Composition and structure of the atmosphere

Atmosphere

• Earth’s atmosphere is a thin layer of gas held around the surface by gravity.

• 90% of the atmosphere’s mass is within 15 km of the earth’s surface– Earth's radius is about 6400 km – The atmosphere is like a layer

of paint on a basketball

Atmosphere

• The atmosphere protects life on Earth by:

absorbing ultraviolet solar radiation,

warming the surface through heat retention

(greenhouse effect),

And reducing temperature extremes

between day and night (the diurnal

temperature variation).

• The Mass of the atmosphere:

It is about 5.15×1018 kg, three quarters of

which is within about 11 km of the surface.

Atmosphere

The Earth's atmosphere is characterized by variations of

temperature and pressure with height.

In fact, the variation of the average temperature profile with

altitude is the basis for distinguishing the layers of the

atmosphere.

Troposphere. The lowest layer of the

atmosphere, extending from the Earth's

surface up to the tropopause, which is at

10-15 km altitude depending on latitude

and time of year (Higher near equator

and lower towards poles); characterized

by decreasing temperature with height;

rapid vertical mixing (baring BL).

Stratosphere. Extends from the

tropopause to the stratopause (From ~

45 to 55 km altitude); temperature

increases with altitude.

Mesosphere. Extends from the

stratopause to the mesopause (From ~

80 to 90 km altitude); temperature

decreases with altitude to the

mesopause, which is the coldest point in

the atmosphere.

Atmospheric Structure

Composition of the Atmosphere

Atmosphere

• The very cold temperature of the tropopause layer at the top of

the troposphere serves as a barrier that causes water vapor to

condense to ice so that it cannot reach altitudes at which it

would photodissociate through the action of intense high-

energy ultraviolet radiation. If this happened, the hydrogen

produced would escape the earth’s atmosphere and be lost.

• The atmospheric layer directly above the troposphere is the

stratosphere, in which the temperature rises to a maximum of

about -2˚C with increasing altitude.

Atmosphere

• The heating effect is caused by the absorption of ultravioletradiation energy by ozone.

• Ozone serves as a natural atmospheric filter to prevent this lightfrom reaching the surface, thereby protecting Earth's life fromdamages.

• The upper regions of the mesosphere and higher define aregion, called the exosphere, from which molecules and ionscan completely escape the atmosphere.

• Extending to the far outer reaches of the atmosphere is the thermosphere, in which the highly rarified gas reaches temperatures as high as 1200˚C by the absorption of very energetic radiation of wavelengths less than approximately 200 nm by gas species in this region.

Atmosphere

In Thermosphere temperatures increase again with altitude due to

absorption of strong UV solar radiation by N2 and O2. The

troposphere and stratosphere account together for 99.9% of total

atmospheric mass and are the domains of main interest from an

environmental perspective.

The fraction of total atmospheric weight located above altitude z is

P(z)/P(0). At 80 km altitude the atmospheric pressure is down to

0.01 hPa, meaning that 99.999% of the atmosphere is below that

altitude.

It has become common use in atmospheric chemistry to describe volume mixing

ratios by the following units:

Volume mixing ratio= 1mL/1m3 =1 mL/106 mL = 10-6

E.g., 1 mL SO2 in 106 mLof Air

parts per million (ppm) 10-6 μmol mol-1

parts per billion (ppb) 10-9 nmol mol-1

parts per trillion (ppt) 10-12 pmol mol-1

These quantities are sometimes distinguished by an added v (for volume) and m

(for mass), that is,

Ppmv parts per million by volume

Ppmm parts per million by mass

Another common unit used to measure the concentration is the

unit of mass per unit volume. The interconversion of these units is

simple as exemplified later.

Unit Conversion

Ideal Gas Equation

The ideal gas law is the equation of state of a hypothetical ideal gas. It is a

good approximation to the behavior of many gases. The ideal gas law is often

written as:

P V = nRT

where the letters denote pressure, volume, amount, the ideal gas constant,

and temperature of the gas, respectively.

The value of R can be expressed in a number of units, the most useful in

environmental chemistry relate to litres and atmospheres. Therefore:

It is helpful to estimate the volume of gas at any temperature and Pressure,

The most useful value to remember is at STP

what is the concentration of 1ppm by volume of Ozone expressed in g/m3 at 25

C and 750 mm Hg pressure?

