Environ. Sci. Techno]. 1995, 29, 1990-1997
Organic Contaminants in the Atmosphere D I A N N E L. P O S T E R , + , * RAYMOND M. HOFF,” AND JOEL E. BAKER*,§ Chesapeake Biological Laboratory, The Center for Environmental and Estuarine Studies, The University of Maryland System, P.O. Box 38, Solomons, Maryland 20688, Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, and Center for Atmospheric Research Experiments, Atmospheric Environment Service, Environment Canada, Egbert, Ontario LOL 1N0, Canada
A method using a Berner five-stage, low-pressure cascade impactor was developed to measure the particle-size distributions of polycyclic aromatic hydrocarbons (PAHs) in the atmosphere. Possible sampling artifacts that may occur when sampling semivolatile organic chemicals with this low-pressure particle impactor (e.g., absorption of vapors onto sampler surfaces or volatilization of analytes from collected particles under reduced pressures within the impactor) were quantified for a variety of PAHs in the laboratory. At 20 “C, these artifacts were insignificant for the collection of PAHs with vapor pressures less than lo-* atm. Several samples collected at a rural site (Egbert, Ontario) and an urban location (Chicago, IL) demonstrate the utility of the Berner impactor to collect PAHs under a variety of field conditions. At both the rural and urban locations, sufficient quantities of particles were collected in the five aerodynamic equivalent diameter size ranges (0.04-0.14, 0.14-0.49, 0.49-1.7, 1.7-6, and 6-21 p m ) during 12 h to provide detectable levels of PAHs. PAHs were most often associated with particles less than 1.7 p m in both the rural and urban areas based on a limited number of observations ( N = 5).
Introduction Although particle size plays a critical role in human health effects (1-5) and greatly influences the atmospheric residence time and removal mechanisms of particle* Corresponding author e-mail address:
[email protected]. Department of Chemistry and Biochemistry. * Present address: Analytical Research Division,Chemical Science and Technology Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899. l 1 Center for Atmospheric Research Experiments. 5 Chesapeake Biological Laboratory. +
1990 1 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 8,1995
associated contaminants (eg.,wet scavenging, dry deposition; 6-10), little is known about the particle-size distributions of semivolatile organic contaminants in rural or urban atmospheres. Many semivolatileorganic contaminants, such as polycyclic aromatic hydrocarbons (PAHs), are classified as hazardous air pollutants due to their carcinogenic properties (11,12)and pose asignificantthreat to human health when associatedwith submicron particles (5).Atmospheric particles less than 2 pm are respirable (1-5) and are enriched in PAHs (3,13-24). However, PAH size distributions measured in previous studies may be biased due to sampling artifacts resulting from the use of particle-sizefractionatingsamplers designed mainly for the collection of nonvolatile species (20). These artifacts include the adsorption of low molecular weight organic gases onto the collection substrates during sampling (25) and volatilizationof particle-associated species under the subatmospheric pressures within the samplers (26). In addition to these artifacts, the relatively low flow rates of many samplers (e&, 30 Llmin; 3,131 require longsampling times (e.g., days) in order to collect adequate sample for analysis. Changing meteorological conditions (Le., temperature, wind direction) and enhanced chemical transformations during these long sample collections likely hinder the interpretation of the resulting data. Changes in meteorology and temperature during extended sampling intervals alter contaminant concentrations and vaporparticle distributions and may lead to changes in particlesize distributions. Chemical reactions of parent PAHs with atmospheric pollutants such as ozone and nitrogendioxides also compromises sampling of particle-associated PAHs (20, 27-30).
Currently, there are no routine methods available for the measurement of size distributions of semivolatile organic compounds that have been shown to avoid either the gaseous sorption or volatilization artifacts. Recently, Berner and colleagues (31) developed a five-stage lowpressure impactor with sufficiently high flow rates (80 Llmin), which may be suitable for the collection of semivolatileorganic chemicals from the atmosphere. This sampler uses foil collection substrates rather than glass fiber filters, which may decrease sampling artifacts. The Berner impactor collects particles in discrete spots, and the particles are embedded in the foil, which limits the amount of particle bounce as the loading on the foil increases (32). We thoroughly evaluated this impactor and its ability to isolate semivolatileparticle-associated PAHs as a function of particle size in a detailed laboratory study. Objectives of this study were as follows: (i) to document the overall importance and relative magnitude of gaseous absorption to “clean”and “dirty”aluminum foil collection substrates, (ii) to measure volatilizationlosses of particleassociated volatile and semivolatile species from the collection substrates under sampling conditions, and (iii) to verify in the field that the impactor can quantitatively sample sufficient masses of atmospheric particles for the analysis of PAH size distributions. In this paper, we present the results of the laboratory evaluation and the particlesize distributions of several PAHs in rural and urban environments. The latter were measured with the Berner impactor to strictly verify the sampler’s performance and
0013-936W95/0R29-1990$09.00/0
0 1995 American Chemical Society
TABLE 1
PAH Measurements as a Function of Particle Size pafiicle impactor
no. of cuts
min cut sirakm)
horizontal elutriator casella system Andersen high-volume Andersen high-volume
2 5 na 5
~2.3 10.14
Hering low pressure Andersen high-volume Sierra high-volume Andersen high-volume EGAl80 system Sierra high-volume#235 Hering LPI Berner low pressures a
flow rata
plate substrates
(m3/min) na
