Unit 06 · Air, Atmosphere & Climate Change

Clean air is common ground. EPA reports that from 1970 to 2025 total emissions of the six principal air pollutants fell 78% while the economy, driving, energy use and population all grew, and it credits state and federal rules and technological advances for much of a separate decline in air toxics. Families still debate which rules are worth their costs, and the work is not finished: EPA estimates about 115 million people lived in counties above a primary air standard in 2025.

The troposphere contains most weather and usually cools with altitude; stratospheric ozone absorbs ultraviolet energy and helps produce warming with altitude there. Uneven solar heating and rotation help organize global wind patterns. An inversion can suppress vertical mixing near the surface, raising local pollutant concentrations without itself establishing a global climate trend.

For a top-of-atmosphere mean budget, net energy gain is incoming sunlight minus reflected sunlight minus outgoing infrared. Greenhouse gases absorb and re-emit infrared; they do not create energy. Persistent positive imbalance increases stored energy, but temperature response also depends on heat capacity, circulation and feedbacks.

CO₂ and methane can act as forcings when their abundance changes; water-vapor increase is an important warming feedback. Ranking gases requires a specified metric, abundance and time horizon. Climate also changes for natural reasons: NASA notes that lower solar activity and more volcanic activity helped trigger the Little Ice Age, and ENSO shifts heat from year to year. NASA tests those alternatives against the evidence and attributes modern warming mainly to human influence. Families disagree about how large the human share is and what policy should follow, and this unit takes no policy position. The synthetic graph here does not independently establish attribution.

Normal tropical Pacific trade winds push surface water west and permit eastern upwelling. El Niño weakens this pattern; La Niña strengthens it. ENSO redistributes heat and changes regional precipitation and ecosystems. It is not a substitute explanation for the long-term externally forced warming trend, nor a perfectly periodic clock.

A Keeling-type record can show both a multiyear rise and a seasonal cycle influenced by land-plant uptake and respiration. Compare the same season across years. Six deliberately generated points are not the actual Keeling record and do not support attribution, a complete carbon budget or an extrapolated climate sensitivity.

To test where added CO₂ comes from, compare candidate sources by size and by what happens to their carbon afterward. Convert a carbon mass to CO₂ by multiplying by 44/12. USGS puts all volcanoes at 0.13–0.44 billion tonnes of CO₂ a year, less than 1% of human emissions. Wildfires can be very large: NASA estimates the 2023 Canadian fires released about 640 million tonnes of carbon in five months, more than Russia or Japan emitted from fossil fuels in 2022. As forests regrow they reabsorb that carbon, while fossil-fuel CO₂ is not readily offset by natural processes. The Global Carbon Budget reports that over the past decade land and ocean absorbed about half of human emissions. If natural sources were driving the rise, the air would gain more CO₂ than people add, not about half as much.

Ground-level ozone is a secondary pollutant formed through NOx/VOC reactions in sunlight. Stratospheric ozone depletion involves ozone-depleting chemicals and UV chemistry; it is not the greenhouse mechanism. SO₂ and NOx can contribute to acid deposition. PM₂.₅ is defined by particle size, not one chemical composition; indoor and outdoor exposures can differ.

A time-weighted concentration uses sum(concentration × hours)/total hours. It is not inhaled dose unless breathing rates and uptake are also modeled, and it is not an AQI: AQI uses pollutant-specific averaging rules and breakpoints. Do not generate smoke, alter ventilation deliberately, or expose anyone to pollutants to obtain observations.

Wildfire smoke ties forests, climate and clean air together. EPA lists fires among the direct sources of fine particles, so smoke can raise PM₂.₅ exposure indoors and out. NASA cites USDA findings that the number, severity and size of wildfires have increased in recent decades, with contributing factors that include extended drought, past fire-management strategies, invasive species and homes built in formerly less developed areas. Forest management, fuel reduction and planning for smoky days are practical steps whatever one concludes about climate; each has costs and tradeoffs a community has to weigh.

Dissolving CO₂ changes seawater carbonate chemistry: more hydrogen ions reduce carbonate available to many shell-building organisms. A change from pH 8.2 to 8.1 means hydrogen-ion activity increases by 10^0.1, about 26%, while the water is still alkaline. This is not a 0.1% change. Biological responses depend on species and other conditions, and ocean warming is a separate stressor.

[noise-energy] Noise levels do not average arithmetically: Traffic, construction and industrial or vessel noise can interfere with communication and affect humans or wildlife, depending on frequency, duration and context. Decibels are logarithmic: a ten-decibel rise is ten times the corresponding intensity, not ten percent. Equivalent level Leq = 10 log10[sum(t×10^(L/10))/sum(t)] for compatible time intervals and weighting. A-weighted air levels and underwater sound levels use different conventions and reference pressures and must not be directly compared.

[noise-energy] Noise levels do not average arithmetically: Model and provenance assumptions: The synthetic air records are compatible A-weighted levels at one receptor, constant within three hours and one hour. The additional two-source example assumes incoherent equal sources with the same geometry. Calculations are silent; no noise generation or exposure measurement is assigned.

