Observations and model calculations of trace gas scavenging in a dense Saharan dust plume during MINATROC

Abstract. An intensive field measurement campaign was performed in July/August 2002 at the Global Atmospheric Watch station Izaña on Tenerife to study the interaction of mineral dust aerosol and tropospheric chemistry (MINATROC). A dense Saharan dust plume, with aerosol masses exceeding 500 µg m^-3, persisted for three days. During this dust event strongly reduced mixing ratios of RO_x (HO₂, CH₃O₂ and higher organic peroxy radicals), H₂O₂, NO_x (NO and NO₂) and O₃ were observed. A chemistry boxmodel, constrained by the measurements, has been used to study gas phase and heterogeneous chemistry. It appeared to be difficult to reproduce the observed HCHO mixing ratios with the model, possibly related to the representation of precursor gas concentrations or the absence of dry deposition. The model calculations indicate that the reduced H₂O₂ mixing ratios in the dust plume can be explained by including the heterogeneous removal reaction of HO₂ with an uptake coefficient of 0.2, or by assuming heterogeneous removal of H₂O₂ with an accommodation coefficient of 5x10^-4. However, these heterogeneous reactions cannot explain the low RO_x mixing ratios observed during the dust event. Whereas a mean daytime net ozone production rate (NOP) of 1.06 ppb_v/hr occurred throughout the campaign, the reduced RO_x and NO_x mixing ratios in the Saharan dust plume contributed to a reduced NOP of 0.14-0.33 ppb_v/hr, which likely explains the relatively low ozone mixing ratios observed during this event.