The terrestrial vegetation emits large amounts of volatile organic compounds (VOC) into the atmosphere, which on oxidation produce secondary organic aerosol (SOA). By acting as cloud condensation ...nuclei (CCN), SOA influences cloud formation and climate. In a warming climate, changes in environmental factors can cause stresses to plants, inducing changes of the emitted VOC. These can modify particle size and composition. Here we report how induced emissions eventually affect CCN activity of SOA, a key parameter in cloud formation. For boreal forest tree species, insect infestation by aphids causes additional VOC emissions which modifies SOA composition thus hygroscopicity and CCN activity. Moderate heat increases the total amount of constitutive VOC, which has a minor effect on hygroscopicity, but affects CCN activity by increasing the particles' size. The coupling of plant stresses, VOC composition and CCN activity points to an important impact of induced plant emissions on cloud formation and climate.
Little experimentally explored and understood are the complex dynamics of microstructure formation by ice‐templating when aqueous solutions or slurries are directionally solidified (freeze cast) into ...cellular solids. With synchrotron‐based, time‐resolved X‐ray tomoscopy it is possible to study in situ under well‐defined conditions the anisotropic, partially faceted growth of ice crystals in aqueous systems. Obtaining one full tomogram per second for ≈270 s with a spatial resolution of 6 µm, it is possible to capture with minimal X‐ray absorption, the freezing front in a 3% weight/volume (w/v) sucrose‐in‐water solution, which typically progresses at 5–30 µm s−1 for applied cooling rates of Ċ$\dot{C}$ = 1–10 °C min−1. These time and length scales render X‐ray tomoscopy ideally suited to quantify in 3D ice crystal growth and templating phenomena that determine the performance‐defining hierarchical architecture of freeze‐cast materials: a complex pore morphology and “ridges”, “jellyfish cap”, and “tentacle”‐like secondary features, which decorate the cell walls.
Time‐resolved X‐ray tomography, also termed tomoscopy, reveals how partially faceted, anisotropic ice‐crystal growth and morphological instabilities template the hierarchical architecture of freeze‐cast materials. The formation of performance‐defining structural features at two hierarchical levels, such as a complex pore morphology and “ridges”, “jellyfish cap”‐, and “tentacle”‐like features on the cell wall surfaces, is observed. Transient and permanent phenomena are discovered.
Recent studies have recognised highly oxygenated organic molecules (HOMs) in the atmosphere as important in the formation of secondary organic aerosol (SOA). A large number of studies have focused on ...HOM formation from oxidation of biogenically emitted monoterpenes. However, HOM formation from anthropogenic vapours has so far received much less attention. Previous studies have identified the importance of aromatic volatile organic compounds (VOCs) for SOA formation. In this study, we investigated several aromatic compounds, benzene (C6H6), toluene (C7H8), and naphthalene (C10H8), for their potential to form HOMs upon reaction with hydroxyl radicals (OH). We performed flow tube experiments with all three VOCs and focused in detail on benzene HOM formation in the Jülich Plant Atmosphere Chamber (JPAC). In JPAC, we also investigated the response of HOMs to NOx and seed aerosol. Using a nitrate-based chemical ionisation mass spectrometer (CI-APi-TOF), we observed the formation of HOMs in the flow reactor oxidation of benzene from the first OH attack. However, in the oxidation of toluene and naphthalene, which were injected at lower concentrations, multi-generation OH oxidation seemed to impact the HOM composition. We tested this in more detail for the benzene system in the JPAC, which allowed for studying longer residence times. The results showed that the apparent molar benzene HOM yield under our experimental conditions varied from 4.1 % to 14.0 %, with a strong dependence on the OH concentration, indicating that the majority of observed HOMs formed through multiple OH-oxidation steps. The composition of the identified HOMs in the mass spectrum also supported this hypothesis. By injecting only phenol into the chamber, we found that phenol oxidation cannot be solely responsible for the observed HOMs in benzene experiments. When NOx was added to the chamber, HOM composition changed and many oxygenated nitrogen-containing products were observed in CI-APi-TOF. Upon seed aerosol injection, the HOM loss rate was higher than predicted by irreversible condensation, suggesting that some undetected oxygenated intermediates also condensed onto seed aerosol, which is in line with the hypothesis that some of the HOMs were formed in multi-generation OH oxidation. Based on our results, we conclude that HOM yield and composition in aromatic systems strongly depend on OH and VOC concentration and more studies are needed to fully understand this effect on the formation of HOMs and, consequently, SOA. We also suggest that the dependence of HOM yield on chamber conditions may explain part of the variability in SOA yields reported in the literature and strongly advise monitoring HOMs in future SOA studies.
