IN-SITU MEASUREMENTS OF THE NO2 NO RATIO FOR TESTING ATMOSPHERIC PHOTOCHEMICAL MODELS/

Simultaneous in situ measurements of NO2, NO, O3, ClO, pressure and te mperature have been made for the first time, presenting a unique oppor tunity to test our current understanding of the photochemistry of the lower stratosphere. Data were collected from several flights of the ER -2 aircraft at mid-latitudes in May 1993 during NASA's Stratospheric P hotochemistry, Aerosols and Dynamics Expedition (SPADE). The daytime r atio of NO2/NO remains fairly constant at 19 km with a typical value o f 0.68 and standard deviation of +/- 0.17. The ratio observations are compared with simple steady-state calculations based on laboratory-mea sured reaction rates and modeled NO2 photolysis rates. At each measure ment point the daytime NO2/NO with its measurement uncertainty overlap the results of steady-state calculations and associated uncertainty. However, over all the ER-2 flights examined, the model systematically overestimates the ratio by 40% on average. Possible sources of error a re examined in both model and measurements. It is shown that more accu rate laboratory determinations of the NO + O3 reaction rate and of the NO2 cross-sections in the 200-220 K temperature range characteristic of the lower stratosphere would allow for a more robust test of our kn owledge of NOx photochemistry by reducing significant sources of uncer tainties in the interpretation of stratospheric measurements. The pres ent measurements are compared with earlier observations of the ratio a t higher altitudes.