1. The problem asks to explain the essence of carbon dioxide in environmental engineering and to understand the concept of dissolved oxygen. 2. Carbon dioxide (CO2) in environmental engineering is crucial because it is a greenhouse gas that influences climate change by trapping heat in the atmosphere. It is also involved in processes such as photosynthesis, carbon cycling, and is a key factor in assessing air quality and the impact of industrial emissions. 3. Dissolved oxygen (DO) refers to the amount of oxygen that is present in water, which is essential for the survival of aquatic life. It is an important indicator of water quality because it shows how much oxygen is available for organisms in water bodies, and it affects biochemical processes such as aerobic respiration and the breakdown of organic matter. The final answers are: - Essence of carbon dioxide in environmental engineering relates to its role in greenhouse gas effects, carbon cycling, and environmental impact assessment. - Dissolved oxygen indicates the level of oxygen dissolved in water, crucial for aquatic ecosystem health and water quality evaluation.