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Atmospheric Environment (v.39, #21)
The ion chemistry and the source of PM2.5 aerosol in Beijing by Ying Wang; Guoshun Zhuang; Aohan Tang; Hui Yuan; Yele Sun; Shuang Chen; Aihua Zheng (pp. 3771-3784).
Daily PM2.5 aerosol samples were collected at five sites in Beijing for a 3-year period from 2001 to 2003. Concentrations of the water-soluble ions (SO42−, NO3−, Cl−, F−, PO43−, NO2−, CH3COO−, HCOO−, MSA, C2O42−, NH4+, Ca2+, K+, Mg2+, Na+) and 23 elements were measured for a total of 334 samples. A relatively even spatial distribution throughout Beijing and a significant seasonal variation were observed. SO42−, NO3−, Cl−, NH4+, Ca2+, and K+ were the major ions and existed mainly in the form of (NH4)2SO4, NH4NO3, NaCl, KCl, and CaCl2 in aerosol particles. Most ions showed high concentrations in winter and low in summer. Secondary ions, mainly SO42−, NO3−, exhibited high concentrations in both summer and winter due to the secondary transformation accelerated under high humidity and strong solar radiation in summer and the higher concentration of SO2 from coal burning and the lower removal rate in winter. The formations of SO42− and NO3− were determined largely by temperature and NH4+, respectively. Temperature, relative humidity, rainwater frequency, and air mass origin might be the main factors regulating the aerosol distribution. Crustal ions exhibited sporadic but high peaks in spring due to the intrusion of dust from west and northwest of China. The Ca2+/Al ratio was used to indicate the mixing of different dust sources. Factor analysis showed that the secondary formation of coal/biomass burning products, crust, industrial and traffic emissions were the major sources of the fine aerosols in Beijing. Traffic source became more significant with motorization in recent years.
Keywords: Water-soluble part; Ions; Speciation; Seasonal variation; Secondary transformation; Sources
Hydrocarbon emissions from industrial release events in the Houston-Galveston area and their impact on ozone formation by Cynthia Folsom Murphy; David T. Allen (pp. 3785-3798).
Ambient measurements have shown that ozone formation in the Houston-Galveston area of Texas is frequently much more rapid than in other urban areas. One of the contributing factors is believed to be short-term episodic or “event� emissions from industrial facilities, particularly releases that contain significant mass fractions of highly reactive volatile organic compounds (HRVOCs). In this work, time series analyses are used to compare average annual flow rates for air pollutant emissions with those released during reported emission events. The results indicate that the magnitude and frequency of HRVOC event emissions are an important element in accurately reflecting ozone precursor emission patterns in the Houston-Galveston area, particularly in Harris, Brazoria, Galveston, and Chambers counties. More than 50% of the reported episodic (event) emissions of HRVOCs are ethene and approximately a third are propene; the remainders are isomers of butene and 1,3-butadiene. Most events last less than 24h. The mass released in an event can vary from a few hundred to more than 100,000lb, and the dominant type of industrial source is chemical manufacturers (SIC 2869). Daily emissions from a single facility can vary from annual average emissions by multiple orders of magnitude at a frequency of several times a year. Because there are so many facilities in the Houston-Galveston area, HRVOC emission variability of this magnitude can be expected daily, at some time and some location in the Houston-Galveston area. If the emission variability occurs at times and locations where atmospheric conditions are conducive to ozone formation, both ambient data and photochemical modeling indicate that industrial emission events can lead to elevated concentrations of ozone. Specifically, peak, area-wide ozone concentration can be increased by as much as 100ppb for large HRVOC emission events.
Keywords: Emission inventory; Point sources; VOC emissions; Episodic emissions; Highly reactive volatile organic compounds (HRVOC); Ozone; Ethene; Propene; Butene; Butadiene
A wintertime study of PAHs in fine and coarse aerosols in São Paulo city, Brazil by Christine Bourotte; M.-C. Maria-Cristina Forti; Satie Taniguchi; Bicego Marcia Caruso Bícego; Paulo Andrade Lotufo (pp. 3799-3811).