Solution:

We know at STP 1 mol of ozone occupies 22.41 L

Using Ideal gas equation

Where P1 = 760 mm Hg, T1 = 273 K and P2 = 750 mm Hg , T2= 298 K

We get:

V2= 24.79 L

Mol wt of Ozone = 48

Now In these conditions 24.79 L contains 48 g of Ozone

So 1 L (1000 mL) will contain g of Ozone

I ml will contain g of Ozone

This 1 mL of Ozone in 1 m-3 of air can be expressed as

Four rough categories of

atmospheric scales of

motion:

1. Microscale. Phenomena

occurring on scales of the

order of 0-100 m, such as

the meandering and

dispersion of a chimney

plume

2. Mesoscale. Phenomena

occurring on scales of tens

to hundreds of kilometers,

such as land-sea breezes,

and fronts.

3. Synoptic Scale. Motions of whole weather systems, on scales of

hundreds to thousands of kilometers.

4. Global Scale. Phenomena occurring on scales exceeding 5 000 km.

Fig: Spatial and temporal scales of

variability for atmospheric constituents.

Atmospheric Scales Of Motion

Spatial scales characteristic of various atmospheric

chemical phenomena are given in the Table

There is more or less of a continuum between

(1) urban and regional air pollution,

(2) the aerosol haze associated with regional air pollution and aerosol-climate interactions,

(3) greenhouse gas increases and stratospheric ozone depletion,

(4) tropospheric oxidative capacity and stratospheric ozone depletion.

Fate of air pollutants and role of

atmosphere

Hazardous air pollutants

• Photolysis

• Chemical reaction with OH radical, nitrate radical and O3.

• Reaction with OH radical and O3 is predominant.

• Photolysis is chemical fragmentation or rearrangement of a chemical

upon the adsorption of light of appropriate wavelength. Photolysis is

only important during daytime only for those chemicals that strongly

absorb light.

• Formaldehyde and acetaldehyde form as a result.

• Major removal mechanism is OH abstraction of addition.

• Product of reaction would be CO2 and CO.

• Atmospheric lifetime generally less than 1 day.

Nitrogen Dioxide

Nitrogen Dioxide

Photochemical oxidants• Unlike other pollutants the photochemical

oxidants results entirely from atmospheric

reaction. Thus they are called secondary

pollutants.

• O3 is primary photochemical oxidant.

• O3 formation is basically attributed to the

nitrogen dioxide photolytic cycle.

• Hydrocarbon modifies this cycle by reacting

with atomic oxygen to form free radicals

(highly reactive organic species)

Sulfur oxides• Sulfur oxides may be both primary and

secondary pollutants.

• Power plants, industries, volcanoes and

ocean emit SO2, SO3, and SO42- directly as

primary pollutants.

• Biological decay process and some industrial

sources emit H2S, which is oxidized to for

SO2.

• 0.125 Pg (Natural sources) and 45 Tg

(anthropogenic sources)

Problem

An coal power plant burning coal at the rate

of 1.00 kg/s. If the analysis of coal reveals a

sulfur content of 3% what is the annual rate

of emission of SO2. sulfur content of the ash

5% of input sulfur

Fate of SO2 in the atmosphere

IIT Delhi-

2009-10

Section 7 – Chemical Aspects

of Air Pollution29

Particulate MatterParticles come in different

shapes and sizes

Particle sizes

• Ultra-fine particles (<0.1 µm)

• Fine particles (0.1 to 2.5 µm)

• Coarse particles (2.5 to 10 µm)

PM10

Carbon chain agglomeratesCrustal material

AREP GAW, WMO Report

31

• NaCl – salt is found in PM near sea coasts and after de-icing materials are applied

• Organic Carbon (OC) – consists of hundreds of separate compounds containing mainly carbon, hydrogen, and oxygen

• Elemental Carbon (EC) – composed of carbon without much hydrocarbon or oxygen. EC is black, often called soot.

• Liquid Water – soluble nitrates, sulfates, ammonium, sodium, other inorganic ions, and some organic material absorb water vapor from the atmosphere

Particulate Matter

Composition

Chow and Watson (1997)

• Geological Material – suspended dust

consists mainly of oxides of Al, Si, Ca, Ti,

Fe, and other metal oxides

• Ammonium – ammonium bisulfate,

sulfate, and nitrate are most common

• Sulfate – results from conversion of SO2

gas to sulfate-containing particles

• Nitrate – results from a reversible

gas/particle equilibrium between ammonia

(NH3), nitric acid (HNO3), and particulate

ammonium nitrate

32

Winds

Precipitation

Temperature

Relative humidity

Winds

Winds Temperature

Solar radiation

Vertical mixing

Clouds, fog

Temperature

Relative humidity

Solar radiation

condensation and

coagulation

photochemical

production cloud/fog

processes

gases condense onto

particles cloud/fog

processes

Measurement

Issues

• Inlet cut points• Vaporization of nitrate

H2O, VOCs

• Adsorption of VOCs

• Absorption of H2O

transpo

rtsedimentation

(dry deposition)

