Jenny Rissler

Hygroscopic properties of submicrometer atmospheric aerosol particles measured with H-TDMA instruments in various environments—a review

The hygroscopic properties play a vital role for the direct and indirect effects of aerosols on climate, as well as the health effects of particulate matter (PM) by modifying the deposition pattern of inhaled particles in the humid human respiratory tract. Hygroscopic Tandem Differential Mobility Analyzer (H-TDMA) instruments have been used in field campaigns in various environments globally over the last 25 yr to determine the water uptake on submicrometre particles at subsaturated conditions. These investigations have yielded valuable and comprehensive information regarding the particle hygroscopic properties of the atmospheric aerosol, including state of mixing. These properties determine the equilibrium particle size at ambient relative humidities and have successfully been used to calculate the activation of particles at water vapour supersaturation. This paper summarizes the existing published H-TDMA results on the sizeresolved submicrometre aerosol particle hygroscopic properties obtained from ground-based measurements at multiple marine, rural, urban and free tropospheric measurement sites. The data is classified into groups of hygroscopic growth indicating the external mixture, and providing clues to the sources and processes controlling the aerosol. An evaluation is given on how different chemical and physical properties affect the hygroscopic growth.

Overview of the inorganic and organic composition of size-segregated aerosol in Rondônia, Brazil, from the biomass-burning period to the onset of the wet season

The aerosol characterization experiment performed within the Large-Scale Biosphere-Atmosphere Experiment in Amazonia-Smoke, Aerosols, Clouds, Rainfall and Climate (LBA-SMOCC) field experiment carried out in Rondonia, Brazil, in the period from September to November 2002 provides a unique data set of size-resolved chemical composition of boundary layer aerosol over the Amazon Basin from the intense biomass-burning period to the onset of the wet season. Three main periods were clearly distinguished on the basis of the PM10 concentration trend during the experiment: (1) dry period, with average PM10 well above 50 mu g m(-3); (2) transition period, during which the 24-hour-averaged PM10 never exceeded 40 mu g m(-3) and never dropped below 10 mg m(-3); (3) and wet period, characterized by 48-hour-averaged concentrations of PM10 below 12 mu g m(-3) and sometimes as low as 2 mu g m(-3). The trend of PM10 reflects that of CO concentration and can be directly linked to the decreasing intensity of the biomass- burning activities from September through November, because of the progressive onset of the wet season. Two prominent aerosol modes, in the submicron and supermicron size ranges, were detected throughout the experiment. Dry period size distributions are dominated by the fine mode, while the fine and coarse modes show almost the same concentrations during the wet period. The supermicron fraction of the aerosol is composed mainly of primary particles of crustal or biological origin, whereas submicron particles are produced in high concentrations only during the biomass-burning periods and are mainly composed of organic material, mostly water-soluble, and similar to 10% of soluble inorganic salts, with sulphate as the major anion. Size-resolved average aerosol chemical compositions are reported for the dry, transition, and wet periods. However, significant variations in the aerosol composition and concentrations were observed within each period, which can be classified into two categories: (1) diurnal oscillations, caused by the diurnal cycle of the boundary layer and the different combustion phase active during day (flaming) or night (smouldering); and (2) day-to-day variations, due to alternating phases of relatively wet and dry conditions. In a second part of the study, three subperiods representative of the conditions occurring in the dry, transition, and wet periods were isolated to follow the evolution of the aerosol chemical composition as a function of changes in rainfall rate and in the strength of the sources of particulate matter. The chemical data set provided by the SMOCC field experiment will be useful to characterize the aerosol hygroscopic properties and the ability of the particles to act as cloud condensation nuclei.

Cloud‐nucleating properties of the Amazonian biomass burning aerosol: Cloud condensation nuclei measurements and modeling

The cloud-nucleating properties of the atmospheric aerosol were studied in an area under strong influence of vegetation burning. The measurements were part of Large-Scale Biosphere Atmosphere Experiment in Amazonia-Smoke Aerosols, Clouds, Rainfall and Climate (LBA-SMOCC) and were carried out at a ground site located in the state of Rondonia in southwestern Amazonia, Brazil, September to November 2002, covering the dry season, a transition period, and the onset of the wet season. The concentrations of cloud condensation nuclei (CCN) were measured with a static thermal gradient CCN counter for supersaturations ranging between 0.23 and 1.12%. As a closure test, the CCN concentrations were predicted with a time resolution of 10 min from measurements of the dry particle number size distribution (3-850 nm, Differential Mobility Analyzer (DMPS)) and hygroscopic growth at 90% relative humidity (Hygroscopic Tandem Differential Mobility Analyzer (H-TDMA)). No chemical information was needed. The predicted and measured CCN concentrations were highly correlated (r(2)=0.97-0.99), and the predictions were only slightly lower than those measured, typically by 15-20%. Parameterizations of the predicted CCN concentrations are given for each of the three meteorological periods. These are based on averages taken during the afternoon hours when the measurements at ground level were representative for the aerosol entering the base of convective clouds. Furthermore, a more detailed parameterization including the mixing state of the aerosol is given, where the hygroscopic properties are expressed as the number of soluble ions or nondissociating molecules per unit volume dry particle. (Less)

