environmental toxicology: chemical aspects atmospheric ... · atmospheric chemistry (1)...

Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross

Environmental toxicology: chemical aspects

Atmospheric chemistry (1)

Fundamentals of atmospheric chemistry




The atmosphere is the thin layer of mixed gases covering Earth′s surface. Dry air (up to

several kilometers altitude) contains (volume %):

nitrogen (N2): 78.08%

oxygen (O2): 20.95%

argon (Ar): 0.934%

carbon dioxide (CO2): 0.039% (= 390 ppm)

noble gases in traces: neon (Ne, 1.81810-3%), krypton (Kr, 1.1410-4%), helium (He,

5.2410-4%), and xenon (Xe, 8.710-6%)

Atmospheric air may contain 0.1 – 3% water by volume, with a normal range of 1 – 3%.




RELEVANT COMPONENTS OF ATMOSPHERIC CHEMISTRY

gaseous oxides

atmospheric methane

hydrocarbons (photochemical smog)

particulate matter (PM)

primary and secondary pollutants (e.g. H2SO4, NO2)

The characteristics of the atmosphere are determined by the balance of energy and

mass transfer processes.




Importance of the atmosphere

The atmosphere is a shield which protects life on Earth from the hostile environment of outer

space:

absorbs most of the cosmic rays from outer space and protects organisms from their

effects;

absorbs most of the electromagnetic radiation from the sun, in particular by filtering out

damaging UV radiation;

absorbs partially the IR radiation, thus stabilizing Earth′s temperature.

It provides CO2 for plant photosynthesis, O2 for respiration, and N2 used by nitrogen-fixing

bacteria and ammonia-manufacturing plants to produce chemically-bound nitrogen, an

essential component of life molecules. It transports water from oceans to land.




Electromagnetic spectrum




Solar spectrum http://www.intechopen.com/




UVA 400 nm - 320 nm

UVB 320 nm - 290 nm

UVC 290 nm - 100 nm

Solar energy flux: 1.34 *103 W/m2




http://www.theozonehole.com

Stratification of atmosphere (Earth diameter= 12.742 km)




Stratification of the atmosphere

In order to understand atmospheric chemistry and air pollution, it is important to have an

overall appreciation of the physical and chemical characteristics of the atmosphere.

M = 28.97 g/mol (average) in the troposphere

1. Density decreases sharply with increasing altitude as a consequence of the gas laws

and gravity: more than 99% of the total mass of the atmosphere is found within

approximately 30 km of Earth′s surface.

2. Pressure decreases as an approximately exponential function of altitude (in absence of

mixing, at T constant).

3. Temperature varies irregularly 𝑃ℎ = 𝑃0𝑒−𝑀𝑔ℎ

𝑅𝑇

At 8 km p = 39% of p0




- Altitude

- Season

- Time of the day

- Solar activity





Stratification of

the atmosphere




Stratification of

the atmosphere

Consider that….

Particles in the atmosphere (dependent

upon pressure)

Mean free path at see level: 10-6 cm

Mean free path at 500 km: 106 cm




Atmosphere is divided into layers on the basis of temperature.

TROPOSPHERE

from sea level to an altitude of 10-16 km;

temperature decreases with increasing

altitude (15° to -56°C at the tropopause);

air expansion: cooling

homogeneous composition of major gases

(other than water) resulting from constant

mixing (greek: tropos) by circulating air

masses.

Adiabatic lapse rate (ALR)* 9.8K/km

Released heat of vaporization (water

condensation) reduces ALR to 6.5 km-1

* rate of temperature change occurring within a rising or descending air parcel





STRATOSPHERE

the tropopause layer at the top of the

troposphere serves as a barrier that

causes water vapor to condense to ice

Otherwise photodissociation: escape of

H2 to space;

temperature rises to about -2°C with

increasing altitude (up to 50 km);

the heating effect is caused by the

absorption of UV radiation energy by

ozone, the major gaseous component.





