The technique of differential optical absorption spectroscopy has been used to determine the nitrate radical (NO3) concentration in the remote marine boundary layer. The instrument was deployed in campaigns at Mace Head on the west coast of Ireland and on the north coast of Tenerife. A comprehensive set of NO3 measurements under a wide variety of conditions was obtained. For instance, nighttime NO3 levels at Mace Head ranged from 1 to 5 ppt in the clean marine atmosphere and from 1 to 40 ppt in semipolluted continental air masses. The nightly averaged NO3 lifetime varied from less than 2 min to 4 hours. At Tenerife, where there was less variability in conditions, nighttime NO3 ranged from 1 to 20 ppt, with nightly averaged lifetimes between 4 and 34 min. A photochemical box model, fully constrained by measurements of species that control the formation and removal of NO3, was then employed to determine the major loss mechanisms of the radical. This shows that NO3 in the clean marine air masses is very sensitive to small increases in the concentrations of dimethyl sulphide (DMS) and nonmethane hydrocarbons and that the radical is rarely in chemical steady state. At Tenerife, 80--90% of NO3 was removed by reaction with DMS. However, in continental air masses with little marine influence, indirect losses of NO3 via dinitrogen pentoxide (N2O5) usually dominate. It appears that in much of the North Atlantic, NO3 is a more efficient sink for DMS compared to the hydroxyl radical (OH) during the day. ¿ 2000 American Geophysical Union