The Polycyclic aromatic hydrocarbons PAHs (naphthalene, acenaphthylene, acenaphthene, phenanthrene, fluoranthene, pyrene, benzo(a)anthracene, chrysene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene, indeno(1,2,3-cd)pyrene and benzo(ghi)perylene) concentrations in fine (PM2.5) and coarse (PM2.5–10) atmospheric particulate matter were measured at São Paulo city, Brazil. The dominant PAH compounds were indeno(1,2,3-cd)pyrene, benzo(ghi)perylene and benzo(b)fluoranthene for both the fractions. The calculated ratios of some specific PAHs were in close agreement with those attributed to direct emissions of car exhaust. The factor analysis for PM2.5 produced four factors: Factor 1 was attributed to diesel emissions, Factor 2 was attributed to stationary combustion source, Factor 3 was attributed to vehicular emissions and Factor 4 to natural gas combustion and biomass burning. For PM2.5–10, Factor 1 was attributed to vehicular emissions and Factor 2 was attributed to a mixture of combustion sources such as natural gas combustion, incineration emissions and oil combustion. Also, vehicular emissions were assumed to be the major source of PAHs in the São Paulo city atmosphere.
Keywords: Air pollution; Urban particles; Polycyclic aromatic hydrocarbons; Tropical region; São Paulo city
An examination of the urban dispersion curves derived from the St. Louis dispersion study by Akula Venkatram (pp. 3813-3822).
We have reanalyzed the data collected during the St. Louis urban dispersion study (McElroy, J.L., Pooler, F., 1968a. The St. Louis Dispersion Study—Instrumentation, Procedures, and Data Tabulations, vol. I. National Air Pollution Control Administration, Pub. No. APTD-68-12, US DHEW, Arlington (352pp); McElroy, J.L., Pooler, F., 1968b. The St. Louis Dispersion Study—Analysis, vol. II. National Air Pollution Control Administration, Pub. No. AP-53, US DHEW, Arlington (50pp)) to address two questions: (1) How well do the empirical Briggs Urban (BU) curves (Briggs, G.A., 1973. Diffusion estimation for small emissions. ERL, ARL USAEC Report ATDL-106, US Atomic Energy Commission, Oak Ridge, TN), referred to as the McElroy–Pooler dispersion curves in industrial source complex, describe the observations that were used to derive them? (2) Can observed meteorological information yield dispersion curves and concentration estimates that are comparable to those based on the empirical BU curves? We find that the BU curves provide an adequate description of the observed horizontal spreads for both daytime and evening trials but underestimate the vertical spreads during daytime conditions. When the centerline ground-level concentrations based on these curves are compared with corresponding observations, we find that the logarithmic mean of the ratio of the modeled to the observed concentrationmg=1.17, and the 95% confidence interval of the ratio,Cp/Co, is 0.15–9.1 during the daytime trials. For the evening experiments,mg=0.92, and the 95% confidence interval is 0.23–3.7 for the ratio. When meteorological measurements made in the boundary layer are used to estimate plume spreads, the resulting model performance statistics are better than those obtained with the empirical BU curves. However, this conclusion is tentative because some of meteorological inputs were not measured but estimated from available measurements.
Keywords: St. Louis field experiment; Tracer study; Dispersion model; Data analysis; Barrio Logan; Urban dispersion; Plume spread; Turbulent intensity; Industrial Source Complex (ISC) model; Urban dispersion curves; Model performance statistics; McElroy–Pooler dispersion curves
Modeling indoor coarse particle deposition onto smooth and rough vertical surfaces by Alvin C.K. Lai (pp. 3823-3830).
Accurate modeling of indoor aerosol deposition is an important step for exposure assessment. Indoor particles normally have a very small diffusion coefficient; hence, a surface that behaves as though it were smooth for heat or momentum transfer might be considered as “rough� for deposition. However, particle deposition from an indoor environment is frequently modeled by assuming that indoor surfaces are smooth. Many indoor surfaces have random roughness protrusion scales up to several millimeters, which may significantly affect the deposition loss rate. A new semi-empirical model derived from an electrical analogy was developed and Brownian and turbulent diffusion, gravitational settling onto roughness elements and inertial transport deposition mechanism were taken into account. The model was used to predict deposition onto one smooth and four sandpapers with different roughness scales. The inertia-incorporated model prediction matches well with the experimental results onto the smooth glass surface for coarse particles from 0.9 to 9μm. Predictions for deposition onto sandpapers are less satisfactory and plausible mechanisms considered are further discussed.
