NATS 1780 – Energy & Earth’s Atmosphere: Comprehensive Study Notes

The UV Index

• Introduced in 1992, originating in Canada; internationally recognized 1994, adopted globally 2002
• Purpose: communicate potential health risk from solar UVR to public
  • $\text{Risk} \alpha \text{UV Index} \alpha \text{UV Intensity}$
  • $\text{Health Safety} \propto \frac{1}{\text{UV Index}}$
• Scale (Canada): 0–11+ (higher = more risk)
• Depends on:
  • Stratospheric ozone thickness (daily variability; anthropogenic + natural)
  • Solar elevation angle (greatest at local solar noon; longer path length when Sun low)
  • Cloud cover (thick clouds attenuate UV)
• Dose concept
  • $\text{UV Dose} = \text{UV Index} \times \text{Time of Exposure}$
• Daily pattern: rises after sunrise → peak at local solar noon → zero at night
• Seasonal pattern (Toronto example): maximum near Summer Solstice, minimum near Winter Solstice, moderate at Equinoxes
• Effectiveness: Systematic review (Italia & Rehfuess, 25 studies) – low familiarity & understanding, minimal influence on protective behaviour.

Health Impacts of UV

• Acute: sunburn, photokeratitis
• Chronic: cataracts, skin aging, skin cancer, immunosuppression

Types of UV Radiation

• UVA (315–400 nm): not absorbed by ozone; penetrates deepest; “most concerning”
• UVB (280–315 nm): partially absorbed; causes sunburn; still concerning
• UVC (100–280 nm): totally absorbed by ozone & atmosphere; no ground-level exposure

Sunscreen Technology

• Physical / Mineral (ZnO, TiO2)
  • Reflects & scatters UV; broad spectrum; immediate protection; may leave white cast; good for sensitive skin/children
• Chemical / Organic (avobenzone, oxybenzone, octinoxate…)
  • Absorbs UV → converts to heat; requires 15–30 min to activate; cosmetically elegant
• Both can be broad spectrum (UVA + UVB) depending on formula

Environmental Concerns

• Release pathways: direct wash-off (swimming, boating), wastewater effluent, sludge, landfill leachate, airborne particulates during application
• UV-filter pollutants: large suite of organics (PABA, benzophenones, cinnamates, salicylates, triazines, camphor derivatives, benzimidazoles, phenols) + inorganics (nano/non-nano TiO2, ZnO)
• Ecotoxicological impacts along molecular → community levels (Hodge 2025, Marine Pollution Bull.)

Radiation & Energy Transfer

• Three heat-transfer modes: conduction, convection, radiation
• Key question: How does solar energy reach Earth? → via radiation (EM waves)
• Wave properties: wavelength $\lambda$ & frequency $f$ (inverse relation $c = \lambda f$)
• Microwave oven example: uses 2.45 GHz radiation to excite polar water molecules (heating)

Electromagnetic Spectrum

• Spectrum spans radio (10³ m) → γ-ray (10⁻¹² m)
• Only certain bands penetrate atmosphere (radio, visible, parts of IR)
• Wien’s Displacement Law (1893): $\lambda_{max} = \frac{b}{T}$ with b = 2.898\times10^{-3}\;\text{m·K}
  • Sun’s $T \approx 5{,}773\,\text{K}$ ⇒ $\lambda_{max} \approx 5.02\times10^{-7}\,\text{m} = 502\,\text{nm}$ (blue-green visible)
• Solar spectrum reaching top-of-atmosphere (ToA): ~48 % visible, 7 % UV, 44 % near-IR

Atmospheric Absorption

• Gases absorb strongly in UV (O₃) & IR (H₂O, CO₂, CH₄, N₂O) bands; visible largely passes through “atmospheric window”
• Result: Downwelling long-wave (DWLW) < 50 % of downwelling short-wave (DWSW) measured at surface

Greenhouse Effect

• Natural GE: water vapour dominant; keeps mean surface $\sim 15\,^{\circ}\text{C}$ vs $-18\,^{\circ}\text{C}$ without atmosphere
• Anthropogenic GE: added GHGs (CO₂, CH₄, N₂O) enhance IR trapping → warming
• Correlation vs causation: CO₂ and temperature strongly correlated (HadCRUT/NOAA); causality attributed to radiative forcing physics

