Tropospheric Chemistry and Smog Formation Notes

Particulate Matter (PM): Includes $PM_{10}$ and $PM_{2.5}$ (solid/liquid aerosols); impacts respiratory and cardiovascular health.
Tropospheric Ozone ( $O_3$ ): A secondary pollutant causing plant damage and human health issues.
Nitrogen Oxides ( $NO_x$ ): Consists of $NO$ and $NO_2$ ; key precursors for ozone and secondary aerosols.
Volatile Organic Compounds (VOCs): Originating from biogenic and anthropogenic sources; precursors to ozone and secondary organic aerosols (SOA).
Carbon Monoxide (CO): Result of incomplete combustion; affects the budget of the $"OH$ radical.
Sulfur Dioxide ( $SO_2$ ): Leads to acid rain and sulfate aerosols.
Ammonia ( $NH_3$ ): Agricultural source that reacts with acids to form secondary particulate matter.

General Definition: Atmospheric visibility obscured by particles and droplets; name combined from "smoke" and "fog."
Classical (London) Smog: Linked to coal burning, high $SO_2$ , and carbon soot. Occurs in winter under cold, humid, stagnant conditions. The severe $1952$ London event caused up to $12,000$ fatalities.
Photochemical (Los Angeles) Smog: Driven by sunlight reactions between $NO_x$ and VOCs. Occurs in summer; characterized by high $O_3$ , peroxyacyl nitrates (PANs), and aldehydes.
Temperature Inversion: A warm air layer traps cooler air at the surface, preventing vertical mixing and causing pollutant accumulation near the ground.

Nitric Oxide (NO) Formation: Endothermic reaction during high-temperature combustion: $N_2 + O_2 \rightleftharpoons 2NO$
Ozone Formation Pathways: NO_2 \text{ (hν, } λ < 420\text{ nm)} \rightarrow NO + O(^3P) $O + O_2 + M \rightarrow O_3 + M$
Hydroxyl Radical ("OH) Sources: O_3 \text{ (hν, } λ < 315\text{ nm)} \rightarrow O^* + O_2 $O^* + H_2O \rightarrow 2 "OH$
Mechanism in Polluted Air: Nitrous acid (HONO) accumulates overnight and photolyzes at sunrise, providing an early morning source of $"OH$ radicals.
PAN Formation: Acts as a reservoir for $NO_2$ and a potent eye irritant: $CH_3C(O)OO" + NO_2 \rightleftharpoons CH_3C(O)OONO_2$

VOC-limited (NOx-saturated): Common in urban cores. Reducing VOCs is the most effective way to lower $O_3$ .
NOx-limited: Common in rural areas. Reducing $NO_x$ is the most effective approach.
Ozone Isopleths: Graphs showing $O_3$ concentrations as a function of VOC and $NO_x$ . In VOC-limited areas, reducing $NO_x$ can temporarily increase ozone due to reduced $NO$ titration.

Carbon Monoxide (CO):
- Sources: Incomplete combustion (60–70\text{%}) and oxidation of methane (20–25\text{%}).
- Health: Binds to hemoglobin (Hb) with $≈ 240\times$ higher affinity than $O_2$ , causing hypoxia.
- Sinks: Primarily leads to ozone formation via reaction with $"OH$ : $CO + 2O_2 + h\nu \rightarrow CO_2 + O_3$
Sulfur Dioxide ( $SO_2$ ):
- Oxidation: Converts to sulfuric acid ( $H_2SO_4$ ) via gas-phase or aqueous-phase reactions.
- Control: Fuel switching, Flue Gas Desulfurization (scrubbers), and emission caps.

Classification:
- $PM_{10}$ : Coarse inhalable particles ( $≤ 10\text{ μm}$ ).
- $PM_{2.5}$ : Fine particles ( $≤ 2.5\text{ μm}$ ) from combustion and secondary formation.
Composition: Includes mineral dust, sea salt, sulfates, nitrates, black carbon, trace metals, and Polycyclic Aromatic Hydrocarbons (PAHs).
Toxic Components:
- PAHs: Benzo[a]pyrene is carcinogenic and binds to DNA.
- Trace Metals: Lead (Pb), Cadmium (Cd), and Chromium (Cr) are persistent neurotoxins or carcinogens.
Deposition: Finer particles reach the alveolar region (deep lung) and are linked to cardiovascular disease and lung cancer. The IARC classifies PM as a Group 1 carcinogen.

Question: What about Beirut?
Response: Air circulation flushes the smog; the Seawind/Landwind circulation is a local circulation driven by temperature/density gradients.