Keywords: Aerosol; Deposition; Modeling; Roughness; Turbulence
Development and validation of a simple mineral dust source inventory suitable for modelling in North Central China by F. Lasserre; G. Cautenet; S.C. Alfaro; L. Gomes; J.L. Rajot; S. Lafon; A. Gaudichet; B. Chatenet; M. Maille; H. Cachier; P. Chazette; X.Y. Zhang (pp. 3831-3841).
The quantitatively most important sources of desert dust are mainly located in North Africa (Sahara and Sahel), in the Middle East (Saudi Arabia, Iran, Iraq) and in continental Asia. This paper presents the buildup of a desert dust source database valid for dust models over China and Mongolia. Our method is based upon the use of a mesoscale model—the RAMS model (Colorado State University, USA)—coupled with a DPM (Dust Production Model) already developed (LISA laboratory, France) and qualified for African and Middle-East areas. As the DPM needs surface data not completely available in this area, our approach is based on an iterative method: successive model runs are compared with observations, and the surface mapping of DPM properties is modified so as to get acceptable model results. After this step, a simple but efficient surface inventory of emissive properties of dust by soils is built. To validate it, we simulate a dust storm in April 2002, which was observed and measured in particular by a surface station near Yulin city (600km East of Beijing). In this event, we compare the model issues with measured concentrations and particle size distribution. Our model successive fittings allow us to retrieve the mass concentration and size distribution (at least for radiir<1μm) in good agreement with observations. Moreover, this latter simulation agrees with the importance of the “northern high dust sources� often mentioned in literature, i.e. confirms the importance of the Alxa plateau as a main regional source. During the second event, an amount of about 4.1Mt of dust is raised in a 6-day period, with a peak of 0.2Mt in 6h over only 10000km2. To sum up, our results allow us to outline an easy built mineral dust source database—not needing too sharp pedological “on field� data—including erodibility fraction, soil types, land cover and snow cover, contributing to the development of the “Asian dust databank�.
Keywords: Aerosol; Mesoscale modelling; Dust production model; Transport and mass concentration; Size number distribution
Surface ozone: A likely threat to crops in Yangtze delta of China by Wang Huixiang; C.S. Kiang; Tang Xiaoyan; Zhou Xiuji; W.L. Chameides (pp. 3843-3850).
Observational surface ozone during a 16-month field experiment between 1999 and 2000 at six non-urban sites in the Yangtze Delta of China suggests that enhanced ozone concentrations were most prevalent during the spring, when the 90-day ozone-exposure indices SUM06, AOT40 and 7-h-daytime mean ranged from 27 to 43ppmh (parts per million-hour), 17 to 27ppmh, and 56 to 63ppbv (parts per billion by volume), respectively. These observations, along with dose–response relationships derived from ozone-effects research carried out on crops in the United States and Europe, suggest that the yields of winter wheat grown in the Yangtze Delta in 1999 and 2000 were likely reduced by about 20–30% as a result of damage caused by exposure to ambient ozone.
Keywords: Surface ozone; Yield losses of winter wheat; SUM06; AOT40; Yangtze delta of China; China-MAP project
Inventory of SO2, PM and toxic metals emissions from industrial sources in Greater Mumbai, India by A.D. Bhanarkar; P.S. Rao; D.G. Gajghate; P. Nema (pp. 3851-3864).
A comprehensive and spatial emission inventory was carried out for sulphur dioxide (SO2), particulate matter (PM) and toxic metals from industrial sources in Greater Mumbai, India. Fuel consumption database was developed for industrial sources. Emission factors for various pollutants were compiled from the literature, scrutinized and used appropriately as applicable under Indian conditions. Emissions of SO2, PM and toxic metals were estimated for 2001–02 and extrapolated to 2010. SO2 emissions from fossil fuel combustion covering 215 point sources for 2001–02 were computed as 55.591Ggy−1 whereas those for PM were calculated as 9.794Ggy−1. The total metal emissions from industrial sources were computed as 0.375Ggy−1. Total fossil fuel energy consumption in industrial sector during 2001–02 was 145PJ, which included fuel consumption (29%) in power plants. Amongst the industries, thermal power plants (TPP) were the major source of emissions in the region contributing 27% share towards SO2, 19% PM and 62% metals. Projected scenario for 2010 indicates that there would be substantial reduction in PM and metal emission load while the gaseous pollutant concentration would show a decreasing trend.
