Highly oxygenated organic molecules (HOMs) are a major source of new particles that affect the Earth's climate. HOM production from the oxidation of volatile organic compounds (VOCs) occurs during both the day and night and can lead to new particle formation (NPF). However, NPF involving organic vapors has been reported much more often during the daytime than during nighttime. Here, we show that the nitrate radicals (NO3), which arise predominantly at night, inhibit NPF during the oxidation of monoterpenes based on three lines of observational evidence: NPF experiments in the CLOUD (Cosmics Leaving OUtdoor Droplets) chamber at CERN (European Organization for Nuclear Research), radical chemistry experiments using an oxidation flow reactor, and field observations in a wetland that occasionally exhibits nocturnal NPF. Nitrooxy-peroxy radicals formed from NO3 chemistry suppress the production of ultralow-volatility organic compounds (ULVOCs) responsible for biogenic NPF, which are covalently bound peroxy radical (RO2) dimer association products. The ULVOC yield of α-pinene in the presence of NO3 is one-fifth of that resulting from ozone chemistry alone. Even trace amounts of NO3 radicals, at sub-parts per trillion level, suppress the NPF rate by a factor of 4. Ambient observations further confirm that when NO3 chemistry is involved, monoterpene NPF is completely turned off. Our results explain the frequent absence of nocturnal biogenic NPF in monoterpene (α-pinene)-rich environments.
Ground subsidence caused by permafrost thawing causes the formation of thermokarst ponds, where organic compounds from eroding permafrost accumulate. We photolyzed water samples from two such ponds in Northern Quebec and discovered the emission of volatile organic compounds (VOCs) using mass spectrometry. One pond near peat-covered permafrost mounds was organic-rich, while the other near sandy mounds was organic-poor. Compounds up to C10 were detected, comprising the atoms of O, N, and S. The main compounds were methanol, acetaldehyde, and acetone. Hourly VOC fluxes under actinic fluxes similar to local solar fluxes might reach up to 1.7 nmol C m-2 s-1. Unexpectedly, the fluxes of VOCs from the organic-poor pond were greater than those from the organic-rich pond. We suggest that different segregations of organics at the air/water interface may partly explain this observation. This study indicates that sunlit thermokarst ponds are a significant source of atmospheric VOCs, which may affect the environment and climate via ozone and aerosol formation. Further work is required for understanding the relationship between the pond's organic composition and VOC emission fluxes.
Chemical ionization Orbitrap mass spectrometry (CI-Orbitrap) represents a promising new technique for gas-phase analysis in analytical and atmospheric chemistry mainly due to its very high mass resolving power. In this work, we performed the first side-by-side comparison between a CI-Orbitrap and the widely used atmospheric pressure interface time-of-flight mass spectrometry (CI-APi-TOF) using two different chemical ionization methods, i.e., acetate-ion-based (CH3COO-) and aminium-ion-based (n-C3H7NH3+) schemes. The capability of the CI-Orbitrap at accurately measuring low concentrations of gaseous species formed from the oxidation of α-pinene was explored. Although this study reveals a lack of linearity of the CI-Orbitrap when measuring product ions at very low concentrations (<1 × 106 molecules cm-3), very good agreement between both techniques can be achieved by applying a newly developed linearity correction. It is experimentally shown that the correction function is independent of the reagent ion used. Thus, accurate quantification of organic compounds at concentrations as low as 1 × 105 molecules cm-3 by the CI-Orbitrap can be achieved. Finally, by means of tandem mass spectrometry, the unique capability of the Orbitrap allows the direct determination of the binding energy of cluster ions between analyte and reagent ions, that is needed for the assessment of a chosen ionization scheme.
