Abstract. Oxygenated organic molecules (OOMs) play an important role in the formation of atmospheric aerosols. Due to various analytical challenges in measuring organic vapors, uncertainties remain in the formation and fate of OOMs. The chemical ionization Orbitrap mass spectrometer (CI-Orbitrap) has recently been shown to be a powerful technique able to accurately identify gaseous organic compounds due to its great mass resolving power. Here we present the ammonium ion (NH4+) based CI-Orbitrap as a technique capable of measuring a wide range of gaseous OOMs. The performance of the CI-(NH4+)-Orbitrap was compared with that of state-of-the-art mass spectrometers, including a nitrate ion (NO3−) based CI coupled to an atmospheric pressure interfaced to long time-of-flight mass spectrometer (APi-LTOF), a new generation of proton transfer reaction-TOF mass spectrometer (PTR3-TOF), and an iodide (I−) based CI-TOF mass spectrometer equipped with a Filter Inlet for Gases and AEROsols (FIGAERO-CIMS). The instruments were deployed simultaneously in the Cosmic Leaving OUtdoors Droplets (CLOUD) chamber at the European Organization for Nuclear Research (CERN) during the CLOUD14 campaign in 2019. Products generated from α-pinene ozonolysis across multiple experimental conditions were simultaneously measured by the mass spectrometers. NH4+-Orbitrap was able to identify the widest range of OOMs (i.e., O ≥ 2), from low oxidized species to highly oxygenated volatile organic compounds (HOM). Excellent agreements were found between the NH4+-Orbitrap and the NO3−-LTOF for characterizing HOMs and with the PTR3-TOF for the less oxidized monomeric species. A semi-quantitative information was retrieved for OOMs measured by NH4+-Orbitrap using calibration factors derived from this side-by-side comparison. As other mass spectrometry techniques used during this campaign, the detection sensitivity of NH4+-Orbitrap to OOMs is greatly affected by relative humidity, which may be related to changes in ionization efficiency and/or multiphase chemistry. Overall, this study shows that NH4+ ion-based chemistry associated with the high mass resolving power of the Orbitrap mass analyzer can measure almost all-inclusive compounds. As a result, it is now possible to cover the entire range of compounds, which can lead to a better understanding of the oxidation processes.
Interfaces are ubiquitous in the environment, and in addition many atmospheric key processes, such as gas deposition, aerosol and cloud formation are, at one stage or the other, strongly impacted by physical- and chemical processes occurring at interfaces. Unfortunately, these processes have only been suggested and discussed but never fully addressed because they were beyond reach. We suggest now that photochemistry or photosensitized reactions exist at interfaces, and we will present and discuss their possible atmospheric implications. Obviously, one of the largest interface is the sea-surface microlayer (SML), which is a region lying at the uppermost tens to hundreds of micrometres of the water surface, with physical, chemical and biological properties that differ from those of the underlying sub-surface water. Organic film formation at the sea surface is made possible in the presence of an excess of surface-active material. Hydrophobic surfactant films are typically believed to play the role of a physical barrier to air-sea exchanges, especially at low wind speed. We will show that dissolved organic matter (DOM) can trigger photochemistry at the air-sea interface, releasing unsaturated, functionalized volatile organic compounds (VOCs), including isoprene,... acting as precursors for the formation of organic aerosols, that were thought, up to now, to be solely of biological origin! In addition, we suggest that when arranged at an air/water interface, hydrophobic surfactant can have weak chemical interactions among them, which can trigger the absorption of sunlight and can consequently induce photochemistry at such interfaces. A major question arises from such observations, namely: can the existence of such weak intra- or intermolecular interactions and the subsequent photochemistry be generalized to many other atmospheric objects such as aerosols? This topic will be presented and discussed.
Volatile organic compounds (VOCs), released from both natural and anthropogenic activities, undergo oxidation in the atmosphere to form alkyl peroxy radicals (RO2) that, through subsequent chemistry, form a variety of oxygenated species that may impact both air quality and climate. Among the potential oxygenated species formed are organic nitrates (ON) that can be transported over long distances and contribute to ozone production and particle formation. The atmospheric lifetime of an ON is influenced by its molecular structure. The ON structure impacts both further gas phase reaction (e.g., photolysis and photo-oxidation with OH radical reaction) and gas-to-particle partitioning that may result in multiphase chemistry (e.g., hydrolysis) and secondary organic aerosol (SOA) formation. This study investigates the photo-oxidation of two atmospheric-relevant ON, specifically, hydroxy nitrates, 2-methyl-1-nitrooxy-3-buten-2-ol (C5H9NO4, secondary -ONO2, C5-ON), and 3-methyl-2-nitrooxy-4-penten-3-ol (C6H11NO4, tertiary -ONO2, C6-ON). Significant differences in their reactivity are demonstrated, and the C5-ON underwent faster photolysis, while the C6-ON was more resistant to photolysis, whereas the C6-ON underwent faster degradation during OH-initiated photo-oxidation. These differences in reactivity are attributed to the presence of the extra methyl group on C6-ON. Furthermore, oxidation products that demonstrate the complexity of the reaction processes were detected by chemical ionization Orbitrap mass spectrometry operated in negative mode. Notably, cleavage of the O–N bond and subsequent alkoxy radical chemistry is the dominant pathway during photolysis, whereas OH addition followed by RO2 bimolecular reactions (i.e., RO2 + RO2 or HO2) is the dominant pathway during OH oxidation. This research sheds light on the intricate chemistry of organic nitrates in the atmosphere, emphasizing the role of the molecular structure on their fate.
