show Chapter 10 10-04-2013

7/28/2019 Slide Show Chapter 10 10-04-2013

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haze Figure 10.1


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To understand visibility degradation, basic principles of light scattering in

the atmosphere should be known.

Solar radiation passing through atmosphere is both absorbed and

scattered by gases and particles.

Visibility is related with the absorbtion of visible part of the

electromagnetic radition

The change in the intensity of light when it travels a given distance is

due to these absorption and scattering processes. This reduction in

intensity is generally given by a general extinction relation:

I/Io = e-b

extL

I: the intensity of the light after it traverses the distance L,Io: the intensity at the beginning point of L,bext: the extinction coefficient.

Figure 10.3 shows a beam of light transmitted through the atmosphere.


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Extinction coefficient consists of two terms:

(1) extinction due to gases,

(2) extinction due to particles

bext = bg + bp

Each of these two terms are in turn consist of two terms;

(a)extinction due to absorption,

(b)extinction due to scattering.

Then the total extinction coefficient can be written as:

bext = bag + bsg + bap + bsp


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Absorption of light in the atmosphere is well characterized and mostly

occurs as the absorption of UV light at stratosphere by O3 molecules and

absorption of IR in the troposphere by greenhouse gases.

!!! But, since we are talking of visible light when we talk about visibility

degradation, these absorptions are not important.

The only molecule that absorbs visible radiation in the troposphere is

the NO2 molecule. (see Figure 10.4 and 10.5)


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Gases scatter

visible

light

by a process called Rayleigh scattering.

Scattering of light by gas molecules; the principles are well known but

will not be discussed.

Absorption and scattering of radiation by particles are more complex

process.

Figure 10.6 shows four forms of particle light interaction.


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The scattering of visible light by particles occurs by three different

mechanisms depending on the size of particles.

For very small particles (D wavelength) the scattering is similar to

that of gases (Rayleigh scattering)

For particle diameters comparable to wavelength the scattering

occurs through Mie scattering

For large particles (where D >> wavelength) the scattering occurs

through geometric scattering and can be treated by classical optics.

Each of these scattering mechanisms have different treatments which will

not be discussed.


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Since the wavelength of visible

radiation is about 400 - 500 nm,

and since most of the particles in

the atmosphere is in the

accumulation mode with

diameters comparable to the

wavelength of the visible light,

then one would expect Mie

scattering to be the most

important mechanism in polluted

atmosphere.

Primary particles

Hotvapor

Coagulation

Chain aggregates

Coagulation

Coagulation

Coagulation

LowVolatality

vapor

Homogeneousnucleation

Condensation growthOf nuclei

Droplets

Chemical conversionOf gases to lowVolatility vapors

Wind blown dust

Emissions

Sea spray

Volcanos

Plant particles

RainoutAnd

washoutSedimentation

Transient nuclei orAitken nuclei range

Accumulationrange

Mechanically generatedAerosol range

Fine particles Coarse particles

PArticle diameter (m)0.002

0.01 0.1 1 2 10 100

Coagulation

+

+

+

+


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Among the processes, which attenuate visible

radiation and cause visibility degradation;

absorption and scattering of light by particles

are the most important parameters.


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Back to visibility degradation.

In haze visual range decrease. The visual range is defined as the distance

at which a black object can be distinguished against the horizon.

During daytime, light coming from the object to the observes eye is

scattered out of sight of the observer.

Also, sunlight which is normally out of sight is scattered into the sight,

resulting in dark object appear lighter (reducing the contrast).


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Typical visual ranges are few hundred kms in clean atmosphere and few

kms in pollutes atmosphere.

In urban atmosphere most of the visibility loss is due to scattering of

radiation by particles.

Light scattering is dominated by particles in the accumulation mode.

This is shown in the Figure 10a where light scattering coefficient per unit

volume is plotted against particles diameter. The highest value of bsp is

found in the accumulation mode (0.1 - 1 m range).


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This is also supported in Figure 10b where bsp is plotted against particle

volume. The slope of the line given in this figure depends on the history of

air mass (chemical constituents in it).


