This paper presents measurements of organic aerosols above subtropical WestAfrica during the wet season using data from the UK Facility for AirborneAtmospheric Measurements (FAAM) aircraft. Measurements of biogenic volatileorganic compounds (BVOC) at low altitudes over these subtropical forestswere made during the African Monsoon Multidisciplinary Analysis (AMMA) fieldexperiment during July and August 2006 mainly above Benin, Nigeria andNiger. Data from an Aerodyne Quadrupole Aerosol Mass Spectrometer show amedian organic aerosol loading of 1.07 μg m over tropical WestAfrica, which represents the first regionally averaged assessment of organicaerosol mass (OM) in this region during the wet season. This is broadly inagreement with global model predictions based on partitioning schemes,although there are large uncertainties associated with such estimates. Incontrast our own calculations based on aerosol yields from isoprene andmonoterpenes during chamber studies under represent the OM measured in thisregion on a comparable scale to the under representations of OM bypredictive models in the mid latitudes. As global models rely on similaryield calculations in their global estimates, as our calculations this pointsto further systematic differences between global model estimates andmeasurements of SOA, most likely caused by use of incorrect BVOC emissionrates. The under predictions of OM by our calculations and those in the midlatitudes employ yields extrapolated from chamber data obtained at highermass concentrations – more recent yield data for α-pinene obtainedat ambient concentrations in a flow through chamber(Shilling et al., 2008) show considerably betteragreement with our data.
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