Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 5 6 2005 10.5194/acpd-5-11821-2005 http://www.atmos-chem-phys-discuss.net/5/11821/2005/ http://www.atmos-chem-phys-discuss.net/5/11821/2005/acpd-5-11821-2005.html http://www.atmos-chem-phys-discuss.net/5/11821/2005/acpd-5-11821-2005.pdf 11821 11860 2005-11-18 Effect of humidity on nitric acid uptake to mineral dust aerosol particles A. Vlasenko S. Sjogren E. Weingartner K. Stemmler H. W. Gäggeler M. Ammann markus.ammann@psi.ch Laboratory of Radio- and Environmental chemistry, Paul Scherrer Institute, Villigen, Switzerland Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, Villigen, Switzerland Department of Chemistry and Biochemistry, University of Bern, Bern, Switzerland This study presents the first laboratory observation of HNO3 uptake by airborne mineral dust particles. The model aerosols were generated by dry dispersion of Arizona Test Dust (ATD), SiO2, and by nebulizing a saturated solution of calcium carbonate. The uptake of 13N-labelled gaseous nitric acid was observed in a flow reactor on the 0.2–2 s reaction time scale at room temperature and atmospheric pressure. The amount of reacted nitric acid was found to be a linear function of aerosol surface area. SiO2 particles did not show any significant uptake, while the CaCO3 aerosol was found to be more reactive than the ATD. Due to the smaller uncertainty associated with the reactive surface area in the case of suspended particles as compared to bulk powder samples, we believe that we provide an improved estimate of the uptake kinetics of HNO3 to mineral dust. The uptake coefficient averaged over the first 2 s of reaction time at a concentration of 1012 molecules cm−3 was found to increase with increasing relative humidity, from 0.022±0.007 at 12% RH to 0.113±0.017 at 73% RH , scaling along a water adsorption isotherm. The processing of the dust at 85% RH leads to a water soluble coating on the particles and enhances their hygroscopicity.