What specific chemical reactions occur when sunlight reacts with nitrogen oxides and VOCs to form photochemical smog?

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Photochemical smog forms through a series of complex chemical reactions involving sunlight, nitrogen oxides (NOx), and volatile organic compounds (VOCs).

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Key Chemical Reactions

1. Formation of Nitrogen Dioxide (NO₂):
   • Nitrogen oxides are primarily emitted from vehicle exhausts and industrial processes. In the presence of sunlight, nitrogen monoxide (NO) reacts with oxygen to form nitrogen dioxide (NO₂): $$2\text{NO} + \text{O}_2 \rightarrow 2\text{NO}_2$$
2. Photolysis of Nitrogen Dioxide:
   • NO₂ absorbs sunlight, particularly in the ultraviolet range, and undergoes photolysis to produce nitric oxide (NO) and a highly reactive oxygen atom: $$\text{NO}_2 + h\nu \rightarrow \text{NO} + \text{O}$$
   • This oxygen atom can then combine with molecular oxygen to form ozone (O₃): $$\text{O} + \text{O}_2 \rightarrow \text{O}_3$$
3. Ozone Formation and Consumption:
   • Ozone is a major component of photochemical smog. It can react with NO to reform NO₂ and oxygen: $$\text{NO} + \text{O}_3 \rightarrow \text{NO}_2 + \text{O}_2$$
   • This cycle allows for the accumulation of ozone in the atmosphere.
4. Reactions with Volatile Organic Compounds (VOCs):
   • VOCs, which are emitted from various sources including vehicle exhausts and industrial activities, react with NO₂ in the presence of sunlight to form a variety of secondary pollutants: $$\text{NO}_2 + \text{R} \rightarrow \text{oxygenated products}$$
   • These reactions can lead to the formation of harmful compounds such as peroxyacetyl nitrate (PAN), which is a potent irritant: $$\text{NO}_2 + \text{R} + \text{O}_2 \rightarrow \text{PAN} + \text{other products}$$

Effects of Photochemical Smog

• Health Impacts: Exposure to photochemical smog can cause respiratory issues, eye irritation, and other health problems. The presence of ozone and PAN in the smog exacerbates these effects.
• Environmental Damage: Photochemical smog can damage vegetation, reduce visibility, and contribute to the formation of acid rain. The accumulation of ozone at ground level is particularly harmful to plants and can lead to reduced crop yields.