Aerosol light scattering measurements as a function of relative humidity: a comparison between measurements made at three different sites

The water uptake by fine aerosol particles in the atmosphere has been investigated at three rural National Parks in the United States (Great Smoky Mountains, Grand Canyon and Big Bend National Parks). The relative humidity (RH) of sample aerosols was varied from less than 20% to greater than 90% using Perma Pure drying tubes as the scattering coefficient of the aerosol was measured with a Radiance Research M903 nephelometer. Data from these studies show that growth curves at all the three sites are similar in shape but the magnitude of growth can vary considerably from day to day. The growth curves from Great Smoky Mountains show smooth continuous growth over the entire range of RH, while the growth curves from the Grand Canyon and Big Bend show smooth and continuous growth on some days and deliquescence on other days. Comparing 12-h filter samples of chemical composition data with the aerosol growth curves, we find that higher fractions of soluble inorganic compounds (sulfate and nitrate) produce growth curves of greater magnitude than do higher concentrations of either organic carbon or soil material.     

Semivolatile behavior of dicarboxylic acids and other polar organic species at a rural background site (Nylsvley,RSA)

In this study aerosol samples from the South African savanna were analyzed for their polar organic constituents. Samples were collected with a front/back-up filter tandem system of quartz fiber filters (dual filter strategy). In all samples (n=15) dicarboxylic acids and a variety of phthalates, aldehydes and monocarboxylic acids were observed. Oxalic acid was the dominating compound with an average amount of 79.2 ng m⁻³ on the front filter and 11.3 ng m⁻³ on the back-up filter. The presence of significant concentrations of dicarboxylic acids on the back-up filter was rather unexpected. There are two possible sources to explain the presence of individual compounds on the back-up filter – particle penetration through the front filter or adsorption of compound parts from the gas phase. Interpretation of the data indicates that the dicarboxylic acid concentrations on the back-up filters appear to be caused by the adsorption of gaseous organic species. Dicarboxylic acids semivolatilic behavior is evident with this results. This conclusion refutes the commonly held view that dicarboxylic acids in the atmosphere were associated with the aerosol phase only. Additionally, it was found that the distribution of dicarboxylic acids between the gas and particle phase in the atmosphere is not only dependent on their vapor pressures. The actual gas phase concentration appears to be more determined by the chemical properties of the particles than by pure physical influences. Surprisingly, malonic acid exhibits an anomaly, as it does not show a semivolatile tendency.     

Phase transition behaviour of sodium oleate aerosol particles

Field measurements have shown that organic surfactants are significant components of atmospheric aerosols. While fatty acids, among other surfactants, are prevalent in the atmosphere, the influence of these species on the chemical and physical properties of atmospheric aerosols remains not fully characterized. In order to assess the phase in which particles may exist, a detailed study of the deliquescence of a model surfactant aerosol has been carried out. Sodium oleate was chosen as a surfactant proxy relevant in atmospheric aerosol. Sodium oleate micelle aerosol particles were generated nebulizing a sodium oleate aqueous solution. In this study, the water uptake and phase transition of sodium oleate aerosol particles have been studied in a room temperature aerosol flow tube system (AFT) using Fourier transform infrared (FTIR) spectroscopy. Aerosol morphology and elemental composition were also analysed using scanning electron microscopy/energy dispersive X-ray analysis (SEM/EDX) techniques. The particles are homogeneously distributed as ellipsoidal-shape aggregates of micelles particles with an average size of ∼1.1 μm. The deliquescence by the sodium oleate aerosol particles was monitored by infrared extinction spectroscopy, where the dried aerosol particles were exposed to increasing relative humidity as they passed through the AFT. Observations of the infrared absorption features of condensed phase liquid water enable to determine the sodium oleate deliquescence phase transition at 88±2%.     

