Figure 1: Growth rates in different systems.

Growth rates of 2 nm particles determined with the appearance time method between 1.5 and 2.5 nm (ref. 24) as a function of the measured sulphuric acid (H₂SO₄) concentration with different amounts of ammonia (NH₃) and dimethylamine (DMA) in the chamber. In the red data points, ammonia was present only as an impurity (<35 p.p.t.v. for CLOUD3 campaign, <5 p.p.t.v. for CLOUD4 and CLOUD7); for the blue points NH₃ (100–1,400 p.p.t.v.) was added, and for green points DMA (5–70 p.p.t.v.) was added to the chamber. Squares represent experiments during the CLOUD3 campaign at varying temperatures (T=248–293 K) and relative humidities (RH=10–40%), while circles represent CLOUD4 and stars the CLOUD7 campaign, each at T=278 K, RH=38%. Sample size (n) for each system is given in the legend. The red line is the mass flux growth rate calculated from the sulphuric acid monomer concentration (at T=278 K)¹⁰, and the grey shaded area represents the appearance time growth rate determined from cluster population simulations^25,28 assuming zero cluster evaporation rates and hard-sphere collision rates. A factor of 0.5–3 uncertainty in the collision rates (giving the limits of the shaded area) arises from the possibly non-unity sticking factors, uncertainty in the geometric cross-section of the clusters, and possible dipole–dipole enhancements in the collision rates. A collision enhancement factor of 2.7 (green dashed line) gives a good match between the simulated and measured data points in the sulphuric-acid–DMA system (see also Kürten et al.²⁵).