Keywords: Emission inventory; Industrial sources; PM, SO; 2; Toxic metals
Diurnal patterns in PM2.5 mass and composition at a background, complex terrain site by Roger L. Tanner; Solomon T. Bairai; Kenneth J. Olszyna; Myra L. Valente; Ralph J. Valente (pp. 3865-3875).
Current fine particle NAAQS specify 24h integrated mass measurements as the compliance metric. However, the value of continuous short-time resolution sampling (1h or less) is recognized and being included in US EPA-monitoring strategies. An extensive body of fine mass concentration data has been acquired using continuous PM2.5 monitoring by TEOM at Look Rock, TN, augmented by trace gas measurements and, during enhanced monitoring periods, continuous sampling for aerosol sulfate (2 methods) and elemental (black) carbon. Continuous data are compared to the 24h-averaged values of mass and composition from integrated samplers at the site, and for fine mass and sulfate are found to be excellent (r2=0.97–0.99). We then report the diurnal variations in concentrations from extensive continuous monitoring during 3 summer periods (2000–2002) and for more limited periods throughout calendar year 2001. Levels of the several continuously measured gases and particulate constituents vary in magnitude diurnally according to expected patterns based on their emissions, their formation and loss processes, their lifetimes toward wet and dry deposition, the dynamics of up-slope—down-slope circulation, and solar-driven boundary layer growth and decay. Thus, concentrations of short-lifetime gases have minima at sunrise and maxima in the evening. Long-lived primary species (CO and black carbon) increase in mid-morning as the boundary layer height reaches the site and decay thereafter. Long-lived secondary species (sulfate and fine mass) show little diurnal variability. Factors controlling the observed diurnal variability thus significantly influence the extent and timing of human exposure to pollutant species, as well as affecting visibility impairment in complex terrain environments near the Great Smoky Mountains National Park.
Keywords: Diurnal variability; Trace gases; Continuous aerosol sampling; Fine particle composition; Background complex-terrain sites
Measurement of aerosol particles, gases and flux radiation in the Pico de Orizaba National Park, and its relationship to air pollution transport by Marquez C. Márquez; T. Castro; A. Muhlia; M. Moya; Martinez-Arroyo A. Martínez-Arroyo; Baez A. Báez (pp. 3877-3890).
Continuous atmospheric measurements were carried out at the Pico de Orizaba National Park (PONP), Mexico, in order to evaluate the characteristics and sources of air quality. This action allowed one to identify specific threats for the effective protection of natural resources and biodiversity. Results show the presence of particles and polluted gases transported by winds from the urban zones nearby (cities of Mexico, Puebla and Tlaxcala), as well as their measurable influence on the optical properties of the park environment.Nitrogen dioxide, carbon monoxide and sulfur dioxide show a daily pattern suggesting an influence of pollution generated by anthropogenic processes. Average concentration of SO2 was higher than recorded at the southern part of Mexico City. Ozone concentrations ranging from 0.035 to 0.06ppm suggest residual or background ozone character.Back trajectory analysis of air parcels arriving at the site confirm pollution caused by biomass burning and mass transport from urban zones. The SO42−/TC ratio exhibited values (0.88±0.33) similar to urban areas. Ratios BC/TC and OC/BC for PONP are similar to those reported as influenced by burning emissions of fossil fuels. Typical rural aerosols were also found at the site, and sulfate and ammonium concentrations were correlated. The most predominating mode in surface particles size distribution was at 0.32μm with no significant presence of coarse particles. Total carbon (OC+BC) content of fine particle mass (PM less than 1μm) comprised, on average, 75%.Optical properties retrieved from photometric data show intermittent influence from urban pollution. Time periods with low absorbing particles, great visibility and abundance of small particles alternating with short times with bigger particles and high turbidity indicated by the optical depth.
Keywords: Air pollution transport; Natural protected areas; Pollution impact on natural resources
Particle deposition rates in residential houses by Congrong He; Lidia Morawska; Dale Gilbert (pp. 3891-3899).
