Net Particle Charge Research Articles

Influence of Tripolyphosphate Cross-Linking on the Physical Stability and Lipase Digestibility of Chitosan-Coated Lipid Droplets

The impact of cross-linking chitosan with tripolyphosphate (TPP) on the physical stability and lipase digestibility of chitosan-coated lipid droplets was investigated. Relatively, high TPP levels (>or=0.004 wt %) promoted droplet aggregation and gravitational separation, which was attributed to charge neutralization and interdroplet cross-linking. Cross-linked chitosan-coated lipid droplets were formed at lower TPP levels that were relatively small (d approximately 450 nm), cationic (zeta approximately +60 mV), and stable to particle aggregation and gravitational separation (pH 3, 21 days). However, these droplets were highly unstable at pH 7 because of a reduction in net particle charge and weakened electrostatic repulsion. An in vitro lipid digestion model was used to study the impact of the chitosan coating on the digestibility of lipid droplets by pancreatic lipase (pH 7, bile salts, pancreatic lipase, and 5.0 mM CaCl(2)). The rate of lipid digestion decreased when the lipid droplets were coated with chitosan and decreased further when the chitosan coating was cross-linked with TPP. Indeed, both cross-linked and noncross-linked chitosan coatings were able to prevent lipid digestion under conditions simulating the small intestine. This study has important implications for the design of structured emulsions with controlled lipid digestibility and for the targeted delivery of lipophilic functional components to specific regions within the gastrointestinal tract.

Biopolymer coated clay particles for the adsorption of tungsten from water

Biopolymer coated clay (i.e. chitosan coated montmorillonite clay called biosorbent) particles were synthesized. The effects of tungsten concentration in feed water (20–500 ppm) and water pH (4, 5.5 and 6.4) on the zeta potential of adsorbent particles, tungsten removal, and adsorption equilibrium were studied using biosorbent and natural clay. Coating clay particles with chitosan shifts the net surface charge of clay from negative to positive and the point of zero charge (PZC) of clay from 2.8 and 5.8. The net surface charge of biosorbent particles decreases with an increase in the tungsten concentration of feed water because the positively charged sites are consumed by the adsorption of tungsten anions. Biosorbent was found to be much more effective than natural clay for the removal of tungsten. The tungsten removal efficiencies of both biosorbent and natural clay decrease with an increase in the pH level and an increase in the tungsten concentration of the feed water. Adsorption equilibrium studies show that tungsten removal is the highest at pH 4. Adsorption equilibrium data were modeled with the Langmuir, Temkin, and Freundlich isotherms and the model parameters were obtained. Adsorption oftungsten species on both biosorbent and natural clay seemed to obey Langmuir isotherm within the range of concentrations and pH investigated. The maximum tungsten adsorption capacities of chitosan, biosorbent and natural clay were found to be 632 mg tungsten/g of chitosan, 23.9 mg tungsten/g of biosorbent, and 5.45 mg tungsten/g of natural clay at pH 4, respectively. The tungsten species adsorption on biosorbent was found to be governed by the ionic attraction between the protonated surface groups on chitosan and the negatively charged tungsten species. Whereas, the tungsten species adsorption on natural clay is governed by the positively charged clay particle edges formed by broken bonds of Al-O and Si-O.

Surface charge heterogeneity of kaolinite in aqueous suspension in comparison with montmorillonite

