Torsten Berndt

A large source of low-volatility secondary organic aerosol

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.

The Role of Sulfuric Acid in Atmospheric Nucleation

Little Things Do Matter Gas-phase sulfuric acid is important during atmospheric particle formation, but the mechanisms by which it forms new particles are unclear. Laboratory studies of the binary nucleation of sulfuric acid with water produce particles at rates that are many orders of magnitude too small to explain the concentration of sulfuric acid particles found in the atmosphere. Sipilä et al. (p. 1243) now show that gas-phase sulfuric acid does, in fact, undergo nucleation in the presence of water at a rate fast enough to account for the observed abundance of sulfuric acid particles in the atmosphere. These particles, which contain 1 to 2 sulfuric acid molecules each, were not detectable previously, owing to their small size, with diameters as small as 1.5 nanometers. Gas-phase sulfuric acid and water react fast enough to account for the concentration of atmospheric sulfuric acid particles. Nucleation is a fundamental step in atmospheric new-particle formation. However, laboratory experiments on nucleation have systematically failed to demonstrate sulfuric acid particle formation rates as high as those necessary to account for ambient atmospheric concentrations, and the role of sulfuric acid in atmospheric nucleation has remained a mystery. Here, we report measurements of new particles (with diameters of approximately 1.5 nanometers) observed immediately after their formation at atmospherically relevant sulfuric acid concentrations. Furthermore, we show that correlations between measured nucleation rates and sulfuric acid concentrations suggest that freshly formed particles contain one to two sulfuric acid molecules, a number consistent with assumptions that are based on atmospheric observations. Incorporation of these findings into global models should improve the understanding of the impact of secondary particle formation on climate.

A new atmospherically relevant oxidant of sulphur dioxide

Atmospheric oxidation is a key phenomenon that connects atmospheric chemistry with globally challenging environmental issues, such as climate change, stratospheric ozone loss, acidification of soils and water, and health effects of air quality. Ozone, the hydroxyl radical and the nitrate radical are generally considered to be the dominant oxidants that initiate the removal of trace gases, including pollutants, from the atmosphere. Here we present atmospheric observations from a boreal forest region in Finland, supported by laboratory experiments and theoretical considerations, that allow us to identify another compound, probably a stabilized Criegee intermediate (a carbonyl oxide with two free-radical sites) or its derivative, which has a significant capacity to oxidize sulphur dioxide and potentially other trace gases. This compound probably enhances the reactivity of the atmosphere, particularly with regard to the production of sulphuric acid, and consequently atmospheric aerosol formation. Our findings suggest that this new atmospherically relevant oxidation route is important relative to oxidation by the hydroxyl radical, at least at moderate concentrations of that radical. We also find that the oxidation chemistry of this compound seems to be tightly linked to the presence of alkenes of biogenic origin.

Production of extremely low volatile organic compounds from biogenic emissions: Measured yields and atmospheric implications