Daily & Seasonal Energy Balance

• Insolation (visible) passes atmosphere, absorbed by land/ocean (≈47 % of incoming)
• Absorbed energy re-emitted as IR; troposphere heated from ground up
• Diurnal cycle: solar noon → maximum insolation, but $T_{\text{max}}$ occurs later due to thermal inertia (radiative transfer time-lag)
  • Cloud cover modulates DWSW (EMOS May 26 2025 example shows DWSW deficit during cloud passage; $T_{\text{max}}$ ~2.5 h earlier than DWSW = UWLW)
• Night-time radiational cooling: continued IR emission without solar input
• Seasonal cycle driven by axial tilt → latitude-dependent insolation (CHELSA global map, winter vs summer solstice patterns)

Planetary Energy Budget (Trenberth 2009)

• Incoming SW: $341\,\text{W m}^{-2}$
  • Reflected (albedo): $101.9\,\text{W m}^{-2}$ (clouds + surface)
  • Absorbed by surface: $161\,\text{W m}^{-2}$
  • Absorbed by atmosphere: $79\,\text{W m}^{-2}$
• Surface emits $396\,\text{W m}^{-2}$ IR; atmosphere back-radiates $333\,\text{W m}^{-2}$ downward
• Net imbalance ~ $0.9\,\text{W m}^{-2}$ (positive) → warming

Energy Balance Climate Model (EBCM)

• Pedagogical spreadsheet model built for NATS 1780 based on process-flow diagrams (Ahrens Fig 2.15, 2.16)
• Balances energy at each node; useful for “what-if” scenarios (ice age, geoengineering, altered solar constant, pandemic emissions drop)
• Example scenario – COP-21 Paris Agreement
  • $\Delta[CO_2] = 146\,\text{ppm}$ (≈2.2 ppm yr⁻¹ reaching 2081)
  • +3 % increase in atmosphere → IR re-emission back to surface
  • Predicts $\Delta T \approx 2^{\circ}\text{C}$ warming
• Limitations: “toy” level – lacks full physics, chemistry, equations, spatial resolution

Preliminary & Key Takeaways

• UVA & UVB constitute primary health risks; UVC fully filtered by ozone
• UV Index communicates risk but behaviour change remains limited
• Visible light (44 %) is the dominant energy input driving atmospheric circulation; UV only 7 % of solar flux
• Earth’s surface absorbs visible → emits IR; greenhouse gases trap part of this IR, warming troposphere
• Tropospheric heating is bottom-up
• Rising GHG concentrations ⇒ $\Delta T \propto [\text{GHG}]$ → global warming
• Energy balance frameworks (daily, seasonal, planetary) enable quantitative climate reasoning; simple models (EBCM) illustrate scenario impacts
• Only anthropogenic enhancement of GE is problematic; natural GE is essential
• Water vapour, though strongest GHG, is controlled by temperature-dependent feedbacks (question deferred)

Frequently Tested Concepts / Sample MCQ Answers

• Daily UV pattern: C (Increases from sunrise, peaks at local solar noon, disappears at sunset)
• Incorrect energy-budget statement slide 40: C (Insolation is converted into IR via radiative transfer) – actually correct → so the intended incorrect one is D? (Check original; teacher flags heating from ground up as correct)
• EBCM incorrect statement slide 74: E (EBCM can be applied to real-world examples such as Paris Agreement) – considered illustrative only

Key Equations

• $\text{UV Dose} = \text{UV Index} \times t$
• $\lambda_{max} = \dfrac{b}{T}$ with b = 2.898\times10^{-3}\,\text{m·K}
• $\Delta T \propto [\text{GHG}]$ (qualitative alpha-statement)

Ethical / Practical Implications

• Sunscreen chemicals pose emerging marine pollution threat – calls for greener formulations & policy
• Public-health messaging (UV Index) must improve to translate awareness into behaviour change
• Mitigating climate change requires curbing anthropogenic GHGs; models (even simple) inform policy (Paris Agreement, geoengineering debates)