Heterogeneous reactivity of NO and HNO3 on mineral dust in the presence of ozone

The co-adsorption and reaction of NO and O-3 on authentic Saharan dust surfaces was investigated in a Knudsen reactor at approximate to296 K. NO was consumed on the dust surface in the presence of O3 and thereby partially converted to NO2, which was released to the gas-phase. An empirical expression for the dependence of NO2 production, P-NO2, on the O-3 and NO concentrations and dust surface area could be formulated, allowing the importance of this process in the atmosphere to be assessed: P-NO2 = C-a' A(dust) b(O3)[O-3](g) b(NO)[NO](g)/1 + b(O3)[O-3](g) 1 + b(NO)[NO](g), where A(dust) is the dust surface area per cubic centimetre of air, C-a' = 1.4 x 10(9) s(-1) cm(-2), b(O3) = 4.0 x 10(-12) cm(3), and b(NO) = 5.4 x 10(-13) cm(3). Even at high tropospheric dust concentrations, this production rate is too low to significantly supplement that from NO oxidation by O-3 and HO2. Experiments to examine chemical ageing of dust by ozone showed that HNO3 uptake was not influenced by the presence of 100 ppb O-3 (STP), and HNO3 uptake coefficients were similar to those previously derived.