Alfred Wiedensohler

Performance Characteristics of a High-Sensitivity, Three-Wavelength, Total Scatter/Backscatter Nephelometer

Abstract As designed in the 1940s by Beuttell and Brewer, the integrating nephelometer offers a direct method of measuring light scattering by airborne particles without assumptions about particle composition, shape, or physical state. A large number of such instruments have been deployed; however, only a limited number of validation experiments have been attempted. This paper reports a set of closure experiments in which a gas-calibrated nephelometer is used to measure the scattering coefficient of laboratory-generated particles of known size and refractive index. Specifically, it evaluates the performance of a high-sensitivity, three-wavelength, total scatter/backscatter integrating nephelometer (TSI, Inc., model 3563). Sources of uncertainty associated with the gas-calibration procedure, with photon-counting statistics, and with nonidealities in wavelength and angular sensitivity are investigated. Tests with particle-free gases indicate that noise levels are well predicted by photon-counting statistics...

Size distribution, mass concentration, chemical and mineralogical composition and derived optical parameters of the boundary layer aerosol at Tinfou, Morocco, during SAMUM 2006

During the SAMUM 2006 field campaign in southern Morocco, physical and chemical properties of desert aerosols were measured. Mass concentrations ranging from 30μgm−3 for PM2.5 under desert background conditions up to 300 000μgm−3 for total suspended particles (TSP) during moderate dust storms were measured. TSP dust concentrations are correlated with the local wind speed, whereasPM10 andPM2.5 concentrations are determined by advection from distant sources. Size distributions were measured for particles with diameter between 20 nm and 500μm (parametrizations are given). Two major regimes of the size spectrum can be distinguished. For particles smaller than 500 nm diameter, the distributions show maxima around 80 nm, widely unaffected of varying meteorological and dust emission conditions. For particles larger than 500 nm, the range of variation may be up to one order of magnitude and up to three orders of magnitude for particles larger than 10μm. The mineralogical composition of aerosol bulk samples was measured by X-ray powder diffraction. Major constituents of the aerosol are quartz, potassium feldspar, plagioclase, calcite, hematite and the clay minerals illite, kaolinite and chlorite. A small temporal variability of the bulk mineralogical composition was encountered. The chemical composition of approximately 74 000 particles was determined by electron microscopic single particle analysis. Three size regimes are identified: for smaller than 500 nm in diameter, the aerosol consists of sulphates and mineral dust. For larger than 500 nm up to 50μm, mineral dust dominates, consisting mainly of silicates, and—to a lesser extent—carbonates and quartz. For diameters larger than 50μm, approximately half of the particles consist of quartz. Time series of the elemental composition show a moderate temporal variability of the major compounds. Calcium-dominated particles are enhanced during advection from a prominent dust source in Northern Africa (Chott El Djerid and surroundings). The particle aspect ratio was measured for all analysed particles. Its size dependence reflects that of the chemical composition. For larger than 500 nm particle diameter, a median aspect ratio of 1.6 is measured. Towards smaller particles, it decreases to about 1.3 (parametrizations are given). From the chemical/mineralogical composition, the aerosol complex refractive index was determined for several wavelengths from ultraviolet to near-infrared. Both real and imaginary parts show lower values for particles smaller than 500 nm in diameter (1.55–2.8 × 10−3i at 530 nm) and slightly higher values for larger particles (1.57–3.7 × 10−3i at 530 nm).

