| 1. |
- Algaba, Juan-Carlos, et al.
(författare)
-
Broadband Multi-wavelength Properties of M87 during the 2017 Event Horizon Telescope Campaign
- 2021
-
Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 911:1
-
Forskningsöversikt (refereegranskat)abstract
- In 2017, the Event Horizon Telescope (EHT) Collaboration succeeded in capturing the first direct image of the center of the M87 galaxy. The asymmetric ring morphology and size are consistent with theoretical expectations for a weakly accreting supermassive black hole of mass ∼6.5 × 109 M o˙. The EHTC also partnered with several international facilities in space and on the ground, to arrange an extensive, quasi-simultaneous multi-wavelength campaign. This Letter presents the results and analysis of this campaign, as well as the multi-wavelength data as a legacy data repository. We captured M87 in a historically low state, and the core flux dominates over HST-1 at high energies, making it possible to combine core flux constraints with the more spatially precise very long baseline interferometry data. We present the most complete simultaneous multi-wavelength spectrum of the active nucleus to date, and discuss the complexity and caveats of combining data from different spatial scales into one broadband spectrum. We apply two heuristic, isotropic leptonic single-zone models to provide insight into the basic source properties, but conclude that a structured jet is necessary to explain M87's spectrum. We can exclude that the simultaneous γ-ray emission is produced via inverse Compton emission in the same region producing the EHT mm-band emission, and further conclude that the γ-rays can only be produced in the inner jets (inward of HST-1) if there are strongly particle-dominated regions. Direct synchrotron emission from accelerated protons and secondaries cannot yet be excluded.
|
|
| 2. |
- King, Martin D., et al.
(författare)
-
Oxidation of oleic acid at the air-water interface and its potential effects on cloud critical supersaturations
- 2009
-
Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 11:35, s. 7699-7707
-
Tidskriftsartikel (refereegranskat)abstract
- The oxidation of organic films on cloud condensation nuclei has the potential to affect climate and precipitation events. In this work we present a study of the oxidation of a monolayer of deuterated oleic acid (cis-9-octadecenoic acid) at the air-water interface by ozone to determine if oxidation removes the organic film or replaces it with a product film. A range of different aqueous sub-phases were studied. The surface excess of deuterated material was followed by neutron reflection whilst the surface pressure was followed using a Wilhelmy plate. The neutron reflection data reveal that approximately half the organic material remains at the air-water interface following the oxidation of oleic acid by ozone, thus cleavage of the double bond by ozone creates one surface active species and one species that partitions to the bulk (or gas) phase. The most probable products, produced with a yield of similar to(87 +/- 14)%, are nonanoic acid, which remains at the interface, and azelaic acid (nonanedioic acid), which dissolves into the bulk solution. We also report a surface bimolecular rate constant for the reaction between ozone and oleic acid of (7.3 +/- 0.9) x 10(-11) cm(2) molecule s(-1). The rate constant and product yield are not affected by the solution sub-phase. An uptake coefficient of ozone on the oleic acid monolayer of similar to 4 x 10(-6) is estimated from our results. A simple Kohler analysis demonstrates that the oxidation of oleic acid by ozone on an atmospheric aerosol will lower the critical supersaturation needed for cloud droplet formation. We calculate an atmospheric chemical lifetime of oleic acid of 1.3 hours, significantly longer than laboratory studies on pure oleic acid particles suggest, but more consistent with field studies reporting oleic acid present in aged atmospheric aerosol.
|
|
| 3. |
- King, Martin D., et al.
