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In (b) and (c), triangles represent α-pinene only and circles represent α-pinene + isoprene conditions. The overall systematic scale uncertainty of HOMs of +78 % and −68 % and of J for ±47 % is not shown. C data for α-pinene only and α-pinene + isoprene systems.Thick solid black and red lines represent power-law fits to +25 Red solid and dashed-dotted lines are power-law fits to J gcr and J n in the presence of isoprene at +5 Black solid and dashed-dotted lines are parameterisations of J gcr and J n from Kirkby et al. Colour shows isoprene-to-monoterpene carbon ratio (R). C all data points are with UV light on.Magenta edges indicate UV-illuminated conditions at +5

Small grey points were taken from Kirkby et al. (a) Triangles represent J gcr and circles J n. Relative humidity is 38 % for all data points. HOM total is defined as the sum of C 5, C 10, C 15 and C 20 carbon classes. Pure biogenic nucleation rates at 1.7 nm diameter (a) and growth rates (b, c) against total HOM concentration with and without isoprene added at +5 and +25 Therefore the formation rate of organic aerosol in a particular region of the atmosphere under study will vary according to the precise ambient conditions. Species that reduce the C₂₀ yield, such as NO, HO₂ and as we show isoprene, can thus effectively reduce biogenic nucleation and early growth.

termination emerges as the critical step that determines the highly oxygenated organic molecule (HOM) distribution and the corresponding nucleation capability.

) reduces particle formation in our chemical system rather than enhances it as previously proposed, since it increases isoprene-derived RO₂.

We further show that increased hydroxyl radical (OH However, above 3.2 nm, C₁₅ dimers contribute to secondary organic aerosol, and the growth rates are unaffected by isoprene. Compared with pure monoterpene conditions, isoprene reduces nucleation rates at 1.7 nm (depending on the isoprene ∕ monoterpene ratio) and approximately halves particle growth rates between 1.3 and 3.2 nm.

) arising from monoterpenes or isoprene.

The dimers (termed C₂₀ and C₁₅, respectively) are produced by termination reactions between pairs of peroxy radicals (RO₂ Here, in experiments performed under atmospheric conditions in the CERN CLOUD chamber, we show that isoprene reduces the yield of highly oxygenated dimers with 19 or 20 carbon atoms – which drive particle nucleation and early growth – while increasing the production of dimers with 14 or 15 carbon atoms. Previous studies have shown that isoprene suppresses new-particle formation from monoterpenes, but the cause of this suppression is under debate. Monoterpenes are emitted mainly by trees, frequently together with isoprene (C₅H₈), which has the highest global emission of all organic vapours. Recent studies show that monoterpene (C₁₀H₁₆) oxidation yields highly oxygenated products that can nucleate with or without sulfuric acid. Nucleation of atmospheric vapours produces more than half of global cloud condensation nuclei and so has an important influence on climate.