Atmospheric Chemistry

Earth's atmosphere: N₂ (78%), O₂ (21%), Ar (0.93%), CO₂ (0.042%), trace gases. Troposphere (0-12 km): weather, most atmospheric chemistry. Stratosphere (12-50 km): ozone layer. Ozone layer: O₃ at 20-30 km altitude — absorbs 97-99% of UV-B radiation (280-315 nm). Chapman cycle: O₂ + hv → 2O, O + O₂ → O₃, O₃ + hv → O₂ + O, O₃ + O → 2O₂. Ozone depletion: CFCs (chlorofluorocarbons) — Cl catalytically destroys O₃ (each Cl atom destroys ~100,000 O₃ molecules). Montreal Protocol (1987): phased out CFCs — ozone recovery expected by ~2066. Nobel Prize 1995 (Crutzen, Molina, Rowland). Greenhouse effect: CO₂, CH₄, N₂O, H₂O absorb infrared radiation → warming. Pre-industrial CO₂: 280 ppm → 2026: ~425 ppm (51% increase). Methane: 2.5× pre-industrial levels, 80× more potent than CO₂ over 20 years. Sources: wetlands, agriculture (rice paddies, livestock), fossil fuels, permafrost thaw. Photochemical smog: NOₓ + VOCs + sunlight → ozone (ground-level, harmful) + PAN + other oxidants. Acid rain: SO₂ + NOₓ → H₂SO₄ + HNO₃ — pH <5.6, damages ecosystems, buildings, infrastructure.

Water Chemistry

Water quality parameters: pH, dissolved oxygen (DO), biochemical oxygen demand (BOD), chemical oxygen demand (COD), total dissolved solids (TDS), turbidity. Eutrophication: excess nutrients (N, P) from agriculture → algal blooms → O₂ depletion → aquatic dead zones. >500 dead zones worldwide (Gulf of Mexico, Baltic Sea, Chesapeake Bay). Heavy metals: Pb, Hg, Cd, As, Cr — bioaccumulate in food chains. Mercury methylation: Hg → CH₃Hg⁺ (methylmercury) by anaerobic bacteria — 10,000× more toxic, biomagnifies in fish. Minamata disease (1956): methylmercury poisoning from industrial discharge — 2,000+ deaths. Lead: removed from gasoline (1970s-2000s), but legacy contamination persists in soil, paint, pipes (Flint, Michigan water crisis, 2014). Arsenic: natural contamination in groundwater affects 140+ million people (Bangladesh, India, Southeast Asia). Microplastics: particles <5 mm, found in 83% of tap water samples worldwide, in human blood and placentas. Per- and polyfluoroalkyl substances (PFAS): "forever chemicals" — persistent, bioaccumulative, found in drinking water of 200+ million Americans. Emerging contaminants: pharmaceuticals, hormones, nanoparticles — conventional water treatment doesn't fully remove them.

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Soil and Remediation

Soil contamination: industrial chemicals, heavy metals, pesticides, petroleum hydrocarbons, radioactive materials. Brownfields: contaminated former industrial sites — 450,000+ in the US alone. Persistent Organic Pollutants (POPs): DDT, PCBs, dioxins — Stockholm Convention (2001) bans or restricts 30+ POPs. Bioaccumulation: concentration increases up the food chain — DDT: water (0.000003 ppm) → phytoplankton → fish → osprey (25 ppm) — 10 million-fold concentration. Bioremediation: using microorganisms to degrade contaminants — bacteria break down petroleum hydrocarbons (Deepwater Horizon cleanup). Phytoremediation: plants absorb, concentrate, or degrade pollutants — hyperaccumulators (Thlaspi caerulescens accumulates Zn, Cd). Mycoremediation: fungi degrade pollutants — white-rot fungi break down persistent organics. Chemical remediation: chemical oxidation (Fenton's reagent: Fe²⁺ + H₂O₂), permeable reactive barriers (zero-valent iron for chlorinated solvents). Soil washing: physical/chemical extraction of contaminants. Electrokinetic remediation: electric field mobilizes contaminants in low-permeability soils. Life Cycle Assessment (LCA): evaluating environmental impact of products from cradle to grave.

Green Chemistry

Green chemistry: designing chemical products and processes that minimize hazardous substances — Paul Anastas and John Warner (1998). Twelve principles: prevention > treatment, atom economy, less hazardous synthesis, safer chemicals, safer solvents, energy efficiency, renewable feedstocks, reduce derivatives, catalysis, degradable design, real-time analysis, safer chemistry for accident prevention. Atom economy: % of reactant atoms incorporated into desired product (vs. waste). Ideal: 100% (rearrangements, additions). E-factor: kg waste per kg product — pharmaceutical industry: 25-100 (high waste), bulk chemicals: <1-5. Green solvents: water, supercritical CO₂, ionic liquids, deep eutectic solvents — replacing toxic organic solvents. Biocatalysis: enzymes as catalysts — high selectivity, mild conditions, water as solvent. Flow chemistry: continuous reactors — better heat/mass transfer, safer for hazardous reactions, smaller waste streams. Renewable feedstocks: biomass-derived chemicals replacing petrochemicals — PLA from corn starch, bio-succinic acid. Carbon capture and utilization (CCU): converting CO₂ into chemicals, fuels, building materials. Circular chemistry: designing molecules for recycling, degradation, or repurposing at end of life. Green chemistry metrics: process mass intensity (PMI), cumulative energy demand (CED), sustainability assessment tools.

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Environmental Chemistry: Chemistry of Our Planet