Impact of RACM2, halogen chemistry, and updated ozone deposition velocity on hemispheric ozone predictions

Abstract

We incorporate the Regional Atmospheric Chemistry Mechanism (RACM2) into the Community Multiscale Air Quality (CMAQ) hemispheric model and compare model predictions to those obtained using the existing Carbon Bond chemical mechanism with the updated toluene chemistry (CB05TU). The RACM2 enhances monthly mean ozone by 2–10 ppbv in polluted areas compared to the CB05TU while reducing mean ozone by 2–6 ppbv in remote areas. We develop an effective halogen reaction that can consume ozone over the gulfs and oceans. The current CMAQ model uses substantially lower ozone deposition velocity over water compared to observed data. We modify the CMAQ deposition velocity to account for the enhanced deposition due to chemical interactions between ozone and oceanic iodide. The effective halogen reaction and enhanced deposition velocity reduce monthly mean ozone by 2–8 ppbv over water. The majority of the reduction occurs via the halogen reaction. A comparison of model predictions with available observed profile reveals that the RACM2 over-predicts surface ozone in polluted areas while improving the comparison in remote areas. Model predictions with the halogen chemistry and enhanced deposition velocity compare better with the observed data.