126 research outputs found
A fast and easy-to-implement inversion algorithm for mobility particle size spectrometers considering particle number size distribution information outside of the detection range
Multiple-charge inversion is an essential procedure to convert the raw mobility distributions recorded by mobility particle size spectrometers, such as the DMPS or SMPS (differential or scanning mobility particle sizers), into true particle number size distributions. In this work, we present a fast and easy-to-implement multiple-charge inversion algorithm with sufficient precision for atmospheric conditions, but extended functionality. The algorithm can incorporate size distribution information from sensors that measure beyond the upper sizing limit of the mobility spectrometer, such as an aerodynamic particle sizer (APS) or an optical particle counter (OPC). This feature can considerably improve the multiple-charge inversion result in the upper size range of the mobility spectrometer, for example, when substantial numbers of coarse particles are present. The program also yields a continuous size distribution from both sensors as an output. The algorithm is able to calculate the propagation of measurement errors, such as those based on counting statistics, into on the final particle number size distribution. As an additional aspect, the algorithm can perform all inversion steps under the assumption of non-spherical particle shape, including constant or size-dependent shape factors
A concept of an automated function control for ambient aerosol measurements using mobility particle size spectrometers
An automated function control unit was developed to regularly check the
ambient particle number concentration derived from a mobility particle size
spectrometer as well as its zero-point behaviour. The function control allows
unattended quality assurance experiments at remote air quality monitoring or
research stations under field conditions. The automated function control also
has the advantage of being able to get a faster system stability response
than the recommended on-site comparisons with reference instruments. The
method is based on a comparison of the total particle number concentration
measured by a mobility particle size spectrometer and a condensation particle
counter while removing diffusive particles smaller than 20 nm in diameter.
In practice, the small particles are removed by a set of diffusion screens,
as traditionally used in a diffusion battery. Another feature of the
automated function control is to check the zero-point behaviour of the
ambient aerosol passing through a high-efficiency particulate air (HEPA)
filter.
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The performance of the function control is illustrated with the aid of a
1-year data set recorded at Annaberg-Buchholz, a station in the Saxon air
quality monitoring network. During the period of concern, the total particle
number concentration derived from the mobility particle size spectrometer
slightly overestimated the particle number concentration recorded by the
condensation particle counter by 2 % (grand average). Based on our first
year of experience with the function control, we developed tolerance criteria
that allow a performance evaluation of a tested mobility particle size
spectrometer with respect to the total particle number concentration. We
conclude that the automated function control enhances the quality and
reliability of unattended long-term particle number size distribution
measurements. This will have beneficial effects for intercomparison studies
involving different measurement sites, and help provide a higher data
accuracy for cohort health and climate research studies
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Ground-based off-line aerosol measurements at Praia, Cape Verde, during the Saharan Mineral Dust Experiment: Microphysical properties and mineralogy
A large field experiment of the Saharan Mineral Dust Experiment (SAMUM) was performed in Praia, Cape Verde, in
January and February 2008. This work reports on the aerosol mass concentrations, size distributions and mineralogical
composition of the aerosol arriving at Praia. Three dust periods were recorded during the measurements, divided by
transitional periods and embedded in maritime-influenced situations. The total suspended particle mass/PM10/PM2.5
were 250/180/74μg/m3 on average for the first dust period (17–21 January) and 250/230/83μg/m3 for the second (24–26
January). The third period (28 January to 2 February) was the most intensive with 410/340/130 μg/m3. Four modes were
identified in the size distribution. The first mode (50–70 nm) and partly the second (700–1100 nm) can be regarded as
of marine origin, but some dust contributes to the latter. The third mode (2–4 μm) is dominated by advected dust, while
the intermittently occurring fourth mode (15–70 μm) may have a local contribution. The dust consisted of kaolinite
(dust/maritime period: 35%wt./25%wt.),K-feldspar (20%wt./25%wt.), illite (14%wt./10%wt.), quartz (11%wt./8%wt.),
smectites (6%wt./4%wt.), plagioclase (6%wt./1%wt.), gypsum (4%wt./7%wt.), halite (2%wt./17%wt.) and calcite
(2%wt./3%wt.)
