Complex sentence: The eruption released about of sulfur dioxide, about half the quantity released by the 1980 eruption of Mount St. Helens, and was sufficient to cause a noticeable increase in atmospheric opacity.
Split into simpler sentences: The eruption released about of sulfur dioxide, about half the quantity released by the 1980 eruption of Mount St. Helens. This amount of sulfur dioxide was sufficient to cause a noticeable increase in atmospheric opacity.