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Since mass and volume are related through density, values of bsp is

expected to be related with the mass as well as volume.

The Figure 10c shows the relation between fine and coarse particle

mass and bsp.

There is a good correlation between the bsp and fine mass. But

coarse mass is not correlated with the bsp.

This also confirms that light scattering is dominated by fine particles.

This is purely an optical effect.


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The bsp also depends on the chemical composition of particles.

Because the Mie scattering depends on the refractive index of

particles. The refractive index in turn is related to chemical

composition.

Since the chemical composition of coarse and fine particles is different, this

would also contribute to observed difference in the correlation of fine mass

with bsp, but lack of similar correlation between coarse mass and bsp.


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Water vapor in the atmosphere strongly affects light scattering

characteristics of aerosols.

Most of the aerosols are hydroscopic, so they take up water depending

the relative humidity.

Water vapor increase the size and mass of particles and reduce the

refractive index. The net result is increase in the light scattering.

This is shown in Figure 10d where liquid water content of aerosols are plotted

against bscat.


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Particles not only scatter, but also absorb visible light.

The most important contributor to light absorption is black or elemental

carbon which is produced in combustion processes.

The contribution of light absorption by carbon on total light extinctionchanges geographically depending on the distribution of combustion

sources.

Wood-burning and diesels are the main sources of elemental carbon. In

urbanindustrial areas bap is 50 - 100% of bsp. In rural areas bap


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Since black carbon both absorbs and scatter visible radiation, it tends to

play a proportionally much greater role in the light extinction than its

contribution to particulate mass suggest.

E.g., in Denver elemental carbon accounts for about 15% of the particle

mass, but accounts for about 35% of total light extinction.


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FORMATION OF ATMOSPHERIC HAZE

Haze:

Reduced visibility caused by the presence of particles and NO2 in the

atmosphere.

For haze to occur sizes of particles should be between 0.1 m and 1.0m. Sources of such particles can be natural as in the case of bluehaze over the mountains in morning hours, or anthropogenic as in the

case of pollution over urban areas.

The main components of atmospheric haze is the sulfate (mostly (NH4)2SO4

particles) and nitrate particles.


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In polluted atmosphere, SO4/NO3 was approximately 3/2 in 1970s and 80s.

But with actions taken to reduce SO2 emissions in late 80s, SO4

concentrations in the atmosphere also decreased (and decreasing).

Currently the ratio is approximately equal to 1.0

Other components in the atmospheric haze are the soot carbon, fine fly

ash particles and organic aerosols.


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These are anthropogenic particles and dominate atmospheric haze in

urban areas.

But their role is limited in the haze in the regional scale.

Since anthropogenic particles such as soot carbon can both scatter and

absorb radiation, if their concentration increase and they become

important in the regional and global scales, they may have significant

effect on the climate and earth energy balance.


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Particle Formation in the Atmosphere

Primary particles

directly emitted from sources.

But secondary particles

formed in the atmosphere from their precursors. How?


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In the formation of secondary particles molecules in the gas phase has to

transform into solid particles. This process can occur by three processes:


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Absorption

Involves dissolution of gaseous molecules inliquid (in the atmosphere,mostly water).

Chemical process that convert gas molecule into another one withmuch less vapor pressure

Then droplet evaporates leaving behind a particle.

One important mechanism of SO4formation is called liquid phase oxidationof SO2.

In this mechanism, SO2 first dissolve in water droplets such as cloudor fog droplets

In the droplet it oxidizes to SO4 by H2O2

Then when cloud or fog droplet evaporates, SO4 particle is leftbehind (either in the form of H2SO4, or (NH4)2SO4).

This mechanism depends on the solubility of the precursor gas inwater (SO

2is highly soluble).


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Nucleation

Molecules in the gas phase grow into clusters and clusters grow into

particles.

The condensable species that will eventually form into particles are

formed from gas precursors and they are in the gas phase initially.

If the product has sufficiently low vapor pressure they can saturatequickly and start to form molecular aggregates called clusters. For these

clusters to form and grow a condition called supersaturation has to be

reached.