Evaluation of alternative filter media for particulate matter emission testing of residential wood heating devices

The performance of Teflon-coated glass fiber filter media (Pallflex Emfab TX40) is evaluated for particulate matter (PM) sampling of residential wood heating devices in a dilution tunnel. Thirty samples of varying duration and PM loading and concentration were collected from an U.S. Environmental Protection Agency (EPA) Method 28 dilution tunnel using dual Method 5G sample trains with untreated glass fiber and Emfab filters. Filters were weighed soon after the end of sampling and again the next day after equilibration at 35% relative humidity (RH). PM concentrations from both types of filters agreed very well with 1-day equilibration, demonstrating that Emfab filters are appropriate for use in measuring PM from residential wood burning appliances in a dilution tunnel and have performance equal to or better than the glass fiber filter media. Agreement between filter media without equilibration was erratic, with PM from glass fiber filter samples varying from slightly less than the Emfab samples to as much as 2.8 times higher. Some of the glass fiber filters lost substantial mass with equilibration, with the highest percent loss at lower filter mass loadings. Mass loss for Emfab samples was a small percentage of the mass and very consistent across the range of mass loadings. Taken together, these results may indicate water uptake on the glass fiber media that is readily removed with 1-day equilibration at moderate RH conditions.

Implications: EPA regulations now allow the use of either glass fiber or Teflon filter media for wood appliance PM emission testing. Teflon filter media minimizes the potential for acid-gas PM artifacts on glass fiber filters; this is important as EPA moves toward the use of locally sourced cordwood for testing that may have higher sulfur content. This work demonstrates that the use of Teflon-coated glass fiber filters can give similar PM measurement results to glass fiber filters after 1 day of equilibration. With no equilibration, measured PM from glass fiber filters was usually higher than from Teflon-coated glass fiber filters.     

Experimental Verification of Diffusion Battery Theory

The diffusion battery, an assembly of circular tubes or rectangular channels, is one of the best devices available for measuring the size and size distribution of submicron aerosols in the diameter range 0.002 to 0.2 µ m. The performance of these batteries is known from molecular diffusion theory, but until now has not been checked experimentally in this size range because of the lack of the necessary monodisperse aerosols. Experimental measurements on singly charged monodisperse aerosols from 0.01 µm to 0.1 µ m are described using a General Electric and a Pollak condensation nucleus counter to measure the aerosol penetration through the stages of a set of portable diffusion batteries in series. Particle sizes in the range tested could be selected at will by adjusting the voltage of an electric mobility classifier. The fraction of aerosol of a given size passing through each battery stage was found to agree closely with the penetration calculated from molecular diffusion theory for that size. This shows that the theory is correct and confirms that the aerosol produced by the electric mobility classifier was monodisperse. In addition, it was found that the difference in penetration between a charged versus a neutralized aerosol was insignificant except for the smallest aerosols used.     

Characterization of activated carbon fiber filters for pressure drop, submicrometer particulate collection, and mercury capture

The use of activated carbon fiber (ACF) filters for the capture of particulate matter and elemental Hg is demonstrated. The pressure drop and particle collection efficiency characteristics of the ACF filters were established at two different face velocities and for two different aerosols: spherical NaCl and combustion-generated silica particles. The clean ACF filter specific resistance was 153 kg m-2 sec-1. The experimental specific resistance for cake filtration was 1.6 x 10(6) sec-1 and 2.4 x 10(5) sec-1 for 0.5- and 1.5-micron mass median diameter particles, respectively. The resistance factor R was approximately 2, similar to that for the high-efficiency particulate air filters. There was a discrepancy in the measured particle collection efficiencies and those predicted by theory. The use of the ACF filter for elemental Hg capture was illustrated, and the breakthrough characteristic was established. The capacity of the ACF filter for Hg capture was similar to other powdered activated carbons.     

Organic and inorganic sampling artefacts assessment

During collection on filter-based aerosol samplers, organic and inorganic aerosol compounds both contribute to positive and negative artefacts, significantly affecting chemical analyses results for single species and PM mass concentrations. Up to now, studies on organic or inorganic artefacts have been conducted in Europe but very scarce data are available for both in a single study.The field study was carried out in Milan, which is located in the Po valley (Northern Italy) one of the major pollution hot spots in Europe. As sampling artefacts depend on many factors, such as filter type, face velocity, sampling duration, and ambient conditions, in this field study two different filter types have been considered (i.e. quartz fibre filters and Teflon filters) for the assessment of both inorganic and organic artefacts during two different seasons (performing also some samplings at different flow-rates).Results showed that positive artefacts due to OC adsorption on quartz filters accounted for 39% of the OC measured concentration in summer, and 23% in winter. Negative artefact due to nitrate volatilisation by the filters was 51% on Teflon and 22% on the quartz filters in summer, and no or negligible losses were observed in winter. A significant improvement in the PM mass comparability obtained in parallel samplings on different filters was obtained taking into account the artefact estimates performed in this study.     

