Identifying Criegee intermediates as potential oxidants in the troposphere

Abstract and poster presented at the AGU Fall meeting, San Francisco 2015.Criegee intermediates (CI) are formed during the ozonolysis of unsaturated compounds and have been intensively studied in the last few years due to their possible role as oxidants in the troposphere. Stabilised CI (SCI) are now known to react very rapidly, k(298 K) = 10-12 to 10-10 cm3 molecule-1 s-1, with a large number of trace gases (SO2, NO2, organic acids, water dimers). Still, it remains challenging to assess their effective oxidative capacity, as CI chemistry is complex, spans a large range of rate coefficients for different SCI conformers reacting with water dimers and trace gases, and in addition no reliable measurement technique able to detect ambient SCI concentrations is currently available. In this study, we examine the extensive dataset from the HUMPPA-COPEC 2010 and the HOPE 2012 field campaigns, aided by literature data, to estimate the abundance of SCI in the lower troposphere. The budget of SCI is analyzed using four different approaches: 1) based on an observed yet unexplained H2SO4 production; 2) from the measured concentrations of unsaturated volatile organic compounds (VOC); 3) from OH reactivity measurements; 4) from the unexplained production rate of OH. A SCI concentration range between 5 x 103 and 2 x 106 molecule cm-3 is calculated for the two environments. The central weighted estimate of the SCI concentration over the boreal forest of ~ 5 x 104 molecules cm-3 implies a significant impact on the conversion of SO2 into H2SO4. In addition, we present measurements obtained using our inlet pre-injector laser-induced fluorescence assay by gas expansion technique (IPI-LIF-FAGE) for the above-mentioned campaigns. A recent laboratory study performed with the same instrumental setup showed that the IPI-LIF-FAGE system is sensitive to the detection of the OH formed from unimolecular decomposition of SCI. Building on these measurements, the background OH (OHbg) measured during the two field campaigns is investigated in comparison with many other trace gases to assess if the observations in controlled conditions are transferable to ambient conditions

Identifying Criegee intermediates as potential oxidants in the troposphere

Discussion started 24/10/16We analysed the extensive dataset from the HUMPPA-COPEC 2010 and the HOPE 2012 field campaigns in the boreal forest and rural environments of Finland and Germany, respectively, and estimated the abundance of stabilised Criegee intermediates (SCI) in the lower troposphere. Based on laboratory tests, we propose that the background OH signal observed in our IPI-LIF-FAGE instrument during the afore-mentioned campaigns is caused at least partially by SCI. This hypothesis is based on observed correlations with temperature and with concentrations of unsaturated volatile organic compounds and ozone. The background OH concentration also complements the previously underestimated production rate of sulfuric acid and is consistent with its scavenging through the addition of sulphur dioxide. A central estimate of the SCI concentration of ~ 5 × 104 molecules cm−3 (with an order of magnitude uncertainty) is calculated for the two environments. This implies a very low ambient concentration of SCI, though, over the boreal forest, significant for the conversion of SO2 into H2SO4. The large uncertainties in these calculations, owing to the many unknowns in the chemistry of Criegee intermediates, emphasise the need to better understand these processes and their potential effect on the self-cleaning capacity of the atmosphere

Solar eclipse demonstrating the importance of photochemistry in new particle formation

Solar eclipses provide unique possibilities to investigate atmospheric processes, such as new particle formation (NPF), important to the global aerosol load and radiative balance. The temporary absence of solar radiation gives particular insight into different oxidation and clustering processes leading to NPF. This is crucial because our mechanistic understanding on how NPF is related to photochemistry is still rather limited. During a partial solar eclipse over Finland in 2015, we found that this phenomenon had prominent effects on atmospheric on-going NPF. During the eclipse, the sources of aerosol precursor gases, such as sulphuric acid and nitrogen-containing highly oxidised organic compounds, decreased considerably, which was followed by a reduced formation of small clusters and nanoparticles and thus termination of NPF. After the eclipse, aerosol precursor molecule concentrations recovered and reinitiated NPF. Our results provide direct evidence on the key role of the photochemical production of sulphuric acid and highly oxidized organic compounds in maintaining atmospheric NPF. Our results also explain the rare occurrence of this phenomenon under dark conditions, as well as its seemingly weak connection with atmospheric ions.Peer reviewe

Sources and sinks driving sulfuric acid concentrations in contrasting environments : implications on proxy calculations

Sulfuric acid has been shown to be a key driver for new particle formation and subsequent growth in various environments, mainly due to its low volatility. However, direct measurements of gas-phase sulfuric acid are oftentimes not available, and the current sulfuric acid proxies cannot predict, for example, its nighttime concentrations or result in significant discrepancies with measured values. Here, we define the sources and sinks of sulfuric acid in different environments and derive a new physical proxy for sulfuric acid to be utilized in locations and during periods when it is not measured. We used H2SO4 measurements from four different locations: Hyytiala, Finland; Agia Marina, Cyprus; Budapest, Hungary; and Beijing, China, representing semi-pristine boreal forest, rural environment in the Mediterranean area, urban environment and heavily polluted megacity, respectively. The new proxy takes into account the formation of sulfuric acid from SO2 via OH oxidation and other oxidation pathways, specifically via stabilized Criegee intermediates. The sulfuric acid sinks included in the proxy are its condensation sink (CS) and atmospheric clustering starting from H2SO4 dimer formation. Indeed, we found that the observed sulfuric acid concentration can be explained by the proposed sources and sinks with similar coefficients in the four contrasting environments where we have tested it. Thus, the new proxy is a more flexible and an important improvement over previous proxies. Following the recommendations in this paper, a proxy for a specific location can be derived.Peer reviewe

