Forests emit large quantities of volatile organic compounds (VOCs) to the atmosphere. Their condensable oxidation products can form secondary organic aerosol, a significant and ubiquitous component of atmospheric aerosol, which is known to affect the Earth's radiation balance by scattering solar radiation and by acting as cloud condensation nuclei. The quantitative assessment of such climate effects remains hampered by a number of factors, including an incomplete understanding of how biogenic VOCs contribute to the formation of atmospheric secondary organic aerosol. The growth of newly formed particles from sizes of less than three nanometres up to the sizes of cloud condensation nuclei (about one hundred nanometres) in many continental ecosystems requires abundant, essentially non-volatile organic vapours, but the sources and compositions of such vapours remain unknown. Here we investigate the oxidation of VOCs, in particular the terpene α-pinene, under atmospherically relevant conditions in chamber experiments. We find that a direct pathway leads from several biogenic VOCs, such as monoterpenes, to the formation of large amounts of extremely low-volatility vapours. These vapours form at significant mass yield in the gas phase and condense irreversibly onto aerosol surfaces to produce secondary organic aerosol, helping to explain the discrepancy between the observed atmospheric burden of secondary organic aerosol and that reported by many model studies. We further demonstrate how these low-volatility vapours can enhance, or even dominate, the formation and growth of aerosol particles over forested regions, providing a missing link between biogenic VOCs and their conversion to aerosol particles. Our findings could help to improve assessments of biosphere-aerosol-climate feedback mechanisms, and the air quality and climate effects of biogenic emissions generally.

Nucleation of aerosol particles from trace atmospheric vapours is thought to provide up to half of global cloud condensation nuclei 1 . Aerosols can cause a net cooling of climate by scattering sunlight and by leading to smaller but more numerous cloud droplets, which makes clouds brighter and extends their lifetimes 2 . Atmospheric aerosols derived from human activities are thought to have compensated for a large fraction of the warming caused by greenhouse gases 2 . However, despite its importance for climate, atmospheric nucleation is poorly understood. Recently, it has been shown that sulphuric acid and ammonia cannot explain particle formation rates observed in the lower atmosphere 3 . It is thought that amines may enhance nucleation 4-16 , but until now there has been no direct evidence for amine ternary nucleation under atmospheric conditions. Here we use the CLOUD (Cosmics Leaving OUtdoor Droplets) chamber at CERN and find that dimethylamine above three parts per trillion by volume can enhance particle formation rates more than 1,000-fold compared with ammonia, sufficient to account for the particle formation rates observed in the atmosphere. Molecular analysis of the clusters reveals that the faster nucleation is explained by a base-stabilization mechanism involving acid-amine pairs, which strongly decrease evaporation. The ion-induced contribution is generally small, reflecting the high stability of sulphuric acid-dimethylamine clusters and indicating that galactic cosmic rays exert only a small influence on their formation, except at low overall formation rates. Our experimental measurements are well reproduced by a dynamical model based on quantum chemical calculations of binding energies of molecular clusters, without any fitted parameters. These results show that, in regions of the atmosphere near amine sources, both amines and sulphur dioxide should be considered when assessing the impact of anthropogenic activities on particle formation.The primary vapour responsible for atmospheric nucleation is thought to be sulphuric acid (H 2 SO 4 ), derived from the oxidation of sulphur dioxide. However, peak daytime H 2 SO 4 concentrations in the atmospheric boundary layer are about 10 6 to 3 3 10 7 cm 23 (0.04-1.2 parts per trillion by volume (p.p.t.v.)), which results in negligible binary homogeneous nucleation of H 2 SO 4 -H 2 O (ref. 3). Additional species such as ammonia or amines 4,5 are therefore necessary to stabilize the embryonic clusters and decrease evaporation. However, ammonia cannot account for particle formation rates observed in the boundary layer 3 and, despite numerous field and laboratory studies [6][7][8][9][10][11][12][13][14][15][16] , amine ternary nucleation has not yet been observed under atmospheric conditions. Amine emissions are dominated by anthropogenic activities (mainly animal husbandry), but about 30% of emissions are thought to arise from the breakdown of organic matter in the oceans, and 20% from biomass burning and soil 8,17 . Atmospheric measurements of gasphase amines ...

Atmospheric nucleation is the dominant source of aerosol particles in the global atmosphere and an important player in aerosol climatic effects. The key steps of this process occur in the sub-2-nanometer (nm) size range, in which direct size-segregated observations have not been possible until very recently. Here, we present detailed observations of atmospheric nanoparticles and clusters down to 1-nm mobility diameter. We identified three separate size regimes below 2-nm diameter that build up a physically, chemically, and dynamically consistent framework on atmospheric nucleation--more specifically, aerosol formation via neutral pathways. Our findings emphasize the important role of organic compounds in atmospheric aerosol formation, subsequent aerosol growth, radiative forcing and associated feedbacks between biogenic emissions, clouds, and climate.

Highly oxygenated organic molecules (HOM) are formed in the atmosphere via autoxidation involving peroxy radicals arising from volatile organic compounds (VOC). HOM condense on pre-existing particles and can be involved in new particle formation. HOM thus contribute to the formation of secondary organic aerosol (SOA), a significant and ubiquitous component of atmospheric aerosol known to affect the Earth’s radiation balance. HOM were discovered only very recently, but the interest in these compounds has grown rapidly. In this Review, we define HOM and describe the currently available techniques for their identification/quantification, followed by a summary of the current knowledge on their formation mechanisms and physicochemical properties. A main aim is to provide a common frame for the currently quite fragmented literature on HOM studies. Finally, we highlight the existing gaps in our understanding and suggest directions for future HOM research.