Significance Extremely low volatility organic compounds (ELVOC) are suggested to promote aerosol particle formation and cloud condensation nuclei (CCN) production in the atmosphere. We show that the capability of biogenic VOC (BVOC) to produce ELVOC depends strongly on their chemical structure and relative oxidant levels. BVOC with an endocyclic double bond, representative emissions from, e.g., boreal forests, efficiently produce ELVOC from ozonolysis. Compounds with exocyclic double bonds or acyclic compounds including isoprene, emission representative of the tropics, produce minor quantities of ELVOC, and the role of OH radical oxidation is relatively larger. Implementing these findings into a global modeling framework shows that detailed assessment of ELVOC production pathways is crucial for understanding biogenic secondary organic aerosol and atmospheric CCN formation. Oxidation products of monoterpenes and isoprene have a major influence on the global secondary organic aerosol (SOA) burden and the production of atmospheric nanoparticles and cloud condensation nuclei (CCN). Here, we investigate the formation of extremely low volatility organic compounds (ELVOC) from O3 and OH radical oxidation of several monoterpenes and isoprene in a series of laboratory experiments. We show that ELVOC from all precursors are formed within the first minute after the initial attack of an oxidant. We demonstrate that under atmospherically relevant concentrations, species with an endocyclic double bond efficiently produce ELVOC from ozonolysis, whereas the yields from OH radical-initiated reactions are smaller. If the double bond is exocyclic or the compound itself is acyclic, ozonolysis produces less ELVOC and the role of the OH radical-initiated ELVOC formation is increased. Isoprene oxidation produces marginal quantities of ELVOC regardless of the oxidant. Implementing our laboratory findings into a global modeling framework shows that biogenic SOA formation in general, and ELVOC in particular, play crucial roles in atmospheric CCN production. Monoterpene oxidation products enhance atmospheric new particle formation and growth in most continental regions, thereby increasing CCN concentrations, especially at high values of cloud supersaturation. Isoprene-derived SOA tends to suppress atmospheric new particle formation, yet it assists the growth of sub-CCN-size primary particles to CCN. Taking into account compound specific monoterpene emissions has a moderate effect on the modeled global CCN budget.

Gas-phase ozonolysis of α-pinene: gaseous products and particle formation

Abstract The gas-phase ozonolysis of α -pinene has been studied in a stopped-flow system at 295±2 K and 950 mbar of synthetic air as well as under flow conditions at 295±0.5 K and 1000 mbar of synthetic air. Gaseous products were analyzed using on-line GC-MS/FID and FT-IR measurements. The formation of new particles ( d p⩾3 nm ) was followed by means of a differential mobility particle sizer system and an ultra-fine condensation particle counter. First, the reaction of OH radicals with c -hexane was reinvestigated. Products were c -hexanol with a yield of 0.35±0.06 and c -hexanone with a yield of 0.53±0.06. The rate coefficients of the consecutive reaction of OH radicals with c -hexanol and c -hexanone of (2.7±0.4)×10 −11 and (6.1±0.9)×10 −12 cm 3 molecule −1 s −1 , respectively, were obtained. From the reaction of OH radicals with c -hexanol, a c -hexanone yield of 0.58±0.07 was observed. Using the c -hexane scavenger technique, an OH radical yield of 0.68±0.10 was measured for the reaction of O 3 with α -pinene applicable for H 2 O concentrations of ∼1.5×10 15 and 2.6×10 17 molecule cm −3 . The formation yield of pinonaldehyde was found to be strongly dependent on the experimental conditions. Generally, the pinonaldehyde yield decreased for increasing α -pinene conversion. In the presence of an OH radical scavenger, maximum pinonaldehyde yields were 0.42±0.05 and 0.32±0.04 for [H 2 O]∼1.5×10 15 and 2.6×10 17 molecule cm −3 , respectively. Under the present conditions the pinonaldehyde formation cannot be described via the reaction of a Criegee intermediate with H 2 O. This finding is corroborated by measurements of the co-product H 2 O 2 . The formation yield of α -pinene oxide of 0.03±0.015 was unaffected by experimental conditions. Newly formed particles were measured for a relatively wide range of experimental conditions. Particle formation was only detectable for an α -pinene conversion above 3×10 11 molecule cm −3 . The results of the present study suggest that the formation of new particles from the pure O 3 + α -pinene reaction is unlikely under atmospheric conditions.

Formation of phenol and carbonyls from the atmospheric reaction of OH radicals with benzene.

