Measurement of prenucleation molecular clusters in the NH₃, H₂SO₄, H₂O system

The molecular cluster ions HSO 4 − (H 2 SO 4 ) n− 1 (NH 3 ) m corresponding to the neutral species (H 2 SO 4 ) n (NH 3 ) 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 NH 3 and H 2 SO 4 concentrations were 2 × 10 9 cm −3 , i.e, ∼100 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 H 2 SO 4 molecules at 275 K. The results are consistent with the species (H 2 SO 4 ) 2 NH 3 playing an important role in the formation of new particles in the atmosphere.