Use of two different acidic aerosol samplers to measure acidic aerosols in Hsinchu, Taiwan

Acidic aerosol concentrations measured by an annular denuder system (ADS) and a honeycomb denuder system (HDS) in Hsinchu, Taiwan, were compared. Aerosols were also sampled by a MOUDI (micro-orifice uniform deposit impactor) and analyzed by an ion chromatograph to determine the size distributions of different species. Using the measured aerosol size distribution, theoretical analysis showed that positive HNO3 artifact due to volatilization of NH4NO3 is generally negligible for both samplers. Comparing two different denuder samplers, the average concentration of HNO3 measured by the ADS was found to be lower than that measured by the HDS, while the difference between the two samplers for the average concentration of other species was found to be within +/- 15%. A possible cause of the difference in HNO3 concentrations is due to a greater loss of HNO3 in the cyclone used by the ADS than in the impactor used by the HDS. The study also showed incomplete absorption of the evaporated HCl and HNO3 from the particles on the Teflon filter by the first nylon filter in the filter pack of the ADS. Collection efficiency and capacity of HCl and HNO3 by the nylon filters need further investigation.     

基本单分散气溶胶透过率测试中真实值的计算

通过对高效、超高效滤料透过率测试台工作原理的分析得到:利用基本单分散粒子作为气溶胶测得的透过率关系与滤料真正的透过率-粒径曲线有区别,实际上是所使用实验气溶胶的综合透过率。详细研究了真正透过率与实验透过率的关系、误差和偏离产生原因,最终提出了一种独特的计算方法,实现了根据实验得到的表观透过率-粒径关系,就可以计算出真实透过率-粒径曲线,并得到了采用较为理想的实验气溶胶测试出的真实透过率的实际验证,这将大大提高高效、超高效滤料性能测试台的精度,拓展了其应用范围,为高效滤料的过滤理论和应用研究提供了比较理想的测试手段。     

Source apportionment of surfactants in marine aerosols at different locations along the Malacca Straits

This study aims to determine the source apportionment of surfactants in marine aerosols at two selected stations along the Malacca Straits. The aerosol samples were collected using a high volume sampler equipped with an impactor to separate coarse- and fine-mode aerosols. The concentrations of surfactants, as methylene blue active substance and disulphine blue active substance, were analysed using colorimetric method. Ion chromatography was employed to determine the ionic compositions. Principal component analysis combined with multiple linear regression was used to identify and quantify the sources of atmospheric surfactants. The results showed that the surfactants in tropical coastal environments are actively generated from natural and anthropogenic origins. Sea spray (generated from sea-surface microlayers) was found to be a major contributor to surfactants in both aerosol sizes. Meanwhile, the anthropogenic sources (motor vehicles/biomass burning) were predominant contributors to atmospheric surfactants in fine-mode aerosols.     

Influence of the presence of heavy metals and surface-active compounds on the sorption of bisphenol A to sediment

The effects of different heavy metals (Cd, Pb), cationic surfactants cetyltrimethylammonium bromide (CTAB), anionic surfactant sodium dodecylbenzenesulfonate (SDBS) and the chemistry of the solution (pH and ionic strength) on the sorption of bisphenol A (BPA) to sediment were studied. Results showed that the presence of Cd and Pb caused a significant increase on the sorption of BPA to sediment and the sorption isotherms were in good agreement with Freundlich equation. The effect of surfactants on the adsorption of BPA onto sediment was found to strongly depend on the type of the surfactants. The presence of CTAB promoted BPA sorption and the amount of BPA adsorbed onto sediment increased linearly with concentration of CTAB. In contrast, the presence of anionic surfactant (SDBS) caused a slight reduction on the sorption of BPA. It was also found that the sorption behavior of BPA was affected by solution pH and ionic strength. The larger amount of BPA was absorbed with higher ionic strength and lower pH. This study may provide important insights into the understanding of the transport and fate of BPA in the environment.     

