Fractionation analysis of transport engine-generated soot particles with respect to hygroscopicity

Combustion-derived particles emitted by various transport engines are analyzed with respect to their morphology and composition. Soot particles collected behind an aircraft gas turbine engine combustor, in the diesel engine exhaust pipe, and at the end of ship pipe demonstrate heterogeneous morphological structures and different chemical identities. Individual particle analysis allows the characterization of emitted soot particles into three fractions, namely C-O, C-O-S, and Fe-C-O, with the dominant presence of chain soot agglomerates, irregular internally mixed soot, and particles of distinct morphology, respectively. The influence of oxygen and sulfur concentrations on the ability of soot to uptake water is proposed as a method for categorizing transport engine-generated particles. Categories of hydrophobic and hydrophilic particles within non-hygroscopic soots can be defined based on the amount of water uptake relative to several reference laboratory-made soots comparable to transport engine emissions. This method of characterizing hydrophobic/hydrophilic and hygroscopic soots in combustion emissions will help to determine the potential impacts of the combustion emissions on CCN/IN formation in the atmosphere.     

Effect of soot on immersion freezing of water and possible atmospheric implications

More than 3000 water droplets with soot particles immersed were monitored individually to identify the effect of heterogeneous freezing and establish the link between soot properties and freezing efficiency. A set of soot samples from many combustion sources, including an aircraft engine combustor were examined. The list of key soot properties affecting freezing were determined. It appears that soot effectiveness in causing the droplets to freeze mainly depends on the soot behaviour in water defined by the mass density of soot agglomerates, size and wetting. When large mass particles were homogeneously distributed through droplets and were stable to sedimentation effects median freezing temperatures of such droplets were significantly warmer than pure water droplets. The long-term presence of large soot agglomerates in water leads to an increase in the freezing efficiency over time because of the slow kinetics of wetting. Small sizes of soot agglomerates stimulate aggregative instability in droplets with large soot mass which decreases the total soot surface area and the freezing temperature. The highest freezing efficiency is found for homogeneously distributed soot agglomerates of high porosity. A gradual increase of the freezing temperature in correspondence with number density of active oxygen-containing sites is observed for soots having surfaces of low or intermediate polarity. However, highly soluble surface compounds, such as sulfates, may be easily dissolved in water leading to a decrease in the potential freezing efficiency. This was the case found for aircraft engine combustor generated soot. Therefore relatively hydrophobic soot with a significant number of oxygen-containing functional groups but not covered totally by hydrophilic sites and soluble compounds is proposed to act as the most efficient immersion freezing nuclei in the atmospheric cloud droplets.     

Hygroscopic behavior of aerosol particles from biomass fires using environmental transmission electron microscopy

We used both a conventional transmission electron microscope and an environmental transmission electron microscope (ETEM) to determine morphology, composition, and water uptake of 80 individual aerosol particles collected from the young smoke of flaming and smoldering fires during SAFARI-2000, a comprehensive air quality campaign in southern Africa. Six representative carbonaceous particle types are described, including soot, tar balls, and heterogeneously internally mixed particles containing C with S-, K-, Mg- or Na-rich inorganic phases. The hygroscopic behavior of these particles over the range 0–100% relative humidity (RH) was studied in detail. Soot and tar balls did not take up water, whereas the mixed organic–inorganic particles took up water between 55 and 100% RH, the exact value depending on the composition of their water-soluble phases. The inorganic phase appeared to determine the hygroscopic properties of all mixed organic–inorganic particles. Thus, incorporation of inorganic plant material or reactions with inorganic atmospheric components can dramatically alter the hygroscopic properties of carbonaceous particles in smoke plumes. The fraction of these mixed organic–inorganic particles plausibly increases with time, which will modulate the effects of smoke on radiative budgets.     

On the potential importance of sulfur-induced activation of soot particles in nascent jet aircraft exhaust plumes

The chemical conversion of emitted SO₂ and SO₃ to gaseous H₂SO₄ in nascent aircraft exhaust plumes and subsequent adsorption of the fully oxidized sulfur species on the surfaces of emitted combustion aerosols (soot) is investigated. Results are presented for the mass fractions of SO₃ and H₂SO₄ acquired per soot particle early in the plume, suggesting that sulfur-induced activation is an efficient pathway to increase the ability of exhaust soot emitted at altitude to host heterogeneous chemical reactions and to trigger the formation of cirrus clouds.     

