Abstract. The heterogeneous reaction between O 3 and authentic Saharan dust surfaces was investigated in a Knudsen reactor at ≈ 296 K. O 3 was destroyed on the dust surface and O 2 was formed with conversion efficiencies of 1.0 and 1.3 molecules O 2 per O 3 molecule destroyed for unheated and heated samples, respectively. No O 3 desorbed from exposed dust samples, showing that the uptake was irreversible. The uptake coefficients for the irreversible destruction of O 3 on (unheated) Saharan dust surfaces depended on the O 3 concentration and varied between 3.5 × 10 −4 and 5.5 × 10 −6 for the initial uptake coefficient (γ 0 ≈ 3 × 10 −5 at 30 ppbv O 3 STP) and between 4.8 × 10 −5 and 2.2 × 10 −6 for the steady-state uptake coefficient (γ ss ≈ 7 × 10 −6 at 30 ppbv O 3 STP). At very high O 3 concentrations the surface was deactivated, and O 3 uptake ceased after a certain exposure period. Sample re-activation (i.e. de-passivation) was found to occur over periods of hours, after exposure to O 3 had ceased, suggesting that re-activation processes play a role both in the laboratory and in the atmosphere.

The heterogeneous reaction between HNO 3 and various authentic and synthetic mineral dust/mineral oxide surfaces has been investigated using a low-pressure Knudsen reactor operating at 298 K. The surfaces used were Saharan dust from Cape Verde, Arizona dust, CaCO 3 , and Al 2 O 3 . In all cases, a large irreversible uptake was observed. An uptake coefficient of γ ) (11 ( 3) × 10 -2 was determined for Saharan dust, and γ ) (6 ( 1.5) × 10 -2 was obtained for Arizona dust. The uptake coefficients for HNO 3 on heated CaCO 3 and on unheated CaCO 3 are given by γ ) (10 ( 2.5) × 10 -2 and (18 ( 4.5) × 10 -2 , respectively, and are in good agreement with previous results. CO 2 and H 2 O were formed as gas-phase products. Measurements of the uptake coefficient of HNO 3 on grain-size selected samples of Al 2 O 3 , γ ) (13 ( 3.3) × 10 -2 , and systematic variation of sample mass enabled us to show that the geometrical surface area of the dust sample is appropriate for calculation of uptake coefficients in these experiments. The high reactivity of HNO 3 toward dust samples highlights the potentially important role of mineral dust in redistributing nitrate from the gaseous to the particulate phase and modifying tropospheric photochemical oxidation cycles.

Abstract. The interaction of mineral dust with N 2 O 5 was investigated using both airborne mineral aerosol (using an aerosol flow reactor with variable relative humidity) and bulk samples (using a Knudsen reactor at zero humidity). Both authentic (Saharan, SDCV) and synthetic dust samples (Arizona test dust, ATD and calcite, CaCO 3 ) were used to derive reactive uptake coefficients (γ ). The aerosol experiments (Saharan dust only) indicated efficient uptake, with e.g. a value of γ (SDCV)=(1.3±0.2)×10 −2 obtained at zero relative humidity. The values of γ obtained for bulk substrates in the Knudsen reactor studies are upper limits due to assumptions of available surface area, but were in reasonable agreement with the AFT measurements, with: γ (SDCV)=(3.7±1.2)×10 −2 , γ (ATD)=(2.2±0.8)×10 −2 and γ (CaCO 3 )=(5±2)×10 −2 . The errors quoted are statistical only. The results are compared to literature values and assessed in terms of their impact on atmospheric N 2 O 5 .

The heterogeneous reaction between and various authentic dust, mineral and clay mineral surfaces has HNO 3 been investigated by means of the Knudsen reactor technique at B296 K. Authentic dust surfaces from both the Saharan and Chinese dust regions were investigated, as were the minerals and clay minerals : kaolinite, ripidolite, illite, illite/smectite, Ca-montmorillonite, Na-montmorillonite, palygorskite, dolomite and orthoclase. A modiÐed Knudsen reactor design and concentrations as low as 109 cm~3 helped HNO 3 eliminate experimental artefacts. In all cases a large and irreversible uptake was observed, with uptake coefficients ranging from 8.1 ] 10~2 (orthoclase) to 19.6 ] 10~2 (palygorskite). These results strongly suggest that mineral dust can modify tropospheric photochemical cycles involving and NOx NO y .