Role of convective transport on tropospheric ozone chemistry revealed by aircraft observations during the wet season of the AMMA campaign

Abstract

Abstract. During the wet season of the African Monsoon Multidisciplinary Analyses (AMMA) campaign, airborne measurements of several chemical species were made onboard the French Falcon-20 (FF20) aircraft. The scientific flights were planned in order to document, on one hand the regional distribution of trace gas species related to the oxidizing capacity of the troposphere, and on the other hand their spatial variability in the outflow of mesoscale convective systems (MCSs). The main objectives of this paper are the analysis of the main transport processes responsible for the observed variability, and the discussion of differences and similarities related to the convective transport by 4 different MCSs. This work is needed before using this data set for future studies of the convective transport of chemical species or for modeling work in the frame of the AMMA project. Regarding the regional distribution, five air masses types have been identified using the Lagrangian particle dispersion model FLEXPART, and by considering relationship between the measured trace gas concentrations (O3, CO, NOx, H2O, and hydroperoxides). This paper specifically discusses the advantage of hydroperoxide measurements in order to document the impact of recent or aged convection. The highest values of O3 are found to be related to transport from the subtropical tropopause region into the mid-troposphere at latitudes as low as 10° N. The lowest ozone values have been always explained by recent uplifting from the monsoon layer where O3 is photochemically destroyed. Regarding the analysis of the MCS outflow, the CO and H2O2 enhancements are related to the age and the southernmost position of the MCS. The analysis of the long range transport of the air masses where convection occurred, shows a connection with the Persian Gulf emissions for the largest CO concentrations in MCS outflow. However for our observations, Lagrangian particle dispersion modelling shows that this possible source is always modified by the convective transport of CO from the African lower troposphere when the air masses encounter a convective system at latitudes below 10° N.