Multi-generation OH oxidation as a source for highly oxygenated organic molecules from aromatics

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 Julich 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.Peer reviewe

Secondary organic aerosol reduced by mixture of atmospheric vapours

Secondary organic aerosol contributes to the atmospheric particle burden with implications for air quality and climate. Biogenic volatile organic compounds such as terpenoids emitted from plants are important secondary organic aerosol precursors with isoprene dominating the emissions of biogenic volatile organic compounds globally. However, the particle mass from isoprene oxidation is generally modest compared to that of other terpenoids. Here we show that isoprene, carbon monoxide and methane can each suppress the instantaneous mass and the overall mass yield derived from monoterpenes in mixtures of atmospheric vapours. We find that isoprene 'scavenges' hydroxyl radicals, preventing their reaction with monoterpenes, and the resulting isoprene peroxy radicals scavenge highly oxygenated monoterpene products. These effects reduce the yield of low-volatility products that would otherwise form secondary organic aerosol. Global model calculations indicate that oxidant and product scavenging can operate effectively in the real atmosphere. Thus highly reactive compounds (such as isoprene) that produce a modest amount of aerosol are not necessarily net producers of secondary organic particle mass and their oxidation in mixtures of atmospheric vapours can suppress both particle number and mass of secondary organic aerosol. We suggest that formation mechanisms of secondary organic aerosol in the atmosphere need to be considered more realistically, accounting for mechanistic interactions between the products of oxidizing precursor molecules (as is recognized to be necessary when modelling ozone production).Peer reviewe

Photochemical Smog in China: Scientific challenges and implications for air quality policies

Severe air-pollution events in many parts of China pose a major threat to health and ecosystems [1]. China’s air pollution is concentrated to economically developed areas, such as Beijing–Tianjin– Hebei (BTH) and Pearl–River–Delta (PRD) [2,3]. The situation has received considerable attention in international and national media, including its secondary societal and economic impacts such as lowered productivity, reduced investments and loss of professionals who have the choice of residing elsewhere. Large efforts are today underway from the government to improve the situation by measures to reduce primary emissions (see Airborne Pollution Prevention and Control Action Plan (2013–2017), available at http://www.gov.cn). This will also affect secondary pollutants such as ozone (O3) and particulate matter (PM) but how and to what magnitude are uncertain. The photochemically induced secondary pollutants will add to any severe local urban air pollution but require a very different approach for abatements. In this perspective view, we will address the complexity of photochemical smog while acknowledging the urge for similar descriptions on local urban air pollution as described elsewhere [1–4].Swedish Research Council [639-2013-6917]; National Basic Research Program from Ministry of Science and Technology, China [2013CB228503]; National Natural Science Foundation of China [91544214, 91544226]; Hong Kong Research Grants Council [C-5022-14G]SCI(E)中国科学引文数据库(CSCD)EDITORIAL MATERIAL4401-403

Size-dependent hygroscopicity parameter ( κ ) and chemical composition of secondary organic cloud condensation nuclei

Secondary organic aerosol components (SOA) contribute significantly to the activation of cloud condensation nuclei (CCN) in the atmosphere. The CCN activity of internally mixed submicron SOA particles is often parameterized assuming a size-independent single-hygroscopicity parameter κ. In the experiments done in a large atmospheric reactor (SAPHIR, Simulation of Atmospheric PHotochemistry In a large Reaction chamber, Jülich), we consistently observed size-dependent κ and particle composition for SOA from different precursors in the size range of 50 nm–200 nm. Smaller particles had higher κ and a higher degree of oxidation, although all particles were formed from the same reaction mixture. Since decreasing volatility and increasing hygroscopicity often covary with the degree of oxidation, the size dependence of composition and hence of CCN activity can be understood by enrichment of higher oxygenated, low-volatility hygroscopic compounds in smaller particles. Neglecting the size dependence of κ can lead to significant bias in the prediction of the activated fraction of particles during cloud formation

Enrichment of 13C in diacids and related compounds during photochemical processing of aqueous aerosols: New proxy for organic aerosols aging

To investigate the applicability of compound specific stable carbon isotope ratios (delta C-13) of organics in assessment of their photochemical aging in the atmosphere, batch UV irradiation experiments were conducted on two ambient (anthropogenic and biogenic) aerosol samples in aqueous phase for 0.5-120 h. The irradiated samples were analyzed for delta C-13 of diacids, glyoxylic acid (omega C-2) and glyoxal. delta C-13 of diacids and related compounds became larger with irradiation time (i.e., aging), except for few cases. In general, delta C-13 of C-2-C-4 diacids showed an increasing trend with decreasing chain length. Based on delta C-13 of diacids and related compounds and their relations to their concentrations, we found that C-2 and C-3 are enriched with C-13 during the photochemical decomposition and production from their higher homologues and oxoacids. Photochemical breakdown of higher (>= C-3) to lower diacids is also important in the enrichment of C-13 in C3-C9 diacids whereas their production from primary precursors causes depletion of C-13. In case of omega C-2 and glyoxal, their photochemical production and further oxidation to highly oxygenated compounds both cause the enrichment of C-13. This study reveals that delta C-13 of diacids and related compounds can be used as a proxy to trace the aging of organic aerosols during long-range atmospheric transport