(författare)
-
The reaction of oleic acid monolayers with gas-phase ozone at the air water interface : the effect of sub-phase viscosity, and inert secondary components
- 2020
-
Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : ROYAL SOC CHEMISTRY. - 1463-9076 .- 1463-9084. ; 22:48, s. 28032-28044
-
Tidskriftsartikel (refereegranskat)abstract
- Organic films that form on atmospheric particulate matter change the optical and cloud condensation nucleation properties of the particulate matter and consequently have implications for modern climate and climate models. The organic films are subject to attack from gas-phase oxidants present in ambient air. Here we revisit in greater detail the oxidation of a monolayer of oleic acid by gas-phase ozone at the air-water interface as this provides a model system for the oxidation reactions that occur at the air-water interface of aqueous atmospheric aerosol. Experiments were performed on monolayers of oleic acid at the air-liquid interface at atmospherically relevant ozone concentrations to investigate if the viscosity of the sub-phase influences the rate of the reaction and to determine the effect of the presence of a second component within the monolayer, stearic acid, which is generally considered to be non-reactive towards ozone, on the reaction kinetics as determined by neutron reflectometry measurements. Atmospheric aerosol can be extremely viscous. The kinetics of the reaction were found to be independent of the viscosity of the sub-phase below the monolayer over a range of moderate viscosities, eta/eta water = 1.0-7.2, demonstrating no involvement of aqueous sub-phase oxidants in the rate determining step. The kinetics of oxidation of monolayers of pure oleic acid were found to depend on the surface coverage with different behaviour observed above and below a surface coverage of oleic acid of similar to 1 x 10(18) molecule m(-2). Atmospheric aerosol are typically complex mixtures, and the presence of an additional compound in the monolayer that is inert to direct ozone oxidation, stearic acid, did not significantly change the reaction kinetics. It is demonstrated that oleic acid monolayers at the air-water interface do not leave any detectable material at the air-water interface, contradicting the previous work published in this journal which the authors now believe to be erroneous. The combined results presented here indicate that the kinetics, and thus the atmospheric chemical lifetime for unsaturated surface active materials at the air-water interface to loss by reaction with gas-phase ozone, can be considered to be independent of other materials present at either the air-water interface or in the aqueous sub-phase.
|
|
| 4. |
|
|
| 5. |
- Pfrang, Christian, et al.
(författare)
-
Ozonolysis of methyl oleate monolayers at the air-water interface: oxidation kinetics, reaction products and atmospheric implications
- 2014
-
Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084. ; 16:26, s. 13220-13228
-
Tidskriftsartikel (refereegranskat)abstract
- Ozonolysis of methyl oleate monolayers at the air-water interface results in surprisingly rapid loss of material through cleavage of the C=C bond and evaporation/ dissolution of reaction products. We determine using neutron reflectometry a rate coefficient of (5.7 +/- 0.9) x 10(-10) cm(2) molecule(-1) s(-1) and an uptake coefficient of similar to 3 x 10(-5) for the oxidation of a methyl ester monolayer: the atmospheric lifetime is similar to 10 min. We obtained direct experimental evidence that <2% of organic material remains at the surface on atmospheric timescales. Therefore known long atmospheric residence times of unsaturated fatty acids suggest that these molecules cannot be present at the interface throughout their ageing cycle, i.e. the reported atmospheric longevity is likely to be attributed to presence in the bulk and viscosity-limited reactive loss. Possible reaction products were characterized by ellipsometry and uncertainties in the atmospheric fate of organic surfactants such as oleic acid and its methyl ester are discussed. Our results suggest that a minor change to the structure of the molecule (fatty acid vs. its methyl ester) considerably impacts on reactivity and fate of the organic film.
|
|
| 6. |
- Sebastiani, Federica, et al.
(författare)
-
Night-time oxidation at the air-water interface : co-surfactant effects in binary mixtures
- 2022
-
Ingår i: Environmental Science. - : Royal Society of Chemistry. - 2634-3606. ; :6, s. 1324-1337
-
Tidskriftsartikel (refereegranskat)abstract
- The ageing of organic-coated aqueous aerosols at night is investigated by reacting NO3 with binary surfactant mixtures floating on water. The surfactants are oleic acid (OA), methyl oleate (MO) and stearic acid (SA). Deuterated surfactants mixed with hydrogenous surfactants were studied using neutron reflectometry to determine the reaction kinetics of organic two-component monolayers with NO3 at the air-water interface for the first time. We measured the rate coefficients for OA monolayers, mixed with hydrogenous co-surfactant MO or SA to be (3 +/- 1) x 10-8 cm2 per molecule per s or (3.6 +/- 0.9) x 10-8 cm2 per molecule per s and MO monolayers mixed with hydrogenous co-surfactant OA or SA to be (0.7 +/- 0.4) x 10-8 cm2 per molecule per s or (3 +/- 1) x 10-8 cm2 per molecule per s. The initial desorption lifetimes of NO3, taud,NO3,1, were 8 +/- 3 ns, 14 +/- 4 ns, 12 +/- 3 ns and 21 +/- 10 ns. The approximately doubled desorption lifetime for MO-SA compared to the other mixtures is consistent with a more accessible double bond associated with the larger area per molecule of MO in the presence of SA facilitating NO3 attack. The significantly slower reactive loss of MO-OA compared to a MO monolayer demonstrates that multi-component surfactant mixtures need to be studied in addition to single-component monolayers. Such a retarded decay would cause the residence time to change from ca. 4 to 22 minutes associated with increased transport distances of surfactant species together with any other pollutants that may be protected underneath the surfactant film.
|
|
| 7. |
- Sebastiani, Federica, et al.
