UNIT 8: CHEMISTRY OF THE ENVIRONMENT
Lesson 01: The Chemistry of the Atmosphere
A. INTRODUCTION
The atmosphere is crucial for sustaining life on Earth, regulating climate, and influencing weather patterns.
This lesson explores complex interactions between chemical processes and atmospheric phenomena.
Goals of the Lesson:
Understand the atmosphere's structure, including various layers.
Discuss implications of the troposphere's lapse rate for life.
Explore the essential components of the atmosphere.
Highlight atmospheric processes that affect air quality, public health, and environmental sustainability.
Investigate the role of stratospheric ozone, aerosols on climate, and issues surrounding air pollution.
Analyze chemistry behind environmental challenges such as photochemical smog, acid rain, and the greenhouse effect.
Equip students to assess implications and explore innovative solutions related to atmospheric chemistry.
Foster awareness of environmental stewardship and sustainability in engineering practices.
B. LEARNING OBJECTIVES
Describe characteristics of different atmosphere layers.
Discuss changes in the lapse rate at the troposphere and effects on life on Earth.
Enumerate components of the atmosphere.
Describe impacts of tropospheric processes on air quality.
Discuss the workings of stratospheric ozone.
Describe how aerosols affect Earth's temperature and climate.
Enumerate ten groups of air pollutants and their sources and effects.
Discuss contribution of incomplete internal engine combustion to air pollution.
Describe how greenhouse gases enhance the greenhouse effect.
Discuss chemical reactions involved in photochemical smog, ozone destruction, and acid rain formation.
C. DISCUSSION OF THE LESSON
Air and the Atmosphere
Air: A homogeneous mixture of gases essential for sustaining life, primarily composed of dioxygen (O₂).
Atmosphere: A thin layer of low-density fluid extending several hundred kilometers from Earth’s surface.
Layers of the Atmosphere
Troposphere:
Lowest layer; extends from ground to 1-15 km.
Weather and air pollution occur here.
Temperature decreases with altitude (lapse rate) at 5 to 6 K/km.
Adverse phenomenon: Thermal Inversion when cold air is overlain by warmer air, trapping pollutants near the surface.
Prolonged inversions can lead to health issues, like lung inflammation.
Tropopause:
Layer of constant temperature between troposphere and stratosphere.
Stratosphere:
Above tropopause; temperature increases with altitude, peaks at about 273K at 50 km.
Layer suppresses vertical air motions.
Stratopause:
Layer of constant temperature between stratosphere and mesosphere.
Mesosphere:
Temperature pattern similar to troposphere, reaching mesopause (85 km).
Atmosphere thick enough to slow burning meteors.
Mesopause: Layer of constant temperature between mesosphere and thermosphere.
Thermosphere:
Lapse rate of 5K/km.
Thermopause: Layer of constant temperature between thermosphere and exosphere.
Exosphere:
Outermost layer; extends to 10,000 km; where satellites orbit.
Components of the Atmosphere
The atmosphere contains a steady mixture of gases, primarily nitrogen (N₂) and oxygen (O₂), which are vital for life.
Processes in the troposphere affect stratospheric composition; inert trace gases like CFCs and methyl chloride can migrate upward if unreactive.
Free radicals play a crucial role in atmospheric chemistry, notably in pollution formation.
Key Chemical Processes
Tropospheric Processes
Free Radicals Origins: Mainly from ozone photolysis by UV radiation, producing radical species like OH.
Example Reaction: ext{O}3 + ext{hv} ightarrow 0 + ext{H}2 ext{O}
ightarrow 2 ext{·OH}
Radical reactions lead to pollutant formation such as carbon monoxide (CO) and nitric acid (HNO₃).
Hydroxyl (·OH): Major oxidant in the troposphere, surpassing O₂ in reaction kinetic favorability.
Stratospheric Processes
**Ozone Layer Functions: ** Protects life by filtering UV radiation.
Photodissociation Reactions of Ozone:
ext{O}3 + ext{hv} ightarrow ext{O}2 + 0 ext{·} // Converts UV to heat, maintaining stratospheric warmth.
