This paper reports direct measurements of the nitrate radical (NO3) in the lower free troposphere. The measurements were carried out at Iza¿a de Tenerife (2300 m altitude) in the Canary Islands during May 1994, using the technique of differential optical absorption spectroscopy (DOAS). The average nighttime NO3 concentration in very clean air from the mid-Atlantic was found to be 8 ppt, with a maximum observed concentration of about 20 ppt. Combining the NO3 data with ancillary measurements of NO2 and O3 in a model shows that there are no important NO3 scavengers in a part of the troposphere that is characterized by a very low relative humidity and aerosol particle count, and where the concentration of NO2 is too small for significant quantities of N2O5 to form. On occasion, the presence of a trace concentration of NO or an organic species such as α-pinene is required to explain the observations. The lifetime of NO3 at night was in excess of 2 hours, much longer than measured hitherto in the tropospheric boundary layer.¿ 1997 American Geophysical Union