(författare)
-
Nighttime oxidation of surfactants at the air-water interface : effects of chain length, head group and saturation
- 2018
-
Ingår i: Atmospheric Chemistry And Physics. - : European Geosciences Union (EGU). - 1680-7316 .- 1680-7324. ; 18:5, s. 3249-3268
-
Tidskriftsartikel (refereegranskat)abstract
- Reactions of the key atmospheric nighttime oxidant NO3 with organic monolayers at the air-water interface are used as proxies for the ageing of organic-coated aqueous aerosols. The surfactant molecules chosen for this study are oleic acid (OA), palmitoleic acid (POA), methyl oleate ( MO) and stearic acid ( SA) to investigate the effects of chain length, head group and degree of unsaturation on the reaction kinetics and products formed. Fully and partially deuterated surfactants were studied using neutron reflectometry (NR) to determine the reaction kinetics of organic monolayers with NO3 at the air-water interface for the first time. Kinetic modelling allowed us to determine the rate coefficients for the oxidation of OA, POA and MO monolayers to be (2.8 +/- 0.7) x 10(-8), (2.4 +/- 0.5) x 10(-8) and (3.3 +/- 0.6) x 10(-8) cm(2) molecule(-1) s(-1) for fitted initial desorption lifetimes of NO3 at the closely packed organic monolayers, tau(d), NO3, 1, of 8.1 +/- 4.0, 16 +/- 4.0 and 8.1 +/- 3.0 ns, respectively. The approximately doubled desorption lifetime found in the best fit for POA compared to OA and MO is consistent with a more accessible double bond associated with the shorter alkyl chain of POA facilitating initial NO3 attack at the double bond in a closely packed monolayer. The corresponding uptake coefficients for OA, POA and MO were found to be (2.1 +/- 0.5) x 10(-3), (1.7 +/- 0.3) x 10(-3) and (2.1 +/- 0.4) x 10(-3), respectively. For the much slower NO3-initiated oxidation of the saturated surfactant SA we estimated a loss rate of approximately (5 +/- 1) x 10(-12) cm(2) molecule(-1) s(-1), which we consider to be an upper limit for the reactive loss, and estimated an uptake coefficient of ca, (5 +/- 1) x 10(-7). Our investigations demonstrate that NO3 will contribute substantially to the processing of unsaturated surfactants at the air-water interface during nighttime given its reactivity is ca. 2 orders of magnitude higher than that of O-3. Furthermore, the relative contributions of NO3 and O-3 to the oxidative losses vary massively between species that are closely related in structure: NO3 reacts ca. 400 times faster than O-3 with the common model surfactant oleic acid, but only ca. 60 times faster with its methyl ester MO. It is therefore necessary to perform a case-by-case assessment of the relative contributions of the different degradation routes for any specific surfactant. The overall impact of NO3 on the fate of saturated surfactants is slightly less clear given the lack of prior kinetic data for comparison, but NO3 is likely to contribute significantly to the loss of saturated species and dominate their loss during nighttime. The retention of the organic character at the air-water interface differs fundamentally between the different surfactant species: the fatty acids studied (OA and POA) form products with a yield of similar to 20% that are stable at the interface while NO3=initiated oxidation of the methyl ester MO rapidly and effectively removes the organic character (<= 3% surface-active products). The film-forming potential of reaction products in real aerosol is thus likely to depend on the relative proportions of saturated and unsaturated surfactants as well as the head group properties. Atmospheric lifetimes of unsaturated species are much longer than those determined with respect to their reactions at the air-water interface, so they must be protected from oxidative attack, for example, by incorporation into a complex aerosol matrix or in mixed surface films with yet unexplored kinetic behaviour.
|
|
| 8. |
- Seddon, Annela M., et al.
