Elemental compositions of organic aerosol (OA) particles provide useful constraints on OA sources, chemical evolution, and effects. The Aerodyne high-resolution time-of-flight aerosol mass ...spectrometer (HR-ToF-AMS) is widely used to measure OA elemental composition. This study evaluates AMS measurements of atomic oxygen-to-carbon (O : C), hydrogen-to-carbon (H : C), and organic mass-to-organic carbon (OM : OC) ratios, and of carbon oxidation state (OS C) for a vastly expanded laboratory data set of multifunctional oxidized OA standards. For the expanded standard data set, the method introduced by Aiken et al. (2008), which uses experimentally measured ion intensities at all ions to determine elemental ratios (referred to here as "Aiken-Explicit"), reproduces known O : C and H : C ratio values within 20% (average absolute value of relative errors) and 12%, respectively. The more commonly used method, which uses empirically estimated H2O+ and CO+ ion intensities to avoid gas phase air interferences at these ions (referred to here as "Aiken-Ambient"), reproduces O : C and H : C of multifunctional oxidized species within 28 and 14% of known values. The values from the latter method are systematically biased low, however, with larger biases observed for alcohols and simple diacids. A detailed examination of the H2O+, CO+, and CO2+ fragments in the high-resolution mass spectra of the standard compounds indicates that the Aiken-Ambient method underestimates the CO+ and especially H2O+ produced from many oxidized species. Combined AMS-vacuum ultraviolet (VUV) ionization measurements indicate that these ions are produced by dehydration and decarboxylation on the AMS vaporizer (usually operated at 600 degree C). Thermal decomposition is observed to be efficient at vaporizer temperatures down to 200 degree C. These results are used together to develop an "Improved-Ambient" elemental analysis method for AMS spectra measured in air. The Improved-Ambient method uses specific ion fragments as markers to correct for molecular functionality-dependent systematic biases and reproduces known O : C (H : C) ratios of individual oxidized standards within 28% (13%) of the known molecular values. The error in Improved-Ambient O : C (H : C) values is smaller for theoretical standard mixtures of the oxidized organic standards, which are more representative of the complex mix of species present in ambient OA. For ambient OA, the Improved-Ambient method produces O : C (H : C) values that are 27% (11%) larger than previously published Aiken-Ambient values; a corresponding increase of 9% is observed for OM : OC values. These results imply that ambient OA has a higher relative oxygen content than previously estimated. The OS C values calculated for ambient OA by the two methods agree well, however (average relative difference of 0.06 OS C units). This indicates that OS C is a more robust metric of oxidation than O : C, likely since OS C is not affected by hydration or dehydration, either in the atmosphere or during analysis.
Secondary organic aerosol (SOA) and oxidized primary organic aerosol (OPOA) were produced in laboratory experiments from the oxidation of fourteen precursors representing atmospherically relevant ...biogenic and anthropogenic sources. The SOA and OPOA particles were generated via controlled exposure of precursors to OH radicals and/or O3 in a Potential Aerosol Mass (PAM) flow reactor over timescales equivalent to 1–20 days of atmospheric aging. Aerosol mass spectra of SOA and OPOA were measured with an Aerodyne aerosol mass spectrometer (AMS). The fraction of AMS signal at m/z = 43 and m/z = 44 (f43, f44), the hydrogen-to-carbon (H/C) ratio, and the oxygen-to-carbon (O/C) ratio of the SOA and OPOA were obtained, which are commonly used to characterize the level of oxidation of oxygenated organic aerosol (OOA). The results show that PAM-generated SOA and OPOA can reproduce and extend the observed f44–f43 composition beyond that of ambient OOA as measured by an AMS. Van Krevelen diagrams showing H/C ratio as a function of O/C ratio suggest an oxidation mechanism involving formation of carboxylic acids concurrent with fragmentation of carbon-carbon bonds. Cloud condensation nuclei (CCN) activity of PAM-generated SOA and OPOA was measured as a function of OH exposure and characterized as a function of O/C ratio. CCN activity of the SOA and OPOA, which was characterized in the form of the hygroscopicity parameter κorg, ranged from 8.4×10−4 to 0.28 over measured O/C ratios ranging from 0.05 to 1.42. This range of κorg and O/C ratio is significantly wider than has been previously obtained. To first order, the κorg-to-O/C relationship is well represented by a linear function of the form κorg = (0.18±0.04) ×O/C + 0.03, suggesting that a simple, semi-empirical parameterization of OOA hygroscopicity and oxidation level can be defined for use in chemistry and climate models.