An analogous study to 2:1 type montmorillonite [Tombácz, E., Szekeres, M., 2004. Colloidal behavior of aqueous montmorillonite suspensions: the specific role of pH in the presence of indifferent electrolytes. Appl. Clay Sci. 27, 75–94.] was performed on 1:1 type kaolinite obtained from Zettlitz kaolin. Clay minerals are built up from silica tetrahedral (T) and alumina octahedral (O) layers. These lamellar particles have patch-wise surface heterogeneity, since different sites are localized on definite parts of particle surface. pH-dependent charges develop on the surface hydroxyls mainly at edges besides the permanent negative charges on silica basal plane due to isomorphic substitutions. Electric double layers (edl) with either constant charge density on T faces (silica basal planes) or constant potential at constant pH on edges and O faces (hydroxyl-terminated planes) form on patches. The local electrostatic field is determined by the crystal structure of clay particles, and influenced by the pH and dissolved electrolytes. The acid–base titration of Na-kaolinite suspensions showed analogous feature to montmorillonite. The initial pH of suspensions and the net proton surface excess vs. pH functions shifted to the lower pH with increasing ionic strength indicating the presence of permanent charges in both cases, but these shifts were smaller for kaolinite in accordance with its much lower layer charge density. The pH-dependent charge formation was similar, positive charges in the protonation reaction of (Si–O) Al–OH sites formed only at pHs below ∼ 6–6.5, considered as point of zero net proton charge (PZNPC) of kaolinite particles. So, oppositely charged surface parts on both clay particles are only below this pH, therefore patch-wise charge heterogeneity exists under acidic conditions. Electrophoretic mobility measurements, however, showed negative values for both clays over the whole range of pH showing the dominance of permanent charges, and only certain decrease in absolute values, much larger for kaolinite was observed with decreasing pH below pH ∼ 6. The charge heterogeneity was supported by the pH-dependent properties of dilute and dense clay suspensions with different NaCl concentrations. Huge aggregates were able to form only below pH ∼ 7 in kaolinite suspensions. Coagulation kinetics measurements at different pHs provided undisputable proofs for heterocoagulation of kaolinite particles. Similarly to montmorillonite, heterocoagulation at pH ∼ 4 occurs only above a threshold electrolyte concentration, which was much smaller, only ∼ 1 mmol l − 1 NaCl for kaolinite, than that for montmorillonite due to the substantial difference in particle geometry. The electrolyte tolerance of both clay suspensions increased with increasing pH, pH ∼ 6–6.5 range was sensitive, and even a sudden change occurred above pH ∼ 6 in kaolinite. There was practically no difference in the critical coagulation concentration of kaolinite and montmorillonite (c.c.c.∼ 100 mmol l − 1 NaCl) measured in alkaline region, where homocoagulation of negatively charged lamellae takes place. Rheological measurements showed shear thinning flow character and small thixotropy of suspensions at and above pH ∼ 6.7 proving the existence of repulsive interaction between uniformly charged particles in 0.01 M NaCl for both clays. The appearance of antithixotropy, the sudden increase in yield values, and also the formation of viscoelastic systems only at and below pH ∼ 6 verify the network formation due to attraction between oppositely charged parts of kaolinite particles. Under similar conditions the montmorillonite gels were thixotropic with significant elastic response.

Net charge of a conducting microsphere embedded in a thermal plasma

The knowledge of the net charge of conducting spheres has gained much interest in the context of high power laser interaction with clusters, dust, impurities, aerosols, and small liquid droplets. Contrary to a general supposition based on linearized plasma quasineutrality it is shown that in electronic thermal equilibrium spheres of one Debye length radius lose almost all free electrons, hence are highly charged. Quasineutrality is approximately reached for pellet sizes R exceeding 30 times the Debye length. In order to gain an estimate of the net charge of nonspherical particles the equilibrium charge is determined in slab geometry for the same parameters.

A general gel layer model for the transport of colloids and macroions in dilute solution

A general boundary element methodology for studying the dilute solution transport of rigid macroions that contain gel layers on their outer surfaces is developed and applied to several model systems. The methodology can be applied to particles of arbitrary size, shape, charge distribution, and gel layer geometry. Account is also taken of the steady state distortion of the ion atmosphere from equilibrium, which makes it applicable to the transport of highly charged structures. The coupled field equations (Poisson, ion-transport, low-Reynolds-number Navier–Stokes, and Brinkman) are solved numerically and from this, transport properties (diffusion constants, electrophoretic mobilities, excess viscosities) can be computed. In the present work, the methodology is first applied to a gel sphere model over a wide range of particle charge and the resulting transport properties are found to be in excellent agreement with independent theory under those conditions where independent theory is available. It is then applied to several prolate spheroidal models of a particular silica sol sample in an attempt to identify possible solution structures. A single model, that is able to account simultaneously for all of the transport behavior, which does not undergo significant conformational change with salt concentration, could not be found. A model with a thin (⩽1-nm) gel layer at high salt content that expands on going to low salt content is able to explain the salt dependence of the intrinsic viscosity, but not the electrophoretic mobility. However, a model with a fairly thick (2-nm) gel layer at high salt content, which expands slightly (2.5-nm) at low salt content, is in fairly good agreement with experiment. In addition, the influence of particle charge and the presence of a gel layer on the Scheraga–Mandelkern parameter are examined. This parameter is proportional to the product of the translational diffusion constant and the cube root of the intrinsic viscosity. It is found to be very robust with regard to net particle charge as well as properties of the gel layer.

