Photochemical smog was first observed in

Options

A. London
B. Los Angeles
C. Paris
D. Tokyo

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks where photochemical smog was first observed. The options given are London, Los Angeles, Paris, and Tokyo.

2. **Identifying the Correct Answer**: Photochemical smog was first observed in Los Angeles in the 1940s. This is the correct answer among the options provided.

3. **Defining Photochemical Smog**: Photochemical smog is a type of air pollution that results from the chemical reactions between sunlight, nitrogen oxides, and volatile organic compounds in the atmosphere.

4. **Characteristics of Photochemical Smog**: It is characterized by a yellow-brown haze and is often associated with the presence of ground-level ozone and particulate matter.

5. **Causes of Photochemical Smog**: The primary contributors to photochemical smog include emissions from automobiles and pollutants released from power plants, particularly those that burn coal.

6. **Conclusion**: Based on the information, the answer to the question is Los Angeles, as it was the first city where photochemical smog was identified due to the high number of automobiles and coal-burning power plants.