Mixing states of cloud interstitial particles between water-soluble and insoluble materials at Mt. Tateyama,Japan: Effects of meteorological conditions

Mixing states of cloud interstitial particles between water-soluble and insoluble materials apparently differ under various cloud-forming conditions. To study the mixing states of cloud interstitial particles, we made observations at Mt. Tateyama, Japan (2300 m a.s.l.) during June 2007 using fog (> 10 μm)-cut inlets. Number concentrations of dried particles (0.3–0.5 μm diameter) selected for less-grown (LG) particles (particles smaller than 0.56 μm diameter at 88% relative humidity) were used to quantify tendencies of the growth characteristics of cloud interstitial particles. Size-segregated soot mass concentrations (< 0.4 and < 1.1 μm) were also measured for cloud interstitial particles. Three samples of cloud interstitial LG particles at 88% RH were investigated for water-soluble and insoluble components using dialysis (extraction) of water-soluble materials with transmission electron microscopy (TEM). For one TEM sample with high fractions of the LG particles and high soot mass concentrations under high precipitation (2–6 mm/h), most particles (0.1–0.5 μm) were found to be water insoluble. More than half of the water-insoluble particles were considered to be soot particles showing chain aggregations of electron-opaque spherules. Regarding the other two TEM samples with low fractions of the LG particles under less intense precipitation (ca. 1 mm/h), most particles were partly water soluble. The scavenging process in the precipitating cloud can change the population of particles left behind, preferentially leaving insoluble particles according to cloud formation conditions.     

Characterisation of Soot Emitted by Domestic Heating,Aircraft and Cars Using Diesel or Biodiesel

The characterisation of aggregates, like soot, firstly requires the determination of the size distribution of the primary particles. The primary particle size of combustion generated aggregates depends upon the combustion environment and the formation conditions, such as temperature, pressure and fuel-to-air ratio, among others. Since the combustion characteristics are different in the different types of burners, the characterisation of primary particles may offer the possibility to distinguish soot from different sources. In this paper, we present the signature of the primary particles and the aggregates of soot emitted by cars using diesel or biodiesel, by domestic heating, and by aircraft exhausts, which can be considered as the major sources as derived from measurements on transmission electron micrographs. The size distributions of all aggregates types with different aerodynamic diameter were log-normal and quasi-monodisperse. The size distribution of the primary particles for soot emitted by different sources showed minor differences. However, a comparison between the diameter of the primary particles and those obtained using a standard method for carbon black revealed discrepancies. The median diameter of the primary particles was combined with the median number of primary particles in an aggregate to calculate the relative particle surface area available for adsorption. In a similar way, the relative specific surface area was determined. The surface area was measured using the Brunauer-Emmett-Teller (B.E.T.) nitrogen adsorption method and the relative surface area available for adsorption was calculated.     

The Effect of Temperature and Humidity on the Reaction of Ozone with Combustion Soot: Implications for Reactivity near the Tropopause

The effect of temperature (296–238 K) on the reaction of combustion soot (n-hexane) with ozone at low concentration (6–8 ppm) has been measured. Long optical path FTIR spectroscopy has revealed the rate law for ozone loss beyond initial stages, second order in O₃, to be the same over this range of conditions. The reaction rate is 3.5 times lower at 238 K than at 296 K, and reveals an activation energy of 12.9 ± 0.5 kJ mol^–1. The effect of humidity on the reaction has been estimated using its recently determined rate law dependence (p^0.2). These data, differing from O₃ reaction kinetics obtained from other types of carbonaceous particles used as surrogates for atmospheric soot, have implications for the role of combustion soot in atmospheric chemistry. Any involvement of aircraft soot in ozone depletion near the tropopause, for example, should be estimated using these temperature and humidity dependences.     

Isolating and identifying atmospheric ice-nucleating aerosols: a new technique

Laboratory studies examined two key aspects of the performance of a continuous-flow diffusion chamber (CFD) instrument that detects ice nuclei (IN) concentrations in air samples: separating IN from non-IN, and collecting IN aerosols to determine chemical composition. In the first study, submicron AgI IN particles were mixed in a sample stream with submicron non-IN salt particles, and the sample stream was processed in the CFD at −19°C and 23% supersaturation with respect to ice. Examination of the residual particles from crystals nucleated in the CFD confirmed that only AgI particles served as IN in the mixed stream. The second study applied this technique to separate and analyze IN and non-IN particles in a natural air sample. Energy-dispersive X-ray analyses (EDS) of the elemental composition of selected particles from the IN and non-IN fractions in ambient air showed chemical differences: Si and Ca were present in both, but S, Fe and K were also detected in the non-IN fraction.     

