The physics of water-in-oil emulsion droplet microexplosion/puffing has been investigated using high-fidelity interface-capturing simulation. Varying the dispersed-phase (water) sub-droplet size/location and the initiation location of explosive boiling (bubble formation), the droplet breakup processes have been well revealed. The bubble growth leads to local and partial breakup of the parent oil droplet, i.e., puffing. The water sub-droplet size and location determine the after-puffing dynamics. The boiling surface of the water sub-droplet is unstable and evolves further. Finally, the sub-droplet is wrapped by boiled water vapor and detaches itself from the parent oil droplet. When the water sub-droplet is small, the detachment is quick, and the oil droplet breakup is limited. When it is large and initially located toward the parent droplet center, the droplet breakup is more extensive. For microexplosion triggered by the simultaneous growth of multiple separate bubbles, each explosion is local and independent initially, but their mutual interactions occur at a later stage. The degree of breakup can be larger due to interactions among multiple explosions. These findings suggest that controlling microexplosion/puffing is possible in a fuel spray, if the emulsion-fuel blend and the ambient flow conditions such as heating are properly designed. The current study also gives us an insight into modeling the puffing and microexplosion of emulsion droplets and sprays.

PMCAMx-2008, a three dimensional chemical transport model (CTM), was applied in Europe to quantify the changes in fine particle (PM_2.5) concentration in response to different emission reductions as well as to temperature increase. A summer and a winter simulation period were used, to investigate the seasonal dependence of the PM_2.5 response to 50% reductions of sulfur dioxide (SO₂), ammonia (NH₃), nitrogen oxides (NO_x), anthropogenic volatile organic compounds (VOCs) and anthropogenic primary organic aerosol (POA) emissions and also to temperature increases of 2.5 and 5 K. Reduction of NH₃ emissions seems to be the most effective control strategy for reducing PM_2.5, in both periods, resulting in a decrease of PM_2.5 up to 5.1 μg m⁻³ and 1.8 μg m⁻³ (5.5% and 4% on average) during summer and winter respectively, mainly due to reduction of ammonium nitrate (NH₄NO₃) (20% on average in both periods). The reduction of SO₂ emissions decreases PM_2.5 in both periods having a significant effect over the Balkans (up to 1.6 μg m⁻³) during the modeled summer period, mainly due to decrease of sulfate (34% on average over the Balkans). The anthropogenic POA control strategy reduces total OA by 15% during the modeled winter period and 8% in the summer period. The reduction of total OA is higher in urban areas close to its emissions sources. A slight decrease of OA (8% in the modeled summer period and 4% in the modeled winter period) is also predicted after a 50% reduction of VOCs emissions due to the decrease of anthropogenic SOA. The reduction of NO_x emissions reduces PM_2.5 (up to 3.4 μg m⁻³) during the summer period, due to a decrease of NH₄NO₃, causing although an increase of ozone concentration in major urban areas and over Western Europe. Additionally, the NO_x control strategy actually increases PM_2.5 levels during the winter period, due to more oxidants becoming available to react with SO₂ and VOCs. The increase of temperature results in a decrease of PM_2.5 in both periods over Central Europe, mainly due to a decrease of NH₄NO₃ during summer (18%) and fresh POA during wintertime (35%). Significant increases of OA are predicted during the summer due mainly to the increase of biogenic VOC emissions. On the contrary, OA is predicted to decrease in the modeled winter period due to the dominance of fresh POA reduction and the small biogenic SOA contribution to OA. The resulting increase of oxidant levels from the temperature rise lead to an increase of sulfate levels in both periods, mainly over North Europe and the Atlantic Ocean. The substantial reduction of PM_2.5 components due to emissions reductions of their precursors outlines the importance of emissions for improving air quality, while the sensitivity of PM_2.5

