Diffusion-controlled toluene reference material for VOC emissions testing: International interlaboratory study

The measurement of volatile organic compound (VOC) emissions from building products and materials by manufacturers and testing laboratories, and the use of the test results for labeling programs, continue to expand. One issue that hinders wide acceptance for chamber product testing is the lack of a reference material to validate test chamber performance. To meet this need, the National Institute of Standards and Technology (NIST) and Virginia Tech (VT) have developed a prototype reference material that emits a single VOC similar to the emissions of a diffusion-controlled building product source with a dynamic emissions profile. The prototype material has undergone extensive testing at NIST and a pilot interlaboratory study (ILS) with four laboratories. The next development step is an evaluation of the prototype source in multiple-sized chambers of 14 laboratories in seven countries. Each laboratory was provided duplicate specimens and a test protocol. Study results identified significant issues related to the need to store the source at a subzero Celsius temperature until tested and possible inconsistencies in large chambers. For laboratories using a small chamber and meeting all the test method criteria, the results were very encouraging with relative standard deviations ranging from 5% to 10% across the laboratories.

Implications: Currently, the chamber performance of laboratories conducting product VOC emissions testing is assessed through interlaboratory studies (ILS) using a source with an unknown emission rate. As a result, laboratory proficiency can only be based on the mean and standard deviation of emission rates measured by the participating ILS laboratories. A reference material with a known emission rate has the potential to provide an independent assessment of laboratory performance as well as improve the quality of interlaboratory studies. Several international laboratories with different chamber testing systems demonstrated the ability to measure the emission rate of such a reference material within an acceptable measurement uncertainty.     

Consistency of ozone and nitrogen oxides standards at tropospherically relevant mixing ratios

The absolute accuracy and long-term precision of atmospheric measurements hinge on the quality of the instrumentation and calibration standards. To assess the consistency of the ozone (O3) and nitrogen oxides (NO(x)) standards maintained at the National Institute of Standards and Technology (NIST), these standards were compared through the gas-phase titration of O3 with nitric oxide (NO). NO and O3 were monitored using chemiluminescence and UV absorption, respectively. Nitrogen dioxide (NO2) was monitored directly by laser-induced fluorescence and indirectly by catalytic conversion to NO, followed by chemiluminescence. The observed equivalent loss of both NO and O3 and the formation of NO2 in these experiments was within 1% on average over the range of 40-200 nmol mol(-1) of NO in excess O3, indicating that these instruments, when calibrated with the NIST O3 and NO standards and the NO2 permeation calibration system, are consistent to within 1% at tropospherically relevant mixing ratios of O3. Experiments conducted at higher initial NO mixing ratios or in excess NO are not in as good agreement. The largest discrepancies are associated with the chemiluminescence measurements. These results indicate the presence of systematic biases under these specific conditions. Prospects for improving these experiments are discussed.     

Demonstration of a global modeling methodology to determine the relative importance of local and long-distance sources

A global three-dimensional (3D) transport–dispersion model was used to simulate Krypton-85 (⁸⁵Kr) background concentrations at five sampling locations along the US east coast during 1982–1983. The samplers were established to monitor the ⁸⁵Kr plume downwind of the Savannah river plant (SRP), a nuclear fuel reprocessing facility. The samplers were located 300–1000 km downwind of the SRP. In the original analyses of the measurements, a constant background concentration, representing an upper-limit and different for each sampling station, was subtracted from the measurements to obtain the part of the measurement representing the SRP plume. The use of a 3D global model, which includes all major ⁸⁵Kr sources worldwide, was able to reproduce the day-to-day concentration background variations at the sampling locations with correlation coefficients of 0.36–0.46. These 3D model background predictions, without including the nearby SRP source, were then subtracted from the measured concentrations at each sampler, the result representing the portion of the measurement that can be attributed to emissions from the SRP. The revised plume estimates were a factor of 1.3–2.4 times higher than from the old method using a constant background subtraction. The greatest differences in the SRP plume estimates occurred at the most distant sampling stations.     