Abstract. We analysed the biogenic volatile organic compound (BVOC) emissions from rapeseed leaf litter and their potential to create secondary organic aerosols (SOAs) under three different conditions, i.e., (i) in the presence of UV light irradiation, (ii) in the presence of ozone, and (iii) with both ozone and UV light. These experiments were performed in a controlled atmospheric simulation chamber containing leaf litter samples, where BVOC and aerosol number concentrations were measured for 6 d. Our results show that BVOC emission profiles were affected by UV light irradiation which increased the summed BVOC emissions compared to the experiment with solely O3. Furthermore, the diversity of emitted VOCs from the rapeseed litter also increased in the presence of UV light irradiation. SOA formation was observed when leaf litter was exposed to both UV light and O3, indicating a potential contribution to particle formation or growth at local scales. To our knowledge, this study investigates, for the first time, the effect of UV irradiation and O3 exposure on both VOC emissions and SOA formation for leaf litter samples. A detailed discussion about the processes behind the biological production of the most important VOC is proposed.
The photosensitized chemistry of three aromatic ketones (xanthone, flavone, and acetophenone) and also of secondary organic aerosols (SOAs) arising from the photo-oxidation of naphthalene was investigated by means of transient absorption spectroscopy.
Abstract We investigated the photosensitizing properties of secondary organic aerosol (SOA) formed during the hydroxyl radical (OH) initiated oxidation of naphthalene. This SOA was injected into an aerosol flow tube and exposed to UV radiation and gaseous volatile organic compounds or sulfur dioxide (SO 2 ). The aerosol particles were observed to grow in size by photosensitized uptake of d‐limonene and β‐pinene. In the presence of SO 2 , a photosensitized production (0.2–0.3 µg m −3 h −1 ) of sulfate was observed at all relative humidity (RH) levels. Some sulfate also formed on particles in the dark, probably due to the presence of organic peroxides. The dark and photochemical pathways exhibited different trends with RH, unraveling different contributions from bulk and surface chemistry. As naphthalene and other polycyclic aromatics are important SOA precursors in the urban and suburban areas, these dark and photosensitized reactions are likely to play an important role in sulfate and SOA formation.
Organosulfates derived from monoterpene oxidation (MT-OSs) are ubiquitously abundant in atmospheric aerosol particles. However, to date, potential sampling artifacts in MT-OS detection and quantification have been largely ignored. Here, it is demonstrated that collecting aerosol particles from α-pinene oxidation onto filters in the presence of SO2 strongly increased OS numbers and particle mass-normalized abundances by up to 72% and 3 orders of magnitude, respectively. Similarly, at much lower SO2 levels (i.e., atmospheric conditions), an average decrease of 43% in MT-OSs abundances was detected during a field study when SO2 was removed during particle sampling. Additionally, a tremendous increase in mass spectrometric ion signals of OSs was observed when particles were analyzed directly (i) by electrospray ionization (ESI) in the presence of S(VI) and (ii) by extractive ESI (EESI) in the presence of S(IV). Our findings raise concerns on the extent of the recently suggested formation of MT-OSs from S(IV) and organic peroxides, as none of these studies accounted for sampling artifacts from SO2. Furthermore, SO2 artifacts likely affected also earlier studies on MT-OSs. In future studies, SO2 artifacts should be considered more carefully for classical filter sampling and also for emerging analytical techniques.
While acknowledged as key components in the formation of new particles in the atmosphere, the accurate characterization of gaseous (highly) oxygenated organic compounds remains challenging and requires analytical developments. Earlier studies have successfully used the nitrate ion (NO3–) based chemical ionization (CI) coupled to atmospheric pressure interface time-of-flight mass spectrometry (CI-APi-TOF) for monitoring these compounds. Despite many breakthroughs in recent years, the CI-APi-TOF has many limitations, preventing for instance the unambiguous ion identification of overlapping peaks. To tackle this analytical challenge, we developed a CI interface coupled to an ultrahigh-resolution Orbitrap mass spectrometer (CI-Orbitrap). We show that the CI-Orbitrap has similar sensitivity and selectivity as the CI-APi-TOF, but with over an order of magnitude higher mass resolving power (up to 140 000). Equally importantly, the CI-Orbitrap allows tandem mass spectrometry, providing the possibility for structural elucidation of the highly oxygenated organic molecules (HOM). As a proof of concept, we characterized HOM formed during the ozonolysis of two biogenic compounds (α-pinene and limonene), under different environmental conditions in a flow reactor. The CI-Orbitrap exhibited high sensitivity to both HOM and radical species, while easily separating ions of different elemental composition in cases where the more common TOF applications would not have been able to distinguish all ions. Our tandem mass spectrometry analyses revealed distinct fingerprint spectra for all the studied HOM. Overall, the CI-Orbitrap is an extremely promising instrument, and it provides a much-needed extension to ongoing research on HOM, with potential to impact also many other fields within atmospheric chemistry.