Effects of acid-washing filter treatment on quantification of aerosol organic compounds

The tests of standard mixtures and four sets of atmospheric particulate samples showed that an acid-wash (AW) pretreatment of fluorocarbon-coated glass fiber filters prior to aerosol sampling enhanced the quantifiable organic compounds for more than 29% (or 66 ng m⁻³); in particular, 47–273 ng m⁻³ (21–366%) more water-soluble organic compounds (WSOCs) were measured. When the acid-pretreated filters were employed, up to nine more organic species were measured in the individual daily samples. Because the acid pretreatment reduced the metal contaminants in the glass fiber filters, using the AW filters for aerosol sampling allows higher extraction recoveries of organic compounds. Since the fingerprinting compounds were more accurately determined when the aerosol samples were collected on the AW filters, better assessment of emission sources and toxicity of air pollutants can be obtained.     

Design of a cost-effective weighing facility for PM2.5 quality assurance

The U.S. Environmental Protection Agency (EPA) Quality Assurance (QA) Guidance Document 2.12: Monitoring PM2.5 in Ambient Air Using Designated Reference or Class I Equivalent Methods (Document 2.12) requires conditioning of PM2.5 filters at 20-23 degrees C and 30-40% relative humidity (RH) for 24 hr prior to gravimetric analysis. Variability of temperature and humidity may not exceed +/-2 degrees C and +/-5% RH during the conditioning period. The quality assurance team at EPA Region 2's regional laboratory designed a PM2.5 weighing facility that operates well within these strict performance requirements. The traditional approach to meeting the performance requirements of Document 2.12 for PM2.5 filter analysis is to build a walk-in room, with costs typically exceeding $100,000. The initial one-time capital cost for the laboratory at EPA's Edison, NJ, facility was approximately $24,000. Annual costs [e.g., National Institute of Standards and Technology (NIST) recertifications and nitrogen replacement cylinders used for humidity control] are approximately $500. The average 24-hr variabilities in temperature and RH in the Region 2 weighing chamber are small, +/-0.2 degrees C and +/-0.8% RH, respectively. The mass detection limit for the PM2.5 weighing system of 47-mm stretched Teflon (lab blank) filters is 6.3 microg. This facility demonstrates an effective and economical example for states and other organizations planning PM2.5 weighing facilities.     

Optical properties of the San Joaquin Valley aerosol collected during the 1995 integrated monitoring study

Optical, filter chemistry, and cascade impactor data collected during the winter intensive of the IMS95 Study in the San Joaquin Valley (SJV) of California were analyzed to determine the light-extinction efficiency of aerosol species. Regression of light scattering by particles (b_sp) measured by a heated nephelometer without a size selective inlet against PM_2.5 front filter mass gave a scattering efficiency of 3.67±0.05 m²/g with an R² (fraction of variance explained) of 0.94. Division of the aerosol into two components and applying two different corrections to the filter data for nitrate and organic carbon on the backup filter gave scattering efficiencies of 3.7±0.3 or 4.1±0.2 m²/g for the salts composed of sulfate, nitrate, and ammonium and 2.9±0.2 or 3.1±0.2 m²/g for all other species with R² of 0.985 and 0.986. The ambient b_sp measured by an open nephelometer was a simple function of PM_2.5 mass and relative humidity (RH), giving R² of 0.90 and 0.88 for two different RH sensors. Variations in PM_2.5 size distribution and composition did not have an important effect on ambient b_sp. The RH data from each sensor were repeatable enough to show the existence of a simple dependence of aerosol water uptake on RH, but RH sensor calibration uncertainties prevented determining this dependence. Inversion of MOUDI cascade impactor data gave sulfate and nitrate mass median diameters (MMD) between 0.4 and 0.8 μm. Mie scattering calculations based on MOUDI data provided humidity-dependent extinction efficiencies for the principal aerosol chemical species. These efficiencies combined with particle filter data showed that ammonium nitrate was the dominant contributor to wintertime light extinction. Source apportionment showed that light extinction was dominated by emissions sources contributing to the formation of secondary species, especially nitrate. These wintertime data are not expected to apply to summertime in the SJV.     