wet

deposition

Mechanic

al• Sea

salt• Dust

Combusti

on• Motorvehicles

• Industrial• Fires

Other gaseous• Biogenic• Anthropoge

nic

Particles• NaCl• Crust

al

Particles• Soot• Metals• OC

Gases• NOx

• SO2

• VOCs• NH3

Gases• VOCs• NH3

• NOx

PM Transport/Loss

PM Sample

Formation CollectionSources Emissions

Chemical Processes

Meteorological Processes

Particulate Matter Chemistry

AREP GAW, WMO Report

Comparison of CPCB (India) and WHO Standards for Particulate Air Pollution

R. Gopalaswami et al. A Study on Effects of Weather, Vehicular Traffic and Other Sources of Particulate Air Pollution on the City

of Delhi for the Year 2015. Journal of Environment Pollution and Human Health, 2016, Vol. 4, No. 2, 24-41. doi:10.12691/jephh-

4-2-1

Component of Particulate Air

Pollution24-Hr Annual

CPCB (India) WHO CPCB WHO

Particulate matter (PM10),

µg/m3

100 µg/m3

AQI 174

Unhealthy

50 µg/m3

AQI 137

Unhealthy to

Sensitive

Groups

60 µg/m3

AQI = 153,

Unhealthy

20 µg/m3

AQI =68

Moderate

Particulate matter (PM2.5),

µg/m3

60 µg/m3

AQI 153

Unhealthy

25 µg/m3

AQI =78,

moderate

AQI =112,

Unhealthy to

sensitive groups

10 µg/m3

AQI = 42,

Good

Table . Monthly Average Particulate Air Pollution (PM 2.5) over a 3-year period in Delhi and Beijing

R. Gopalaswami et al. A Study on Effects of Weather, Vehicular Traffic and Other Sources of Particulate Air Pollution on the City

of Delhi for the Year 2015. Journal of Environment Pollution and Human Health, 2016, Vol. 4, No. 2, 24-41. doi:10.12691/jephh-

4-2-1

Season Month

Measured Values of Particulate Air Pollution (PM 2.5) In

Micrograms/ cubic-metre

2013 2014 2015

Site Not Specified Site Not SpecifiedSite at

Chanak

yap uri

Delhi Beijing Delhi Beijing Delhi

Winter and Early

Summer

Jan 230 100 200 120 184

Feb 120 120 150 150 139

Summer

Mar 115 115 100 105 71

Apr 100 70 100 100 62

May 130 80 110 80 74

Jun 100 100 100 80 55

Monsoon (Rainy) Season

in India

Jul 60 70 90 85 33

Aug 60 70 90 60 33

Sep 70 70 70 70 50

Winter

Oct 100 100 130 130 105

Nov 270 110 240 100 236

Dec 240 90 230 100 249

Venkataraman et al. Atmos. Chem. Phys., 18, 8017–8039, 2018 https://doi.org/10.5194/acp-18-8017-2018

Ozone

• Presence of O3 in the upper atmosphere

20-40 km and up provides a barrier to UV

radiation.

• Small amount that do seep through

provide you with your summer tan.

• To much UV will cause skin cancer.

• O2 also serves as barrier to UV radiation,

it absorbs only over a narrow band centred

at a wave length of 0.2 m.

Ozone destruction• Photoreaction of O3 and O3 destruction

by chlorofluorocarbon

Ozone destruction

Helsinki Declaration

• Eight countries met in Helsinki, Finland in the spring of 1989 and Helsinki

declaration took place as follows.

• All joint the 1985 Vienna Convention for the protection of O3 layerand the

follow up Montreal protocol.

• Phase out production and consumption of O3 depleting CFCs no later than

2000.

• Phase out production and consumption as soon as feasible of halons and

such chemicals as carbon tetrachloride and methyl chloroform that also

contribute to O3 depletion.

• Commit themselves to accelerated development of environmentally

acceptable alternative chemical and technology.

• Make relevant scientific information, research results and training available

to developing countries.

Alternative to CFCs

• Hydrofluorocarbons (HFCs)

• Hydrochlorofluorocarbons (HCFCs)

• In contrast to CFCs HFCs and HCFCs contain one or more C-H

bonds. This makes them susceptible to attack by OH radicals in the

lower atmosphere.

• HFCs do not contain chlorine they do not have the O3 depletion

potential.

• HCFCs contains chlorine, this chlorine is not transported to the

stratosphere because of OH scavenging in the troposphere is

relatively efficient.