Final Report: Distribution of H+ and Trace Metals in Ultrafine Ambient AerosolEPA Grant Number: R824791
Title: Distribution of H+ and Trace Metals in Ultrafine Ambient Aerosol
Institution: New York University Medical Center
EPA Project Officer: Chung, Serena
Project Period: October 1, 1995 through September 30, 1998
Project Amount: $589,560
RFA: Air Pollutants (1995) Recipients Lists
Research Category: Air Quality and Air Toxics , Air
Objective:The stated goals of this research were to: (1) develop a field method to measure the number concentration of acidic airborne ultrafine particles and their associated metal content, and (2) devise a personal monitor for the evaluation of individual exposure to ultrafine acid aerosols based on the detectors developed for the field method.
Summary/Accomplishments (Outputs/Outcomes):The research largely has achieved the specific aims defined in the original plan. We have: (1) developed a method to determine the number concentration of ultrafine ambient acid particles based on ultrathin iron-film detectors; (2) designed and constructed a prototype monitor, based on diffusion deposition of ultrafine acidic particles onto the metal-coated detectors, which with further development, can be worn by an individual to measure personal exposure to acidic aerosols; (3) assembled a field sampling system for ambient air that utilizes state-of-the-art methods in particle sampling technology to measure the number concentration of acidic ultrafine particles and their associated metal content; and (4) measured the distribution of H+ and trace metals in the ultrafine ambient aerosol at a relatively clean regional background site in Sterling Forest (Tuxedo, NY) in air masses originating in source regions in Midwest, Southeast, and Eastern Canada.
Recent epidemiological evidence suggests that the number of ambient ultrafine acid particles, rather than ambient mass, is an important determining factor affecting lung injury, but no data currently are available on the size distribution or number concentration, largely because there have been no suitable methods for measuring these important quantities. The results of this research will allow widespread collection of these data for use in population exposure studies.
Number Concentration of Ultrafine Ambient Acid Particles. We have developed and tested a method to determine size distributed number concentration of ultrafine (d<100 nm) ambient total and acid particles using iron nanofilm detectors. Reaction sites are produced when acid deposits onto the nanofilms, with one reaction site produced by each acid droplet. Detectors were prepared by vapor deposition of iron onto 12-mm diameter glass cover slips. These detectors were tested, responses characterized, and methods for analysis developed. Based on the initial results, an improved nanofilm detector was developed for ambient air sampling.
Ambient particles are collected by deposition onto 5 mm x 5 mm silicon chips coated with a 25 nm thin iron film, deployed in a newly developed diffusion monitor. The concentration of particles on the surface of the collectors was quantified as a function of size by scanning force microscopy (SFM). Airborne number concentrations then were calculated based on calibrated deposition efficiency measurements and the relationship between reaction spot size and the size of the droplet causing the reaction.
Ultrafine Diffusion Monitor. A widening channel low-flow diffusion monitor was developed for the collection of a time-integrated sample of ultrafine ambient particles. The particles entering the monitor deposit by diffusion onto the walls of the channel. The objective was to design a diffusion sampler for environmental monitoring, in which the smaller particles would be efficiently removed from the air stream and the collection of larger particles would be increased over that expected in a channel of uniform width. The monitor was designed to incorporate multiple iron nanofilm coated detectors as well as nonreactive (silicon) particle collectors. The nonreactive detectors were used to collect a known sample of all ultrafine particles, and the iron nanofilm detectors were used to quantitate those particles that are acidic. The deposition efficiency of particles decreases differently for particles of different diameter with increasing distance from the inlet; therefore, placement of the detectors at different locations along the channel allowed replicate calculation of the air concentration for different particle size ranges. After obtaining the particle size distribution on the detectors, the particle size distribution in air was calculated based on the known collection efficiency for each particle size.
The prototype sampler was made of aluminum and was a flat, rectangular channel with a height of 1.0 mm. The width of the channel was 12 mm at the inlet, increasing to 150 mm at the exit. The overall length of the sampler was 500 mm. The design flow rate was 20 cm3 min-1. The particles deposited by diffusion on the walls of the channel. Exact theoretical equations are not yet available for this kind of a sampler. Predictions for the deposition efficiencies (DEs) of ultrafine particles along the channel were obtained by stepwise calculations using the equations for diffusion of particles in rectangular channels. The calculations showed a local deposition minimum at 100?200 mm and a local maximum at 200?300 mm. The locations of the minimum and maximum depended on the particle size. The experimental data obtained using monodisperse fluorescein particles with diameters from 20 to 200 nm supported the predictions. In addition, the deposition per unit area was almost constant along the channel beyond 150 mm.
Field Sampling System. A field sampling system was designed for the measurement of the number concentration of acidic nuclei and their associated metal content, as well as for size-fractionated particle-associated acid, sulfate, nitrate, and ammonium. The system utilized a Micro-Orifice Impactor (MOI) and an Electrical Aerosol Sampler (EAS) for separating and collecting particles smaller than 100 nm onto the metal-film detector. Thus, it was only necessary to count, not size, the reaction spots on these metal-film detectors to obtain the total number concentration of acidic (and other) nuclei mode particles. A polycarbonate filter collector in parallel with the EAS was used to collect the size-selected particles for chemical analysis. Acid neutralization by ambient NH3 and retention of gaseous HNO3 and SO2 by sorption or reaction with the filter or the particulate matter are known artifacts of particulate acidity sampling. To minimize these artifacts, two sets of annular glass denuders preceded the inlet of the MOI.