[noise-energy] Noise levels do not average arithmetically: Uncertainty and inference limits: Leq hides short peaks and frequency content. It is not an exposure limit, clinical assessment or proof of ecological harm. These air calculations cannot be applied numerically to the linked underwater examples without changing reference, medium and weighting.

[control-mass] A lower concentration may be dilution, not removal: Fabric filters and electrostatic precipitators capture particles, while sulfur-gas scrubbers or NOx reduction systems address different chemical pathways. Captured material may become a residue needing management; it does not disappear. Removal efficiency should compare inlet and outlet mass rates under compatible gas-volume conventions. Dilution can lower concentration without lowering mass discharge. Bypass time can reduce shift-wide performance even when the operating device works well.

[control-mass] A lower concentration may be dilution, not removal: Model and provenance assumptions: Synthetic PM inlet/outlet concentrations use the same dry standard-volume basis; outlet flow is larger because of added clean gas. Flow and concentration are fixed within each interval. The device runs seven hours and bypasses one, with no other particle source or storage change.

[control-mass] A lower concentration may be dilution, not removal: Uncertainty and inference limits: With outlet concentration 8–12 instead of 10 mg/m³ and fixed flows, operating removal ranges 90.4–85.6%. Instrument bias, flow normalization and bypass logs need review. No stack, roadside or hazardous air sampling is assigned.

[acid-deposition] Rain acidity needs amount as well as pH: Atmospheric transformation of SO₂ and nitrogen oxides can contribute to sulfuric and nitric acids transported and deposited away from sources. Wet deposition carries dissolved material; dry deposition occurs without precipitation. pH describes hydrogen-ion activity, not total acid delivered. For this dilute unbuffered model only, approximate [H⁺] = 10^(−pH) mol/L and multiply by rain volume per area. Buffered soils or waters can consume hydrogen ions, so free-H⁺ load is not total acidity or ecological damage.

[acid-deposition] Rain acidity needs amount as well as pH: Model and provenance assumptions: Two synthetic storms reach one square meter. One mm rain equals one liter/m². Use common temperature, ideal dilute concentrations and no reactions during mixing. The model does not include sulfate/nitrate speciation, alkalinity or dry deposition.

[acid-deposition] Rain acidity needs amount as well as pH: Uncertainty and inference limits: Measurement on a logarithmic scale means pH ±0.1 changes modeled H⁺ by factors 10^∓0.1, not by ±0.1%. Carbonate reactions and ionic activity can invalidate the simple mixture estimate. No collection or acid-handling procedure is assigned.

[smog-regimes] Precursor reductions need a chemistry and meteorology context: Photochemical smog includes secondary pollutants formed in reactions involving nitrogen oxides, volatile organic compounds and sunlight. Ozone is not directly emitted in the same way as many precursors. NO can remove local ozone by titration, while NO₂ photochemistry and peroxy radicals can support net production. In a VOC-limited setting, a NOx reduction can raise a local peak; in a NOx-limited setting it can lower it. The response depends on chemistry, transport and meteorology, not one universal percentage rule.

[smog-regimes] Precursor reductions need a chemistry and meteorology context: Model and provenance assumptions: These synthetic response cards stipulate two chemistry regimes, with weather and sunlight held fixed within each paired comparison. NOx and VOC values are scenario inputs, not measurements or a mechanistic chemical model fitted here. Ozone is the modeled maximum one-hour concentration in ppb.

[smog-regimes] Precursor reductions need a chemistry and meteorology context: Uncertainty and inference limits: Different weather, mixing and precursor mixtures can reverse or weaken a response. A modeled one-hour peak is not regulatory attainment or an eight-hour standard comparison. These cards illustrate a hypothesis to test with vetted records, not instructions to produce smog or sample polluted air.

[ocean-heat] Accumulated marine heat stress and ecosystem response: Ocean warming changes stratification, oxygen solubility, habitat ranges and marine heatwaves; it is not identical to acidification. Corals can lose symbiotic algae under sustained heat stress, but bleaching is not always immediate death. NOAA’s Degree Heating Week method accumulates HotSpots at least 1 °C above a local maximum monthly mean over the preceding twelve weeks. Here constant-within-week records make daily accumulation divided by seven equivalent to summing qualifying degree-weeks.

[ocean-heat] Accumulated marine heat stress and ecosystem response: Model and provenance assumptions: All four synthetic weeks last seven days; the previous eight weeks in the twelve-week window have zero qualifying stress. Maximum monthly mean is fixed at 28 °C. Cover records use the same recognition method one year apart, but are not randomized exposure treatments.

[ocean-heat] Accumulated marine heat stress and ecosystem response: Uncertainty and inference limits: SST error ±0.2 °C can switch a week across the 1 °C inclusion threshold: this window ranges 3.1–5.1 °C-weeks. Cover change can reflect storms, disease or sampling as well as heat. The indicator is not a NOAA observation and does not itself measure bleaching, mortality or attribution.