Characteristics of aerosol particles formed during grate combustion of moist forest residue

The characteristics of aerosol particles formed during combustion of moist forest residue were studied as a function of load in a I MW moving grate boiler and at almost full load in a similar larger 6 MW boiler. The coarse (1 mum 12) were K, S and Cl in the fine mode and Ca, K and S in the coarse mode. The dominant ions in the fine mode were K+, SO42- and CO32-. The fine mode particles had hygroscopic growth factors of around 1.65 at RH=90%, with a deliquescence point at a relative humidity between 30% and 60%. It was assessed that K2CO3 is responsible for the low deliquescence point. Fine mode particles of a given dry diameter had similar chemical composition

Deposition of biomass combustion aerosol particles in the human respiratory tract.

Smoke from biomass combustion has been identified as a major environmental risk factor associated with adverse health effects globally. Deposition of the smoke particles in the lungs is a crucial factor for toxicological effects, but has not previously been studied experimentally. We investigated the size-dependent respiratory-tract deposition of aerosol particles from wood combustion in humans. Two combustion conditions were studied in a wood pellet burner: efficient (“complete”) combustion and low-temperature (incomplete) combustion simulating “wood smoke.” The size-dependent deposition fraction of 15-to 680-nm particles was measured for 10 healthy subjects with a novel setup. Both aerosols were extensively characterized with regard to chemical and physical particle properties. The deposition was additionally estimated with the ICRP model, modified for the determined aerosol properties, in order to validate the experiments and allow a generalization of the results. The measured total deposited fraction of particles from both efficient combustion and low-temperature combustion was 0.21–0.24 by number, surface, and mass. The deposition behavior can be explained by the size distributions of the particles and by their ability to grow by water uptake in the lungs, where the relative humidity is close to saturation. The experiments were in basic agreement with the model calculations. Our findings illustrate: (1) that particles from biomass combustion obtain a size in the respiratory tract at which the deposition probability is close to its minimum, (2) that particle water absorption has substantial impact on deposition, and (3) that deposition is markedly influenced by individual factors.

Effective Density Characterization of Soot Agglomerates from Various Sources and Comparison to Aggregation Theory

Soot particle (black carbon) morphology is of dual interest, both from a health perspective and due to the influence of soot on the global climate. In this study, the mass-mobility relationships, and thus effective densities, of soot agglomerates from three types of soot emitting sources were determined in situ by combining a differential mobility analyzer (DMA) and an aerosol particle mass analyzer (APM). High-resolution transmission electron microscopy was also used. The soot sources were diesel engines, diffusion flame soot generators, and tapered candles, operated under varying conditions. The soot microstructure was found to be similar for all sources and settings tested, with a distance between the graphene layers of 3.7–3.8 Å. The particle specific surface area was found to vary from 100 to 260 m2/g. The particle mass-mobility relationship could be described by a power law function with an average exponent of 2.3 (±0.1) for sources with a volatile mass fraction <10% and primary particle sizes of 11–29 nm. The diesel exhaust from a heavy duty engine at idling had a substantially higher volatile mass fraction and a higher mass-mobility exponent of 2.6. The mass-mobility exponent was essentially independent of the number of primary particles in the range covered (Npp = 10–1000). Despite the similar exponents, the effective density varied substantially from source to source. Two parameters were found to alter the effective density: primary particle size and coating mass fraction. A correlation was found between primary particle size and mass-mobility relationship/effective density and an empirical expression relating these parameters is presented. The effects on the DMA-APM results of doubly charged particles and DMA agglomerate alignment were investigated and quantified. Finally, the dataset was compared to three theoretical approaches describing agglomerate particles’ mass-mobility relationship. Copyright 2013 American Association for Aerosol Research

Hygroscopic Behavior of Aerosol Particles Emitted from Biomass Fired Grate Boilers