A high-resolution time-of-flight chemical-ionization mass spectrometer (HR-ToF-CIMS) using Iodide-adducts has been characterized and deployed in several laboratory and field studies to measure a suite of organic and inorganic atmospheric species. The large negative mass defect of Iodide, combined with soft ionization and the high mass-accuracy (<20 ppm) and mass-resolving power (R>5500) of the time-of-flight mass spectrometer, provides an additional degree of separation and allows for the determination of elemental compositions for the vast majority of detected ions. Laboratory characterization reveals Iodide-adduct ionization generally exhibits increasing sensitivity toward more polar or acidic volatile organic compounds. Simultaneous retrieval of a wide range of mass-to-charge ratios (m/Q from 25 to 625 Th) at a high frequency (>1 Hz) provides a comprehensive view of atmospheric oxidative chemistry, particularly when sampling rapidly evolving plumes from fast moving platforms like an aircraft. We present the sampling protocol, detection limits and observations from the first aircraft deployment for an instrument of this type, which took place aboard the NOAA WP-3D aircraft during the Southeast Nexus (SENEX) 2013 field campaign.

The prompt formation of highly oxidized organic compounds in the ozonolysis of cyclohexene (C6H10) was investigated by means of laboratory experiments together with quantum chemical calculations. The experiments were performed in borosilicate glass flow tube reactors coupled to a chemical ionization atmospheric pressure interface time-of-flight mass spectrometer with a nitrate ion (NO3(-))-based ionization scheme. Quantum chemical calculations were performed at the CCSD(T)-F12a/VDZ-F12//ωB97XD/aug-cc-pVTZ level, with kinetic modeling using multiconformer transition state theory, including Eckart tunneling corrections. The complementary investigation methods gave a consistent picture of a formation mechanism advancing by peroxy radical (RO2) isomerization through intramolecular hydrogen shift reactions, followed by sequential O2 addition steps, that is, RO2 autoxidation, on a time scale of seconds. Dimerization of the peroxy radicals by recombination and cross-combination reactions is in competition with the formation of highly oxidized monomer species and is observed to lead to peroxides, potentially diacyl peroxides. The molar yield of these highly oxidized products (having O/C > 1 in monomers and O/C > 0.55 in dimers) from cyclohexene ozonolysis was determined as (4.5 ± 3.8)%. Fully deuterated cyclohexene and cis-6-nonenal ozonolysis, as well as the influence of water addition to the system (either H2O or D2O), were also investigated in order to strengthen the arguments on the proposed mechanism. Deuterated cyclohexene ozonolysis resulted in a less oxidized product distribution with a lower yield of highly oxygenated products and cis-6-nonenal ozonolysis generated the same monomer product distribution, consistent with the proposed mechanism and in agreement with quantum chemical modeling.

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Speciated particle-phase organic nitrates (pONs) were quantified using online chemical ionization MS during June and July of 2013 in rural Alabama as part of the Southern Oxidant and Aerosol Study. A large fraction of pONs is highly functionalized, possessing between six and eight oxygen atoms within each carbon number group, and is not the common first generation alkyl nitrates previously reported. Using calibrations for isoprene hydroxynitrates and the measured molecular compositions, we estimate that pONs account for 3% and 8% of total submicrometer organic aerosol mass, on average, during the day and night, respectively. Each of the isoprene-and monoterpenes-derived groups exhibited a strong diel trend consistent with the emission patterns of likely biogenic hydrocarbon precursors. An observationally constrained diel box model can replicate the observed pON assuming that pONs (i) are produced in the gas phase and rapidly establish gasparticle equilibrium and (ii) have a short particle-phase lifetime (∼2-4 h). Such dynamic behavior has significant implications for the production and phase partitioning of pONs, organic aerosol mass, and reactive nitrogen speciation in a forested environment. O rganic nitrates (ONs; ON = RONO 2 + RO 2 NO 2 ) are an important reservoir, if not sink, of atmospheric nitrogen oxides (NO x = NO + NO 2 ). ONs formed from isoprene oxidation alone are responsible for the export of 8-30% of anthropogenic NO x out of the US continental boundary layer (1, 2). Regional NO x budgets and tropospheric ozone (O 3 ) production are, therefore, particularly sensitive to uncertainties in the yields and fates of ON (3-6). The yields implemented in modeling studies are determined from laboratory experiments, in which only a few of the first generation gaseous ONs or the total gas-phase ONs and particle-phase organic nitrates (pONs) have been quantified, whereas production of highly functionalized ONs capable of strongly partitioning to the particle phase have been inferred (7-11) or directly measured in the gas phase (12). Addition of a nitrate (-ONO 2 ) functional group to a hydrocarbon is estimated to lower the equilibrium saturation vapor pressure by 2.5-3 orders of magnitude (13). Thus, ON formation can enhance particle-phase partitioning of semivolatile species in regions with elevated levels of nitrogen oxides, contributing to secondary organic aerosol (SOA) growth (8). However, highly time-resolved measurements of speciated ON in the particle phase have been lacking.We use a recently developed high-resolution time-of-flight chemical ionization mass spectrometer (HRToF-CIMS) using iodide-adduct ionization (14) with a filter inlet for gases and aerosols (FIGAERO) (15) that allows alternating in situ measurements of the molecular SignificanceWe present online field observations of the speciated molecular composition of organic nitrates in ambient atmospheric particles utilizing recently developed high-resolution MS-based instrumentation. We find that never-before-identified low-volatility organi...