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

Abstract

[1] 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.

Aerosol transport and wet scavenging in deep convective clouds: A case study and model evaluation using a multiple passive tracer analysis approach

“…They found the anvil air to be composed of about 64% PBL and 36% free tropospheric air. Similarly, Borbon et al [2012] found 60% PBL and 40% free troposphere entrainment contributions to UT outflow air. Our estimated contributions from the surface to 3 km layer (47% for observations and 35% for model) are somewhat smaller than the PBL contribution estimates from these two other studies, while our estimated contributions from the surface to 7 km layers (77% for observations and 58% for model) are somewhat higher.…”

“…Although our four-layer transport analysis is based on an entraining plume framework, equation (7) can be viewed as a four-layer extension of the simple two-layer mixture model [Cohan et al, 1999] and the more recent three-layer mixture model [Borbon et al, 2012] that arbitrarily adds a term to the two-layer mixture model to account for entrainment from the midlevel free troposphere. In our application of the two-layer method, we chose to set the low-level inflow and UT-environment mixing ratios equal to the layer 1 and layer 4 mixing ratios used with our four-layer analysis framework, respectively, although they could be defined somewhat differently.…”

“…Aerosol and trace gas wet scavenging and transport have been commonly inferred by comparing aerosol and gas mixing ratios in anvil outflow and lower troposphere inflow air [e.g., Barth et al, 2007;Bertram et al, 2007] (for trace gases). The commonly used two-component approach estimates the efficiency of trace gas scavenging based on observations by assuming that the mixing ratio of an insoluble trace gas in the fresh convective outflow results from a mixture of the boundary layer and upper troposphere background mixing ratios [e.g., Cohan et al, 1999;Borbon et al, 2012]. Using this observation-based approach, Borbon et al [2012] found that entrainment of free tropospheric air contributed 40% of the convective upper tropospheric air for the studied mesoscale convective system cases.…”

“…The commonly used two-component approach estimates the efficiency of trace gas scavenging based on observations by assuming that the mixing ratio of an insoluble trace gas in the fresh convective outflow results from a mixture of the boundary layer and upper troposphere background mixing ratios [e.g., Cohan et al, 1999;Borbon et al, 2012]. Using this observation-based approach, Borbon et al [2012] found that entrainment of free tropospheric air contributed 40% of the convective upper tropospheric air for the studied mesoscale convective system cases. The scavenging ratio is another observation-based approach used to characterize or predict precipitation chemistry and wet deposition [Barrie, 1992;Hicks, 2005].…”

The formaldehyde budget as seen by a global-scale multi-constraint and multi-species inversion system

“…Considering that OMI data integrate the atmospheric column of HCHO, comparison with aircraft observations are of particular interest. The airborne HCHO measurements made during INTEX-B (Intercontinental Chemical Transport Experiment B) (Singh et al, 2009;Fried et al, 2011) Multidisciplinary Analysis) (Reeves et al, 2010;Borbon et al, 2012) . We compute the ratio of the posterior to the prior bias between modeled and observed concentrations.…”