Particle number size distributions in a street canyon and their transformation into the urban-air background: measurements and a simple model study

Abstract Car traffic is one of the main anthropogenic aerosol sources in modern cities. The characterization of these emissions is important for describing the quality of urban air. Measurements in a street canyon in a German urban area were made. Maximum number concentrations occurred during morning hours from Monday to Friday when the traffic density is highest. The maximum of the number size distribution measured during rush hour near a busy city street was at a particle diameter of 15 nm . This differs significantly from size distributions directly measured in vehicle exhaust (vehicles placed on chassis dynamometers used for vehicle emissions certification), typically about 50 nm . The size distributions measured in the urban area depended on the distance to the nearest road. With increasing distance, the maximum of the size distribution increased, and the total number concentration decreased. This seems to be a result of particle growth due to processes such as coagulation and condensation, and dilution with the surrounding air. To clarify the transformation of the particle number size distributions measured in a street canyon into the urban-air background, a sectional aerosol model was used to calculate the evolution of the number size distribution, and included the effect of condensation, coagulation, dilution, and continuous entrainment of freshly emitted particles yielding good agreement with measurements.

New particle formation in the continental boundary layer: Meteorological and gas phase parameter influence

New particle formation in the polluted continental boundary layer was studied, based on 1.5-year observations of the particle size distribution, meteorological and gas phase parameters. Events of new particle formation involving significant ultrafine particle number concentrations (>104 cm−3 in the size range 3–11 nm) were observed on 20% of all days, pointing out that a frequent particle production from gaseous precursors can occur despite the relatively high pre-existing particle surface area in the area of investigation. The maximum in the observed particle size distributions was mostly above 3 nm, suggesting the actual particle nucleation to take place upwind of the measurement site. A particle growth analysis yielded 2.3±1.4 h as an upper limit of the time for the particles to grow from the critical cluster size till the observation of the peak in ultrafine number concentration. On 80% of the significant events of new particle formation (though not on all), SO2 concentrations increased considerably (by an average factor of 7), most likely by entrainment from aloft. Particle surface area was, on average, higher on event days compared to non-event days, indicating only a weak competition between condensation onto the pre-existing particle surface area and the new particle formation process. The highest statistical correlation was found between the events of new particle formation and solar radiation, indicating a high degree of meteorological control.

Atmospheric particle number size distribution in central Europe: Statistical relations to air masses and meteorology

Atmospheric particle number size distributions determined over 1.5 years at a central European site were statistically analyzed in terms of their relation to time of day, season, meteorology, and synoptic-scale air masses. All size distributions were decomposed into lognormal particle modes corresponding to the accumulation, Aitken, aged nucleation, and nucleation modes. The concentration of nucleation mode particles ( 30 nm) lacked such diurnal behavior, and proved to be indicative of different synoptic-scale air mass types. Over 70% of the time, air masses of Atlantic origin and maritime character prevailed, showing obvious signs of anthropogenic influence most of the time (accumulation mode: 500 cm−3; Aitken mode: 2300 cm−3). During a limited period of time (10%), however, continentally aged air with significantly enhanced concentrations of aerosol was observed (accumulation mode: 1200 cm−3; Aitken mode: 3300 cm−3). These air masses were advected from source regions in Russia, and eastern, southeastern, and central Europe, mainly under anticyclonic and high-pressure influence. The analysis provides a refined picture of the behavior of the particle number size distribution and provides parameterizations that are representative for a variety of air masses in Europe and thus suitable for future climate modeling applications.

