NEW INSIGHTS ON OH - MEASUREMENTS AROUND AND IN CLOUDS

The first airborne measurements of OH performed in the lower free trop osphere are presented. Measurements performed above clouds yielded lar ge [OH], with values of 8-15 x 10(6) molecule cm(-3), compared with va lues of 3-5 x 10(6) molecule cm(-3) measured in cloud free regions. Me asurements performed inside a cloud also yielded relatively high [OH], with values of 7.5 x 10(6) molecule cm(-3), yet a factor of 2 lower t han 15 x 10(6) molecule cm(-3) measured just above the cloud. Steady s tate model calculations using Eppley scaled photolysis rates underesti mate [OH] above clouds and overestimate [OH] inside clouds. Actinic fl uxes of 2.5 times clear sky values (obtained from theoretical calculat ions) bring model values into agreement with those measured above clou ds, while OH loss onto cloud droplets needs to be included for calcula tions inside clouds.