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

Abstract

In situ measurements of formaldehyde (CH 2 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 2 O by mesoscale convective systems (MCS) in the tropical upper troposphere (UT), (2) to evaluate the scavenging efficiency (SE) of CH 2 O by MCS and (3) to quantify the impact of CH 2 O on UT photooxidant production downwind of MCS. The intercomparison of CH 2 O measurements is first tested, providing a unique and consistent 3‐D‐spatially resolved CH 2 O database in background and convective conditions. While carbon monoxide (CO) is vertically uplifted by deep convection up to 12 km, CH 2 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 2 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 2 O is due to its photochemical production rather than to its direct transport. Model results indicate that CH 2 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 2 O, ozone and HOx.