Ozone transport by mesoscale convective storms in western Senegal

During the rainy period of August and September 2006, a study was conducted to investigate the prevailing diurnal patterns of ozone in rural western Senegal. Ground-level ozone was continuously measured with gas analyzers. During selected days, ozone and atmospheric thermodynamic measurements were also made in the troposphere. Ground-level ozone exhibited diurnal variations in response to the local photochemical processes and vertical transport. Maximum ground-level ozone mixing ratios reached 30 parts per billion on a volume basis (ppbv) during 1200–1600 h, local time. Deviations from the expected diurnal ozone patterns were observed during most rainy days. The observed deviations were manifested as ozone increases which were associated with downdrafts of mesoscale convective storms. The magnitude of the ozone increases depended on the strength of convective velocities which exceeded 10 m s−1. Downdrafts increased the ground-level ozone by 10–30 ppbv. Based on field observation in Senegal and elsewhere, a conceptual model is presented to explain the influences of convective storms on the redistribution of ozone in the continental troposphere.