Aerosol light scattering measurements as a function of relative humidity

The hygroscopic nature of atmospheric fine aerosol was investigated at a rural site in the Great Smoky Mountains National Park during July and August 1995. Passing the sample aerosol through an inlet, which housed an array of Perma Pure diffusion dryers, controlled the sample aerosol's relative humidity (RH). After conditioning the aerosol sample in the inlet, the light scattering coefficient and the aerosol size distribution were simultaneously measured. During this study, the conditioned aerosol's humidity ranged between 5% < RH < 95%. Aerosol response curves were produced using the ratio bspw/bspd; where bspw is the scattering coefficient measured at some RH greater than 20% and bspd is the scattering coefficient of the "dry" aerosol. For this work, any sample RH values below 15% were considered dry. Results of this investigation showed that the light scattering ratio increased continuously and smoothly over the entire range of relative humidity. The magnitude of the ratio at a particular RH value, however, varied considerably in time, particularly for RH values greater than approximately 60%. Curves of the scattering coefficient ratios as a function of RH were generated for each day and compared to the average 12-hour chemical composition of the aerosol. This comparison showed that for any particular RH value the ratio was highest during time periods of high sulfate concentrations and lowest during time periods of high soil or high organic carbon concentrations.     

Effectiveness of In-Room Air Filtration and Dilution Ventilation for Tuberculosis Infection Control

Abstract

Tuberculosis (TB) is a public health problem that may pose substantial risks to health care workers and others. TB infection occurs by inhalation of airborne bacteria emitted by persons with active disease. We experimentally evaluated the effectiveness of in-room air filtration systems, specifically portable air filters (PAFs) and ceiling-mounted air filters (CMAFs), in conjunction with dilution ventilation, for controlling TB exposure in high-risk settings. For each experiment, a test aerosol was continuously generated and released into a full-sized room. With the in-room air filter and room ventilation system operating, time-averaged airborne particle concentrations were measured at several points. The effectiveness of in-room air filtration plus ventilation was determined by comparing particle concentrations with and without device operation. The four PAFs and three CMAFs we evaluated reduced room-average particle concentrations, typically by 30% to 90%, relative to a baseline scenario with two air-changes per hour of ventilation (outside air) only. Increasing the rate of air flow recirculating through the filter and/or air flow from the ventilation did not always increase effectiveness. Concentrations were generally higher near the emission source than elsewhere in the room. Both the air flow configuration of the filter and its placement within the room were important, influencing room air flow patterns and the spatial distribution of concentrations. Air filters containing efficient, but non-high efficiency particulate air (HEPA) filter media were as effective as air filters containing HEPA filter media.     

Loss of PM2.5 nitrate from filter samples in central California

Evaporative loss of particulate matter (with aerodynamic diameter < 2.5 microm, [PM2.5]) ammonium nitrate from quartz-fiber filters during aerosol sampling was evaluated from December 3, 1999, through February 3, 2001, at two urban (Fresno and Bakersfield) and three nonurban (Bethel Island, Sierra Nevada Foothills, and Angiola) sites in central California. Compared with total particulate nitrate, evaporative nitrate losses ranged from < 10% during cold months to > 80% during warm months. In agreement with theory, evaporative loss from quartz-fiber filters in nitric acid denuded samplers is controlled by the ambient nitric acid-to-particulate nitrate ratio, which is determined mainly by ambient temperature. Accurate estimation of nitrate volatilization requires a detailed thermodynamic model and comprehensive chemical measurements. For the 14-month average of PM2.5 acquired on Teflon-membrane filters, measured PM2.5 mass was 8-16% lower than actual PM2.5 mass owing to nitrate volatilization. For 24-hr samples, measured PM2.5 was as much as 32-44% lower than actual PM2.5 at three California Central Valley locations.     

