4. Please briefly describe the three main factors for the formation of photochemical smog in metropolitan areas. (5 pts)

8. There was an experiment that 20 L of air samples from the atmosphere were collected. The atmospheric pressure was 1.0 atm and the temperature was 25℃. After the air samples were dried in the laboratory, the volume of the air remained 19.5 L. What is originally the weight percentage of water in the air sample? (10 pts)