PRESSURE AND TEMPERATURE-DEPENDENCE OF THE GAS-PHASE REACTION OF SO3 WITH H2O AND THE HETEROGENEOUS REACTION OF SO3 WITH H2O H2SO4 SURFACES/

The gas-phase reaction of SO3 with H2O and the heterogeneous reaction of SO3 with H2O-H2SO4 surfaces have been studied in a fast flow reacto r coupled to a chemical ionization mass spectrometer (CIMS) for specie s detection. The gas-phase reaction was studied under turbulent flow c onditions over the pressure range from 100 to 760 Torr N-2 and the tem perature range from 283 to 370 K. The loss rate of SO3 was measured un der pseudo-first-order conditions; it exhibits a second-order dependen ce on water vapor concentration and has a strong negative temperature dependence. The first-order rate coefficient for the SO3 loss by gas-p hase reaction shows no significant pressure dependence and can be expr essed as k(1)(s(-1)) = 3.90 x 10(-41) exp(6830.6/T)[H2O](2) where [H2O ] is in units of molecule cm(-3) and T is in Kelvin. The overall uncer tainty of our experimentally determined rate coefficients is estimated to be +/-20%. At sufficiently low SO3 concentrations (<10(12) molecul e cm(-3)) the rate coefficient is independent of the initial SO3 level , as expected for a gas-phase reaction mechanism involving one SO3 and two H2O molecules. However, at higher concentrations and lower temper atures, increased rate coefficients were observed, indicating a fast h eterogeneous reaction after the onset of binary homogeneous nucleation of acid hydrate clusters leading to particle formation, which was ver ified by light-scattering experiments. The heterogeneous loss of SO3 t o the reactor walls has also been investigated under low pressure (1.1 -12.5 Torr) laminar flow conditions. The loss rate is highly dependent on the humidity of the surface. In the presence of excess water the r eactive sticking coefficient approaches unity and the wall loss rate i s gas diffusion limited; under dry conditions it approaches zero, as e xpected. The atmospheric implications of the homogeneous and heterogen eous SO3-water reaction are discussed.