The EAS is a two-stage electrostatic precipitator, one charging and one collecting stage, that is designed to collect particles in a random and uniform manner onto a 2.5 cm x 12.7 cm collection surface area for microscopic evaluation of the deposited particles. The iron nanofilm detectors were placed over the collection area of the EAS to obtain uniform particle deposits. The fraction of the particles that precipitate in the charging section is known to be particle-size-dependent, but is a reproducible factor for most aerosols. The reported collection efficiency is about 60 percent for 28 nm particles and decreases to 50 percent for particles smaller than 20 nm. The particle collection efficiency of the EAS, Ef, for different particle sizes is being confirmed experimentally using monodisperse aerosols for a range of particle sizes. These results will be used for correcting the sample number concentration when sizing and counting particles. A Micro-Orifice Uniform Deposit Impactor (MOUDI) was used simultaneously to collect size-fractionated samples to be analyzed for metal content.
Field Sampling. The sampling system was deployed at a relatively clean regional background site in Sterling Forest (Tuxedo, NY) and used to collect samples of air masses originating in source regions in Midwest, Southeast, and Eastern Canada. The field sampling complex is an assemblage of the individual sampling systems described earlier. Samplers are housed outdoors in a shielded stainless steel hood. A 4-inch diameter stainless steel duct extends 3 feet above the roof. Samples were drawn through ports located within the duct. The pumps and sensitive electronic modules were housed inside a shed adjacent to the stainless steel hood. Temperature, relative humidity, barometric pressure, and weather conditions were monitored.
Twenty five sets of samples were collected from March 1998 through February 1999. The sampling duration was 4 days for 20 of the 25 sets and 5 days for the remaining sets. For each of these periods, samples were collected on the MOI for ion analysis, and the aerosol that penetrated the 100 nm stage of the MOI was collected on the iron nanofilms. The penetrating particles were precipitated onto the detectors deployed in an EAS. The MOUDI samples for size-fractionated metal analysis were collected for 21 of the 25 sampling periods. To date, analysis of the size-fractionated samples for H+, sulfate, and ammonium have been completed. The samples collected on the lower stages of the MOI were analyzed by ion chromatography. Higher ambient concentrations of ammonium, sulfate, and acid (as H+) were measured during the spring and the summer than during the fall and winter. In all seasons, the highest concentrations measured were of the ammonium and the lowest concentrations measured were of the hydrogen.
Total ion concentrations for the 25 samples ranged from 13.50 nmol/m3 to 73.56 nmol/m3 for sulfate, from 23.85 nmol/m3 to 139.03 nmol/m3 for ammonium and from 0.98 nmol/m3 to 18.30 nmol/m3 for hydrogen, for the different sampling periods. As expected, nitrate concentrations were very low. A high correlation (r2 = 0.933) was observed between the total sulfate and total ammonium measured. The slope of a line indicating the correlation of total ammonium and total sulfate is lower than the slope indicating a 2:1 ratio. A 2:1 ratio indicates total acid neutralization in the form of ammonium sulfate. Residual unneutralized acid still exists in the fine ambient PM sampled. The correlation of the sum of ammonium and hydrogen versus sulfate (r2 = 0.944) brings the slope closer to a 2:1 ratio. For most samples, the highest ambient sulfate, hydrogen, and ammonium concentrations were measured at the 0.41 µm midpoint diameter size fraction. An additional small peak also exists in the 0.04 µm size range. This additional peak indicates that the submicrometer size distribution of sulfate, hydrogen, and ammonium is bimodal. Although ambient concentrations varied from one season to another, the overall profile is similar.
The remaining analyses could not be completed during this grant period. Subsequent funding for field validation of the iron nanofilm detectors has been obtained from the Health Effects Institute, Cambridge, MA, and SPM analyses of the nanofilm detectors are in progress. The samples collected on the MOUDI for analysis of the metal content of the size-fractionated samples are to be analyzed by x-ray fluorescence (XRF). The analyses are expected to be completed using a new XRF unit purchased by the New York University-EPA PM10 Center. These data will be reported when they become available.
The following four students and Post-Doctoral Fellows were trained under this project: Yair Hazi (Ph.D. candidate), Paulina Zolotarevsky (M.S.), Wei Lei (Post-Doctoral Fellow), and Maire Heikkinen (Post-Doctoral Fellow).
Journal Articles on this Report : 2 Displayed | Download in RIS Format
|Other project views:||All 13 publications||2 publications in selected types||All 2 journal articles|
||Cohen BS, Li W, Xiong JQ, Lippmann M. Detecting H+ in ultrafine ambient aerosol using iron nano-film detectors and scanning probe microscopy. Applied Occupational and Environmental Hygiene 2000;15(1):80-89.||
||Hazi Y, Heikkinen MSA, Cohen BS. Size distribution of acidic sulfate ions in fine ambient particulate matter and assessment of source region effect. Atmospheric Environment 2003;37(38):5403-5413.||