Circulating electronegatively charged low‐density lipoprotein in patients with angiographically documented coronary artery disease

There is growing experimental evidence to suggest the role of oxidatively modified low‐density lipoprotein (LDL) in the initiation and progression of atherosclerosis. The oxidation of lipoprotein moiety causes modification of positively charged lysine residues and results in negative net charge of lipoprotein particles. Objective: To measure the amount of circulating electronegatively charged LDL particles (LDL–) in plasma of patients with angiographically documented coronary artery disease (CAD). Methods: Thirty patients were assigned to the study group (CAD+) and 10 patients to the control group (Ctrl). LDL– was quantitated in homogeneous LDL fractions obtained by ultracentrifugation, using ion exchange high performance liquid chromatography. Plasma lipids were measured using enzymatic kits. Results: The CAD+ group had significantly higher levels of LDL– in the whole LDL fraction (7.66±1.92 vs. 5.14±0.84%, p=0.0003). Moreover the CAD+ group had significantly higher levels of total cholesterol (255.4±35.1 vs. 210.4±22.4 mg/dL), LDL cholesterol (154.5±26.9 vs. 122.4±21.1 mg/dL) and significantly lower levels of high‐density lipoprotein (HDL) cholesterol (40.4±9.4 vs. 51.0±11.5 mg/dL). LDL– remained significantly higher in the CAD+ group after adjustment for total cholesterol, LDL cholesterol and HDL cholesterol (6.3 vs. 5.14% at p=0.0095). There is a trend towards a positive correlation between LDL– levels and LDL cholesterol in the control group (Spearman R=0.55 at p=0.098). Conclusions: Electronegatively charged LDL appears to be an additional hallmark of coronary artery disease, independently of established lipid risk factors. The trend towards a positive correlation between LDL cholesterol concentration and the level of LDL– in the control group may reflect the susceptibility of LDL cholesterol to autoxidation, Moreover, this may indicate other oxidative mechanisms in coronary artery disease. Nonetheless, further studies assessing the prognostic value of electronegatively charged LDLs are necessary.

Circulating electronegatively charged low-density lipoprotein in patients with angiographically documented coronary artery disease

AbstractThere is growing experimental evidence to suggest the role of oxidatively modified low‐density lipoprotein (LDL) in the initiation and progression of atherosclerosis. The oxidation of lipoprotein moiety causes modification of positively charged lysine residues and results in negative net charge of lipoprotein particles. Objective: To measure the amount of circulating electronegatively charged LDL particles (LDL–) in plasma of patients with angiographically documented coronary artery disease (CAD). Methods: Thirty patients were assigned to the study group (CAD+) and 10 patients to the control group (Ctrl). LDL– was quantitated in homogeneous LDL fractions obtained by ultracentrifugation, using ion exchange high performance liquid chromatography. Plasma lipids were measured using enzymatic kits. Results: The CAD+ group had significantly higher levels of LDL– in the whole LDL fraction (7.66±1.92 vs. 5.14±0.84%, p=0.0003). Moreover the CAD+ group had significantly higher levels of total cholesterol (255.4±35.1 vs. 210.4±22.4 mg/dL), LDL cholesterol (154.5±26.9 vs. 122.4±21.1 mg/dL) and significantly lower levels of high‐density lipoprotein (HDL) cholesterol (40.4±9.4 vs. 51.0±11.5 mg/dL). LDL– remained significantly higher in the CAD+ group after adjustment for total cholesterol, LDL cholesterol and HDL cholesterol (6.3 vs. 5.14% at p=0.0095). There is a trend towards a positive correlation between LDL– levels and LDL cholesterol in the control group (Spearman R=0.55 at p=0.098). Conclusions: Electronegatively charged LDL appears to be an additional hallmark of coronary artery disease, independently of established lipid risk factors. The trend towards a positive correlation between LDL cholesterol concentration and the level of LDL– in the control group may reflect the susceptibility of LDL cholesterol to autoxidation, Moreover, this may indicate other oxidative mechanisms in coronary artery disease. Nonetheless, further studies assessing the prognostic value of electronegatively charged LDLs are necessary.Atherosclerosiscoronary artery diseaseelectronegative low‐density lipoprotein