Consistency of Ozone and Nitrogen Oxides Standards at Tropospherically Relevant Mixing Ratios

Abstract

The absolute accuracy and long‐term precision of atmospheric measurements hinge on the quality of the instrumentation and calibration standards. To assess the consistency of the ozone (O₃) and nitrogen oxides (NO_x) standards maintained at the National Institute of Standards and Technology (NIST), these standards were compared through the gas‐phase titration of O3 with nitric oxide (NO). NO and O₃ were monitored using chemiluminescence and UV absorption, respectively. Nitrogen dioxide (NO₂) was monitored directly by laser‐induced fluorescence and indirectly by catalytic conversion to NO, followed by chemiluminescence. The observed equivalent loss of both NO and O₃ and the formation of NO₂ in these experiments was within 1% on average over the range of 40–200 nmol mol^?1 of NO in excess O3, indicating that these instruments, when calibrated with the NIST O₃ and NO standards and the NO₂ permeation calibration system, are consistent to within 1% at tropospherically relevant mixing ratios of O₃. Experiments conducted at higher initial NO mixing ratios or in excess NO are not in as good agreement. The largest discrepancies are associated with the chemiluminescence measurements. These results indicate the presence of systematic biases under these specific conditions. Prospects for improving these experiments are discussed.     

Evaluation of a Test Method for Measuring Indoor Air Emissions from Dry-Process Photocopiers

ABSTRACT

A large chamber test method for measuring indoor air emissions from office equipment was developed, evaluated, and revised based on the initial testing of four dry-process photocopiers. Because all chambers may not necessarily produce similar results (e.g., due to differences in sink effects, temperature and humidity control, air exchange, pollutant monitoring, and measurement biases), a preliminary four-laboratory evaluation of the revised test method was conducted. To minimize variability, the evaluation used a single dry-process photocopier that was shipped to each of the four laboratories along with supplies (i.e., toner and paper).

The results of this preliminary four-laboratory evaluation demonstrate that the test method was used successfully in the different chambers to measure emissions from dry-process photocopiers. Differences in chamber design and construction appeared to have had minimal effect on the results for the volatile organic compounds (VOCs). Perhaps more important than the chamber itself is the sample analysis as identified by duplicate samples that were analyzed by a different laboratory. Percent relative standard deviation (%RSD) was used to provide a simplistic view of interlaboratory precision for this evaluation. Excluding problems with suspected analytical bias observed from one of the laboratories, the precision was excellent for the VOCs with RSDs of less than 10% in most cases. Less precision was observed among the laboratories for aldehydes/ketones (RSD of 23.2% for formaldehyde). The precision for ozone emission rates among three of the laboratories was excellent (RSD of 7.9%), but emission rates measured at the fourth laboratory were much higher.     

Aircraft measurement of ozone turbulent flux in the atmospheric boundary layer

In May 1995, the “Chimie-Creil 95” experiment was undertaken in the north of France. The field data are first used to validate the methodology for airborne measurement of ozone flux. A certain number of methodological problems due to the location of the fast ozone sensor inside the airplane are, furthermore discussed. The paper describes the instrumentation of the ARAT (Avion de Recherche Atmosphérique et de Télédétection), an atmospheric research and remote-sensing aircraft used to perform the airborne measurements, the area flown over, the meteorological conditions and boundary layer stability conditions. These aircraft measurements are then used to determine ozone deposition velocity and values are proposed for aerodynamic, bulk transfer coefficients (ozone and momentum). The paper also establishes the relationship between the normalised standard deviation and stability parameters (z/L) for ozone, temperature, humidity and vertical velocity. The laws obtained are then presented.     

