CHEM3120 · Environmental and Analytical Chemistry
Atmospheric Chemistry & the Carbon/Nitrogen Cycles
Block 2 (Lectures 9-16) of CHEM3120 covers the atmosphere: the carbon and nitrogen cycles, the greenhouse effect and Earth's radiative energy balance (blackbody emission, Wien's law), photochemical smog and NOx, and CFCs and the ozone hole. The examinable calculations use formula-sheet relationships (Wien's law, Stefan-Boltzmann), and the mechanisms — why CO2 is an effective greenhouse gas, how HFCs are removed before reaching the stratosphere — appear as Part A short-answer.
What this chapter covers
- 01Blackbody radiation: any body at T > 0 emits; the peak wavelength shortens as T rises
- 02Wien's law λ_max · T = 2.898×10^6 K·nm; Earth's emission peaks in the infrared
- 03Greenhouse effect: Earth's IR emission overlaps the CO2 bending-mode absorption
- 04Why CO2 outweighs water vapour as a greenhouse gas near Earth's emission peak
- 05Radiative energy balance and the IPCC picture; Stefan-Boltzmann q = 4πr²σT⁴
- 06Photochemical smog: N2 + O2 → 2NO in engines (morning peak), O3 downstream at midday
- 07Ozone and CFCs: HFC replacements removed in the troposphere by R-H + OH· → R· + H2O
- 08Carbon and nitrogen cycles: nitrogen fixation, Haber-Bosch benefits and runoff drawbacks
Wien's law: Earth's peak thermal emission and why CO2 matters
- +1Rearrange Wien's law for the peak wavelength: λ_max = (2.898×10^6 K·nm) ÷ T.
- +1Substitute T = 288 K: λ_max = 2.898×10^6 ÷ 288 = 1.006×10^4 nm ≈ 10 µm — this lies in the infrared, not the visible.
- +1Interpretation: Earth's emission peaks in the IR, overlapping the region where CO2 vibrational modes (its bending mode) absorb strongly, so CO2 intercepts outgoing IR and re-radiates it back — an effective greenhouse gas even though it is far less abundant than water vapour, because it absorbs where Earth radiates most.
Key terms
- Blackbody radiation
- The thermal radiation emitted by any object at a temperature above absolute zero; hotter bodies emit more and peak at shorter wavelengths.
- Wien's law
- λ_max · T = 2.898×10^6 K·nm: the wavelength of peak emission is inversely proportional to absolute temperature. Cool Earth peaks in the IR; the hot Sun peaks in the visible.
- Greenhouse effect
- Absorption of Earth's outgoing infrared radiation by atmospheric gases (notably CO2 and water vapour) that then re-radiate energy downward, warming the surface.
- Stefan-Boltzmann law
- The total power radiated by a body scales as T⁴ (q = 4πr²σT⁴, σ = 5.67×10^-8 W m^-2 K^-4); the basis of Earth's radiative energy balance.
- Nitrogen fixation
- Conversion of atmospheric N2 into biologically accessible nitrogen oxides or hydrides; achieved industrially by the Haber-Bosch process, which boosts crop yields but whose runoff can cause algal blooms.
- Photochemical smog
- Air pollution in which NO from combustion (N2 + O2 → 2NO, peaking at morning rush hour) is processed photochemically to produce ozone downstream around midday.
Atmospheric Chemistry & the Carbon/Nitrogen Cycles FAQ
Why is CO2 a more important greenhouse gas than water vapour, despite being less abundant?
Because of WHERE it absorbs. Earth's thermal emission peaks in the infrared near 10 µm (Wien's law at ~288 K), and CO2's vibrational modes absorb strongly in that region, so CO2 intercepts a large share of the outgoing longwave radiation. Water vapour absorbs more at shorter IR wavelengths where Earth radiates comparatively little, so per molecule near the emission peak CO2 is disproportionately effective.
How do CFC replacements avoid destroying stratospheric ozone?
The hydrofluorocarbon replacements keep a C-H bond, and that is the key. In the troposphere the hydroxyl radical attacks it, R-H + OH· → R· + H2O, breaking the molecule down before it can drift up to the stratosphere. The original CFCs had no such handle, survived to the stratosphere, and released chlorine that catalytically destroyed ozone.
Why does NO peak in the morning but ozone at midday?
NO is made thermally in hot engines, N2 + O2 → 2NO, so it tracks traffic and peaks at the morning rush hour. Ozone is not emitted directly; it forms downstream through sunlight-driven photochemistry acting on NOx and hydrocarbons, which takes time and needs strong sunlight, so O3 builds to a midday peak after the NO.
Can AI help me with the atmospheric-chemistry calculations in CHEM3120?
Yes. Sia can take you through a Wien's-law or Stefan-Boltzmann calculation step by step, check your unit conversions (nm to µm, K to power), and rehearse the mechanism answers — greenhouse overlap, CFC breakdown, smog timing — in the short-answer style. It explains the method and checks your reasoning; it does not do graded work for you, and University of Sydney academic-integrity rules apply.
Exam move
The atmosphere block mixes quantitative and conceptual marks, so prepare both. On the numbers, drill the formula-sheet relationships — Wien's law and Stefan-Boltzmann — as clean one-line rearrangements with careful units (K, nm, µm, W m^-2). On the concepts, rehearse the set-piece explanations: why CO2 outweighs water vapour near Earth's emission peak, how HFCs are removed by OH· before the stratosphere, and why NO peaks in the morning while O3 peaks at midday. Learn the nitrogen-fixation and Haber-Bosch trade-off (yield versus runoff) as a benefit/drawback pair. This is a large block spanning several lectures, so spread revision across the semester rather than cramming. Confirm the exam date, room and permitted materials on Canvas and the exam timetable.
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