Oxidation by hydroxyl radical (OH) and ozonolysis are the two major pathways of daytime biogenic volatile organic compound (BVOC) oxidation and secondary organic aerosol (SOA) formation. In this ...study, we investigated the particle formation of several common monoterpenes (α-pinene, β-pinene and limonene) by OH-dominated oxidation, which has seldom been investigated. OH oxidation experiments were carried out in the SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction) chamber in Jülich, Germany, at low NOx (0.01 ~ 1 ppbV) and low ozone (O3) concentration (< 20 ppbV). OH concentration and total OH reactivity (kOH) were measured directly, and through this the overall reaction rate of total organics with OH in each reaction system was quantified. Multi-generation reaction process, particle growth, new particle formation (NPF), particle yield and chemical composition were analyzed and compared with that of monoterpene ozonolysis. Multi-generation products were found to be important in OH-dominated SOA formation. The relative role of functionalization and fragmentation in the reaction process of OH oxidation was analyzed by examining the particle mass and the particle size as a function of OH dose. We developed a novel method which quantitatively links particle growth to the reaction rate of OH with total organics in a reaction system. This method was also used to analyze the evolution of functionalization and fragmentation of organics in the particle formation by OH oxidation. It shows that functionalization of organics was dominant in the beginning of the reaction (within two lifetimes of the monoterpene) and fragmentation started to play an important role after that. We compared particle formation from OH oxidation with that from pure ozonolysis. In individual experiments, growth rates of the particle size did not necessarily correlate with the reaction rate of monoterpene with OH and O3. Comparing the size growth rates at the similar reaction rates of monoterpene with OH or O3 indicates that, generally, OH oxidation and ozonolysis had similar efficiency in particle growth. The SOA yield of α-pinene and limonene by ozonolysis was higher than that of OH oxidation. Aerosol mass spectrometry (AMS) shows SOA elemental composition from OH oxidation follows a slope shallower than −1 in the O / C vs. H / C diagram, also known as Van Krevelen diagram, indicating that oxidation proceeds without significant loss of hydrogen. SOA from OH oxidation had higher H / C ratios than SOA from ozonolysis. In ozonolysis, a process with significant hydrogen loss seemed to play an important role in SOA formation.
