Laboratory Studies of H2So4/H2O Binary Homogeneous Nucleation from the So2+Oh Reaction: Evaluation of the Experimental Setup and Preliminary Results01/01/2008
Binary homogeneous nucleation (BHN) of sulphuric acid and water (H2SO4/H2O) is one of the most important atmospheric nucleation processes, but laboratory observations of this nucleation process are very limited and there are also large discrepancies between different laboratory studies. The difficulties associated with these experiments include wall loss of H2SO4 and uncertainties in estimation of H2SO4 concentration ([H2SO4]) involved in nucleation. We have developed a new laboratory nucleation setup to study H2SO4/H2O BHN kinetics and provide relatively constrained [H2SO4] needed for nucleation. H2SO4 is produced from the SO2+OH→HSO3 reaction and OH radicals are produced from water vapor UV absorption. The residual [H2SO4] were measured at the end of the nucleation reactor with a chemical ionization mass spectrometer (CIMS). Wall loss factors (WLFs) of H2SO4 were estimated by assuming that wall loss is diffusion limited and these calculated WLFs were in good agreement with simultaneous measurements of the initial and residual [H2SO4] with two CIMSs. The nucleation zone was estimated from numerical simulations based on the measured aerosol sizes (particle diameter, Dp) and [H2SO4]. The measured BHN rates (J) ranged from 0.01–220 cm−3 s−1at the initial and residual [H2SO4] from 108−1010 cm−3, a temperature of 288 K and relative humidity (RH) from 11–23%; J increased with increasing [H2SO4] and RH. J also showed a power dependence on [H2SO4] with the exponential power of 3–8. These power dependences are consistent with other laboratory studies under similar [H2SO4] and RH, but different from atmospheric field observations which showed that particle number concentrations are often linearly dependent on [H2SO4]. These results, together with a higher [H2SO4] threshold (108–109 cm−3) needed to produce the unit Jmeasured from the laboratory studies compared to the atmospheric conditions (106–107 cm−3), imply that H2SO4/H2O BHN alone is insufficient to explain atmospheric aerosol formation and growth. Particle growth rates estimated from the measured aerosol size distributions, residence times (tr), and [H2SO4] were 100–500 nm h−1, much higher than those seen from atmospheric field observations, because of the higher [H2SO4] used in our study.