Abstract. The Orbitrap mass spectrometer has recently been proved to be a powerful instrument to accurately measure gas-phase and particle-phase organic compounds with a greater mass resolving power than other widely-used online mass spectrometers in atmospheric sciences. We develop an open-source software tool (Orbitool, https://orbitrap.catalyse.cnrs.fr) to facilitate the analysis of long-term online Orbitrap data. Orbitool can average long-term data while maintaining the mass accuracy by re-calibrating each mass spectrum, identify chemical compositions and isotopes of measured signals, and export time series and mass defect plots. The noise reduction procedure in Orbitool can separate signal peaks from noise and greatly reduce the computational and storage expenses. Chemical-ionization Orbitrap data from laboratory experiments on ozonolysis of monoterpenes and ambient measurements in urban Shanghai were used to successfully test Orbitool. For the test dataset, the average mass accuracy was improved from
Abstract. Volatile organic compounds (VOCs) and volatile inorganic compounds (VICs) provide critical information across many scientific fields including atmospheric chemistry, soil, and biological processes. Chemical ionization (CI) mass spectrometry has become a powerful tool for tracking these chemically complex and temporally variable compounds in a variety of laboratory and field environments. It is particularly powerful with time-of-flight mass spectrometers, which can measure hundreds of compounds in a fraction of a second and have enabled entirely new branches of VOC/VIC research in atmospheric and biological chemistry. To accurately describe each step of these chemical, physical, and biological processes, measurements across the entire range of gaseous products is crucial. Recently, chemically comprehensive gas-phase measurements have been performed using many CI mass spectrometers deployed in parallel, each utilizing a different ionization method to cover a broad range of compounds. Here we introduce the recently developed Vocus AIM (Adduct Ionization Mechanism) ion-molecule reactor (IMR), which samples trace vapors in air and ionizes them via chemical ionization at medium pressures. The Vocus AIM supports the use of many different reagent ions of positive and negative polarity and is largely independent of changes in the sample humidity. Within the present study, we present the performance and explore the capabilities of the Vocus AIM using various chemical ionization schemes, including Chloride (Cl–), Bromide (Br–), Iodide (I–), Nitrate (NO3–), Benzene cations (C6H6+), Acetone dimers ((C3H6O)2H+), and Ammonium (NH4+) reagent ions primarily in laboratory and flow tube experiments. We report the technical characteristics, operational principles, and compare its performance in terms of time response, humidity dependence, and sensitivity to that of previous chemical ionization approaches. This work demonstrates the benefits of the Vocus AIM reactor which provides a versatile platform to characterize VOCs and VICs in real time at trace concentrations.
In subarctic regions, rising temperature and permafrost thaw lead to the formation of thermokarst ponds, where organics from eroding permafrost accumulate. Despite its environmental significance, limited knowledge exists regarding the photosensitivity of permafrost-derived carbon in these ponds. In this study, laboratory experiments were conducted to explore the photochemical transformations of organic matter in surface water samples from thermokarst ponds from different environments in northern Quebec, Canada. One pond near Kuujjuarapik is characterized by the presence of a collapsing palsa and is therefore organically rich, while the other pond near Umiujaq is adjacent to a collapsing lithalsa and thus contains fewer organic matters. Photobleaching occurred in the Umiujaq sample upon irradiation, whereas the Kuujjuarapik sample exhibited an increase in light absorbance at wavelength related to aromatic functionalities, indicating different photochemical aging processes. Ultrahigh-resolution mass spectrometry analysis reveals that the Kuujjuarapik sample preferentially photoproduced highly unsaturated CHO compounds with great aromaticity, while the irradiated Umiujaq sample produced a higher proportion of CHON aromatics with reduced nitrogen functionalities. Overall, this study illustrates that the photochemical reactivity of thermokarst pond water varies with the source of organic matter. The observed differences in reactivity contribute to an improved understanding of the photochemical emission of volatile organic compounds discovered earlier. Further insights into the photoinduced evolutions in thermokarst ponds may require the classification of permafrost-derived carbon therein.
The sea surface microlayer extensively covers the Earthâs oceans and is host to numerous organic and biogenic compounds which also concentrate there. Many bulk and surface-bound organic materials, such as humic acids, are photosensitizers and, thus have the potential to trigger unique chemistry when irradiated by sunlight. It is well recognized that the exchange of gases and particles with the atmosphere are impacted by the presence of the sea surface microlayer, however, the exact mechanisms which accomplish this are not fully understood. Here, we present a laboratory study on VOC emission due to photochemical production at the sea surface microlayer followed by secondary organic aerosol (SOA) generation. These data are valuable to the assessment of VOC and SOA atmospheric budgets and increase our fundamental understanding of their production.
