Transport and chemistry of formaldehyde by mesoscale convective systems in West Africa during AMMA 2006

Abstract

In situ measurements of formaldehyde (CH O) onboard four European research aircraft in August 2006 as part of the African Monsoon Multidisciplinary Analysis (AMMA) experiment in West Africa are used (1) to examine the redistribution of CH O by mesoscale convective systems (MCS) in the tropical upper troposphere (UT), (2) to evaluate the scavenging efficiency (SE) of CH O by MCS and (3) to quantify the impact of CH O on UT photooxidant production downwind of MCS. The intercomparison of CH O measurements is first tested, providing a unique and consistent 3-D-spatially resolved CH O database in background and convective conditions. While carbon monoxide (CO) is vertically uplifted by deep convection up to 12 km, CH O is also affected by cloud processing as seen from its ratio relative to CO with altitude. A new observation-based model is established to quantify the SE of CH O. This model shows that convective entrainment of free tropospheric air cannot be neglected since it contributes to 40% of the convective UT air. For the 4 studied MCS, SE shows a large variability within a 4% to 39% range at a relative standard deviation of 30%, which is consistent with MCS features. A time-dependent photochemical box model is applied to convective UT air. After convection, 60% of CH O is due to its photochemical production rather than to its direct transport. Model results indicate that CH O directly injected by convection does not impact ozone and HOx production in the tropical UT of West Africa. NOx and anthropogenic hydrocarbon precursors dominate the secondary production of CH O, ozone and HOx.