Gradual ozone reduction by anthropogenic compounds (Cl, Br) leads to ozone hole issues.
Aerosols
Solid and liquid particulates that impact atmospheric conditions, including temperature and climate, e.g., soot from fossil fuel combustion.
Air Pollutants
Primary Pollutants: Emitted directly into the atmosphere.
Secondary Pollutants: Products of chemical reactions among primary pollutants.
Ten Groups of Air Pollutants by US EPA:
Carbon Dioxide (CO₂) and Carbon Monoxide (CO):
CO₂: From complete combustion; vital but toxic in excess, potentially displacing oxygen.
CO: From incomplete combustion; toxicity and indirect hydrocarbon transformations.
NOx (Nitrogen Oxides): NO, NO₂, and N₂O; from combustion and natural phenomena like lightning.
SOx (Sulfur Oxides): SO₂ and SO₃; from sulfur fuel oxidations.
Photochemical Oxidants: Secondary pollutants like ozone and PAN formed from other pollutants.
Particulates and Aerosols: Small particles from natural and anthropogenic sources, notably PM2.5 and PM10.
Metal and Metalloid Compounds: Toxic metals generated through industrial activities.
PAHs (Polycyclic Aromatic Hydrocarbons): Produced during combustion of organic material.
Volatile Organic Compounds (VOCs): Precursors to secondary pollutants, released during fuel combustion or industrial processes.
Halogenated Hydrocarbons: Includes chlorinated compounds from various processes, related to ozone layer depletion.
Radionuclides: Formed naturally or through nuclear activities, detrimental in excess.
Internal Engine Combustion Process
Combustion of fuels leads to primary pollutants (CO, NOx, VOCs, SO₂).
Ideal combustion equations are of fundamental importance for engineering considerations.
CnHm + (n + 1) ext{O}_2
ightarrow nCO₂ + (m/2) H₂OIncomplete combustion leads to emissions like CO, while excess oxygen increases NO production:
N₂ + O₂
ightleftharpoons 2 NO2 NO + O₂
ightarrow 2 NO₂S + O₂
ightarrow SO₂
The Greenhouse Effect
A natural occurrence that stabilizes Earth’s surface temperature for life.
Solar radiation penetrates the atmosphere; the Earth absorbs energy and re-emits it.
A portion of the heat is trapped by greenhouse gases (CO₂, methane, etc.), increasing global temperatures.
Addressing the impacts of rising greenhouse gases is essential for mitigating climate change and its effects.
Photochemical Smog Formation
Happens when primary pollutants react ultraviolet light.
Ozone and nitrogen oxides contribute to smog, adversely affecting health and the environment.
Stratospheric Ozone Destruction
Anthropogenic chlorine and bromine compounds degrade ozone levels.
The Montreal Protocol aimed to reduce such compounds to protect the ozone layer.
Acid Rain Formation
Normal rain is slightly acidic, pH 5.7, due to dissolved CO₂.
Polluted rain has lower pH due to sulfur and nitrogen oxides generating larger acid concentrations.
Acid rain adversely affects buildings, vegetation, and water bodies.
D. ACTIVITY: Case Study Analysis on Air Quality Issue
Analyze a specific air quality issue by selecting a case study location.
Research pollutants, their sources, effects, and existing mitigation measures.
Propose engineering solutions including technological or policy-driven initiatives.
E. RUBRICS OF ACTIVITY
Excellent (10-9 points): Comprehensive identification of pollutants and sources with detailed analysis.
Good (8-7 points): Reasonable identification with some depth but lacking full analysis.
Fair (6-5 points): Minimal identification of key pollutants.
Poor (4-0 points): Fails to identify key pollutants or sources.
F. REFERENCES
Brown, Lawrence and Holme, Thomas, (2011) Chemistry for Engineering Students, 2nd Edition, Brooks/Cole Cengage Learning, USA.
Brown, T.L., et al. (2012) Chemistry: The Central Science, 12th Ed., Pearson Education.
Chang, Raymond (2010), Chemistry, 10th Edition, McGraw-Hill Companies, USA.