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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. Lognormal 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
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Optical properties of aerosol mixtures derived from sun-sky radiometry during SAMUM-2
The SAMUM-2 experiment took place in the Cape Verde is lands in January–February 2008. The colocated ground-based and airborne instruments allow the study of desert dust optical and microphysical properties in a closure experiment. The Meteorological Institute of the University of Munich deployed one sun-sky photometer and two tropospheric lidar systems. A travelling AERONET-Cimel sun-sky radiometer was also deployed. During the measurement period the aerosol scenario over Cape Verde mostly consisted of a dust layer below 2 km and a smoke-dust layer above 2–4 km a.s.l. The Saharan dust arrived at the site from the NE, whereas the smoke originated in the African equatorial region. This paper describes the main results of the Sun photometer observations, supported by lidar information. An analysis of the variations in the aerosol optical depth (AOD) in the range 340–1550 nm, the Ångström exponent, volume size distributions and single scattering albedo is presented. The aerosol mixtures are analysed by means of the fine mode fraction of the AOD provided by the sun-sky inversion data and the Spectral Deconvolution Algorithm. The mean AOD (500 nm) was 0.31, with associated low ångström exponent of 0.46. Several types of events were detected within the data set, with prevalence of dust or mixtures as characterized by the Ångstr¨om exponents of extinction and absorption and the fine mode fraction. Aerosol properties derived from sunphotometry were compared to in situ measurements of size distribution, effective radius and single scattering albedo
Regional Saharan dust modelling during the SAMUM 2006 campaign
The regional dust model system LM-MUSCAT-DES was developed in the framework of the SAMUM project. Using the
unique comprehensive data set of near-source dust properties during the 2006SAMUMfield campaign, the performance
of the model system is evaluated for two time periods in May and June 2006. Dust optical thicknesses, number size
distributions and the position of the maximum dust extinction in the vertical profiles agree well with the observations.
However, the spatio-temporal evolution of the dust plumes is not always reproduced due to inaccuracies in the dust
source placement by the model. While simulated winds and dust distributions are well matched for dust events caused
by dry synoptic-scale dynamics, they are often misrepresented when dust emissions are caused by moist convection or
influenced by small-scale topography that is not resolved by the model. In contrast to long-range dust transport, in the
vicinity of source regions the model performance strongly depends on the correct prediction of the exact location of
sources. Insufficiently resolved vertical grid spacing causes the absence of inversions in the model vertical profiles and
likely explains the absence of the observed sharply defined dust layers
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Spectral surface albedo over Morocco and its impact on radiative forcing of Saharan dust
In May-June 2006, airborne and ground-based solar (0.3-2.2 μm) and thermal infrared (4-42 μm) radiation measurements have been performed in Morocco within the Saharan Mineral Dust Experiment (SAMUM). Upwelling and downwelling solar irradiances have been measured using the Spectral Modular Airborne Radiation Measurement System (SMART)-Albedometer. With these data, the areal spectral surface albedo for typical surface types in southeastern Morocco was derived from airborne measurements for the first time. The results are compared to the surface albedo retrieved from collocated satellite measurements, and partly considerable deviations are observed. Using measured surface and atmospheric properties, the spectral and broad-band dust radiative forcing at top-of-atmosphere (TOA) and at the surface has been estimated. The impact of the surface albedo on the solar radiative forcing of Saharan dust is quantified. In the SAMUM case of 19 May 2006, TOA solar radiative forcing varies by 12 W m-2 per 0.1 surface-albedo change. For the thermal infrared component, values of up to +22 W m-2 were derived. The net (solar plus thermal infrared) TOA radiative forcing varies between -19 and +24 W m-2 for a broad-band solar surface albedo of 0.0 and 0.32, respectively. Over the bright surface of southeastern Morocco, the Saharan dust always has a net warming effect. © 2008 The Author Journal compilation © 2008 Blackwell Munksgaard