Nighttime NO3-initiated oxidation of biogenic volatile organic compounds (BVOCs) such as monoterpenes is important for the atmospheric formation and growth of secondary organic aerosol (SOA), which ...has significant impact on climate, air quality, and human health. In such SOA
formation and growth, highly oxygenated organic molecules (HOM) may be
crucial, but their formation pathways and role in aerosol formation have yet to be clarified. Among monoterpenes, limonene is of particular interest for its high emission globally and high SOA yield. In this work, HOM formation in the reaction of limonene with nitrate radical (NO3) was investigated in the SAPHIR chamber (Simulation of Atmospheric PHotochemistry In a large Reaction chamber). About 280 HOM products were identified, grouped into 19 monomer families, 11 dimer families, and 3 trimer families. Both closed-shell products and open-shell peroxy radicals (RO2⚫) were observed, and many of them have not been reported previously. Monomers and dimers accounted for 47 % and 47 % of HOM concentrations, respectively, with trimers making up the remaining 6 %. In the most abundant monomer families, C10H15−17NO6−14, carbonyl products outnumbered hydroxyl products, indicating the importance of RO2⚫ termination by unimolecular dissociation. Both RO2⚫ autoxidation and alkoxy–peroxy pathways were found to be important processes leading to HOM. Time-dependent concentration profiles of monomer products containing nitrogen showed mainly second-generation formation patterns. Dimers were likely formed via the accretion reaction of two monomer RO2⚫, and HOM-trimers via the accretion reaction
between monomer RO2⚫ and dimer RO2⚫. Trimers are suggested to play an important role in new particle formation (NPF) observed in our experiment. A HOM yield of 1.5%-0.7%+1.7% was estimated considering only first-generation products. SOA mass growth could
be reasonably explained by HOM condensation on particles assuming
irreversible uptake of ultra-low volatility organic compounds (ULVOCs),
extremely low volatility organic compounds (ELVOCs), and low volatility
organic compounds (LVOCs). This work provides evidence for the important role of HOM formed via the limonene +NO3 reaction in NPF and growth of SOA particles.
An experimental setup has been developed that allows for capturing up to 25 tomograms s−1 using the white X‐ray beam at the experimental station EDDI of BESSY II, Berlin, Germany. The key points are ...the use of a newly developed, precise and fast rotation stage, a very efficient scintillator and a fast CMOS camera. As a first application, the foaming of aluminium alloy granules at 923 K was investigated in situ. Formation and growth of bubbles in the liquid material were observed and found to be influenced by the limited thermal conductivity in the bulk granules. Changes that took place between two tomographic frames separated in time by 39 ms could be detected and analysed quantitatively.
A setup for time‐resolved in situ tomography with up to 25 tomograms s−1 is implemented at the EDDI beamline, BESSY II, Germany. The capabilities of the methods are demonstrated by analysing the foaming behaviour of a metallic foam granulate.
Isoprene oxidation by nitrate radical (NO3) is a potentially important
source of secondary organic aerosol (SOA). It is suggested that the second
or later-generation products are the more substantial ...contributors to SOA.
However, there are few studies investigating the multi-generation chemistry
of isoprene-NO3 reaction, and information about the volatility of
different isoprene nitrates, which is essential to evaluate their potential
to form SOA and determine their atmospheric fate, is rare. In this work, we
studied the reaction between isoprene and NO3 in the SAPHIR chamber
(Jülich) under near-atmospheric conditions. Various oxidation products
were measured by a high-resolution time-of-flight chemical ionization mass
spectrometer using Br− as the reagent ion. Most of the products
detected are organic nitrates, and they are grouped into monomers (C4
and C5 products) and dimers (C10 products) with 1–3 nitrate
groups according to their chemical composition. Most of the observed
products match expected termination products observed in previous studies,
but some compounds such as monomers and dimers with three nitrogen atoms
were rarely reported in the literature as gas-phase products from isoprene
oxidation by NO3. Possible formation mechanisms for these compounds are
proposed. The multi-generation chemistry of isoprene and NO3 is
characterized by taking advantage of the time behavior of different
products. In addition, the vapor pressures of diverse isoprene nitrates are
calculated by different parametrization methods. An estimation of the vapor
pressure is also derived from their condensation behavior. According to our
results, isoprene monomers belong to intermediate-volatility or
semi-volatile organic compounds and thus have little effect on SOA
formation. In contrast, the dimers are expected to have low or extremely low
volatility, indicating that they are potentially substantial contributors to
SOA. However, the monomers constitute 80 % of the total explained signals
on average, while the dimers contribute less than 2 %, suggesting that the
contribution of isoprene NO3 oxidation to SOA by condensation should be
low under atmospheric conditions. We expect a SOA mass yield of about 5 %
from the wall-loss- and dilution-corrected mass concentrations, assuming that
all of the isoprene dimers in the low- or extremely low-volatility organic
compound (LVOC or ELVOC) range will condense completely.
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