The body manufactures vitamin D3 out of cholesterol in the presence of sunlight. (cheeseslave.com)
You see, my camera skills go kaput when I take pictures without the presence of sunlight. (proteinpow.com)
Formed in the presence of sunlight by water vapor and pollutants like ozone and nitrogen oxides, hydroxyl is hard to measure, because it persists for just a second in the air before it reacts away. (sciencemag.org)