As part of a large study investigating indoor air in residential houses in Brisbane, Australia, the purpose of this work was to quantify the particle deposition rate of size classified particles in the size range from 0.015 to 6μm. Particle size distribution resulting from cooking, repeated under two different ventilation conditions in 14 houses, as well as changes to particle size distribution and PM2.5 concentration as a function of time, were measured using a scanning mobility particle sizer (SMPS), an aerodynamic particle sizer (APS), and a DustTrak. Deposition rates were determined by regression fitting of the measured size-resolved particle number and PM2.5 concentration decay curves, and accounting for air exchange rate.The measured deposition rates were shown to be particle size dependent and they varied from house to house. The lowest deposition rates were found for particles in the size range from 0.2 to 0.3μm for both minimum (air exchange rate: 0.61±0.45h−1) and normal (air exchange rate: 3.00±1.23h−1) ventilation conditions. The results of statistical analysis indicated that ventilation condition (measured in terms of air exchange rate) was an important factor affecting deposition rates for particles in the size range from 0.08 to 1.0μm, but not for particles smaller than 0.08μm or larger than 1.0μm. Particle coagulation was assessed to be negligible compared to the two other processes of removal: ventilation and deposition. This study of particle deposition rates, the largest conducted so far in terms of the number of residential houses investigated, demonstrated trends in deposition rates comparable with studies previously reported, usually for significantly smaller samples of houses (often only one). However, the results compare better with studies which, similarly to this study, investigated cooking as a source of particles (particle sources investigated in other studies included general activity, cleaning, artificial particles, etc).
Keywords: Indoor air quality; Indoor particle deposition; Supermicrometer particles; Submicrometer particles; PM; 2.5
Possible transcontinental dust transport from North Africa and the Middle East to East Asia by Taichu Y. Tanaka; Yasunori Kurosaki; Masaru Chiba; Takatsugu Matsumura; Tomohiro Nagai; Akihiro Yamazaki; Akihiro Uchiyama; Nobumitsu Tsunematsu; Kenji Kai (pp. 3901-3909).
From the evening of 25 March 2003 until 27 March an Asian dust event (Kosa) was observed at a total of 37 routine meteorological observatories in Japan, although no significant dust storms had been observed in the arid regions of China and Mongolia. A numerical simulation with a three-dimensional global aerosol transport model and meteorological observations reveal that the observed mineral dust particles were generated through dust storms in North Africa and the Middle East on 19 March. The simulation predicted that dust particles generated in the Sahara Desert and Arabian Peninsula on 19 March would be transported north of the Tien Shan Mountains in China and arrive over Japan in about 6–7 days. It also indicated that over 50% of the dust particles in Japan on 26–27 March came from North Africa, about 30% from the Middle East, and only about 10% from China in the boundary layer. The simulated result is consistent with polarization lidar and sky radiometer observations, indicating that the simulation is realistic. The simulation indicates that the Kosa phenomenon was caused by a mixture of transported dust and anthropogenic pollutants. The simulation of this dust event suggests the possible importance of dust transport from the Sahara and the Arabian Peninsula to East Asia.
Keywords: Aerosols; Long-range transport; Numerical simulation; Lidar; Sun photometer
Diurnal, weekly, and seasonal variation of hygroscopic properties of submicrometer urban aerosol particles by A. Massling; M. Stock; A. Wiedensohler (pp. 3911-3922).
Hygroscopic properties of aerosol particles at an urban background site in Leipzig, Germany, were measured using a Hygroscopicity-Tandem Differential Mobility Analyzer (H-TDMA) system. Hygroscopic properties of particles with dry diameter of 50nm (Dd50) and 150nm (Dd150) were determined at 92% relative humidity (RH) for a period of eight months in the year 2000.For both Dd, the results revealed two major hygroscopic groups (nearly hydrophobic group with an average growth factor of 1.06±0.04 for Dd50 and 1.09±0.04 for Dd150, and more hygroscopic with 1.44±0.08 for Dd50 and 1.65±0.08 for Dd150) and a less-pronounced third group (less hygroscopic with 1.25±0.08 for Dd50 and 1.33±0.07 for Dd150).A simple solubility model was applied to these growth factors to estimate soluble volume fractions. Based on these estimated fractions, for all three hygroscopic groups the Dd50 particles were composed of smaller amounts of soluble material than were the Dd150 particles; the soluble volume fraction for more hygroscopic particles was only 50% for the Dd50 particles and 70% for the Dd150 particles.Calculated number fractions revealed that 31% of Dd50 particles and 22% of Dd150 particles showed nearly hydrophobic behavior. For both particle sizes, the number fraction of nearly hydrophobic particles showed a characteristic diurnal variation dependent on the two daily rush-hour periods. Also, the number fraction of nearly hydrophobic particles on weekdays was about 10% higher than that on weekends. In winter (November–February), the number fraction of nearly hydrophobic particles showed two maxima that coincided with the two daily rush-hour periods. In summer (May–August), only one distinct maximum during the morning rush-hour period was observed.
