Figure 2. Molecular composition and mass spectra of charged clusters during GCR nucleation events without sulfuric acid.

a, b, Cluster mass defect (difference from integer mass) versus m/z of negatively (a) and positively (b) charged clusters measured with the APi-TOF at 240 p.p.t.v. α-pinene, 34 p.p.b.v. O₃, zero H₂ or HONO, 38% relative humidity, 278 K and [H₂SO₄] below the detection limit (5 × 10⁴ cm⁻³). The values of J_gcr and total HOMs concentration are, respectively, 3.4 cm⁻³ s⁻¹ and 1.7 × 10⁷ cm⁻³ (a), and 3.3 cm⁻³ s⁻¹ and 2.4 × 10⁷ cm⁻³ (b). The mass bands are labelled according to the number of HOM monomer units in the cluster, E_n. Each circle represents a distinct molecular composition and its area represents the counts per second. The most highly oxidized compounds are located at the lower right-hand edge of each band. The dark blue circle represents ions; the light blue circle represents ions. Clusters with fully identified molecular composition are coloured according to their core ion: purple (), green (E⁻) or orange (). Grey circles are unidentified clusters. c, d, Mass spectra from the same events for negative (c) and positive (d) clusters up to m/z = 3,000 Th. A particle of 1.7-nm mobility diameter has a mass of about 1,200 Th. The ‘Nessie’ plot (d) shows that positive-ion-induced nucleation involves HOM dimers alone (E₁. clusters are not seen owing to instrument tuning). The decreasing signal amplitude at larger masses is due to the lower concentration and decreasing detection efficiency of the APi-TOF mass spectrometer (the efficiency versus m/z depends on the instrument tune and polarity).

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