The physical phase state (solid, semi-solid, or liquid) of secondary organic aerosol (SOA) particles has important implications for a number of atmospheric processes. We report the phase state of SOA ...particles spanning a wide range of oxygen to carbon ratios (O / C), used here as a surrogate for SOA oxidation level, produced in a flow tube reactor by photo-oxidation of various atmospherically relevant surrogate anthropogenic and biogenic volatile organic compounds (VOCs). The phase state of laboratory-generated SOA was determined by the particle bounce behavior after inertial impaction on a polished steel substrate. The measured bounce fraction was evaluated as a function of relative humidity and SOA oxidation level (O / C) measured by an Aerodyne high resolution time of flight aerosol mass spectrometer (HR-ToF AMS). The main findings of the study are: (1) biogenic and anthropogenic SOA particles are found to be amorphous solid or semi-solid based on the measured bounced fraction (BF), which was typically higher than 0.6 on a 0 to 1 scale. A decrease in the BF is observed for most systems after the SOA is exposed to relative humidity of at least 80% RH, corresponding to a RH at impaction of 55%. (2) Long-chain alkanes have a low BF (indicating a "liquid-like", less viscous phase) particles at low oxidation levels (BF < 0.2 ± 0.05 for O / C = 0.1). However, BF increases substantially upon increasing oxidation. (3) Increasing the concentration of sulphuric acid (H2SO4) in solid SOA particles (here tested for longifolene SOA) causes a decrease in BF levels. (4) In the majority of cases the bounce behavior of the various SOA systems did not show correlation with the particle O / C. Rather, the molar mass of the gas-phase VOC precursor showed a positive correlation with the resistance to the RH-induced phase change of the formed SOA particles.
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► We measure the temperature of water/oil emulsion droplets during micro-explosion. ► The emulsion droplets were heated by using the Leidenfrost regime. ► The size of the dispersed ...water droplets affects the temperature of micro-explosion. ► Temperature of m-e and size of water droplets are not monotonically correlated. ► Very high metastable temperature of the water were reached before micro-explosion.
The burning of water in oil emulsions is considered as an effective alternative to bring out waste oil because of a significant reduction in carbon monoxide, nitrogen oxides and particulates in the exhaust. These advantages have different origins, an important contribution is provided by the phenomenon of micro-explosion. In this work, the influence of the size of the dispersed water droplets in the micro-explosion phenomenon is studied by the hot plate technique. The variation of the temperature and the evolution of the phenomenon have been investigated using a synchronized thermocouple/high speed imaging system. Three emulsions with the same amount of water (30%mass) but with different distribution of the size of the dispersed water droplets have been tested.
The results show that the size distribution of the dispersed water droplets plays an important role in the phenomenon of micro-explosion. Moreover, some internal phenomenon as the separation process between water and oil seems to affect significantly the phenomenon.