Keywords: Urban aerosol; H-TDMA; Hygroscopic properties; Growth factor; Soluble volume fraction
Two independent methods for mapping bulk deposition in France by Croise Luc Croisé; Erwin Ulrich; Pierre Duplat; Olivier Jaquet (pp. 3923-3941).
Two independent methods—deterministic and geostatistical—were used to estimate the spatial distribution of bulk deposition for nine ions and the associated uncertainties for France.The deterministic models enable bulk deposition estimates using only a few auxiliary variables (precipitation, altitude and the period in the year). The models explain between 38% and 64% of the depositional variability depending on the ions. The compounds can be grouped according to their primary origin: (1) (S-SO42−, N-NO3−, N-NH4+, Ntot, H+) with high deposition levels in the case of heavy precipitation, and (2) (Na+, Cl−, Mg2+) with high deposition levels near the sea. Model validation for 1999, 2000 and 2001 generally shows a close relationship between calculated and measured bulk deposition. However, the models overestimate the values for smaller deposition quantities (S-SO42−, H+). Comparison with EMEP (European Monitoring and Evaluation Program) maps shows good correlation for averages at the country level, despite spatial differences mainly due to very different spatial scales .The geostatistical approach gave good results in spite of the small number (27) of plots in the CATAENAT sub-network. Spatially structured behaviour was identified and quantified for all the compounds. The contribution of one auxiliary variable (precipitation at 2614 “Météo-France� weather stations) appears to be significant for most of the interpolations. The maps produced were globally similar to those obtained with the deterministic approach. However, the deterministic approach slightly overestimated (7–11%) or underestimated (17–24%) bulk deposition depending on the ion. Finally, spatial characterisation of errors and a greater potential make the geostatistical approach more attractive than the deterministic method for monitoring levels above critical loads for acidity.The maps produced were compared with EMEP maps for Europe and critical loads are discussed.
Keywords: Atmospheric bulk deposition; Critical loads; Monitoring; Geostatistics
Dust weather records in Beijing during 1860–1898 AD based on the Diary of Tonghe Weng by Jie Fei; Jie Zhou; Qingyao Zhang; Huizhong Chen (pp. 3943-3946).
The Diary of Tonghe Weng, a famous Chinese historical diary, is found to be an enormously valuable historical resource for historical dust weather researches. It contains a body of daily observations of the dust weather in Beijing during 1860–1898 AD. The continuity and reliability of the dust weather records are evaluated. The data absences amount to a little more than 9%. A total of 349 dusty days are identified, thus the annual average is about 9 days. However, the true average of the annual total of dusty days in Beijing during 1860–1898 AD would be a little more than 9 days due to data absences.
Keywords: Dust weather; Beijing; Historical times; Diary; China
A technical review of the feasibility of producing certified reference materials for the measurement of gaseous pollutants in ambient air by Theo Hafkenscheid; Annarita Baldan; Paul Quincey; Leopoldo Cortez; Florbela Dias; Richard H. Brown; Mike Wright; Michel Gerboles; Kevin Saunders (pp. 3947-3965).
Within the frame of the EU sponsored project ‘Cermatair’ (Certified Reference Materials for the Measurement of Gaseous Pollutants in Ambient Air, contract G6RD-CT-2001-00517) the feasibility of preparing and certifying reference materials for the measurement of sulphur dioxide, nitrogen dioxide, carbon monoxide and benzene in ambient air was studied. The project focused on measurements at concentration levels corresponding to limit values given in EU Ambient Air Quality Directives and covered reference materials for the reference methods specified in these Directives and for alternative methods based on diffusive sampling. State-of-the-art technologies for the production and certification of the reference materials were identified through literature surveys. Limited batches of reference materials were prepared and, wherever appropriate, tested for homogeneity. The reference materials were subsequently tested in small-scale external verifications, performed by 2–4 laboratories other than the preparation laboratory, aimed at identifying possible discrepancies between concentration values from the preparation processes and experimental values. The results of these verifications revealed possible mechanisms of certification (based on preparation or measurements). The remaining materials were subjected to a one-year stability study.
Keywords: Air quality; Quality assurance/quality control; Traceability; Certified reference materials