When the Philippine volcano Mount Pinatubo erupted in June 1991, it delivered the second-largest terrestrial eruption of the 20th century. However, its most mesmerizing secondary effect took place far above the clouds. The explosion injected roughly 20 million tons of sulfur dioxide into the stratosphere, reaching altitudes of over 20 miles.
There, the gas reacted with water vapor to form a global blanket of fine sulfate aerosols. While these microscopic droplets famously cooled the planet by about 0. 5 degrees Celsius over the following year, they also dramatically altered atmospheric optics.
Sunlight hitting the high-altitude sulfate layer underwent intense Rayleigh scattering. Standard Rayleigh scattering makes the sky blue and sunrises red by scattering shorter wavelengths. However, the unique size distribution of Pinatubo's sulfate aerosols produced aerosol optical depth anomalies that enhanced forward scattering in specific wavelengths.
When combined with stratospheric ozone absorption (the Chappuis band), the scatter produced brilliant purple, magenta, and deep violet hues during twilight and sunrise across the globe. Painters, photographers, and meteorologists recorded these prolonged twilight twilights, known as volcanic sunsets and sunrises, well into 1993. The event served as a real-world case study for solar geoengineering, showing how injected stratospheric particles could alter planetary albedo and atmospheric aesthetics simultaneously.