You see sunlight as white, but it's made up as a combination of the colors of the spectrum.
Those particles will tend to scatter away the blue and green light part of the sunlight's spectrum, leaving behind the more-difficult-to-scatter red and orange colors. That's because red and orange lights has a longer wavelength than the blue and greens, making it less affected by particles.
Pollution, ash or smoke can enhance the colors as there's more particles to scatter away the shorter-wavelength part of the spectrum.
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