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Abstract. While the role of highly oxygenated molecules (HOMs) in new particleformation (NPF) and secondary organic aerosol (SOA) formation is not indispute, the interplay between HOM chemistry and atmospheric conditionscontinues to draw significant research attention. During the Influence ofBiosphere-Atmosphere Interactions on the Reactive Nitrogen budget (IBAIRN)campaign in September 2016, profile measurements of neutral HOMs below andabove the forest canopy were performed for the first time at the borealforest SMEAR II station. The HOM concentrations and composition distributionsbelow and above the canopy were similar during daytime, supporting awell-mixed boundary layer approximation. However, much lower nighttime HOMconcentrations were frequently observed at ground level, which was likely dueto the formation of a shallow decoupled layer below the canopy. Near theground HOMs were influenced by the changes in the precursors and oxidants andenhancement of the loss on surfaces in this layer, while the HOMs above thecanopy top were not significantly affected. Our findings clearly illustratethat near-ground HOM measurements conducted under stably stratifiedconditions at this site might only be representative of a small fraction ofthe entire nocturnal boundary layer. This could, in turn, influence thegrowth of newly formed particles and SOA formation below the canopy where thelarge majority of measurements are typically conducted. 

Abstract. Nucleation of atmospheric vapours produces more than half of global cloudcondensation nuclei and so has an important influence on climate. Recentstudies show that monoterpene (C10H16) oxidation yieldshighly oxygenated products that can nucleate with or without sulfuric acid.Monoterpenes are emitted mainly by trees, frequently together with isoprene(C5H8), which has the highest global emission of all organicvapours. Previous studies have shown that isoprene suppresses new-particleformation from monoterpenes, but the cause of this suppression is underdebate. Here, in experiments performed under atmospheric conditions in theCERN CLOUD chamber, we show that isoprene reduces the yield ofhighly oxygenated dimers with 19 or 20 carbon atoms – which drive particlenucleation and early growth – while increasing the production of dimers with14 or 15 carbon atoms. The dimers (termed C20 and C15,respectively) are produced by termination reactions between pairs of peroxyradicals (RO2⚫) arising from monoterpenes or isoprene.Compared with pure monoterpene conditions, isoprene reduces nucleation ratesat 1.7 nm (depending on the isoprene ∕ monoterpene ratio) and approximatelyhalves particle growth rates between 1.3 and 3.2 nm. However, above 3.2 nm,C15 dimers contribute to secondary organic aerosol, and the growth ratesare unaffected by isoprene. We further show that increased hydroxyl radical(OH⚫) reduces particle formation in our chemical system ratherthan enhances it as previously proposed, since it increases isoprene-derivedRO2⚫ radicals that reduce C20 formation.RO2⚫ termination emerges as the critical step that determinesthe highly oxygenated organic molecule (HOM) distribution and the corresponding nucleation capability. Speciesthat reduce the C20 yield, such as NO, HO2 and as we showisoprene, can thus effectively reduce biogenic nucleation and early growth.Therefore the formation rate of organic aerosol in a particular region ofthe atmosphere under study will vary according to the precise ambientconditions. 

A major fraction of atmospheric aerosol particles, which affect both air quality and climate, form from gaseous precursors in the atmosphere. Highly oxygenated organic molecules (HOMs), formed by oxidation of biogenic volatile organic compounds, are known to participate in particle formation and growth. However, it is not well understood how they interact with atmospheric pollutants, such as nitrogen oxides (NO x ) and sulfur oxides (SO x ) from fossil fuel combustion, as well as ammonia (NH 3 ) from livestock and fertilizers. Here, we show how NO x suppresses particle formation, while HOMs, sulfuric acid, and NH 3 have a synergistic enhancing effect on particle formation. We postulate a novel mechanism, involving HOMs, sulfuric acid, and ammonia, which is able to closely reproduce observations of particle formation and growth in daytime boreal forest and similar environments. The findings elucidate the complex interactions between biogenic and anthropogenic vapors in the atmospheric aerosol system.