Measurement of prenucleation molecular clusters in the NH3, H2SO4, H2O system -: art. no. 4158

[1] The molecular cluster ions HSO4- (H2SO4)(n-1) (NH3)(m) corresponding to the neutral species (H2SO4)(n) (NH3)(m) for n = 2 to 6 and m =0 to n-1 have been observed at temperatures up to 285 K. A transverse chemical ionization apparatus was located inside a cooled flow tube where water, sulfuric acid, and ammonia vapors mixed and formed clusters. The complexities of the experimental technique and the interpretation of the results are extensively discussed. Typical NH3 and H2SO4 concentrations were 2 x 10(9) cm(-3), i.e, similar to100 pptv at atmospheric pressure. For these conditions, cluster concentrations were estimated to be a few times 10(6) cm(-3) and the critical, particle-forming cluster likely contained 2 H2SO4 molecules at 275 K. The results are consistent with the species (H2SO4)(2)NH3 playing an important role in the formation of new particles in the atmosphere.