Air pollution and greenhouse gases Knowledge-Based Study

Study notes

1. Overview of Air Pollution

   Air pollution refers to the introduction of harmful particulates, biological molecules, or chemical substances into Earth's atmosphere, leading to adverse effects on living organisms and the built environment. Pollutants are classified into primary and secondary categories based on their origin.

   Primary Pollutants

   Primary pollutants are emitted directly from identifiable sources into the atmosphere:

   • Carbon Monoxide (CO): Produced by the incomplete combustion of fossil fuels and biomass: $$2\text{C}(s) + \text{O}_2(g) ightarrow 2\text{CO}(g)$$ Carbon monoxide binds preferentially to hemoglobin with an affinity approximately 200 times higher than oxygen, forming carboxyhemoglobin and impairing cellular respiration.
   • Sulfur Dioxide ($\text{SO}_2$): Generated primarily during the combustion of sulfur-containing coal and heavy fuel oil, as well as during sulfide ore smelting: $$\text{S}(s) + \text{O}_2(g) ightarrow \text{SO}_2(g)$$
   • Nitrogen Oxides ($\text{NO}_x$): Formed when atmospheric nitrogen and oxygen react under high-temperature combustion conditions, such as inside internal combustion engines: $$\text{N}_2(g) + \text{O}_2(g) ightarrow 2\text{NO}(g)$$
   • Particulate Matter ($\text{PM}_{10}$ and $\text{PM}_{2.5}$): Suspended solid particles and liquid droplets categorized by aerodynamic diameter. $\text{PM}_{2.5}$ particles are respirable, penetrating deeply into pulmonary alveoli and entering the bloodstream.
   • Volatile Organic Compounds (VOCs): Organic chemicals with high vapor pressures at room temperature, including unburned hydrocarbons, industrial solvents, and biogenic terpenes.

   Formation of Secondary Air Pollutants

   Secondary pollutants are not directly emitted; they form in the atmosphere through chemical transformations of primary precursors catalyzed by solar radiation, water, or atmospheric oxidants.

   Acid Deposition

   Sulfur dioxide and nitrogen oxides undergo atmospheric oxidation to form strong mineral acids, resulting in acid rain (precipitation with $\text{pH} < 5.6$):

   • Sulfuric Acid Pathway: $\text{SO}_2$ reacts with hydroxyl radicals ($\cdot\text{OH}$) or aqueous oxidants such as hydrogen peroxide ($\text{H}_2\text{O}_2$) in cloud droplets: $$\text{SO}_2(g) + \text{H}_2\text{O}(l) ightarrow \text{H}_2\text{SO}_3(aq)$$ $$\text{SO}_3(g) + \text{H}_2\text{O}(l) ightarrow \text{H}_2\text{SO}_4(aq)$$
   • Nitric Acid Pathway: Nitrogen dioxide reacts with hydroxyl radicals: $$\text{NO}_2(g) + \cdot\text{OH}(g) ightarrow \text{HNO}_3(g)$$

   Photochemical Smog and Tropospheric Ozone

   Photochemical smog develops in urban atmospheres characterized by high vehicular emissions, sunlight, and thermal inversions:

   1. Photodissociation of $\text{NO}_2$ by ultraviolet light produces atomic oxygen: $$\text{NO}_2 + h\nu ightarrow \text{NO} + \text{O}$$
   2. Atomic oxygen rapidly combines with molecular oxygen to yield ground-level ozone ($\text{O}_3$): $$\text{O} + \text{O}_2 + \text{M} ightarrow \text{O}_3 + \text{M}$$
   3. In the presence of VOCs, peroxy radicals convert $\text{NO}$ back to $\text{NO}_2$ without consuming ozone, causing $\text{O}_3$ and secondary irritants such as peroxyacetyl nitrate (PAN) to accumulate.

   Mechanics of the Greenhouse Effect

   Solar radiation reaches Earth primarily as shortwave ultraviolet and visible light. Earth's surface absorbs this energy and re-emits it as longwave infrared (thermal) radiation. Greenhouse gases (GHGs) absorb specific wavelengths of this outgoing terrestrial infrared radiation due to dipole moment changes during vibrational excitation, trapping thermal energy within the troposphere.

   Principal Greenhouse Gases

   • Water Vapor ($\text{H}_2\text{O}$): The most abundant natural greenhouse gas, acting as a positive feedback mechanism in atmospheric warming.
   • Carbon Dioxide ($\text{CO}_2$): The primary anthropogenic greenhouse gas, released predominantly by fossil fuel combustion, industrial processes (such as cement manufacture), and deforestation.
   • Methane ($\text{CH}_4$): Emitted through enteric fermentation in ruminants, rice cultivation, natural gas extraction, and anaerobic decomposition in landfills. Methane features a Global Warming Potential (GWP) significantly higher than carbon dioxide over a 100-year time horizon.
   • Nitrous Oxide ($\text{N}_2\text{O}$): Produced by the microbial denitrification and nitrification of synthetic nitrogen fertilizers in agriculture, as well as industrial nylon production.
   • Fluorinated Gases (CFCs, HCFCs, HFCs, $\text{SF}_6$): Synthetic compounds exhibiting exceptionally high radiative efficiencies and long atmospheric lifetimes, making their GWP values thousands of times greater than $\text{CO}_2$.

   Mitigating Atmospheric Pollution

   Addressing air quality and climate disruption involves deploying both end-of-pipe emission controls and fundamental green chemistry principles.

   End-of-Pipe Industrial Technologies

   • Three-Way Catalytic Converters: Utilize platinum, palladium, and rhodium catalysts to simultaneously oxidize $\text{CO}$ and unburnt hydrocarbons to $\text{CO}_2$ and $\text{H}_2\text{O}$, while reducing $\text{NO}_x$ to $\text{N}_2$.
   • Flue-Gas Desulfurization (Wet Scrubbing): Removes $\text{SO}_2$ from power plant exhaust streams by reacting acidic gases with basic limestone ($\text{CaCO}_3$) slurries, generating marketable gypsum ($\text{CaSO}_4\cdot 2\text{H}_2\text{O}$): $$\text{SO}_2(g) + \text{CaCO}_3(s) + \frac{1}{2}\text{O}_2(g) + 2\text{H}_2\text{O}(l) ightarrow \text{CaSO}_4\cdot 2\text{H}_2\text{O}(s) + \text{CO}_2(g)$$
   • Electrostatic Precipitators and Baghouses: High-voltage collection plates and fabric filtration systems capture particulate matter from industrial flue gases.

   Green Chemistry and Source Reduction

   • Catalytic Alternative Synthesis: Designing atom-economical industrial pathways that eliminate the generation of VOC solvents and toxic volatile reagents.
   • Renewable and Carbon-Neutral Feedstocks: Transitioning from petrochemical fuels to electrolytic green hydrogen and bio-based platforms.
   • Carbon Capture, Utilization, and Storage (CCUS): Capturing post-combustion $\text{CO}_2$ using amine scrubbers or metal-organic frameworks (MOFs) and converting it catalytically into synthetic fuels, polymers, or cyclic carbonates.