OBSERVATIONS OF THE NITRATE RADICAL IN THE FREE TROPOSPHERE AT IZANA DE TENERIFE

This paper reports direct measurements of the nitrate radical (NO3) in the lower free troposphere. The measurements were carried out at Izan 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 concen tration of about 20 ppt. Combining the NO3 data with ancillary measure ments of NO2 and O-3 in a model shows that there are no important NO3 scavengers in a part of the troposphere that is characterized by a ver y low relative humidity and aerosol particle count, and where the conc entration of NO2 is too small for significant quantities of N2O5 to fo rm. On occasion, the presence of a trace concentration of NO or an org anic species such as alpha-pinene is required to explain the observati ons. The lifetime of NO3 at night was in excess of 2 hours, much longe r than measured hitherto in the tropospheric boundary layer.