Figure 3. Mass and molecular composition of charged clusters during a nucleation event with DMA.
Molecular composition of charged clusters measured by the APi-TOF for Jgcr = 1.2 cm−3 s−1, 4.0 × 106 cm−3 [H2SO4], 11 p.p.t.v. NH3, 9.4 p.p.t.v. DMA, 38% RH and 278 K. a, Negative particles. b, Positive particles. Cluster mass/charge, m/z, defect (difference from integer m/z) is plotted against m/z; each circle represents a distinct molecular composition and its area represents counts s−1. The labels (n, m) indicate the number of sulphuric acid (nSA) and DMA (mDMA) molecules in pure clusters of SA and DMA, including both neutral and charged species. The addition of a single SA (H2SO4) or DMA (C2H7N) molecule to any cluster displaces it on the plot by a vector distance indicated by the grey arrows in b. Red circles represent pure SA clusters; green circles are clusters containing SA and DMA alone; black circles contain ammonia in addition (only appearing in some clusters above m/z = 900); other clusters (mostly containing light organic contaminants) are grey circles. Water molecules evaporate rapidly in the APi-TOF and are not detected (see Supplementary Information).