Laboratory experiments investigated the relationship between oxidation level and hygroscopic properties of secondary organic aerosol (SOA) particles generated via OH radical oxidation in an aerosol ...flow reactor. The hygroscopic growth factor at 90% RH (HGF90%), the CCN activity (κORG,CCN) and the level of oxidation (atomic O:C ratio) of the SOA particles were measured. Both HGF90% and κORG,CCN increased with O:C; the HGF90% varied linearly with O:C, while κORG,CCN mostly followed a nonlinear trend. An average HGF90% of 1.25 and κORG,CCN of 0.19 were measured for O:C of 0.65, in agreement with results reported for ambient data. The κORG values estimated from the HGF90% (κORG,HGF) were 20 to 50% lower than paired κORG,CCN values for all SOA particles except 1,3,5‐trimethylbenzene (TMB), the least hygroscopic of the SOA systems. Within the limitations of instrumental capabilities, we show that differences in hygroscopic behavior among the investigated SOA systems may correspond to differences in elemental composition.
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•An experimental study of micro-explosion by means two different approaches has been carried out.•Temperature of micro-explosion and fall in temperature after micro-explosion are ...evaluated.•Separation of the continuous and dispersed compounds has been observed.•The influence of the thermocouple and the heat transfer are discussed.
Combustion of water in oil μ-emulsions is still considered as a useful technology for the energy conversion of waste oil. One of the most relevant advantages is related to the phenomenon of micro-explosion (μ–e) that produces the secondary atomization of the oil. Several experimental approaches have been proposed in the last years with the aim to characterize the μ–e effect under different conditions. In this paper, an experimental comparison between the two useful approaches is presented. The results obtained with the technique of the Suspended droplet will be related to data present in the literature, obtained through the Leidenfrost technique. Quantitative thermal results such as the μ–e temperature and the fall temperature after μ–e show the most important differences. The important role played by the separation process as coalescence and creaming in both approaches is also discussed.
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•The microchannels emulsifier is able to form a rather stable water–oil emulsion.•WiDE resulted stable after 2 h running into the high pressure common rail system.•WiDE emulsion was ...successfully used in an optical diesel engine.•WiDE implied a reduction in PM and NOx compared to that resulting from diesel oil.•The 2D chemiluminescence technique highlights a reduction in soot formation.
The paper reports the results of an experimental investigation carried out on a prototype optically accessible compression ignition engine fueled with water in diesel emulsion (WiDE) and Diesel only. The effect of WiDE on combustion process evolution and exhaust emissions was investigated through standard engine benchmark and optical diagnostics. 2D chemiluminescent emission measurements centered at 690 nm were carried out during the whole combustion process to discriminate the soot emission from other excited chemical species. The emulsion was produced through a prototype designed microchannels emulsifier that can also work inline. The water concentration was 9.1%v with a small amount (0.2%v) of nonionic surfactant (SPAN80) used to stabilize the emulsion. Tests were performed comparing combustion and exhaust emissions of the reference commercial diesel fuel to the WiDE. For any investigated fuel and operating point, engine tests were carried out changing the injection interval to achieve the same chemical energy as the reference diesel (935 J/str). Compared to Diesel, the WiDE induced an increase in ignition time, enhancing the air/fuel mixing with a simultaneous reduction in both PM and NOx. The digital imaging and 2D chemiluminescence techniques highlighted a reduction in soot formation without significant changes on the soot oxidation rate. The results suggest the use of WiDE as a reliable method to improve NOx-soot trade-off of CI engines.
Reliable characterization of particles freshly emitted from the ocean surface requires a sampling method that is able to isolate those particles and prevent them from interacting with ambient gases ...and particles. Here we report measurements of particles directly emitted from the ocean using a newly developed in situ particle generator (Sea Sweep). The Sea Sweep was deployed alongside R/V Atlantis off the coast of California during May of 2010. Bubbles were generated 0.75 m below the ocean surface with stainless steel frits and swept into a hood/vacuum hose to feed a suite of aerosol instrumentation on board the ship. The number size distribution of the directly emitted, nascent particles had a dominant mode at 55–60 nm (dry diameter) and secondary modes at 30–40 nm and 200–300 nm. The nascent aerosol was not volatile at 230°C and was not enriched in SO4=, Ca++, K+, or Mg++above that found in surface seawater. The organic component of the nascent aerosol (7% of the dry submicrometer mass) volatilized at a temperature between 230 and 600°C. The submicrometer organic aerosol characterized by mass spectrometry was dominated by non‐oxygenated hydrocarbons. The nascent aerosol at 50, 100, and 145 nm dry diameter behaved hygroscopically like an internal mixture of sea salt with a small organic component. The CCN/CN activation ratio for 60 nm Sea Sweep particles was near 1 for all supersaturations of 0.3 and higher indicating that all of the particles took up water and grew to cloud drop size. The nascent organic aerosol mass fraction did not increase in regions of higher surface seawater chlorophyll but did show a positive correlation with seawater dimethylsulfide (DMS).
