Environmental toxicology: chemical aspects Atmospheric ... · Atmospheric chemistry (1)...
Transcript of Environmental toxicology: chemical aspects Atmospheric ... · Atmospheric chemistry (1)...
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
Fundamentals of atmospheric chemistry
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
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%.
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
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.
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
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.
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
Electromagnetic spectrum
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
Solar spectrum http://www.intechopen.com/
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
UVA 400 nm - 320 nm
UVB 320 nm - 290 nm
UVC 290 nm - 100 nm
Solar energy flux: 1.34 *103 W/m2
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
http://www.theozonehole.com
Stratification of atmosphere (Earth diameter= 12.742 km)
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
http://www.theozonehole.com
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
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
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
- Altitude
- Season
- Time of the day
- Solar activity
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
http://www.theozonehole.com
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
http://www.theozonehole.com
Stratification of
the atmosphere
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
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
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
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
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
Atmosphere is divided into layers on the basis of temperature.
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.
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
Atmosphere is divided into layers on the basis of temperature.
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.
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
Atmosphere is divided into layers on the basis of temperature.
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.
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
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).
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
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
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
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.
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
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
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
Chemical and photochemical reactions
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
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)
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
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.
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
Solar spectrum http://www.intechopen.com/
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
Electron excitation (E = hn, n = c/l)
E =
chemwiki.ucdavis.edu
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
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
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
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-
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
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.
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
Ions in the atmosphere (ionosphere, 50 km)
http://www.theozonehole.com
Produced by UV
Effect of magnetic field
Formation of free radicals
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
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.
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
involved in controlling, in the
troposphere, the concentrations of OH∙
involved in controlling the concentrations
of reactants and products
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
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.
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
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.
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
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
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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross
Environmental toxicology: chemical aspects
Atmospheric chemistry (1)
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.