New insights on OH: Measurements around and in clouds

The first airborne measurements of OH performed in the lower free troposphere are presented. Measurements performed above clouds yielded large [OH], with values of 8-15 x 10(6) molecule cm(-3), compared with values of 3-5 x 10(6) molecule cm(-3) measured in cloud free regions. Measurements 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 than 15 x 10(6) molecule cm(-3) measured just above the cloud. Steady state model calculations using Eppley scaled photolysis rates underestimate [OH] above clouds and overestimate [OH] inside clouds. Actinic fluxes of 2.5 times clear sky values (obtained from theoretical calculations) bring model values into agreement with those measured above clouds, while OH loss onto cloud droplets needs to be included for calculations inside clouds.