Ground-based remote sensing measurements of aerosol and ozone in an urban area: A case study of mixing height evolution and its effect on ground-level ozone concentrations

We have estimated the mixing height (MH) and investigated the relationship between vertical mixing and ground-level ozone concentrations in Seoul, Korea, by using three ground-based active remote sensing instruments operating side by side: micro-pulse lidar (MPL), differential absorption lidar (DIAL), and differential optical absorption spectroscopy (DOAS). The MH is estimated from MPL measurements of aerosol extinction profiles by the gradient method under convective conditions. Comparisons of the MHs estimated from MPL and radiosonde measurements show a good agreement (r²=0.99). Continuous MPL measurements with high temporal and vertical resolution reveal the diurnal variations of the MH under convective conditions and the presence of a residual layer during the nighttime. Comprehensive measurements of ozone and aerosol by MPL, DIAL and DOAS during an high ozone episode (24–26 May 2000) in Seoul, Korea, reveal that (1) photochemical ozone production and advection from upwind regions (the western part of Seoul) contribute two peaks of ozone concentrations at the ground around 14:00 and 18:00 local time on 25 May 2000, respectively, and (2) the entrainment and the fumigation processes of ozone aloft in the nighttime residual layer into the ground is a major contributor of high concentrations of ground-level ozone observed on the following day (26 May 2000).     

Operational ozone forecasts for the region of Copenhagen by the Danish Meteorological Institute

The Danish Meteorological Institute (DMI) has developed an operational forecasting system for ozone concentrations in the Atmospheric Boundary Layer; this system is called the Danish Atmospheric Chemistry FOrecasting System (DACFOS). At specific sites where real-time ozone concentration measurements are available, a statistical after-treatment of DACFOS’ results adjusts the next 48 h ozone forecasts. This post-processing of DACFOS’ forecasts is based on an adaptive linear regression model using an optimal state estimator algorithm. The regression analysis uses different linear combinations of meteorological parameters (such as temperature, wind speed, surface heat flux and atmospheric boundary layer height) supplied by the Numerical Weather Prediction model DMI-HIRLAM. Several regressions have been tested for six monitoring stations in Denmark and in England, and four of the linear combinations have been selected to be employed in an automatic forecasting system. A statistical study comparing observations and forecasts shows that this system yields higher correlation coefficients as well as smaller biases and RMSE values than DACFOS; the present post-processing thus improves DACFOS’ forecasts. This system has been operational since June 1998 at the DMI's monitoring station in the north of Copenhagen, for which a new ozone forecast is presented every 6 h on the DMI's internet public homepage.     

Statistical estimation of ozone exposure metrics

Data from recent experiments at North Carolina State University and other locations provide a unique opportunity to study the effect of ambient ozone on the growth of clover. The data consist of hourly ozone measurements over a 140 day growing season at eight sites in the US, coupled with clover growth response data measured every 28 days. The objective is to model an indicator of clover growth as a function of ozone exposure. A common strategy for dealing with the numerous hourly ozone measurements is to reduce these to a single summary measurement, a so-called exposure metric, for the growth period of interest. However, the mean ozone value is not necessarily the best summarization, as it is widely believed that low levels of ozone have a negligible effect on growth, whereas peak ozone values are deleterious to plant growth. There are also suspected interactions with available sunlight, temperature and humidity. A number of exposure metrics have been proposed that reflect these beliefs by assigning different weights to ozone values according to magnitude, time of day, temperature and humidity. These weighting schemes generally depend on parameters that have, to date, been subjectively determined. We propose a statistical approach based on profile likelihoods to estimate the parameters in these exposure metrics.     

Climatology of diurnal trends and vertical distribution of ozone in the atmospheric boundary layer in urban North Carolina

