Free radical, formula NO₃, formed from the reaction of nitrogen dioxide with ozone. NO₃ has a very strong optical absorption in the visible portion of the spectrum. Absorption of solar radiation in this region leads to photodissociation and thus a very short atmospheric lifetime during the day. However, at night NO₃ can build up to substantial levels and react with some families of organic molecules, allowing oxidation chemistry that would not otherwise occur at night. Nitrate reacts to form dinitrogen pentoxide, which can be taken up into aqueous droplets and increase acidity. Optical absorption has been used to detect NO₃ in the atmosphere.