Enhanced growth rate of atmospheric particles from sulfuric acid

In the present-day atmosphere, sulfuric acid is the most important vapour for aerosol particle formation and initial growth. However, the growth rates of nanoparticles (<10 nm) from sulfuric acid remain poorly measured. Therefore, the effect of stabilizing bases, the contribution of ions and the impact of attractive forces on molecular collisions are under debate. Here, we present precise growth rate measurements of uncharged sulfuric acid particles from 1.8 to 10 nm, performed under atmospheric conditions in the CERN (European Organization for Nuclear Research) CLOUD chamber. Our results show that the evaporation of sulfuric acid particles above 2 nm is negligible, and growth proceeds kinetically even at low ammonia concentrations. The experimental growth rates exceed the hard-sphere kinetic limit for the condensation of sulfuric acid. We demonstrate that this results from van derWaals forces between the vapour molecules and particles and disentangle it from charge-dipole interactions. The magnitude of the enhancement depends on the assumed particle hydration and collision kinetics but is increasingly important at smaller sizes, resulting in a steep rise in the observed growth rates with decreasing size. Including the experimental results in a global model, we find that the enhanced growth rate of sulfuric acid particles increases the predicted particle number concentrations in the upper free troposphere by more than 50 %.Peer reviewe

The effect of acid-base clustering and ions on the growth of atmospheric nano-particles

The growth of freshly formed aerosol particles can be the bottleneck in their survival to cloud condensation nuclei. It is therefore crucial to understand how particles grow in the atmosphere. Insufficient experimental data has impeded a profound understanding of nano-particle growth under atmospheric conditions. Here we study nano-particle growth in the CLOUD (Cosmics Leaving OUtdoors Droplets) chamber, starting from the formation of molecular clusters. We present measured growth rates at sub-3 nm sizes with different atmospherically relevant concentrations of sulphuric acid, water, ammonia and dimethylamine. We find that atmospheric ions and small acid-base clusters, which are not generally accounted for in the measurement of sulphuric acid vapour, can participate in the growth process, leading to enhanced growth rates. The availability of compounds capable of stabilizing sulphuric acid clusters governs the magnitude of these effects and thus the exact growth mechanism. We bring these observations into a coherent framework and discuss their significance in the atmosphere.Peer reviewe

Hydroxyl radical-induced formation of highly oxidized organic compounds

Explaining the formation of secondary organic aerosol is an intriguing question in atmospheric sciences because of its importance for Earth’s radiation budget and the associated effects on health and ecosystems. A breakthrough was recently achieved in the understanding of secondary organic aerosol formation from ozone reactions of biogenic emissions by the rapid formation of highly oxidized multifunctional organic compounds via autoxidation. However, the important daytime hydroxyl radical reactions have been considered to be less important in this process. Here we report measurements on the reaction of hydroxyl radicals with a- and b-pinene applying improved mass spectrometric methods. Our laboratory results prove that the formation of highly oxidized products from hydroxyl radical reactions proceeds with considerably higher yields than previously reported. Field measurements support these findings. Our results allow for a better description of the diurnal behaviour of the highly oxidized product formation and subsequent secondary organic aerosol formation in the atmosphere

Quantification of liver perfusion with [(15)O]H(2)O-PET and its relationship with glucose metabolism and substrate levels.: Quantification of liver perfusion with PET

International audienceBACKGROUND/AIMS: Hepatic perfusion plays an important role in liver physiology and disease. This study was undertaken to (a) validate the use of Positron Emission Tomography (PET) and oxygen-15-labeled water ([(15)O]H(2)O) to quantify hepatic and portal perfusion, and (b) examine relationships between portal perfusion and liver glucose and lipid metabolism. METHODS: Liver [(15)O]H(2)O-PET images were obtained in 14 pigs during fasting or hyperinsulinemia. Carotid arterial and portal venous blood were sampled for [(15)O]H(2)O activity; Doppler ultrasonography was used invasively as the reference method. A single arterial input compartment model was developed to estimate portal tracer kinetics and liver perfusion. Endogenous glucose production (EGP) and insulin-mediated whole body glucose uptake (wbGU) were determined by standard methods. RESULTS: Hepatic arterial and portal venous perfusions were 0.15+/-0.07 and 1.11+/-0.34 ml/min/ml of tissue, respectively. The agreement between ultrasonography and [(15)O]H(2)O-PET was good for total and portal liver perfusion, and poor for arterial perfusion. Portal perfusion was correlated with EGP (r=or+0.62, p=0.03), triglyceride (r=or+0.66, p=0.01), free fatty acid levels (r=or+0.76, p=0.003), and plasma lactate levels (r=or-0.81, p=0.0009). CONCLUSIONS: Estimates of liver perfusion by [(15)O]H(2)O-PET compared well with those by ultrasonography. The method allowed to predict portal tracer concentrations which is essential in human studies. Portal perfusion may affect liver nutrient handling