Sticking of protein-coated particles in a shear field

The shear-driven formation of particle doublets at a wall, of latex and emulsion systems, has been studied using a colloidal particle scattering apparatus. In this technique, one particle is fixed on the wall while a second, mobile free particle is made to collide with the fixed particle by the application of the shear field. The effects of pH (in the range 5.5–4.8) and ionic strength (in the range 0.01–0.5 mol dm −3) on the efficiency of irreversible sticking (capture) of the mobile particle to the fixed particle have been studied for systems stabilised by adsorbed films of sodium caseinate, pure α s1-casein or pure β-casein. At low ionic strength (≤0.2 mol dm −3), the efficiency of capture increases with decreasing pH and increasing ionic strength in ways that are consistent with decreasing net charge on the protein and increased screening of the electrostatic repulsion between the particles. The increase in capture efficiency also agrees well with a previously observed increase in viscoelasticity of the corresponding concentrated protein-stabilised emulsions. However, a pronounced drop in the capture efficiency was found at higher ionic strength (0.5 mol dm −3). This effect at high ionic strength, combined with qualitative visual assessment of the behaviour of particles during collisions, conforms to the additional role of tangential interparticle forces in determining the sticking behaviour. It seems that, if the adsorbed protein layer becomes sufficiently thick, even if it has a very low net charge, then particle sticking can be prevented. The sticking phenomenon probably depends on entanglement of the adsorbed layers and on the strength of the protein–protein attractive interactions, as well as on the net particle charge.

Electrostatic and electrosteric stabilization of aqueous slips of 3Y–ZrO 2 powder

Yttria-doped zirconia powder (3Y-ZrO 2) was characterized and dispersed in distilled water. The state of dispersion was evaluated in terms of zeta potential, apparent viscosity and the mean particle size of solid phase in the slip. Zeta potential, apparent viscosity and the mean particle size as a function of pH indicated the pH range of electrostatic stabilization. These results showed that electrostatic stabilization of the slip can be accomplished in low acidic and high basic range of pH. Dissolution of yttria from the powder surface in acidic pH was found to be high and fast, risky to the stability of the slip as well as the microstructure of the sintered body. Electrosteric stabilisation by addition of an anionic polyelectrolyte (PMAA-NH 3) shifted the isoelectric point (IEP) to lower pH. The state of dispersion was further investigated by particle size measurements of the solid phase in the slip. The optimum amount of dispersant is discussed in terms of zeta potential and viscosity. The adsorption of polyelectrolyte is considered at pH 4, native pH of suspension and at pH 10, below and above the IEP of powder and correlated with the dissociation rate of polymer and the net surface charge of particles at given pH. Higher adsorption of dissociated polymer to the positively charged surfaces justifies the higher optimum amount of polyelectrolyte at pH 4.

The effect of cationic surfactants on wetting, colloid stability and flotation of silica

Contact angle, colloid stability and flotation measurements have been carried out for silica as a function of the concentration of cationic surfactants at different salt concentrations. The results are compared with surfactant adsorption and surface charge isotherms. The surfactants used are dodecyl and cetyl pyridinium chloride, and dodecyl trimethyl-ammonium bromide. Colloid instability, contact angles and flotation recovery all pass through a maximum at about the charge compensation point that corresponds for the present system to the iso-electric point and the (near) common intersection point of surfactant isotherms measured at a given pH and different ionic strength values. Before the charge compensation point the surfactant progressively neutralises the negative surface charge by head-on adsorption and simultaneously the surfactant tail causes an increase in hydrophobicity. Beyond the charge compensation point admicelles are formed, leading to a net positive particle charge and an increase in hydrophilicity. In the charge compensation point the maximum of hydrophobicity occurs together with the minimal net particle charge. Therefore, the minimum in the stability and the maximum in the flotation recovery are related to both the low particle charge and the relatively large particle hydrophobicity around the charge compensation point.

Ionization and charge-transfer: basic data for track structure calculations.

It is widely accepted that an understanding of the detailed structure of charged particle tracks is essential for interpreting the mechanistic consequences of energy deposition by high linear energy transfer (LET) radiation. The spatial relationship of events along the path of a charged particle, including excitation, ionization, and charge-transfer, govern subsequent chemical, biochemical, and biological reactions that can lead to adverse biologic effects. The determination of spatial patterns of ionization and excitation relies on a broad range of cross-section data relating the interactions of charged particles to the molecular constituents of the absorbing medium. It is important that these data be absolute in magnitude, comprehensive in scope, and reliable if accurate assessment of track structure parameters is to be achieved. Great strides have been made in the development of this database, understanding the underlying theory, and developing analytic models, particularly for interactions involving electrons and protons with atoms and molecules. The database is less comprehensive for interactions involving heavier charged particles, especially those that carry bound electrons, and for interactions in condensed phase media. Although there has been considerable progress in understanding the physical mechanisms for interactions involving fast heavy ions and atomic targets during the past few years, we still lack sufficient understanding to confidently predict cross-sections for these ions with biologically relevant material. In addition, little is known of the interaction cross-sections for heavy charged particles as they near the end of their track, i.e., for low velocity ions where collision theory is less well developed and where the particle's net charge fluctuates owing to electron capture and loss processes. This presentation focuses on the current status of ionization and charge-transfer data. Compilations, reviews, Internet sources, theoretical models, and recent data applicable to track structure calculations are discussed.