The gas-phase reaction of OH radicals with benzene has been studied in a flow tube operated at 295 +/- 2 K and 950 mbar of synthetic air or O2. Ozonolysis of tetramethylethylene (dark reaction) with a measured OH radical yield of 0.92 +/- 0.08 or photolysis of methyl nitrite in the presence of NO served as the OH sources. For investigations in the presence of NOx, the conditions were chosen so that more than 95% of the OH/benzene adduct reacted with O2 even for the highest NO2 concentration occurring in the experiment. In the absence of NOx, a phenol yield from the reaction of OH radicals with benzene of 0.61 +/- 0.07 was measured by means of long-path FT-IR and UV spectroscopy over a wide range of experimental conditions. This yield was confirmed by measurements performed in the presence of NOx. Detected carbonyls were glyoxal, cis-butenedial and trans-butenedial with formation yields of 0.29 +/- 0.10, 0.08 +/- 0.03 and 0.023 +/- 0.007, respectively, measured in synthetic air and in the presence of NOx. There was no significant difference in the product yields applying both experimental approaches for OH generation (dark reaction or photolysis). Nitrobenzene and o-nitrophenol were detected in traces. The yield of nitrobenzene increased with increasing NOx resulting in a maximum formation yield of 0.007. The detected products in the presence of NOx account for approximately 78% of the reacted carbon. Butenedial yields from benzene degradation are reported for the first time. In the absence of NOx, glyoxal, cis-butenedial and trans-butenedial were also detected, but with distinctly lower yields compared to the experiments with NOx.

Highly Oxidized Multifunctional Organic Compounds Observed in Tropospheric Particles: A Field and Laboratory Study.

Very recent studies have reported the existence of highly oxidized multifunctional organic compounds (HOMs) with O/C ratios greater than 0.7. Because of their low vapor pressure, these compounds are often referred as extremely low-volatile organic compounds (ELVOCs), and thus, they are able to contribute significantly to organic mass in tropospheric particles. While HOMs have been successfully detected in the gas phase, their fate after uptake into particles remains unclear to date. Hence, the present study was designed to detect HOMs and related oxidation products in the particle phase and, thus, to shed light on their fate after phase transfer. To this end, aerosol chamber investigations of α-pinene ozonolysis were conducted under near environmental precursor concentrations (2.4 ppb) in a continuous flow reactor. The chemical characterization shows three classes of particle constituents: (1) intact HOMs that contain a carbonyl group, (2) particle-phase decomposition products, and (3) highly oxidized organosulfates (suggested to be addressed as HOOS). Besides chamber studies, HOM formation was also investigated during a measurement campaign conducted in summer 2013 at the TROPOS research station Melpitz. During this field campaign, gas-phase HOM formation was found to be correlated with an increase in the oxidation state of the organic aerosol.

Gas-phase reaction of OH radicals with phenol

The gas-phase reaction of OH radicals with phenol was investigated in a flow tube in the temperature range of 266–364 K and a pressure of 100 mbar. The product formation was followed by on-line FT-IR spectroscopy and GC-MS measurements. Newly formed particles were detected by means of a low-pressure CPC (condensation particle counter). In the presence of O2, OH radicals were generated via the reaction sequence H + O2 + M → HO2 + M, HO2 + NO → OH + NO2 and in the absence of O2via H + NO2 → OH + NO. For evaluation of a possible competing process, the rate constant for H + phenol was measured, k(H + phenol) = (2.5 ± 1.5) × 10−13 cm3 molecule−1 s−1 (295 ± 2 K, 25 mbar He). Under the experimental conditions used the H-atom reaction does not compete with the reaction of OH radicals with phenol. At 295 K, the product distribution was studied for different O2, NO and NO2 concentrations. Identified products were catechol, o-nitrophenol and p-benzoquinone. Under all experimental conditions catechol represented the main product. The measured dependence of the catechol yield on NO and NO2 for constant O2 concentrations allowed an estimate of the reactivity of the OH/phenol adduct towards O2, NO and NO2, k(adduct + O2)/k(adduct + NO) > 10−3 and k(adduct + O2)/k(adduct + NO2) = (1.4 ± 0.5) × 10−4. For constant gas composition, in the absence of additional NO2, the product distribution was measured for different temperatures. With increasing temperature the catechol yield increased from 0.37 ± 0.06 (266 K) to 0.87 ± 0.04 (364 K). The yields of o-nitrophenol and p-benzoquinone were nearly constant. Below 295 K, with decreasing temperature enhanced formation of newly formed particles was observed. For realistic atmospheric conditions, a catechol yield of 0.73–0.78 (295 K) can be recommended from this study.