Vertical measurements of ozone were made on a 610-m-tall tower located about 15 km southeast of Raleigh, NC, as part of an effort by the state of North Carolina to develop a state implementation plan (SIP) for ozone control in the Raleigh Metropolitan Statistical Area. During summer 1993, 1994, and 1995, ozone was monitored at ground level, 250 m, and 433 m. Boundary layer wind, temperature, and other meteorological variable profiles were determined from balloon soundings. During summer 1996 and 1997, ozone was monitored at ground level, 76 m, 128 m, and 433 m. This paper presents the analysis and discussion of the five-year data. The evolutions of the convective boundary layer during daytime and the stable nocturnal boundary layer (NBL) were found to have marked impacts on ozone concentrations. A strong diurnal pattern, with an afternoon maximum and an early morning minimum, was dominant at ground level, but it was much weaker at elevated levels and insignificant above the NBL at night. Ozone deposition velocities at night during the measurement periods were estimated to range from 0.09 to 0.64 cm/sec. We found evidence of regional transport of ozone and/or its precursors from northwest and north of the site, which may play a role in high ozone events in the Raleigh-Durham area. Ozone concentrations between the various elevated levels were well correlated, while correlations between the ground and upper levels were much weaker. However, a strong correlation was found between the nighttime and early morning ozone concentrations (CR) in the residual layer above the NBL and the maximum ground level concentration (Co max) the following afternoon. Based on this correlation, the latter may be predicted by an observational model Co max = 27.76e 0.016 CR.     

The use of ambient data to corroborate analyses of ozone control strategies

Data from environmental-chamber studies and photochemical box-model simulations were used to evaluate and revise a method for developing a qualitative understanding of the sensitivity of ozone formation at a particular time and place to changes in concentrations of volatile organic compounds (VOC) and oxides of nitrogen (NO_x). The revised method requires measurements of ozone, NO, and either NO_x or NO_y. The sensitivities of the method to biases in measurements were evaluated. The method potentially can be used for qualitative assessment of VOC versus NO_x limitation, comparison with the predictions of grid-based photochemical air-quality models, and evaluation of trends over time in the relative effectiveness of VOC versus NO_x controls.     

Inter-laboratory comparison study on measuring semi-volatile organic chemicals in standards and air samples

Measurements of semi-volatile organic chemicals (SVOCs) were compared among 21 laboratories from 7 countries through the analysis of standards, a blind sample, an air extract, and an atmospheric dust sample. Measurement accuracy strongly depended on analytes, laboratories, and types of standards and samples. Intra-laboratory precision was generally good with relative standard deviations (RSDs) of triplicate injections <10% and with median differences of duplicate samples between 2.1 and 22%. Inter-laboratory variability, measured by RSDs of all measurements, was in the range of 2.8-58% in analyzing standards, and 6.9-190% in analyzing blind sample and air extract. Inter-laboratory precision was poorer when samples were subject to cleanup processes, or when SVOCs were quantified at low concentrations. In general, inter-laboratory differences up to a factor of 2 can be expected to analyze atmospheric SVOCs. When comparing air measurements from different laboratories, caution should be exercised if the data variability is less than the inter-laboratory differences.     

Investigation of the effects of using single-wall carbon nanotubes (SWCNTs) in ozone measurement with passive samplers

Passive samplers are used in air quality monitoring for many years to compete in terms of being economical with continuous measurement systems. In this study, different amounts of single-wall carbon nanotubes (SWCNTs) were added in the impregnation solution of the filters of passive samplers and the effect on the absorption of ozone studied. The results of the measurement of ozone with varying amounts of SWCNTs added to the impregnation solution of the filters of the passive samplers were compared with the results of the continuous ozone measurement system (CS). Measurements were performed for 7 days and 14 days at two different exposure times. The increase of the amount of SWCNTs on the filters of the passive samplers, however, did not have an effect on the measurement of ozone. The measurement results of the passive samplers of the 14-day exposure periods, alternating with the 7-day exposure periods, were lower considerably than the results of the 7-day exposure.

Implications: The accuracy and the use of passive samplers in SWCNTs are expected to provide high measurement results. Observing the effect of the change in the amount of diffusion of pollutants held in the SWCNT is also one of the expected implications.     