Particulate nitrate measurement using nylon filters

Nylon filters are a popular medium to collect atmospheric fine particles in different aerosol monitoring networks, including those operated by the U.S. Environmental Protection Agency and the Interagency Monitoring of Protected Visual Environments (IMPROVE) program. Extraction of the filters by deionized water or by a basic aqueous solution (typically a mixture of sodium carbonate and sodium bicarbonate) is often performed to permit measurement of the inorganic ion content of the collected particles. Whereas previous studies have demonstrated the importance of using a basic solution to efficiently extract gaseous nitric acid collected using nylon filters, there has been a recent movement to the use of deionized water for extraction of particles collected on nylon filters to eliminate interference from sodium ion (Na+) during ion chromatographic analysis of inorganic aerosol cations. Results are reported here from a study designed to investigate the efficiency of deionized water extraction of aerosol nitrate (NO3-) and sulfate from nylon filters. Data were obtained through the conduct of five field experiments at selected IMPROVE sites. Results indicate that the nylon filters provide superior retention of collected fine particle NO3-, relative to Teflon filters, and that deionized water extraction (with ultrasonication) of collected NO3- and sulfate is as efficient, for the situations studied, as extraction using a basic solution of 1.7 mM sodium bicarbonate and 1.8 mM sodium carbonate.     

Surfactant-enhanced release of carbaryl and ethion from two long-term contaminated soils

The potential of five nonionic surfactants, Triton X-100, Brij35, Ethylan GE08, Ethylan CD127, and Ethylan CPG660 for enhancing release of carbaryl and ethion from two long-term contaminated soils was evaluated using the batch method. Incorporation of the surfactants into soils enhanced the release of both pesticides to various extents, which could be related to the type of pesticides and type and the amount of surfactants added. Release of ethion was dramatically enhanced by aqueous concentrations of surfactants above their critical micelle concentration values. This was attributed to solubility enhancement through incorporation of the highly hydrophobic compound within surfactant micelles. A concentration of 10 g L(-1) of various surfactants released >70% of the total ethion from the soil irrespective of the surfactant. For carbaryl, the surfactants were effective at low concentrations and dependence on concentration was lower than in the case of ethion. The ethylan surfactants (GE08, CD127, and CPG660) had a higher potential than Triton X-100 and Brij35 for releasing the pesticides. However, there was still a significant portion of carbaryl (11% of the total) and ethion (17% of the total) left in the soil. Our study also showed that there must be an optimal concentration of each surfactant to maximize the mass transfer of pesticides. At some threshold concentration level, additional surfactant started to inhibit the mass transfer of solute from the soil into the water. The results suggested that surfactants could help remediation of soils polluted by pesticides. The choice of surfactant should be made based on the properties of pesticides.     

Use of Two Different Acidic Aerosol Samplers To Measure Acidic Aerosols in Hsinchu,Taiwan

ABSTRACT

Acidic aerosol concentrations measured by an annular denuder system (ADS) and a honeycomb denuder system (HDS) in Hsinchu, Taiwan, were compared. Aerosols were also sampled by a MOUDI (micro-orifice uniform deposit impactor) and analyzed by an ion chromatograph to determine the size distributions of different species. Using the measured aerosol size distribution, theoretical analysis showed that positive HNO₃ artifact due to volatiliza-tion of NH₄NO₃ is generally negligible for both samplers.4 3Comparing two different denuder samplers, the average concentration of HNO₃ measured by the ADS was found3to be lower than that measured by the HDS, while the difference between the two samplers for the average concentration of other species was found to be within ±15%. A possible cause of the difference in HNO₃ con-3centrations is due to a greater loss of HNO₃ in the cyclone3 used by the ADS than in the impactor used by the HDS. The study also showed incomplete absorption of the evaporated HCl and HNO₃ from the particles on the Teflon3filter by the first nylon filter in the filter pack of the ADS. Collection efficiency and capacity of HCl and HNO₃ by3the nylon filters need further investigation.