In situ detection of tropospheric OH, HO2, NO2, and NO by laser-induced fluorescence in detection chambers at reduced pressures

For detection of OH and HO2, we have met or exceeded most of the goals of the proposal for this grant. We have succeeded in building a prototype instrument for the detection of OH and HO2 that has a detection sensitivity of about 2 x 10 exp 4 (OH molecules/cc)/(ct/s), about five times greater than proposed. The current minimum detectable OH of 1.4 x 10 exp 5 OH molecules/cc (S/N=2; 30 second integration) is less than half that proposed, and will be much lower once scattered light levels are reduced. The instrument displays other important properties. First, interfering signals from other gases and OH losses on the inlet appear to be small under laboratory and field conditions. Second, three different calibration methods, two external to the inlet, give similar results, to within 30 percent. Third, the chemical conversion efficiency of HO2 to OH by addition of reagent NO is better than 90 percent. All of these factors give us great confidence that this technique works. The instrument has gone through several variations that have not affected its potential performance but have affected its use. We were able to undergo the first field trials away from State College in June 1992, when we were able to participate informally in the ROSE experiment in rural Alabama, a year ahead of the proposed schedule for field studies. Because this field experiment was our first and the data analysis was complicated by instrument instabilities, we have not yet released the observations. None-the-less, we have gained insight into instrument design. We have not yet completed all of the proposed work. The calibration systems need to be improved to reduce the 50 percent to 100 percent uncertainties to less than 30 percent. Interference signals from ambient gases and possible losses on the inlet under field conditions need to be quantified. Finally, the detection of NO2 and NO with laser induced fluorescence is only now being seriously pursued.