The saturation ratio (S) is the ratio of the actual pressure of the

gas to its equilibrium vapor pressure. If S>1 the situation is

described as supersaturation.


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E.g., The situation is analogous to the supersaturation in solutions.

If you dissolve sugar (or salt) in hot water you can dissolve more than you

can at room temperature, because solubility increase with temperature.

After you dissolve sugar in hot water if you cool the water to room

temperature, you expect excess dissolved sugar to crystallize and stay at

the bottom. But, crystallization does not occur immediately and solution

stays at the supersaturation state until you add a apiece of something or

stir it. During supersaturation the ratio of sugar concentration to that

of solubility of sugar is >1.

The situation is the same in atmosphere, except instead of concentration you

use vapor pressure.


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Substances with low vapor pressure favor nucleation, because their

equilibrium vapor pressure is low and supersaturation can be reached easily.

Example of this type of particles is the sulfate generated by gas phase

photochemical oxidation of SO2.


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Condensation

Particle formation by condensation occurs when molecules formed

after reaction collides with the existing particles and droplets.

For condensation to occur supersaturation should be reached (S>1)

Since condensation can occur at lower supersaturation levels than

nucleation, it is the dominating mechanism when there are particles.


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Ankara-August 2003


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Figure 10.1. Factors determining visibility in the atmosphere.

Optical Charactersitics ofillumination source

Sun angle, spectrum, intensity as

altered by cloud cover andatmosphere

Optical Charactersitics ofviewed targets

Inherent contrast, spectralreflectance (color), size, shapedistance pattern, hoerizon,brightness

Characteristics of the observer

Psycophysica (eye-brain)response to incoming lighttresholds of perception forcontrast and color change

Sensivity to size, pattern

distribution of color

Subjective judgement ofpercived images

Characteristics of the observer


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Optical Charactersitics of interveningatmosphere

Psycophysica (eye-brain)response to incoming lighttresholds of perception forcontrast and color change

Sensivity to size, patterndistribution of color

Subjective judgement ofpercived images

Optical Charactersitics of illumination source

Sun angle, spectrum, intensity as altered bycloud cover and atmosphere

Optical Charactersitics of viewedtargets

Inherent contrast, spectralreflectance (color), size, shapedistance pattern, hoerizon,brightness


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Figure 10.3.(a) a diagram of extinction of light from a source such as an electric light in a reflector,illustrating (i) transmitted, (ii) scattered, and (iii) absorbed light.

(b) A diagram of daylight visibility illustrating (I) residual light from a target reaching anobserver,(ii) light from a target scattered out of an observers line of sight, (iii) air light from theintervening atmosphere and, (iv) air light constituting horizon sky.


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Figure 10.4. Absorption spectrum of NO2.


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Figure 10.5. Comparison of bext for 0.1 ppm NO2 and Rayleigh scattering by air.


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Figure 10.6. Four forms of particle light interaction.


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Figure 10.7. Scattering and absorption cross-section per unit volumes as a function of particlediameter.


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Figure 10.8. Single particle scattering to mass ratio for particles of four different compositions.


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Figure 10.10. Historic trens in hours of reduced visibility at Phoenix and Tuscon, Arizona,compared to trends in Sox emissions from Arizona smelters.


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Figure 10a. Light scattering coefficient per unit volume vs. particles diameter


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Figure 10a. Light scattering coefficient per unit volume vs. particles diameter


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Figure 10b. bsp vs. particle volume


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Figure 10c. Relation between fine and coarse particle mass and bsp.


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Figure 10d. liquid water content of aerosols are plotted against bscat


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The reduction in contrast is given by Koschimieder equation:

C/Co = e-b

extL

similar to Beers law. Co is the contrast of an object against horizon at the L =

0 and C is the contrast at distance L. The contrast is given by:

C = (Bo/BH) - 1

where Bo is the brightness of the object and BH is the brightness of the

horizon (or background).


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Observers can typically differentiate objects on the horizon if C/Co is 0.02 -

0.05.

If the contrast is 0.02 than the visual range would be

Lv = (ln C/Co)/bext = 3.9/bext

For a contrast of 0.05 the visual range is Lv = 3.0/bext


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