Airborne observations of dust aerosol over the North Atlantic Ocean duringACE 2: Indications for heterogeneous ozone destruction

Aerosol size distribution measurements have been performed in the free trop osphere during the Second Aerosol Characterization Experiment (ACE 2) near Tenerife, Canary Islands, in July 1997. During one measurement flight, on J uly 8, a uniform aerosol layer was encountered between 2.5 and 5.5 km altit ude, characterized by a relatively low Aitken mode particle number concentr ation and high concentrations of accumulation and coarse mode particles? re sulting in a relatively large aerosol surface area and mass, which is estim ated to be about 400 mu g m(-3). Five-day backward trajectories indicate th at the aerosol in this layer was mineral dust originating from arid regions on the North African continent. The dust layer was associated with reduced ozone mixing ratios. Model simulations have been performed with a photoche mical box model including different heterogeneous removal reactions to stud y the interaction between gas phase chemistry and mineral aerosol. The best agreement between the observed and modeled ozone concentrations was obtain ed when heterogeneous removal of ozone and precursor gases on dust aerosol were taken into account. Heterogeneous O-3 loss is estimated at 4 ppbv O-3 per day. Although uncertainties concerning heterogeneous ozone removal rema in, in particular related to the reactive uptake coefficient of O-3, it is likely that the loss of O-3 and precursor gases on mineral dust aerosol sig nificantly reduces the O-3 abundance in large-scale dust plumes.