Observations of the nitrate radical in the free troposphere at Izaña de Tenerife

This paper reports direct measurements of the nitrate radical (NO3) in the lower free troposphere. The measurements were carried out at Izaña 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.