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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)
An episode of extremely high PM concentrations over Central Europe caused by dust emitted over the southern Ukraine
International audienceOn 24 March 2007, the atmosphere over Central Europe was affected by an episode of exceptionally high mass concentrations of aerosol particles, most likely caused by a dust storm in the Southern Ukraine on the preceding day. At ground-based measurement stations in Slovakia, the Czech Republic, Poland and Germany PM10 mass concentrations rose to values between 200 and 1400 ?g m?3. An evaluation of PM10 measurements from 360 monitoring stations showed that the dust cloud advanced along a narrow corridor at speeds of up to 70 km h?1. According to lidar observations over Leipzig, Germany, the high aerosol concentrations were confined to a homogeneous boundary layer of 1800 m height. The wavelength dependence of light extinction using both lidar and sun photometer measurements suggested the dominance of coarse particles during the main event. At a wavelength of 532 nm, relatively high volume extinction coefficients (300?400 Mm?1) and a particle optical depth of 0.65 was observed. In-situ measurements with an aerodynamic particle sizer at Melpitz, Germany, confirmed the presence of a coarse particle mode with a mode diameter >2 ?m, whose maximum concentration coincided with that of PM10. A chemical particle analysis confirmed the dominance of non-volatile and insoluble matter in the coarse mode as well as high enrichments of Ti and Fe, which are characteristic of soil dust. A combination of back trajectory calculations and satellite images allowed to identify the dust source with confidence: On 23 March 2007, large amounts of dust were emitted from dried-out farmlands in the southern Ukraine, facilitated by wind gusts up to 100 km h?1. The unusual vertical stability and confined height of this dust layer as well as the rapid transport under dry conditions led to the conservation of high aerosol mass concentrations along the transect and thus to the extraordinary high aerosol concentrations over Central Europe. Our observations demonstrate the capacity of a combined apparatus of in situ and remote sensing measurements to characterise such a dust with a variety of aerosol parameters. As a conclusion, the description of dust emission, transport and transformation processes needs to be improved, especially when facing the possible effects of further anthropogenic desertification and climate change
An case of extreme particulate matter concentrations over Central Europe caused by dust emitted over the southern Ukraine
On 24 March 2007, an extraordinary dust plume was observed in the Central European troposphere. Satellite observations revealed its origins in a dust storm in Southern Ukraine, where large amounts of soil were resuspended from dried-out farmlands at wind gusts up to 30 m s?1. Along the pathway of the plume, maximum particulate matter (PM10) mass concentrations between 200 and 1400 ?g m?3 occurred in Slovakia, the Czech Republic, Poland, and Germany. Over Germany, the dust plume was characterised by a volume extinction coefficient up to 400 Mm?1 and a particle optical depth of 0.71 at wavelength 0.532 ?m. In-situ size distribution measurements as well as the wavelength dependence of light extinction from lidar and Sun photometer measurements confirmed the presence of a coarse particle mode with diameters around 2?3 ?m. Chemical particle analyses suggested a fraction of 75% crustal material in daily average PM10 and up to 85% in the coarser fraction PM10?2.5. Based on the particle characteristics as well as a lack of increased CO and CO2 levels, a significant impact of biomass burning was ruled out. The reasons for the high particle concentrations in the dust plume were twofold: First, dust was transported very rapidly into Central Europe in a boundary layer jet under dry conditions. Second, the dust plume was confined to a relatively stable boundary layer of 1.4?1.8 km height, and could therefore neither expand nor dilute efficiently. Our findings illustrate the capacity of combined in situ and remote sensing measurements to characterise large-scale dust plumes with a variety of aerosol parameters. Although such plumes from Southern Eurasia seem to occur rather infrequently in Central Europe, its unexpected features highlights the need to improve the description of dust emission, transport and transformation processes needs, particularly when facing the possible effects of further anthropogenic desertification and climate change
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