Laboratory experiments were conducted in a custom-built Teflon reaction chamber with a pure or sea water reservoir containing nonanoic acid, a model surfactant proxy for a surface microlayer, and humic acids. Experiments were performed under irradiation of UV and visible light in a humidified low NOx and low ozone environment. Concentrations of VOCs were measured over time using a proton transfer reaction-time of flight-mass spectrometer (PTR-ToF-MS). Characterization of organic and inorganic compounds in water and aerosol was performed using ion chromatography and liquid chromatography-high resolution-mass spectrometry (LC-HR-MS) utilizing a quadrupole-orbitrap detector. Aerosol size distribution and numbers were continually monitored. Physical and chemical characterization of SOA was investigated with scanning transmission X-ray microscopy coupled with near-edge X-ray absorption fine structure (STXM/NEXAFS) spectroscopy using the PolLux X-ray beamline at the Swiss Light Source.
Production of VOCs was observed while the chamber air and water were irradiated with UV light for the system, nonanoic acid and pure water (background levels of ozone and NOx). Gas phase products include alkenes, aldehydes and dienes, such as isoprene as observed by PTR-ToF-MS in two different ionization modes (H3O+ and NO+) and confirmed from independent experiments in a controlled Quartz reaction cell. Introduction of ozone into the chamber triggered new particle formation followed by condensational growth, likely due to the ozonolysis of present gas phase products having carbon double bonds to form lower volatility compounds. When a salt water system was used containing humic acid and the surfactant, we find that the VOC and SOA yield is further enhanced under irradiation suggesting that photosensitized reactions can influence VOC production. Maximum new particle concentrations ranged from 103-105 cm-3.
A spectroscopic signature of SOA produced in our chamber was acquired with STXM/NEXAFS characterized by oxygenated organic material dominated by the presence of the carboxyl and carbonyl functional groups. Secondary absorption peaks indicating the presence of hydroxyl functionality and carbon double bonding were also present. This method allows for spectral comparison between the generated SOA and known SOA spectra from field and laboratory studies.
We suggest that radicals produced due to photochemistry or photosensitized reaction move to the interface and react with nonanoic acid through hydrogen abstraction. Due to the high concentration of organic in the microlayer, unique chemistry follows involving self-reactions unfavorable in the gas and aqueous phase. Ozonolysis of these products then stimulates SOA formation. These results underscore the significance of photon-induced chemistry at the ocean-atmosphere interface with the potential to significantly impact on VOCs and SOA over the oceans.
The complex refractive index (CRI) is one of the key parameter driving aerosol spectral optical properties and direct radiative effects (DRE). Its value and spectral variation under different conditions, such as anthropogenic− and biogenic−dominated environments and anthropogenic−biogenic mixing situations, remains not fully understood. As a consequence, oversimplified representations of aerosol optical properties are generally used in climate models. Therefore, measurements of aerosol CRI in different environments and their inclusion in models are needed. The field observations from the ACROSS campaign, performed in June-July 2022 in the Ile de France region, are used in this study to deepen the knowledge of aerosol optical properties, aiming to improve the aerosol representation in the CHIMERE model and provide the best constraint for DRE simulations. Measurements obtained both at the Paris city center and the Rambouilllet rural forest sites during ACROSS are considered, in order to explore the CRI variability from anthropogenic−dominated to biogenic−dominated environments, including anthropogenic−biogenic mixing situations. The CRI retrievals at seven different wavelengths, performed by combining the Mie theory with optical and size distribution measurements, are representative of different atmospheric conditions, aerosol loadings as well as type and chemical compositions. In fact, the June-July 2022 period was characterized by highly diversified weather conditions: 1) two strong heatwaves, promoting SOA build-up and favoring the export of the Paris pollution plume towards the forest site; 2) Saharan dust events transported from the upper atmosphere to the ground; 3) biomass burning episode; 4) periods with reduced anthropogenic influence. The CRI retrievals under these different conditions and their link to particulate chemical composition is investigated. Hence, the CRI dataset presented here constitutes a unique dataset from which models can benefit to validate and constrain simulations and DRE estimations, under both urban and biogenic emissions influence. These data, in conjunction with those from the aircraft observations during ACROSS, are used to initialize and perform sensitivity studies on the aerosol DRE, using the CHIMERE−WRF coupled model, the OPTSIM model for the aerosol optical properties and the Rapid Radiative Transfer Model for GCMs (RRTMG).Keywords: Complex refractive index, direct radiative effect, aerosol mixing, urban, forest
Films of organic compounds exposed to the atmosphere are ubiquitously found on surfaces of cloud droplets, aerosol particles, buildings, plants, soils and the ocean. The sea-surface microlayer is one example of organic films that are host to countless biogenic amphiphilic compounds concentrated there with respect to bulk water. Yet, organic materials present in the bulk, such as humic acids and other photosensitizing compounds, can still have a tremendous impact on the surfactants (George, 2015).
