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Air Pollutants, Health Effects, and Green Chemistry – Lecture Review

Particulate Matter (PM)

  • Definition: complex mixture of tiny solid particles + microscopic liquid droplets suspended in air.

  • Size classifications

    • PM_{10}: aerodynamic diameter "approx 10\, \mu m .

    • PM_{2.5}: aerodynamic diameter "approx 2.5\, \mu m (more dangerous due to deeper lung penetration).

  • Health effects

    • Smaller particles (especially PM_{2.5}) reach alveoli, may translocate into bloodstream.

    • Correlated with cardiovascular events (heart attacks, strokes), asthma exacerbation, emergency-room visits, hospitalizations and premature death in sensitive groups.

    • General respiratory irritation; systemic effects once in blood (organs not fully mapped but statistically significant correlations).

  • Environmental effects

    • Reduces visibility (haze).

    • Alters lake/river acidity; disturbs aquatic nutrient balance.

    • Depletes soil nutrients; damages sensitive forests & crops.

  • Practical note: masks help not only against pathogens (e.g., COVID-19) but also PM exposure.

Sulfur Oxides (SO₂ & SO₃)

  • Primary source: coal combustion (coal contains 1-3\% sulfur).

  • Transformation

    • S + O2 \rightarrow SO2 during burning.

    • Atmospheric oxidation: 2\,SO2 + O2 \rightarrow 2\,SO_3 (catalyzed by sunlight, particulates, etc.).

  • Health effects

    • SO_2: colorless, irritating; aggravates cardiovascular disease; forms fine acidic droplets.

    • Permissible exposure limit: 0.03\,ppm (stricter than CO).

  • Environmental role: precursor of acid rain

    • SO3 + H2O \rightarrow H2SO4 (sulfuric acid in raindrops)

    • lowers pH of lakes/rivers, harms fish & aquatic life.

  • Control strategy: prevent release (pre-combustion desulfurization, flue-gas scrubbers) rather than post-release remediation.

Nitrogen Oxides (NOₓ)

  • Two main species: NO (nitric oxide, colorless) and NO_2 (brown).

  • Formation

    • High-temperature combustion (auto engines, coal-fired power plants) enables N2 + O2 \xrightarrow{T>1500\,^\circ C} 2\,NO.

    • Subsequent oxidation: 2\,NO + O2 \rightarrow 2\,NO2.

  • Acid-rain pathway: 2\,NO2 + H2O \rightarrow HNO2 + HNO3

    • nitric acid deposition.

  • Permissible limit: 0.053\,ppm .

  • Additional impacts

    • Lung/eye irritation; formation of acidic aerosols.

    • Atmospheric deposition

    • acidification + nutrient enrichment of ecosystems (eutrophication), especially when excess agricultural NH4NO3 fertilizer is present.

Tropospheric vs. Stratospheric Ozone

  • Stratospheric O_3 ("approx 15-35\,km altitude)

    • Beneficial: absorbs harmful UV-B/C radiation.

  • Tropospheric O_3 (ground-level)

    • Pollutant produced secondarily; no direct emission source.

  • Formation chain (photochemical smog):

    1. NO_2 \xrightarrow{hv} NO + O (sunlight photolysis).

    2. O + O2 \rightarrow O3.

    3. Hydrocarbons (RH) + OH, NO3 radicals form RO2\cdot that perpetuate NO \rightarrow NO2 cycling, sustaining O3 accumulation.

  • Health effects

    • Reduces lung function, causes chest pain, coughing, airway inflammation.

  • Environmental effects

    • Damages leaves; reduces photosynthesis, crop yields; impairs plant reproduction.

    • Acts as a greenhouse gas (climate forcing).

Hydrocarbons & Photochemical Relationships

  • Hydrocarbons (unburned fuel vapors, exhaust) act as volatile organic compounds (VOCs) in smog chemistry.

  • Classroom discussion takeaway: as hydrocarbon concentration rises, O3 formation rises after a photochemical lag; graph often shows inverse/time-shifted relationship—HCs peak early (morning rush) followed by O3 peak (afternoon).

Quantitative Example Problems

  1. Molecules of O_3 in a breath

    • Given: 2 \times 10^{22} total molecules per breath; air quality standard 0.12\,ppm (= 0.12\times10^{-6}).

    • Calculation: 2\times10^{22} \times 0.12\times10^{-6} = 2.4\times10^{15} O_3 molecules (rounded "approx 2\times10^{15}).

  2. Oxygen atoms in that breath

    • Each O_3 contains 3 O atoms

      • 3 \times 2\times10^{15} = 6\times10^{15} O atoms.

    • Conversion factor emphasized: 1\,O_3\text{ molecule} \rightarrow 3\,O\text{ atoms}.

Additional Outdoor & Indoor Pollutants

  • Indoor specific

    • Nicotine, formaldehyde, benzene: emitted from tobacco smoke, incense/candles, wood stoves, poorly vented appliances.

    • Radon (Rn): inert radioactive gas seeping from soil

    • lung cancer risk; mitigated via basement ventilation & test kits.

    • Lead: formerly in paints & gasoline; neurotoxin linked to behavioral impacts & historical crime-rate correlations.

  • Key mitigation: proper appliance installation, ventilation, substitution of hazardous materials, routine indoor-air testing.

Environmental & Health Impact Summary

  • Many pollutants (PM, SO2, NOx, O_3) share overlapping health outcomes—respiratory and cardiovascular morbidity/mortality.

  • Ecosystem effects range from acidification, eutrophication, visibility loss, to impaired plant growth and climate forcing.

  • Complex atmospheric chemistry means one emitted species (e.g., NO_x) can trigger multiple downstream problems (acid rain + ozone).

Sustainability & Green Chemistry Principles

  • Sustainability: meeting present needs without compromising future generations’ ability to meet theirs.

  • Prevention preferable to remediation: cheaper & more effective to avoid emissions than to treat ambient air.

  • Green-chemistry goals (per ACS 12 Principles)

    • Use less energy; design energy-efficient syntheses.

    • Minimize waste/by-products; favor catalytic over stoichiometric reagents.

    • Employ renewable feedstocks; reduce or eliminate toxic reagents.

    • Design for degradation; ensure products break down into innocuous substances.

  • Suggested resources: ACS Green Chemistry webpages; EPA Green Chemistry Challenge winners (career inspiration).

Trends in U.S. Air Quality ("Good News")

  • National data show downward trend for major criteria pollutants (CO, SO2, NOx, lead, PM, ozone precursors).

  • Attributed to