MESOSPHERE

the absence of high levels of radiation-

absorbing species results in the

temperature to decrease up to –92°C at

an altitude around 85 km;

the upper regions define a region called

the exosphere from which molecules

and ions can completely escape the

atmosphere.





THERMOSPHERE

from 85 km up to an altitude of 500 km;

the highly rarified gas reaches

temperatures as high as 1200°C by the

absorption of very energetic radiation

of wavelengths.




Energy transfer in the atmosphere

Incoming solar energy is largely in the visible region of the spectrum.

The shorter wavelength blue solar light is scattered relatively more strongly by molecules and

particles in the upper atmosphere (that is why the sky is blue as it is viewed by scattered light).

Light transmitted through scattering atmospheres appears red (e.g. around sunset sunrise).




The solar energy reaching the atmosphere is 1.34 x 103 W m-2 (19.2 kcal min-1 m-2): solar

constant (also called insolation, which stands for incoming solar radiation).

If all this energy reached Earth’s surface

and were retained, the planet would have

vaporized long ago!

Therefore, complex factors are involved

in maintaining Earth’s heat balance




About half of the solar radiation entering the atmosphere reaches Earth’s surface either

directly or after scattering by clouds, atmospheric gases, or particles.

The remaining half of the radiation is either reflected directly back or absorbed in the

atmosphere, and its energy radiated back into space at a later time as infrared radiation.

Most of the solar energy reaching the surface is absorbed and must be returned to space

in order to maintain heat balance.

In addition, a very small amount of energy (less than 1% of that received from the sun)

reaches Earth’s surface by convection and conduction processes from Earth′s hot mantle,

and this must be lost too.




Energy transport is accomplished by three major mechanisms:

conduction: occurs through the interaction of adjacent atoms or molecules without the

bulk movement of matter (relatively slow transport);

convection: involves the movement of whole masses of air (sensible heat due to the

kinetic energy of molecules, latent heat due to the water vapor which releases heat as it

condenses);

radiation: occurs through electromagnetic radiation in the IR region of the spectrum, it is

the only way in which energy that must be lost from the planet to maintain its heat balance

is ultimately returned to space.

Energy transport




Chemical and photochemical reactions




Chemical and photochemical reactionsAtmospheric chemistry involves the unpolluted atmosphere, highly polluted atmospheres,

and a wide range of situations in between.

Very low concentrations involved

Gaseous atmospheric chemical species can be tentatively classified as follows:

inorganic oxides (CO, CO2, NO2, SO2);

inorganic oxidants (O3, H2O2, HO·, HO2·, ROO·, NO3) and reductants (CO, SO2, H2S);

organics (CH4, alkanes, alkenes, aryl compounds, carbonyls, organic nitrates);

photochemically-active species (NO2, formaldehyde);

acids (H2SO4), bases (NH3), salts (NH4HSO4);

unstable reactive species (excited NO2*, radicals such as HO·)

solid or liquid particles (see dedicated section)




Chemical and photochemical reactions

These are somewhat arbitrary and overlapping classifications.

In addition, both solid and liquid particles in atmospheric aerosols and clouds play a

strong role in atmospheric chemistry as sources and sinks for gas-phase species:

sites for surface reactions (solid particles);

bodies for aqueous-phase reactions (liquid droplets).

The most important species involved in atmospheric chemistry are:

radiant energy from the sun (predominantly UV radiation): provide energy for reactions to

occur;

HO· and NO3· radicals: highly reactive intermediates triggering several atmospheric

chemical reactions.




Solar spectrum http://www.intechopen.com/




Electron excitation (E = hn, n = c/l)

E =

chemwiki.ucdavis.edu

http://chemwiki.ucdavis.edu/Physical_Chemistry/Spectroscopy/Electronic_Spectroscopy/Electronic_Spectroscopy:_Theory




Photochemical processes (I)

Reactions occurring following absorption of a photon of light to produce an electronically

excited species (M*) depend on the way such energy is released back.

