Which layer of the atmosphere is critical for blocking most ultraviolet radiation from the sun?

The ozone layer is critical for blocking most ultraviolet (UV) radiation from the sun due to its unique chemical composition. This layer is found within the stratosphere and contains a high concentration of ozone molecules (O3), which absorb harmful UV radiation effectively. When UV rays from the sun enter the atmosphere, they encounter the ozone molecules, which dissipate the energy by converting it into heat. This protective mechanism is vital for life on Earth, as excessive exposure to UV radiation can lead to skin cancer, cataracts, and other health issues, as well as adversely affecting ecosystems.

The stratosphere, while it contains the ozone layer, is not specifically responsible for the UV-blocking capability; rather, it houses the ozone layer. The troposphere is the lowest layer of the atmosphere and is where weather phenomena occur, but it does not significantly contribute to UV protection. The mesosphere, situated above the stratosphere, plays a part in different atmospheric processes but does not have a significant role in ozone and, consequently, UV protection. Thus, it's the ozone layer that specifically enables the critical blocking of UV radiation from the sun.