A more fundamental contribution, she thinks, may come from the sun itself. When exposed to ultraviolet light, certain molecules “get excited” and form highly reactive compounds called radicals, Ms. Pugliese said. Those radicals then recombine with other nearby molecules, processes that often lead to the creation of aldehydes as well as ketones.
— Silvia Pugliese looks into the smell of line-dried laundry. Read more in How Line-Dried Laundry Gets That Fresh Smell at The New York Times; you can find an abstract of the original study here.