Sulfate-coated dust particles in the free troposphere over Japan

Airborne aerosol collections were performed over Wakasa bay (36°00′N, 135°30′E) in March and Kumano open sea (34°00′N, 136°50′E) and Seto (35°10′N, 137°10′E) in July 2001 at altitudes between 1.0 and 5.8 km. The particles were individually analyzed using transmission electron microscopy (TEM). Relatively large mineral-dust (mostly clay) particles were abundant in the March samples. They also dominated in July in the mid-troposphere higher than 4 km altitude, whereas sea salt and ammonium sulfate were more abundant at lower altitudes. Ca-coated grid samples show many traces of aqueous sulfate droplets. The proportions of former sulfate droplets to the total collected particles apparently increased with increasing relative humidity at the time of sampling. TEM analysis revealed that a significant fraction of these former droplets enclose mineral-dust particles as well as sea salt, soot, and fly ash. Some enclose mixtures of mineral-dust, sea-salt, soot, and fly ash particles. The results provide evidence that mineral dust from the Asian continent could acquire coatings of sulfate while being transported in the free troposphere. The mineral-dust particles probably acquired the sulfate coatings either through heterogeneous uptake of gaseous SO₂ and subsequent oxidation or through coagulation with cloud or fog droplets. The presence of the mixed particles in sulfate droplets also indicates that aggregation of particles of different origins occurred through cloud processing. Such sulfate-coated dust particles would affect cloud formation, precipitation, and chemistry of the free troposphere.     

SEM-EDX Characterisation of Tropospheric Aerosols in the Negev Desert (Israel)

Individual aerosol particles collected in the Negev desert in Israel during a summer and winter campaign in 1996–1997 were analysed by scanning electron microscopy with energy-dispersive X-ray analysis. Hierarchical cluster analysis was performed to interpret the data on the basis of particle diameter and composition. Eleven particle classes (groups) provided clues on sources and/or particle formation. The summer samples were enriched in sulphates and mineral dusts; the winter samples contained more sea salts, aged sea salts, and industrial particles. The fine size fraction below 1 m diameter was enriched in secondary particles and showed evidence of atmospheric processing. The secondary sulphate particles were mainly attributed to long-range transport. A regional conversion from calcite to calcium sulphate occurred during summer. Industrial particles originating from local pollution appeared during winter.     

Sulfate-coated dust particles in the free troposphere over Japan

Airborne aerosol collections were performed over Wakasa bay (36°00′N, 135°30′E) in March and Kumano open sea (34°00′N, 136°50′E) and Seto (35°10′N, 137°10′E) in July 2001 at altitudes between 1.0 and 5.8 km. The particles were individually analyzed using transmission electron microscopy (TEM). Relatively large mineral-dust (mostly clay) particles were abundant in the March samples. They also dominated in July in the mid-troposphere higher than 4 km altitude, whereas sea salt and ammonium sulfate were more abundant at lower altitudes. Ca-coated grid samples show many traces of aqueous sulfate droplets. The proportions of former sulfate droplets to the total collected particles apparently increased with increasing relative humidity at the time of sampling. TEM analysis revealed that a significant fraction of these former droplets enclose mineral-dust particles as well as sea salt, soot, and fly ash. Some enclose mixtures of mineral-dust, sea-salt, soot, and fly ash particles. The results provide evidence that mineral dust from the Asian continent could acquire coatings of sulfate while being transported in the free troposphere. The mineral-dust particles probably acquired the sulfate coatings either through heterogeneous uptake of gaseous SO₂ and subsequent oxidation or through coagulation with cloud or fog droplets. The presence of the mixed particles in sulfate droplets also indicates that aggregation of particles of different origins occurred through cloud processing. Such sulfate-coated dust particles would affect cloud formation, precipitation, and chemistry of the free troposphere.     