Key Points
The ocean is a source of sub 100nm particles to the atmosphere
Hygroscopically the particles behave like an internal mixture of sea salt/organic
Organic mass fraction did not correlate with chlorophyll
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•An analysis on the evaporation of acetic acid and ethylene glycol is presented.•A multiregion approach for the fluid-solid heat transfer is adopted.•The fiber transports heat towards ...the liquid, which distribute it by convection.•The agreement with the experiments is excellent for all the cases.•Internal convection influences the preferential vaporization in the mixture.
A detailed analysis on the evaporation of acetic acid and ethylene glycol droplets is performed experimentally and numerically. The isolated droplet is positioned in a combustion chamber, suspended on a thermocouple and evaporated in buoyancy driven convection, following the thermal history throughout the droplet lifetime. The experiments provide quantitative and qualitative data on the evaporation physics of acetic acid, ethylene glycol and their mixture. The data are then modeled adopting the multiphase CFD code DropletSMOKE++, describing the flow field around the droplet, the heating rate and the evaporation process. The main novelty introduced in this work is a multiregion approach to describe the solid fiber, which allows to model the conjugate heat transfer with the liquid and the gas phase, as well as its impact on the droplet evaporation. DropletSMOKE++ results show a good agreement with the experimental data, regarding both the diameter decay and the liquid temperature, whose internal distribution in the liquid is shown to be highly affected by the heat flux from the fiber (which can contribute up to 30–40% in the total heat flux on the droplet). The effect of the thermocouple on the evaporation rate has been highlighted simulating the same experiments considering the solid as adiabatic, showing in this case a large underprediction of the vaporization rate and confirming the need of a detailed model for the fiber to correctly predict the vaporization phenomenon. The mixture evaporation has been investigated, emphasizing the importance of adopting a detailed thermodynamic model (which includes activity coefficients) and the impact of the mixture non-ideality on the evaporation process. The mixture also exhibits preferential vaporization, facilitated by the internal convection in the liquid phase.
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•The steady state behavior of a MGT at two different load conditions is studied.•Jet-A1 kerosene and blends with sunflower and rapeseed oils are tested.•Small percentage of SVO added ...to Jet-A1 slightly affects MGT emissions.•PM levels are independent of oil concentration in fuel blends but depend on SVO quality.
The paper reports the experimental results concerning emissions from 30kWe commercial micro gas turbine feed with blends of straight vegetable oil with fossil fuel. Both gaseous and particulate emissions were measured at full and partial load for blends 10% v/v and 20% v/v of rapeseed and sunflower oils with JET A1 kerosene. The variations of nitric oxide and carbon monoxide emissions were taken into account to evaluate the combustor behavior at different loads and blends. The effects of the fuel composition on the variations, in terms of concentrations and dimensions, of ultrafine – particulate matter were also evaluated.
NOx and CO emissions are practically insensitive to the composition of the fuel being the differences within the experimental uncertainty of the instrumentation. This demonstrates a similar overall combustion at both partial and full load for the five fuels used (Jet A1 and four SVO blends). On the contrary, particle matter emission is three times for blends of rapeseed oil and more than fifty times greater for blends of sunflower oil with respect to pure Jet A1. The differences are ascribed to the chemical structure of the vegetable oil tested.
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