This study focuses on the hygroscopic properties of submicrometer aerosol particles emitted from two small-scale district heating combustion plants (1 and 1.5 MW) burning two types of biomass fuels (moist forest residue and pellets). The hygroscopic particle diameter growth factor (Gf) was measured when taken from a dehydrated to a humidified state for particle diameters between 30–350 nm (dry size) using a Hygroscopic Tandem Differential Mobility Analyzer (H-TDMA). Particles of a certain dry size all showed similar diameter growth and the Gf at RH = 90% for 110/100 nm particles was 1.68 in the 1 MW boiler, and 1.5 in the 1.5 MW boiler. These growth factors are considerably higher in comparison to other combustion aerosol particles such as diesel exhaust, and are the result of the efficient combustion and the high concentration of alkali species in the fuel. The observed water uptake could be explained using the Zdanovski-Stokes-Robinson (ZSR) mixing rule and a chemical composition of potassium salts only, taken from ion chromatography analysis of filter and impactor samples (KCl, K2SO4, and K2CO3). Agglomerated particles collapsed and became more spherical when initially exposed to a moderately high relative humidity. When diluted with hot particle-free air, the fractal-like structures remained intact until humidified in the H-TDMA. A method to estimate the fractal dimension of the agglomerated combustion aerosol and to convert the measured mobility diameter hygroscopic growth to the more useful property volume diameter growth is presented. The fractal dimension was estimated to be ∼ 2.5.

Effective Density and Mixing State of Aerosol Particles in a Near-Traffic Urban Environment.

In urban environments, airborne particles are continuously emitted, followed by atmospheric aging. Also, particles emitted elsewhere, transported by winds, contribute to the urban aerosol. We studied the effective density (mass-mobility relationship) and mixing state with respect to the density of particles in central Copenhagen, in wintertime. The results are related to particle origin, morphology, and aging. Using a differential mobility analyzer-aerosol particle mass analyzer (DMA-APM), we determined that particles in the diameter range of 50-400 nm were of two groups: porous soot aggregates and more dense particles. Both groups were present at each size in varying proportions. Two types of temporal variability in the relative number fraction of the two groups were found: soot correlated with intense traffic in a diel pattern and dense particles increased during episodes with long-range transport from polluted continental areas. The effective density of each group was relatively stable over time, especially of the soot aggregates, which had effective densities similar to those observed in laboratory studies of fresh diesel exhaust emissions. When heated to 300 °C, the soot aggregate volatile mass fraction was ∼10%. For the dense particles, the volatile mass fraction varied from ∼80% to nearly 100%.

A Potential Soot Mass Determination Method from Resistivity Measurement of Thermophoretically Deposited Soot

Miniaturized detection systems for nanometer-sized airborne particles are in demand, for example in applications for onboard diagnostics downstream particulate filters in modern diesel engines. A soot sensor based on resistivity measurements was developed and characterized. This involved generation of soot particles using a quenched co-flow diffusion flame; depositing the particles onto a sensor substrate using thermophoresis and particle detection using a finger electrode structure, patterned on thermally oxidized silicon substrate. The generated soot particles were characterized using techniques including Scanning Mobility Particle Sizer for mobility size distributions, Differential Mobility Analyzer—Aerosol Particle Mass analyzer for the mass–mobility relationship, and Transmission Electron Microscopy for morphology. The generated particles were similar to particles from diesel engines in concentration, mobility size distribution, and mass fractal dimension. The primary particle size, effective density and organic mass fraction were slightly lower than values reported for diesel engines. The response measured with the sensors was largely dependent on particle mass concentration, but increased with increasing soot aggregate mobility size. Detection down to cumulative mass as small as 20–30 μg has been demonstrated. The detection limit can be improved by using a more sensitive resistance meter, modified deposition cell, larger flow rates of soot aerosol and modifying the sensor surface.

An evaluation and comparison of cloud condensation nucleus activity models: Predicting particle critical saturation from growth at subsaturation

The ability of particles to activate and form cloud droplets influences the functioning of the Earths hydrological cycle. This work links the particle water uptake at subsaturation to the critical supersaturation ratio needed for particles to become cloud condensation nuclei (CCN). Five models using the particle hygroscopic growth at subsaturation for predicting the critical supersaturation needed for droplet activation were applied to a laboratory data set of inorganic and organic compounds and mixtures of them. The data set consisted of hygroscopicity tandem differential mobility analyzer measurements and CCN counter measurements. No chemical composition information was used when applying the models. All models tested were based on modifications of Kohler theory and gave similar results. The agreement between predicted and measured critical supersaturations was good, considering the relatively simple models used. A trend of overestimating the critical supersaturations was observed, typically by similar to 15%. The best performing model gave on average only a 4% offset from experimental values; the model with the largest deviation was offset by 20%. A comparison was made between the number of soluble entities (ions or nondissociating molecules) estimated from the particle hygroscopic growth at 90% relative humidity (RH) and the number estimated from the particle critical supersaturation; a similar to 35% increase was observed in the effective number of entities in solution when going from 90% RH to activation. For many types of aerosols, differences in the model approaches tested do not induce large differences in the predicted critical supersaturation. However, it is most important to follow the recommendations published with the respective models and not use them indiscriminately. (Less)