Comparison of Submicron Particles at a Rural and an Urban Site in the North China Plain during the December 2016 Heavy Pollution Episodes

An extensive field experiment for measurement of physical and chemical properties of aerosols was conducted at an urban site in the Chinese Academy of Meteorological Sciences (CAMS) in Beijing and at a rural site in Gucheng (GC), Hebei Province in December 2016. This paper compares the number size distribution of submicron particle matter (PM1, diameter < 1 μm) between the two sites. The results show that the mean PM1 number concentration at GC was twice that at CAMS, and the mass concentration was three times the amount at CAMS. It is found that the accumulation mode (100–850 nm) particles constituted the largest fraction of PM1 at GC, which was significantly correlated with the local coal combustion, as confirmed by a significant relationship between the accumulation mode and the absorption coefficient of soot particles. The high PM1 concentration at GC prevented the occurrence of new particle formation (NPF) events, while eight such events were observed at CAMS. During the NPF events, the mass fraction of sulfate increased significantly, indicating that sulfate played an important role in NPF. The contribution of regional transport to PM1 mass concentration was approximately 50% at both sites, same as that of the local emission. However, during the red-alert period when emission control took place, the contribution of regional transport was notably higher.     

State of mixing, shape factor, number size distribution, and hygroscopic growth of the Saharan anthropogenic and mineral dust aerosol at Tinfou, Morocco

The Saharan Mineral Dust Experiment (SAMUM) was conducted in May and June 2006 in Tinfou, Morocco. A H-TDMA system and a H-DMA-APS system were used to obtain hygroscopic properties of mineral dust particles at 85% RH. Dynamic shape factors of 1.11, 1.19 and 1.25 were determined for the volume equivalent diameters 720, 840 and 960 nm, respectively.
During a dust event, the hydrophobic number fraction of 250 and 350 nm particles increased significantly from 30 and 65% to 53 and 75%, respectively, indicating that mineral dust particles can be as small as 200 nm in diameter. Log-normal functions for mineral dust number size distributions were obtained from total particle number size distributions and fractions of hydrophobic particles. The geometric mean diameter for Saharan dust particles was 715 nm during the dust event and 570 nm for the Saharan background aerosol.
Measurements of hygroscopic growth showed that the Saharan aerosol consists of an anthropogenic fraction (predominantly non natural sulphate and carbonaceous particles) and of mineral dust particles. Hygroscopic growth and hysteresis curve measurements of the 'more' hygroscopic particle fraction indicated ammonium sulphate as a main component of the anthropogenic aerosol. Particles larger than 720 nm in diameter were completely hydrophobic meaning that mineral dust particles are not hygroscopic.     

Relationship between cloud characteristics and radar reflectivity based on aircraft and cloud radar co-observations

Cloud properties were investigated based on aircraft and cloud radar co-observation conducted at Yitong, Jilin, Northeast China. The aircraft provided in situ measurements of cloud droplet size distribution, while the millimeter-wavelength cloud radar vertically scanned the same cloud that the aircraft penetrated. The reflectivity factor calculated from aircraft measurements was compared in detail with simultaneous radar observations. The results showed that the two reflectivities were comparable in warm clouds, but in ice cloud there were more differences, which were probably associated with the occurrence of liquid water. The acceptable agreement between reflectivities obtained in water cloud confirmed that it is feasible to derive cloud properties by using aircraft data, and hence for cloud radar to remotely sense cloud properties. Based on the dataset collected in warm clouds, the threshold of reflectivity to diagnose drizzle and cloud particles was studied by analyses of the probability distribution function of reflectivity from cloud particles and drizzle drops. The relationship between reflectivity factor (Z) and cloud liquid water content (LWC) was also derived from data on both cloud particles and drizzle. In comparison with cloud droplets, the relationship for drizzle was blurred by many scatter points and thus was less evident. However, these scatters could be partly removed by filtering out the drop size distribution with a large ratio of reflectivity and large extinction coefficient but small effective radius. Empirical relationships of Z-LWC for both cloud particles and drizzle could then be derived.     

The Effect of Air/Fuel Ratio on Properties and Reactivity of Combustion Soots

The dependence of specific properties of black carbon (BC) soots on fuel type and combustion conditions has been studied, and the effects of these properties on soot particle hydration and reaction with ozone determined. Series of soots were prepared from n-hexane, diesel and JP8 aircraft fuels, utilizing a flow combustion system designed for accurate control of the air/fuel ratio in premixed flames. It has been shown that, for each of these, linear relationships exist between the state of soot surface oxidation, surface area, unpaired electron spin density, and air/fuel ratio. The interrelationship of these properties potentially enables the preparation of soots with pre-selected characteristics by establishing combustion conditions for each fuel type. Predictable variations in these soot characteristics have been demonstrated through preparations near sea level and at higher elevation. Soot hydration and its reactivity with ozone are quantitatively related to surface oxidation and, thus, the air/fuel combustion ratio for soot's formation in premixed flames. The effect of relative humidity (RH) on the soot-ozone reaction over the O₃ concentration and RH ranges 2–4.8 ppm and17–78%, respectively, is expressed by–d[O₃]/dt =k[O₃]²[H₂O]^0.2and is directly related to particle surface oxidation.     