(författare)
-
Control of Nanomaterial Self-Assembly in Ultrasonically Levitated Droplets
- 2016
-
Ingår i: The Journal of Physical Chemistry Letters. - 1948-7185. ; 7:7, s. 1341-1345
-
Tidskriftsartikel (refereegranskat)abstract
- We demonstrate that acoustic trapping can be used to levitate and manipulate droplets of soft matter, in particular, lyotropic mesophases formed from selfassembly of different surfactants and lipids, which can be analyzed in a contact-less manner by X-ray scattering in a controlled gas-phase environment. On the macroscopic length scale, the dimensions and the orientation of the particle are shaped by the ultrasonic field, while on the microscopic length scale the nanostructure can be controlled by varying the humidity of the atmosphere around the droplet. We demonstrate levitation and in situ phase transitions of micellar, hexagonal, bicontinuous cubic, and lamellar phases. Thetechnique opens up a wide range of new experimental approaches of fundamentalimportance for environmental, biological, and chemical research.
|
|
| 9. |
- Shepherd, Rosalie H., et al.
(författare)
-
Measurement of gas-phase OH radical oxidation and film thickness of organic films at the air-water interface using material extracted from urban, remote and wood smoke aerosol
- 2022
-
Ingår i: Environmental Science. - : Royal Society of Chemistry (RSC). - 2634-3606. ; 2:4, s. 574-590
-
Tidskriftsartikel (refereegranskat)abstract
- The presence of an organic film on a cloud droplet or aqueous aerosol particle has the potential to alter the chemical, optical and physical properties of the droplet or particle. In the study presented, water insoluble organic materials extracted from urban, remote (Antarctica) and wood burning atmospheric aerosol were found to have stable, compressible, films at the air-water interface that were typically similar to 6-18 angstrom thick. These films are reactive towards gas-phase OH radicals and decay exponentially, with bimolecular rate constants for reaction with gas-phase OH radicals of typically 0.08-1.5 x 10(-10) cm(3) molecule(-1) s(-1). These bimolecular rate constants equate to initial OH radical uptake coefficients estimated to be similar to 0.6-1 except woodsmoke (similar to 0.05). The film thickness and the neutron scattering length density of the extracted atmosphere aerosol material (from urban, remote and wood burning) were measured by neutron reflection as they were exposed to OH radicals. For the first time neutron reflection has been demonstrated as an excellent technique for studying the thin films formed at air-water interfaces from materials extracted from atmospheric aerosol samples. Additionally, the kinetics of gas-phase OH radicals with a proxy compound, the lipid 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) was studied displaying significantly different behaviour, thus demonstrating it is not a good proxy for atmospheric materials that may form films at the air-water interface. The atmospheric lifetimes, with respect to OH radical oxidation, of the insoluble organic materials extracted from atmospheric aerosol at the air-water interface were a few hours. Relative to a possible physical atmospheric lifetime of 4 days, the oxidation of these films is important and needs inclusion in atmospheric models. The optical properties of these films were previously reported [Shepherd et al., Atmos. Chem. Phys., 2018, 18, 5235-5252] and there is a significant change in top of the atmosphere albedo for these thin films on core-shell atmospheric aerosol using the film thickness data and confirmation of stable film formation at the air-water interface presented here.
|
|
| 10. |
- Thompson, Katherine C., et al.
(författare)
-
Reaction of a Phospholipid Monolayer with Gas-Phase Ozone at the Air-Water Interface : Measurement of Surface Excess and Surface Pressure in Real Time
- 2010
-
Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 26:22, s. 17295-17303
-
Tidskriftsartikel (refereegranskat)abstract
- The reaction between gas-phase ozone and monolayers of the unsaturated lipid 1-palmitoy1-2-oleoyl-sn-glycero-3-phosphocholine, POPC, on aqueous solutions has been studied in real time using neutron reflection and surface pressure measurements. The reaction between ozone and lung surfactant, which contains POPC, leads to decreased pulmonary function, but little is known shout the changes that occur to the interfacial material as a result of oxidation. The results reveal that the initial reaction of ozone with POPC leads to a rapid increase in surface pressure followed by a slow decrease to very low values. The neutron reflection measurements, performed on an isotopologue of POPC with a selectively deuterated palmitoyl strand, reveal that the reaction leads to loss of this strand from the air-water interface. suggesting either solubilization of the product lipid or degradation of the palmitoyl strand by a reactive species. Reactions of H-1-POPC on D2O reveal that the headgroup region of the lipids in aqueous solution is not dramatically perturbed by the reaction of POPC monolayers with ozone supporting degradation of the palmitoyl strand rather than solubilization. The results are consistent with the reaction of ozone with the oleoyl strand of POPC at the air water interface leading to the formation of OH radicals. the highly reactive OH radicals produced can then go on to react with the saturated palmitoyl strands leading to the formation or oxidized lipids with shorter alkyl tails.
|
|