Here, we present a laboratory investigation on secondary organic aerosol (SOA) formation from the reaction of ozone with volatile organic compounds (VOCs), originating from photo-induced processes in surfactant-containing water or sea-water. The results underscore the environmental importance of photochemical reactions at the air–water interface to produce SOA precursors in significant amounts, leading to significant particle formation and growth.
Laboratory experiments were conducted in a custom-built 2 m3 Teflon reaction chamber with a pure or sea-water reservoir containing nonanoic acid, a model surfactant proxy for a surface microlayer, and humic acids when desired. In other experiments nonanol was used as a surfactant for comparison. VOC concentrations were measured over time using a proton transfer reaction–time of flight–mass spectrometer (PTR–ToF–MS) as a function of irradiation (UV and visible light) in a humidified low NOx and low ozone environment. Additionally, photochemical gas- and liquid-phase products generated by irradiation of natural riverine biofilms were investigated. Characterization of organic compounds in water and aerosol particles was performed with ion chromatography and liquid chromatography–high resolution mass spectrometry (LC–HRMS) utilizing a quadrupole-orbitrap detector. Aerosol size distribution and numbers were continuously monitored during all experiments.
Production of VOCs occurred while the chamber air and water were irradiated with UV light for the nonanoic acid and pure water system. Gas-phase products included alkenes, aldehydes and dienes, as observed by PTR–ToF–MS in three different ionization modes (H3O+, NO+ and O2+). Aerosol nucleation and growth occurred upon ozone introduction and reaction in the chamber but with lights switched off, further indicating the presence of unsaturated VOCs. Concentrations of aerosol particles were typically on the order of 102–103 cm–3. When nonanoic acid and seawater were present, similar types and concentrations of VOCs were observed. Production of halogenated organic compounds was investigated using O2+ reagent ions. The addition of humic acids typically enhanced photochemical VOC production and aerosol formation, where SOA numbers exceeded 104 cm–3 after ozone injection and particle nucleation. We note that when nonanol was used as a surfactant, VOC and aerosol formation was never observed. This result highlights the potential importance of the carbonyl functionality of the acid group in the photochemical mechanism. The products were not only emitted to the gas phase, but also the bulk water. Using LC–HRMS the same VOCs, observed with the PTR–ToF–MS in the gas phase, were also identified in bulk water samples extracted during and after experiments. In addition, high molecular weight compounds such as highly oxidized organic molecules and molecular recombination products were identified in experiments with the nonanoic acid surfactant.
We suggest that light absorption precedes radical chemistry at the organic surfactant interface. Recent experimental findings in our group, supported by quantum chemical calculations, reveal the possibility of exciting nonanoic acid molecules to their triplet state even under irradiation of 300–320 nm wavelengths. Subsequent inter- and intra-molecular reactions have been proposed generating OH and other radicals leading to a suite of products. At an organic interface neighboring organic molecules are likely to react, especially in molecular layers such as those existing in organic or biogenic films. This proximity of organic molecules is unique to an interface, in contrast to the bulk or gas phase and thus, may also result in unique chemical reaction pathways leading to VOC formation. Fatty acid and organic compound coated surfaces are ubiquitous and photon-induced chemistry at organic interfaces may be important, ultimately influencing VOC flux and SOA formation at the Earth’s surface and aloft.
References
George, C., Ammann, M., D’Anna, B. Donaldson, D. J., Nizkorodov, S. A. (2015), Chemical Reviews 115 (10), 4218–4258. DOI: 10.1021/cr500648z
