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Biogenic vapors form new particles in the atmosphere, affecting global climate. The contributions of monoterpenes and isoprene to new particle formation (NPF) have been extensively studied. However, sesquiterpenes have received little attention despite a potentially important role due to their high molecular weight. Via chamber experiments performed under atmospheric conditions, we report biogenic NPF resulting from the oxidation of pure mixtures of β-caryophyllene, α-pinene, and isoprene, which produces oxygenated compounds over a wide range of volatilities. We find that a class of vapors termed ultralow-volatility organic compounds (ULVOCs) are highly efficient nucleators and quantitatively determine NPF efficiency. When compared with a mixture of isoprene and monoterpene alone, adding only 2% sesquiterpene increases the ULVOC yield and doubles the formation rate. Thus, sesquiterpene emissions need to be included in assessments of global aerosol concentrations in pristine climates where biogenic NPF is expected to be a major source of cloud condensation nuclei.

 

Abstract. New particle formation (NPF) is a significant source of atmosphericparticles, affecting climate and air quality. Understanding the mechanismsinvolved in urban aerosols is important to develop effective mitigationstrategies. However, NPF rates reported in the polluted boundary layer spanmore than 4 orders of magnitude, and the reasons behind this variability are the subject of intense scientific debate. Multiple atmospheric vapours have beenpostulated to participate in NPF, including sulfuric acid, ammonia, aminesand organics, but their relative roles remain unclear. We investigated NPFin the CLOUD chamber using mixtures of anthropogenic vapours that simulatepolluted boundary layer conditions. We demonstrate that NPF in pollutedenvironments is largely driven by the formation of sulfuric acid–baseclusters, stabilized by the presence of amines, high ammonia concentrationsand lower temperatures. Aromatic oxidation products, despite their extremelylow volatility, play a minor role in NPF in the chosen urban environment butcan be important for particle growth and hence for the survival of newlyformed particles. Our measurements quantitatively account for NPF in highlydiverse urban environments and explain its large observed variability. Suchquantitative information obtained under controlled laboratory conditionswill help the interpretation of future ambient observations of NPF rates inpolluted atmospheres. 

Abstract. In the present-day atmosphere, sulfuric acid is the mostimportant vapour for aerosol particle formation and initial growth. However,the growth rates of nanoparticles (<10 nm) from sulfuric acidremain poorly measured. Therefore, the effect of stabilizing bases, thecontribution of ions and the impact of attractive forces on molecularcollisions are under debate. Here, we present precise growth ratemeasurements of uncharged sulfuric acid particles from 1.8 to 10 nm, performedunder atmospheric conditions in the CERN (EuropeanOrganization for Nuclear Research) CLOUD chamber. Our results showthat the evaporation of sulfuric acid particles above 2 nm is negligible,and growth proceeds kinetically even at low ammonia concentrations. Theexperimental growth rates exceed the hard-sphere kinetic limit for thecondensation of sulfuric acid. We demonstrate that this results fromvan der Waals forces between the vapour molecules and particles anddisentangle it from charge–dipole interactions. The magnitude of theenhancement depends on the assumed particle hydration and collisionkinetics but is increasingly important at smaller sizes, resulting in asteep rise in the observed growth rates with decreasing size. Including theexperimental results in a global model, we find that the enhanced growth rate ofsulfuric acid particles increases the predicted particle number concentrationsin the upper free troposphere by more than 50 %. 

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.