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A detailed characterization of air quality in the megacity of Paris (France) during two 1-month intensive campaigns and from additional 1-year observations revealed that about 70% of the urban background fine particulate matter (PM) is transported on average into the megacity from upwind regions. This dominant influence of regional sources was confirmed by in situ measurements during short intensive and longer-term campaigns, aerosol optical depth (AOD) measurements from ENVISAT, and modeling results from PMCAMx and CHIMERE chemistry transport models. While advection of sulfate is well documented for other megacities, there was surprisingly high contribution from long-range transport for both nitrate and organic aerosol. The origin of organic PM was investigated by comprehensive analysis of aerosol mass spectrometer (AMS), radiocarbon and tracer measurements during two intensive campaigns. Primary fossil fuel combustion emissions constituted less than 20% in winter and 40% in summer of carbonaceous fine PM, unexpectedly small for a megacity. Cooking activities and, during winter, residential wood burning are the major primary organic PM sources. This analysis suggests that the major part of secondary organic aerosol is of modern origin, i.e., from biogenic precursors and from wood burning. Black carbon concentrations are on the lower end of values encountered in megacities worldwide, but still represent an issue for air quality. These comparatively low air pollution levels are due to a combination of low emissions per inhabitant, flat terrain, and a meteorology that is in general not conducive to local pollution build-up. This revised picture of a megacity only being partially responsible for its own average and peak PM levels has important implications for air pollution regulation policies

In this study, the application of exhaust gas-assisted fuel reforming in compression ignition engines (CI) has been investigated. Experiments were conducted in a single-cylinder directinjection (DI) diesel engine fueled by conventional diesel and also by a biodiesel mixture. First, the effects of exhaust gas recirculation (EGR) and addition of small amounts of hydrogen on the combustion and exhaust emissions were explored. With the addition of hydrogen, the flow of the main fuel (diesel or biodiesel) was reduced to maintain constant indicated mean effective pressure (IMEP). Thus, in effect the tests involved fuel replacement by hydrogen rather than hydrogen addition. Second, the feasibility of producing hydrogen "on-board" by catalytic exhaust gas fuel reforming was examined by incorporating a laboratory reforming mini reactor in the engine exhaust system. Prototype catalysts and different reaction conditions were examined. The results from the first part of the study showed that partial replacement of the hydrocarbon fuel by hydrogen combined with EGR resulted in simultaneous reductions of smoke and nitrogen oxides emissions (NO x ) without significant changes to engine efficiency. In the second part of the study, it was shown that the amount of hydrogen required to achieve these beneficial effects potentially can be produced by exhaust gas-assisted reforming of the hydrocarbon fuel.

Abstract.A detailed three-dimensional regional chemical transport model (Particulate Matter Comprehensive Air Quality Model with Extensions, PMCAMx) was applied over Europe, focusing on the formation and chemical transformation of organic matter. Three periods representative of different seasons were simulated, corresponding to intensive field campaigns. An extensive set of AMS measurements was used to evaluate the model and, using factor-analysis results, gain more insight into the sources and transformations of organic aerosol (OA). Overall, the agreement between predictions and measurements for OA concentration is encouraging, with the model reproducing two-thirds of the data (daily average mass concentrations) within a factor of 2. Oxygenated OA (OOA) is predicted to contribute 93 % to total OA during May, 87 % during winter and 96 % during autumn, with the rest consisting of fresh primary OA (POA). Predicted OOA concentrations compare well with the observed OOA values for all periods, with an average fractional error of 0.53 and a bias equal to −0.07 (mean error = 0.9 µg m −3 , mean bias = −0.2 µg m −3 ). The model Published by Copernicus Publications on behalf of the European Geosciences Union. C. Fountoukis et al.: Organic aerosol concentration and composition over Europesystematically underpredicts fresh POA at most sites during late spring and autumn (mean bias up to −0.8 µg m −3 ). Based on results from a source apportionment algorithm running in parallel with PMCAMx, most of the POA originates from biomass burning (fires and residential wood combustion), and therefore biomass burning OA is most likely underestimated in the emission inventory. The sensitivity of POA predictions to the corresponding emissions' volatility distribution is discussed. The model performs well at all sites when the Positive Matrix Factorization (PMF)-estimated low-volatility OOA is compared against the OA with saturation concentrations of the OA surrogate species C * ≤ 0.1 µg m −3 and semivolatile OOA against the OA with C * > 0.1 µg m −3 .

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