Influence of updated gas-phase rate constants on modeled tropospheric OH concentrations

The impact of updated rate constants for several key reactions of troposphe ric gas-phase chemistry, namely the reactions of OH with NO2 (reduced by 30 %) and the reaction of HO2 with NO (increased by 20%) are studied using the gas-phase chemical scheme RADM2. Furthermore, the earlier value of the rat e constant of CH4 with OH used in RADM2 is reduced by 30% due to recent lit erature. The updated rate constants influence in particular the calculated budget of HOx and O-3 For trace gas mixes in a rural environment the averag e calculated OH concentration increases by 20%, while HO2 exhibits only a s mall decrease of 2%. The net formation of ozone is about 12% larger. The lo ng-term impact on the globally averaged calculated OH concentration is esti mated in a simple box model to be 48%, i.e. much larger than the short-term effect due to the feedback on the mixing ratios of long-lived precursors a nd reaction partners. The predicted global methane and ozone mixing ratios are unaffected because of compensating effects of the simultaneous change o f all three rate parameters. (C) 2000 Elsevier Science Ltd. All rights rese rved.