HVAC - EPA Certification Practice Exam

Which layer in the Earth's atmosphere is primarily responsible for absorbing UV radiation?

1. Troposphere

2. Stratosphere

3. Mesosphere

4. Thermosphere

The correct answer is: Stratosphere

The stratosphere is the layer of the Earth's atmosphere responsible for absorbing the majority of the Sun's harmful ultraviolet (UV) radiation. This is largely due to the presence of ozone (O3) within this layer, which plays a crucial role in filtering UV radiation. When UV rays hit the ozone molecules, they cause a chemical reaction that absorbs and re-emits the energy as thermal radiation. This process significantly reduces the amount of UV radiation that reaches the Earth's surface, protecting living organisms from potential harm, such as skin cancer and other health issues. The other layers of the atmosphere, while important for various functions, do not have the same capacity to absorb UV radiation as the stratosphere. For example, the troposphere is primarily where weather events occur, but it does not have a significant amount of ozone. The mesosphere and thermosphere, although they comprise critical atmospheric functions and phenomena, are higher up and do not concentrate ozone to the same extent as the stratosphere, making them less effective in UV absorption. Thus, the stratosphere is vital for protecting Earth’s surface from the damaging effects of UV radiation.