A closure study of sub-micrometer aerosol particle hygroscopic behaviour

Abstract The hygroscopic properties of sub-micrometer aerosol particles were studied in connection with a ground-based cloud experiment at Great Dun Fell, in northern England in 1995. Hygroscopic diameter growth factors were measured with a Tandem Differential Mobility Analyser (TDMA) for dry particle diameters between 35 and 265 nm at one of the sites upwind of the orographic cloud. An external mixture consisting of three groups of particles, each with different hygroscopic properties, was observed. These particle groups were denoted less-hygroscopic, more-hygroscopic and sea spray particles and had average diameter growth factors of 1.11–1.15, 1.38–1.69 and 2.08–2.21 respectively when taken from a dry state to a relative humidity of 90%. Average growth factors increased with dry particle size. A bimodal hygroscopic behaviour was observed for 74–87% of the cases depending on particle size. Parallel measurements of dry sub-micrometer particle number size distributions were performed with a Differential Mobility Particle Sizer (DMPS). The inorganic ion aerosol composition was determined by means of ion chromatography analysis of samples collected with Berner-type low pressure cascade impactors at ambient conditions. The number of ions collected on each impactor stage was predicted from the size distribution and hygroscopic growth data by means of a model of hygroscopic behaviour assuming that only the inorganic substances interacted with the ambient water vapour. The predicted ion number concentration was compared with the actual number of all positive and negative ions collected on the various impactor stages. For the impactor stage which collected particles with aerodynamic diameters between 0.17–0.53 μm at ambient relative humidity, and for which all pertinent data was available for the hygroscopic closure study, the predicted ion concentrations agreed with the measured values within the combined measurement and model uncertainties for all cases but one. For this impactor sampling occasion, the predicted ion concentration was significantly higher than the measured. The air mass in which this sample was taken had undergone extensive photochemical activity which had probably produced hygroscopically active material other than inorganic ions, such as organic oxygenated substances.

Hygroscopic properties of aerosol particles in the north-eastern Atlantic during ACE-2

Measurements of the hygroscopic properties of sub-micrometer atmospheric aerosol particles were performed with hygroscopic tandem differential mobility analysers (H-TDMA) at 5 sites in the subtropical north-eastern Atlantic during the second Aerosol Characterization Experiment (ACE-2) from 16 June to 25 July 1997. Four of the sites were in the marine boundary layer and one was, at least occasionally, in the lower free troposphere. The hygroscopic diameter growth factors of individual aerosol particles in the dry particle diameter range 10−440 nm were generally measured for changes in relative humidity (RH) from <10% to 90%. In the marine boundary layer, growth factors at 90% RH were dependent on location, air mass type and particle size. The data was dominated by a unimodal growth distribution of more-hygroscopic particles, although a bimodal growth distribution including less-hygroscopic particles was observed at times, most often in the more polluted air masses. In clean marine air masses the more-hygroscopic growth factors ranged from about 1.6 to 1.8 with a consistent increase in growth factor with increasing particle size. There was also a tendency toward higher growth factors as sodium to sulphate molar ratio increased with increasing sea-salt contribution at higher wind speeds. During outbreaks of European pollution in the ACE-2 region, the growth factors of the largest particles were reduced, but only slightly. Growth factors at all sizes in both clean and polluted air masses were markedly lower at the Sagres, Portugal site due to more proximate continental influences. The frequency of occurrence of less-hygroscopic particles with a growth factor of ca. 1.15 was greatest during polluted conditions at Sagres. The free tropospheric 50 nm particles were predominately less-hygroscopic, with an intermediate growth factor of 1.4, but more-hygroscopic particles with growth factors of about 1.6 were also frequent. While these particles probably originate from within the marine boundary layer, the less-hygroscopic particles are probably more characteristic of lower free tropospheric air masses. For those occasions when measurements were made at 90% and an intermediate 60% or 70% RH, the growth factor G(RH) of the more-hygroscopic particles could be modelled empirically by a power law expression. For the ubiquitous more-hygroscopic particles, the expressions G(RH)=(1-RH/100)-0.210 for 50 nm Aitken mode particles and G(RH)=(1-RH/100)-0.233 for 166 nm accumulation mode particles are recommended for clean marine air masses in the north-eastern Atlantic within the range 0

