Why do newspapers made with groundwood pulp tend to yellow over time?

Newspapers made with groundwood pulp yellow over time primarily because they retain all the chemical constituents of the original wood, most notably lignin.1

Unlike chemical pulping processes that remove lignin to produce more durable paper, mechanical or groundwood pulping retains this complex organic polymer within the fibers. Lignin is a phenolic compound that is highly sensitive to environmental factors; when exposed to light, heat, and the natural aging process, it undergoes chemical changes that cause the paper to discolor and turn yellow. Because groundwood pulp contains these reactive components, the resulting paper is less stable than paper made from purified cellulose.12