Atmospheric NO2:  In Situ Laser-Induced Fluorescence Detection at Parts per Trillion Mixing Ratios

We describe a time-gated laser-induced fluorescence instrument designed for accurate (+/- 5%, 1 sigma), continuous, autonomous, in situ observations of NO2 with the sensitivity (15 ppt/10 s at S/N = 2) and portability necessary to study NO2 anywhere in the troposphere. The technique is advantageous because it is spectroscopically specific and direct in that it does not require conversion of NO2 into another species (e.g., NO) prior to detection, eliminating a class of potential interferences. Performance of the instrument is illustrated with 15 weeks (July-Oct 1998) of observations at the University of California, Blodgett Forest field station located in the foothills of the Sierra Nevada and 4 weeks (June 15-July 15, 1999) in Nashville, TN during the Southern Oxidants Study. Ambient concentrations of NO2 at Blodgett Forest varied from below 50 ppt to 4000 ppt and NO2 ranged from 5 to 50% of the total reactive nitrogen; while in Nashville, TN, concentrations ranged from 1 to 75 ppb.