Laboratory studies of H2SO4/H2O binary homogeneous nucleation from the SO2+OH reaction: evaluation of the experimental setup and preliminary results

摘要

Binary homogeneous nucleation (BHN) of sulphuric acid and water(HSO/HO) is one of the most important atmosphericnucleation processes, but laboratory observations of this nucleation processare very limited and there are also large discrepancies between differentlaboratory studies. The difficulties associated with these experimentsinclude wall loss of HSO and uncertainties in estimation ofHSO concentration ([HSO]) involved in nucleation.We have developed a new laboratory nucleation setup to studyHSO/HO BHN kinetics and provide relatively constrained[HSO] needed for nucleation. HSO is produced fromthe SO+OH→HSO reaction and OH radicals are producedfrom water vapor UV absorption. The residual [HSO] were measuredat the end of the nucleation reactor with a chemical ionization massspectrometer (CIMS). Wall loss factors (WLFs) of HSO wereestimated by assuming that wall loss is diffusion limited and thesecalculated WLFs were in good agreement with simultaneous measurements of theinitial and residual [HSO] with two CIMSs. The nucleation zonewas estimated from numerical simulations based on the measured aerosol sizes(particle diameter, ) and [HSO]. The measured BHN rates() ranged from 0.01–220 cm s at the initial and residual[HSO] from 10−10 cm, a temperature of 288 Kand relative humidity (RH) from 11–23%; increased with increasing[HSO] and RH. also showed a power dependence on[HSO] with the exponential power of 3–8. These power dependencesare consistent with other laboratory studies under similar [HSO]and RH, but different from atmospheric field observations which showed thatparticle number concentrations are often linearly dependent on[HSO]. These results, together with a higher [HSO]threshold (10–10 cm) needed to produce the unit measuredfrom the laboratory studies compared to the atmospheric conditions(10–10 cm), imply that HSO/HO BHNalone is insufficient to explain atmospheric aerosol formation and growth.Particle growth rates estimated from the measured aerosol sizedistributions, residence times (), and [HSO] were100–500 nm h, much higher than those seen from atmospheric fieldobservations, because of the higher [HSO] used in our study.