Which of the pollutant causes aggravation of respiratory diseases, and atmospheric discolouration?

The correct option is Nitrogen dioxide.

Step-by-Step Explanation:

1. Atmospheric Discoloration: Nitrogen dioxide (NO₂) is a highly reactive, reddish-brown gas. When it accumulates in the lower atmosphere, especially in urban environments with heavy traffic, it absorbs light and causes a distinct brown haze. This visible coloration of the horizon is a major contributor to atmospheric discoloration and smog.

2. Aggravation of Respiratory Diseases: NO₂ is a well-known respiratory irritant. When inhaled, it can irritate the mucous membranes of the respiratory tract, leading to airway inflammation. For individuals suffering from chronic respiratory conditions like asthma, bronchitis, or Chronic Obstructive Pulmonary Disease (COPD), exposure to nitrogen dioxide aggravates their symptoms and decreases lung function.

3. Evaluating other options:
- Carbon monoxide (CO) is a colorless, odorless gas that primarily interferes with oxygen transport in the bloodstream by binding to hemoglobin.
- Lead (Pb) is a toxic metal that damages the nervous, reproductive, and cardiovascular systems, but does not cause atmospheric discoloration.
- Ozone (O₃) is a colorless, irritating gas at ground level that affects respiratory health, but does not directly impart color to the atmosphere.