The effect of particle size on cardiovascular disorders — The smaller the worse

BACKGROUND Previous studies observed associations between airborne particles and cardio-vascular disease. Questions, however, remain as to which size of the inhalable particles (coarse, fine, or ultrafine) exerts the most significant impact on health. METHODS For this retrospective study, data of the total number of 23,741 emergency service calls, registered between February 2002 and January 2003 in the City of Leipzig, were analysed, identifying 5326 as being related to cardiovascular incidences. Simultaneous particle exposure was determined for the particle sizes classes <100 nm (UFP), <2.5 μm (PM2.5) and <10 μm (PM10). We used a time resolution of 1 day for both parameters, emergency calls and exposure. RESULTS Within the group of cardiovascular diseases, the diagnostic category of hypertensive crisis showed a significant association with particle exposure. The significant effect on hypertensive crisis was found for particles with a size of <100 nm in diameter and starting with a lag of 2 days after exposure. No consistent influence could be observed for PM2.5 and PM10. The Odds Ratios on hypertensive crisis were significant for the particle size <100 nm in diameter from day 2 post exposure OR=1.06 (95%CI: 1.02-1.10, p=0.002) up to day 7 OR=1.05 (95%CI 1.02-1.09, p=0.005). CONCLUSION Ultrafine particles affect cardiovascular disease adversely, particularly hypertensive crises. Their effect is significant compared with PM2.5 and PM10. It appears necessary, from a public health point of view, to consider regulating this type of particles using appropriate measurands as particle number.

NaCl Aerosol Particle Hygroscopicity Dependence on Mixing with Organic Compounds

Organic compounds in the atmosphere can influence the activation, growth and lifetimes of haze, fog and cloud droplets by changing the condensation and evaporation rates of liquid water by these aqueous aerosol particles. Depending on the nature and properties of the organic compounds, the change can be to enhance or reduce these rates. In this paper we used a tandem differential mobility analyzer (TDMA) to examine the effect of tetracosane, octanoic acid, and lauric acid on the hygroscopic properties of NaCl aerosol particles at relative humidities (RH) between 30 and 95%. These organic compounds have been identified in ambient aerosol particle samples. A slight lowering of the deliquescence relative humidity (DRH) and suppression of hygroscopic growth for the NaCl-organic compound mixtures were observed when compared to pure NaCl particles. The growth of pure NaCl particles was 2.25 in diameter at 85% RH while the growth of the mixed particles was 1.3 to 1.7 in particle diameter at 85% RH with organic mass fraction of 30–50%. This shows that these organic compounds have to be present in rather large mass fractions to effect the hygroscopic behavior to a similar degree observed for ambient aerosol during field measurements. Despite the mixing of the organic material with NaCl, hysteresis was observed for decreasing RH histories, suggesting the formation of metastable droplets. These laboratory results are strikingly similar to ambient field results. For example, if the total organic mass fraction of the particles is between 0.30 and 0.50, the particle growth at 85% RH is about a factor of 1.4 for the laboratory and field measurements. Such reduction in growth compared to the pure inorganic salt is in contradiction to speculations concerning significant effects by organic compounds on cloud condensation nuclei and thus formation on clouds.

Hygroscopic growth of aerosol particles and its influence on nucleation scavenging in cloud: Experimental results from Kleiner Feldberg

The hygroscopic growth of individual aerosol particles has been measured with a Tandem Differential Mobility Analyser. The hygroscopic growth spectra were analysed in terms of diameter change with increasing RH from ≤20% to 85%. The measurements were carried out during the GCE cloud experiment at Kleiner Feldberg, Taunus, Germany in October and November 1990.Two groups of particles with different hygroscopic growth were observed. The less-hygroscopic group had average growth factors of 1.11, 1.04 and 1.02 for particle diameters of 50, 150 and 300 nm, respectively. The more-hygroscopic group had average growth factors of 1.34, 1.34, and 1.37 for the same particle diameters. The average fraction of less-hygroscopic particles was about 50%. Estimates of the soluble fractions of the particles belonging to the two groups are reported.Hygroscopic growth spectra for total aerosol, interstitial aerosol and cloud drop residuals were measured. A comparison of these hygroscopic growths of individual aerosol particles provides clear evidence for the importance of hygroscopic growth in nucleation scavenging. The measured scavenged fraction of particles as a function of diameter can be explained by the hygroscopic growth spectra.