CHARACTERISATION OF BEER PARTICLE CHARGES AND THE ROLE OF PARTICLE CHARGE IN BEER PROCESSING

Streaming current measurement has been shown to be an effective tool in measuring the net charge of beer particles. Using this method, it was demonstrated that non-microbiological particles of a number of beers could be either positively or negatively charged. Copper fining has been shown to significantly affect the net charge of the NMP in the resultant beer. Increasing the copper fining rate enhances the negative net charge of the NMP to a limiting value at which the addition of further copper finings has no effect. This is believed to correspond to the complete removal of all of the positively charged NMP by copper fining. The reduction in pH during fermentation results in the production of further positively charged NMP. The presence of positively charged NMP has been shown to have an adverse effect on isinglass fining performance. These results suggest that choice of raw materials and process conditions play as much of a role in defining fining performance as does careful consideration of fining rates.

Dynamic effect of charged particles on the measuring probe potential

This paper presents a new approach to mathematical modelling of the dynamic effect of the space charge density or net electric charge of solid particles in a certain volume of a transporting pipe (pneumatic pipelines) on the electric potential of any electrostatic, inductive, non-intrusive metal ring probe. Certain relationships between the probe potential and dynamic space charge density or net charge of particles in the pipe volume have been obtained. All the relationships have a very concise, distinct form.

Electric charge measurement in pneumatic installations

Measurements of a net electric charge carried by solid particles travelling along a conveying pipe in pneumatic transport installations are very important especially for monitoring the ESD dust ignition hazard associated with pneumatic transport of solids. The electrostatic, inductive, non-contact method permits rapid and continuous measurements of the net charge of particles flowing in pipes of a special pneumatic transport installation in a quasi-real-time mode. Some preliminary tests were carried out using granulated polypropylene for different mean solids mass flow rates, and their results show certain relationships between the measuring ring probe voltage and the net particle charge, and are presented in this paper. The specific charge was also determined for the known values of the mass flow rate of particles.

Addition of aluminium stearate to enhance sintering of carbonyl iron compacts : K.S. Hwang et al. (Taiwan National University, Taiwan.)

The distribution of charge within the wall fouling region and bulk of a fluidized bed reactor was investigated. Experiments were conducted in a 0.1 m in diameter carbon steel fluidization column under atmospheric conditions. Polyethylene particles were fluidized with extra dry air at 1.5 the minimum fluidization velocity (bubbling flow regime) for 1 h. Using an online Faraday cup measurement technique, the net charge-to-mass ratio (q/m), as well as the size distribution of all particles adhered to the column wall and those in the bulk of the bed was determined. The wall particles were found to be predominantly negatively charged while those which did not adhere to the wall were predominantly positively charged. The charge distribution within each region was then investigated by a custom made charged particle separator that separated the particles according to their charge magnitude and polarity. It was determined that although the net charge of the wall layer particles was negative, a significant amount of positively charged particles existed within each sample and therefore the entire wall particle layer. This suggests that the wall layer was formed through layering between positively and negatively charged particles. Particles in the bulk of the bed also consisted of bipolarly charged particles.

Dispersion and zeta potential of pure clays as related to net particle charge under varying pH, electrolyte concentration and cation type

