^{1}

^{2}

^{2}

^{3}

^{4}

^{5}

^{7}

^{6}

^{1}

We report measurements of the diffusion coefficient of sulfuric acid in humidified air at a range of relative humidities (from ~4 to 70 %), temperatures (278, 288 and 298 K) and initial H<sub>2</sub>SO<sub>4</sub> concentration (from 1 × 10e6 to 1 × 10e8 molec. cm<sup>−3</sup>). The diffusion coefficients were estimated from the sulfuric acid wall loss rate coefficients under laminar flow conditions. The flow conditions were verified with additional fluid dynamics model CFD-FLUENT simulations which also reproduced the loss rate coefficients very well at all three temperatures with the maximum difference of 7 % between the measured and simulated values. The concentration of H<sub>2</sub>SO<sub>4</sub> was measured continuously with chemical ionization mass spectrometer (CIMS) at seven different positions along the flow tube. The wall losses of H<sub>2</sub>SO<sub>4</sub> were determined from the slopes of fits to measured H<sub>2</sub>SO<sub>4</sub> concentrations as a function of the position along the flow tube. The observed wall loss rate coefficients, and hence the diffusion coefficients, were independent of different initial H<sub>2</sub>SO<sub>4</sub> concentrations and different total flow rates. However, the determined diffusion coefficients decreased with increasing relative humidity, as also seen in previous experiments, and had a rather strong power dependence of the diffusion coefficient with respect to temperature, around ∝<i>T</i><sup>5.4</sup>, which is in disagreement with the expected temperature dependency of ~<i>T</i><sup>1.75</sup> observed for other gases and not tested before for sulfuric acid. The effect of relative humidity on the diffusion coefficient is likely due to stronger hydration of H<sub>2</sub>SO<sub>4</sub> molecules and likely also due to the presence of trace impurities such as amines, possibly brought to the system by humidification. Clustering kinetics simulations using quantum chemical data suggest that also the strong temperature dependence of the observed diffusion coefficient might be explained by increased diffusion volume of H<sub>2</sub>SO<sub>4</sub> molecules due to stronger clustering with base-impurities like amines.