Large aerosol particles in cirrus type clouds

Research flights in November 1990 over the central parts of the United States, Wyoming and Colorado, were aimed to the investigation of the properties and microstructure of cirrus clouds (mainly cirrocumulus lenticularis). Among the other parameters measured on board the NCAR Saberliner were the concentration and size distribution of submicron particles and, in some cases, the particle deliquescence. For coarse insoluble particles found inside and outside of cloud elements, size distributions and morphology information were obtained by evaluating inertial impactor samples with an optical microscope and scanning electron microscope. In addition, the coarse particle composition was determined by x-ray energy spectrum analysis. The following conclusions from these measurements are:The large and coarse particle size distribution can be roughly simulated by a log-normal function with the modus around r=0.5 μm. Particle concentrations are very variable between several tenths and several particles per cm³. Particle volume distribution features a distinct maximum around 0.75 μm without a broad plateau which was observed in the case of sampling at lower altitude. Aerosol composition heterogeneity at cirrus cloud level is well documented by the evaluation of the fine particle sampling taken with the UMR sampling system. This heterogeneity can be partly explained by the interaction between aerosol and cloud elements, which is documented by the measured particle size distribution curves inside and outside of cloud elements. Assuming that particle deliquescence is caused by H₂SO₄ and/or by (NH₄)₂SO₄, particle soluble mass fractions were found to be around 30% in the first case and about 40% in the second. The most frequently occurring elements in large and coarse particles at cirrus cloud level were Si, Cl, Ba, S, Ca and C.     

Photochemical Effects in the Heterogeneous Reaction of Soot with Ozone at Low Concentrations

The soot-ozone reaction at low concentrations (ppm O₃) hasbeen examined todetermine any influence of solar radiation on its products and kinetics. Theeffect of simulatedsolar radiation is to change the product distribution towardsCO₂(g), CO (g) and H₂O(g) at theexpense of soot surface functional groups formation. Little effect on theextent or rate ofdiminution of O₃ through this rapid reaction is observed. Theinitial rate laws for formation ofall products remain the same under simulated solar radiation, with changes inthe rate constants(and thus in the relative importance of mechanistic pathways) responsible forthe differingproduct distributions. Decarboxylation of soot surface functionalities hasbeen shown to be onepossible mechanism underlying these effects. Atmospheric soot, particularlythat emitted nearthe tropopause by increasing numbers of subsonic and supersonic aircraft, mayplay a role inozone depletion; the rapid diminution of ozone in soot's presence isunaffected by solarradiation.     

飞机发动机积冰原因探讨

飞机发动机积冰的预报是航空气象预报业务中的难题之一。作者根据飞机发动机的工作特性，结合实际的积冰事例作了积冰成因的初步分析。并讨论了有关预报的问题。     

Size-Resolved Aerosol Characterization for a Polluted Episode at the IfT Research Station Melpitz in Autumn 1997

Inorganic ions, organic carbon (OC), elemental carbon (EC) and a variety of organic single species in airborne particles have been determined at the research station of the Leibniz – Institut für Troposphärenforschung (IfT) in Melpitz (Germany) in autumn 1997.Samples of eight selected measurement events were divided in two groups in order to investigate differences in the chemical composition of particles originating from southwesterly (SW – developed EU countries) or from easterly directions (E – less developed eastern countries). Differences between these two groups were tested statistically by Student's t-test.Five stage cascade impactor samples show nitrate as most abundant in the accumulation mode in the SW group. EC and sulphate show the most abundant mass fractions in the E group. That can be considered as a consequence of domestic coal heating and coal-fired power plant emissions in the region of westerly Poland, northern Czech Republic and easterly Germany. Higher nitrate concentrations in the SW group can be explained by stronger NO _x emissions caused by the leeward plume of the conurbation of Leipzig, as well as by the still higher traffic density in western Germany.The methane sulphonic acid (MSA) mass fraction was higher for SW air masses in accumulation mode particles, probably indicating marine origin. Succinic acid also showed higher mass fractions for the SW group. This could be caused by primary emission in automobile exhaust gases and photochemical formation during transport from SW. Indeed, during SW sampling, solar radiation intensity was higher than during E sampling.The observed differences in the particle composition are an expression of the still existing technology gradient in Europe. Future campaigns could show the development to a joint economy with smaller differences in anthropogenic emissions.