SummaryThe effect of changing pH and electrolyte concentration on the dispersion and zeta potential of Na‐and Ca‐forms of kaolinite, illite and smectite was investigated in relation to changes in their net negative charge.The percentage of dispersible Na‐clay and the percentage increase in net negative charge was positively correlated with pH, but the slopes varied from clay to clay. In general, the net negative charge was the primary factor in clay dispersion, and the pH affected clay dispersion by changing the net charge on clay particles. Na‐smectite had larger net charge at all pHs than Na‐illite and Na‐kaolinite, and it always had larger flocculation values. The role of electrolyte concentration could be due to its effect both on flocculation and variable charge component of the clay minerals. The zeta potential at different pHs also reflected the same trend of clay dispersion with net particle charge.In Ca‐clays the trends were similar to Na‐clays up to pH 7.0. In more alkaline solution CaCO3 formation led to charge reduction on clay particles, resulting in flocculation and reduction of zeta potential. At similar pHs the electrophoretic mobilities of all the clays showed constant potential behaviour. However, the zeta potentials of Ca‐clays were always smaller than those of sodic clays because the clays were more aggregated. Net particle charge was the most important factor in controlling clay dispersion for the whole range of pH and ionic strength and for all types of cations.

Clay dispersion as influenced by pH and net particle charge of sodic soils

The effect of changing pH on the dispersion of clay from sodic soils was investigated in relation to changes in net charge on clay particles. A positive relationship was obtained between pH and the percentage of dispersible clay for each soil clay. The percentage increase in net negative charge was also positively correlated with pH. However, the slopes of these relationships varied between soil clays. In general, the net negative charge was the primary factor in clay dispersion and the pH affected clay dispersion by changing the net charge on clay particles. In comparing the values for pure clay minerals quoted in the literature with soil clays having similar mineralogy, it was found that soil clays had higher flocculation values. This is shown to be due to higher net negative charge on soil clays than the corresponding values for pure clay minerals found in the literature. The effect of soil organic matter in enhancing the net negative charge probably contributes to the higher charge on soil clays. The critical dispersion concentration for clay dispersion from soil aggregates was lower than the flocculation values observed for the separated soil clays. The separated soil clays had high negative charge due to exposure of surfaces which were originally bonded in the aggregates. The dispersive potential of a number of Alfisols, Oxisols, Aridisols (calcareous soils) and Vertisols collected from different parts of Australia was highly correlated with soil pH. The relationship with CEC was poor because CEC was estimated at a pH different to the natural pH of the soil. This study has brought out the importance of pH in the management of dispersive soils.

Organic Matter Role in Illitic Soil Colloids Flocculation: II. Surface Charge

AbstractThe ability to predict the effect of various factors on soil flocculation depends on a knowledge of the mechanisms by which those factors operate. The purpose of this study was to investigate the mechanisms that control the effects of organic matter content, pH, and bivalent cation type on soil flocculation. The electrophoretic mobility of an illitic soil before and after treatment with H2O2 was measured at a soluble Ca or Mg charge fraction of 0.0 to 0.9 at pH 6, and at pH 4 to 9 and a soluble Ca charge fraction of 0.0 or 0.5 The change in the net particle charge as pH was increased from 6 to 8 at a soluble Ca concentration of 0.65 and 2.26 mol m−3 was determined from proton and Ca ion titrations. Electrophoretic mobility was independent of both Ca vs. Mg and organic matter content, which suggests that the influence of these factors on flocculation was caused by steric effects. The effect of pH on soil flocculation and electrophoretic mobility in the presence or absence of organic matter, on the other hand, was consistent with an electrostatic mechanism. The change in net particle charge from pH 6 to 8 became more positive as the soluble Ca concentration was increased, and this result was modeled in terms of competitive binding of H+ and Ca2+ by soil functional groups.

The influence of a corona discharge on mass size distribution and net charge of diesel exhaust particles

The influence of a corona discharge on mass size distribution and net charge of diesel exhaust particles

Acadian ambulance service

Atmospheric ions are produced by many natural and anthropogenic sources and their concentrations vary widely between different environments. There is very little information on their concentrations in different types of urban environments, how they compare across these environments and their dominant sources. In this study, we measured airborne concentrations of small ions, particles and net particle charge at 32 different outdoor sites in and around a major city in Australia and identified the main ion sources. Sites were classified into seven groups as follows: park, woodland, city centre, residential, freeway, power lines and power substation. Generally, parks were situated away from ion sources and represented the urban background value of about 270 ions cm−3. Median concentrations at all other groups were significantly higher than in the parks. We show that motor vehicles and power transmission systems are two major ion sources in urban areas. Power lines and substations constituted strong unipolar sources, while motor vehicle exhaust constituted strong bipolar sources. The small ion concentration in urban residential areas was about 960 cm−3. At sites where ion sources were co-located with particle sources, ion concentrations were inhibited due to the ion-particle attachment process. These results improved our understanding on air ion distribution and its interaction with particles in the urban outdoor environment.