Gas-phase reaction of OH radicals with benzene: products and mechanism

The gas-phase reaction of OH radicals with benzene was studied in O2/He mixtures under flow conditions in the temperature range 276–353 K and at pressures of 100 and 500 mbar using on-line FT-IR spectroscopy and GC-MS measurements. The reaction conditions were chosen so that the initially formed OH/benzene adduct predominantly reacted either with O2 or O3. Under conditions of a predominant reaction of the OH/benzene adduct with O2 the product formation was studied for variable NO concentrations. Identified products were the isomers of hexa-2,4-dienedial, phenol, nitrobenzene, p-benzoquinone and glyoxal. Furan was found in small amounts. For increasing NO concentrations there was a decrease of the phenol yield and the yields of trans,trans-hexa-2,4-dienedial and nitrobenzene increased, resulting in maximum values of 0.36 ± 0.02 and 0.11 ± 0.02, respectively (100 mbar, 295 K). The p-benzoquinone yield of 0.08 ± 0.02 was found to be independent of the NO concentration. The temperature dependence of the phenol yield was measured in the range of 276–353 K for initial ratios of [NO]/[O2] = 1–20 × 10−6 at 500 mbar. For a fixed [NO]/[O2] ratio, a distinct increase of the phenol yield with increasing temperature was observed; initial [NO]/[O2] = 1–1.2 × 10−6, phenol yield: 0.18 ± 0.04 (276 K) and 0.68 ± 0.05 (353 K). Generally, the total yield of carbonylic substances was found to be anti-correlated to the phenol yield. When the OH/benzene adduct reacted with O3, trans,trans-hexa-2,4-dienedial, phenol and formic acid were identified as main products with formation yields of 0.28 ± 0.02, 0.20 ± 0.05 and 0.12 ± 0.02, respectively (100 mbar, 295 K). Further products were p-benzoquinone, CO and unidentified carbonylic substances. For the different experimental conditions, reaction mechanisms are proposed explaining the formation of the observed products. A simple model describing the temperature and NOx-dependence of the phenol yield is presented.

Molecular-scale evidence of aerosol particle formation via sequential addition of HIO3

Homogeneous nucleation and subsequent cluster growth leads to the formation of new aerosol particles in the atmosphere. The nucleation of sulfuric acid and organic vapours is thought to be responsible for the formation of new particles over continents, whereas iodine oxide vapours have been implicated in particle formation over coastal regions. The molecular clustering pathways that are involved in atmospheric particle formation have been elucidated in controlled laboratory studies of chemically simple systems, but direct molecular-level observations of nucleation in atmospheric field conditions that involve sulfuric acid, organic or iodine oxide vapours have yet to be reported. Here we present field data from Mace Head, Ireland, and supporting data from northern Greenland and Queen Maud Land, Antarctica, that enable us to identify the molecular steps involved in new particle formation in an iodine-rich, coastal atmospheric environment. We find that the formation and initial growth process is almost exclusively driven by iodine oxoacids and iodine oxide vapours, with average oxygen-to-iodine ratios of 2.4 found in the clusters. On the basis of this high ratio, together with the high concentrations of iodic acid (HIO3) observed, we suggest that cluster formation primarily proceeds by sequential addition of HIO3, followed by intracluster restructuring to I2O5 and recycling of water either in the atmosphere or on dehydration. Our study provides ambient atmospheric molecular-level observations of nucleation, supporting the previously suggested role of iodine-containing species in the formation of new aerosol particles, and identifies the key nucleating compound.