(Kinetics matters)

Physical quenching followed by dissipation of the energy as heat:

O2* + M O2 + M (higher translational energy)

Dissociation of the excited molecule (atomic oxygen in the upper atmosphere):

O2* O + O

Direct reaction with another species:

O2* + O3 2 O2 + O




Luminescence consisting of loss of energy by the emission of electromagnetic radiation

(fluorescence, phosphorescence):

NO2* NO2 + hn’

Intermolecular energy transfer:

O2* + Na O2 + Na*

Intramolecular transfer (energy transferred within a molecule):

XY* XY§

Spontaneous isomerization.

Photoionization through loss of an electron:

N2* N2+ + e-




Photochemical processes

UV radiations can also:

produce ionic species;

produce atoms or groups of atoms with unpaired electrons called free radicals (M•)

Free radicals are quite reactive (therefore, they generally have short lifetimes) and tend to

give rise to chain reactions.

Electromagnetic radiation absorbed in the IR region lacks the energy to break chemical

bonds, but does cause the receptor molecules to gain vibrational and rotational energy.

The energy absorbed as infrared radiation is ultimately dissipated as heat and raises the

temperature of the whole atmosphere.




Ions in the atmosphere (ionosphere, 50 km)


Produced by UV

Effect of magnetic field

Formation of free radicals




Hydroxyl and hydroperoxyl radicals

The hydroxyl radical HO· is the single most important reactive intermediate species in

atmospheric chemical processes and can be formed by several mechanisms.

Photolysis of water at higher altitudes:

H2O + hn HO· + H·

Photolysis of ozone in the relatively unpolluted troposphere:

O3 + hn O2 + O*

O* + H2O 2 HO·

In the presence of organic matter, HO· is produced as an intermediate in the formation of

photochemical smog.




involved in controlling, in the

troposphere, the concentrations of OH∙

involved in controlling the concentrations

of reactants and products




The hydroxyl radical HO· is most frequently removed from the troposphere by reaction with

carbon monoxide or methane:

CO + HO· CO2 + H

CH4 + HO· H3C· + H2O

The hydrogen atom reacts with oxygen to produce hydroperoxyl radical HOO·, which can

undergo several reactions including chain termination:

HOO· + HO· H2O + O2 / HOO· + HOO· H2O2 + O2

The highly reactive methyl radical reacts with oxygen to form methylperoxyl radical H3COO·

which undergoes further reactions.




Acid-base reactions

The atmosphere is normally slightly acidic because of the presence of a low level of carbon

dioxide which dissolves in atmospheric water droplets and dissociates slightly:

CO2(g) CO2(aq) (CO2·H2O)

CO2(aq)·H2O HCO3- + H+

Atmospheric sulfur dioxide forms a somewhat stronger acid when it dissolves in water:

SO2(g) + H2O HSO3- + H+

In terms of pollution, strongly acidic HNO3 and H2SO4 formed by the atmospheric oxidation of

NOx, SO2 and H2S are much more important because they lead to the formation of acid rain.




Given the generally acidic pH of the atmosphere, basic species are relatively less common.

The most important basic species in the atmosphere is gas-phase ammonia, arising from

biodegradation of nitrogen-containing biological matter and from bacterial reduction of nitrate:

NO3- + 2 {CH2O} + H+ NH3(g) + 2 CO2 + H2O

Ammonia is particularly important as a base in the air because it is the only water soluble

base present at significant levels in the atmosphere.

Dissolved in atmospheric water droplets, it plays a strong role in neutralizing atmospheric

acids.

Acid-base reactions




Summary:

1. Chemical reactions occurring in the atmosphere are almost exclusively photochemical or

photochemically-triggered reactions in gas phase.

2. Main atmospheric reactions and phenomena involve:

• particles

• inorganic gaseous pollutants

• organic gaseous pollutants

• photochemical smog, greenhouse effect, acid rain.

environmental toxicology: chemical aspects atmospheric ... · atmospheric chemistry (1)...

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