Interaction between nitrogen and phosphorus cycles in mining-affected aquatic systems—experiences from field and laboratory measurements

The main objectives of this study were to (a) study the interaction between N and P cycles in mining-affected aquatic systems and (b) to quantify release rates of sedimentary soluble reactive phosphorus (SRP) that may be related to this interaction. Sediment cores and water from Lake Bruträsket (Boliden, northern Sweden) were collected and a time series of water sampling and flow measurements was conducted in the Brubäcken stream connected to the lake. Factors affecting SRP release were studied in a sediment incubation experiment and water column experiments. Field and laboratory measurements indicated that pH and dissolved oxygen are two important factors for SRP release. At the end of the low-oxygen incubation, an SRP concentration of 56 μg?L^?1 resulted in a sedimentary flux of 1.1 mg SRP?m^?2?day^?1. This is ～10 times higher than the flux of 0.12 mg SRP?m^?2?day^?1 obtained from depth integration of vertical SRP profiles measured in the lake, and ～100 times higher than the external flux of 0.014 mg SRP?m^?2?d^?1 into the lake (based on catchment area). Field measurements indicated that oxidation of organic matter and mining-related chemicals (ammonium and thiosulphates) may result in increased internal SRP flux from the sediment. Increased P loading in the lake as a result of low-oxygen conditions could change water column total nitrogen/total phosphorus ratios from 27 to 17, consequently changing the lake from being P-limited to be co-limited by N and P. The obtained findings point to possible interaction between the cycles of nitrogen (oxygen consumption) and P (flux from sediment) that may be important for nutrient regulation in mine water recipients.     

Comparison of modelled and measured ozone concentrations and meteorology for a site in south-west Sweden: implications for ozone uptake calculations

Measurements of ground-level ozone concentrations and meteorology (temperature, vapour pressure deficit (VPD), solar radiation) at the monitoring site Ostad (south-west Sweden) were compared to data from the corresponding grid in the EMEP photo-oxidant model for 1997, 1999 and 2000. The influence of synoptic weather on the agreement between model and measurements was studied. Implications of differences between modelled and observed inputs for ozone flux calculations for wheat and potato were investigated. The EMEP model output of ozone, temperature and VPD correlated well with measurements during daytime. Deviations were larger during the night, especially in calm conditions, attributed to local climatological conditions at the monitoring site deviating from average conditions of the grid. These differences did not lead to significant differences in calculated ozone uptake, which was reproduced remarkably well. The uptake calculations were sensitive to errors in the ozone and temperature input data, especially when including a flux threshold.     

Current status and future needs for biological and environmental reference materials certified for methylmercury compounds.

A good quality control/quality assurance programme should be implemented in all environmental or health related studies on mercury and its organic compounds, particularly, for monomethylmercury (MeHg) which is the most toxic mercury compound. This can be achieved initially by analysing suitable certified reference materials (CRMs), which are available from various producers such as the National Institute of Standards and Technology (NIST) from USA, National Institute of Environmental Studies (NIES), National Research Council of Canada (NRCC), Standards, Measurements and Testing programme (SM&T) of the European Commission, and the International Atomic Energy Agency (IAEA). It is well understood that these materials are not covering present needs, as most of them are of the marine origin, while many laboratories are conducting research and monitoring in terrestrial ecosystems and fresh water environment. In addition, CRMs for human exposure assessment, such as blood, urine, and hair at several levels of concentrations are still lacking. Therefore, many other actions should be undertaken to achieve, improve and/or maintain quality of data, including participation in interlaboratory studies, proficiency testing and production of laboratory reference materials. A review of these actions has shown that MeHg compounds determination in samples such as soil, sediment and water is rather difficult and the results are also method dependent. In addition, it has been shown that some of the most frequently employed analytical methods may be a subject to spurious MeHg formation in the presence of high concentrations of inorganic mercury and organic matter. These findings have put a number of previous data on MeHg in question and consequently prompt actions were undertaken by a number of well experienced laboratories and producers of CRMs. So far, it is shown that the results obtained by various laboratories using different analytical techniques agree well with certified values in all RMs certified for MeHg. This suggests that comparability of data can be achieved, which however is not a guarantee of the true values.     

Evaluation of the reaction artifacts in an annular denuder-based sampler resulting from the heterogeneous ozonolysis of naphthalene

The heterogeneous ozonolysis of naphthalene adsorbed on XAD-4 resin was studied using an annular denuder technique. The experiments involved depositing a known quantity of naphthalene on the XAD-4 resin and then measuring the quantity of the solid naphthalene that reacted away under a constant flow of gaseous ozone (0.064 to 4.9 ppm) for a defined amount of time. All experiments were performed at room temperature (26 to 30 °C) and atmospheric pressure. The kinetic rate coefficient for the ozonolysis reaction of naphthalene adsorbed on XAD-4 resin is reported to be (10.1?±?0.4)?×?10^?19 cm³ molecule^?1 s^?1 (error is 2σ, precision only). This value is five times greater than the currently recommended literature value for the homogeneous gas phase reaction of naphthalene with ozone. The obtained rate coefficient is used to evaluate reaction artifacts from field concentration measurements of naphthalene, acenaphthene, and phenanthrene. The observed uncertainties associated with field concentration measurements of naphthalene, acenaphthene, and phenanthrene are reported to be much higher than the uncertainties associated with the artifact reactions. Consequently, ozone reaction artifact appears to be negligible compared to the observed field measurement uncertainty results.     

Accuracy and traceability concepts applied to environmental speciation analysis

Traceability issues are of increasing concern in all fields where chemical measurements form the basis for decisions. The concepts of accuracy and traceability as applied to environmental analysis are, however, still prone to misunderstandings, which has been recently illustrated by controversial discussions among the analytical community with respect to accuracy and traceability issues in the area of speciation analysis. Analytical techniques used for the determination of chemical species (speciation analysis) are generally based on a succession of steps (e.g. extraction derivatisation, separation, detection) which are all prone to various sources of systematic errors. Many speciation measurement techniques have been studied within the last decade through interlaboratory studies and certification of reference materials. These collaborative efforts have been understood as being directed towards strive for accuracy (trueness and precision). It has been recognised recently that the achievements actually enabled mostly to establish reference points (e.g. certified values in reference materials) which does not necessarily correspond to 'true values' but rather offer a mean for laboratories to compare their data internationally and, hence, achieve traceability. This ambiguity still generates confusion and misunderstandings among the scientific community. This communication discusses this issue, focusing on analytical measurements only. Extending discussions on general traceability issues would imply an examination of steps prior to laboratory work (sampling, storage, etc.) which is beyond the scope of this contribution.     

Hydrocarbon Measurements During the 1992 Southern Oxidants Study Atlanta Intensive: Protocol and Quality Assurance

Abstract

A major component of the Southern Oxidants Study (SOS) 1992 Atlanta Intensive was the measurement of atmospheric nonmethane hydrocarbons. Ambient air samples were collected and analyzed by a network of strategically located automated gas chromatography (GC) systems (field systems). In addition, an extensive canister sampling network was deployed. Combined, more than 3000 chromatograms were recorded. The SOS science team targeted for quantitative analysis 56 compounds which may be substantial contributors to ozone formation or used as air mass tracers. A quality assurance program was instituted to ensure that good measurements were being made throughout the network for each target compound. Common, high-quality standards were used throughout the network. The performance of individual field systems was evaluated during the intensive through the analysis of challenge mixtures. This methodology helped to identify and correct analytical problems as they arose.     

Suitability of Nicotiana tabacum 'Bel W3' for biomonitoring ozone in São Paulo, Brazil

Nicotiana tabacum 'Bel W3' is a widely used sensitive bioindicator for ambient ozone, but it is rarely used in tropical countries. Our goal was to determine the suitability of this plant for biomonitoring ozone in the city of S?o Paulo by evaluating the relationships between leaf necroses and ozone under field conditions and measurements of chlorophyll a fluorescence and antioxidants in plants exposed to different concentrations of ozone in closed chambers. While a weak linear relationship between leaf injury and ozone concentrations (R(2)=0.10) was determined in the field, a strong linear relationship was observed in the chamber experiments. Maximum leaf injury was observed in plants submitted to 40 ppb, which coincided with a significant decrease in fluorescence and total ascorbic acid. The relationship between leaf damage observed in the field and ozone was improved when the concentrations were limited to 40 ppb (R(2)=0.28).