Environ. Sci. Technol. 1999, 33, 3324-3331
Aerosol Size Distributions of Polycyclic Aromatic Hydrocarbons in Urban and Over-Water Atmospheres J O H N H . O F F E N B E R G †,‡.§ A N D J O E L E . B A K E R * ,† Chesapeake Biological Laboratory, The University of Maryland Center for Environmental Science, Solomons, Maryland 20688, and Department of Chemistry and Biochemistry, The University of Maryland, College Park, Maryland 20742
Aerosol mass size distributions of 41 polycyclic aromatic hydrocarbons (PAHs) were measured during 20 different 12-h periods in urban Chicago and over Lake Michigan during July 1994 and January 1995. Geometric mean aerodynamic equivalent diameters (GMDs) range from 0.72 to 2.39 µm for particulate matter and from 0.33 to 9.85 µm for individual PAHs. GMDs of the less volatile PAHs are larger in the urban atmosphere than over the water during the summer. Geometric standard deviations of the particle size distributions, however, are larger at the urban location for many PAHs, indicating a broader mass size distributions. GMDs of unsubstituted PAHs (except perylene) are well correlated with their log subcooled liquid vapor pressures (p°l, Pa), following the form: GMD ) mg log p°l + bg . Values for mg and bg range from 0.03 to 0.88 and from 0.83 to 8.80, respectively. Higher molecular weight PAHs are sorbed to the finest sized aerosols, but more volatile PAHs are associated with larger particles. The slope (mg) and intercept (bg) of these regressions are interdependent in these field data and follow the model: bg ) mhmg + bh, where mh ) 9.55, bh ) 0.61, and r 2 ) 0.98, suggesting that all GMD vs log p°l regressions for a class of semivolatile compounds tend to intersect at the same point (-mh, bh). This may allow the size distributions of the entire class of PAHs to be estimated by measuring the distribution of one PAH that is sufficiently removed from this intersection point. PAH size distributions change downwind of urban emission sources due to selective deposition of larger aerosols during atmospheric transport.
Introduction Particle size is the major determining factor in the atmospheric behavior of aerosol particles and controls the residence time and removal mechanisms of aerosol-bound contaminants from the troposphere (1-12). Particle size greatly influences inhalation exposure and, therefore, the human health effects of the aerosol-bound contaminants (13-18). However, reliable measurements of semivolatile organic contaminant size distributions in urban and rural atmospheres are difficult due to the complexities of sensitive * Corresponding author e-mail:
[email protected]; telephone: (410)326-7205; fax: (410)326-7341. † Chesapeake Biological Laboratory. ‡ Department of Chemistry and Biochemistry. § Current address: Statoil Research Centre, N-7005, Trondheim, Norway. 3324
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and unbiased methods (19, 20). Many semivolatile organic contaminants, such as polycyclic aromatic hydrocarbons (PAHs), are classified as hazardous air pollutants (21, 22) due to their suspected carcinogenicity. They pose a significant human health risk when associated with small particles (1618) due to deposition in the lung. Additionally, atmospheric deposition of these persistent organic contaminants, via both wet and dry processes, contributes significant loadings of semivolatile organic contaminants to the Great Lakes, the Chesapeake Bay, and other surface waters (23-29). Often these atmospheric inputs dominate the loadings to rural and remote environments, including the Arctic (30-33). Polycyclic aromatic hydrocarbons are 2-8-ring compounds often formed in combustion processes. Major PAH sources to the atmosphere include motor vehicles, home heating, fossil fuel combustion in energy and industrial processes, biomass burning, and municipal and medical incinerators (e.g., refs 34-36). Ambient atmospheric PAH concentrations range greatly, with higher concentrations most often found in urban atmospheres and significantly lower concentrations in rural and remote regions (29, 37-41). Higher total suspended particle (TSP) concentrations are usually observed in urban locations relative to those in rural and remote locations (42). Along with these decreases in concentration come changes in the particle size distributions of TSP along urban to rural transects (43). Typically, urban aerosol mass size distributions exhibit a bimodal character with modal diameters in the submicron and supermicrometer sizes (42-44). During transit downwind of urban emission sources, the distribution becomes unimodal due to dry particle deposition of the coarsest fraction. The deposition rate is a function of particle aerodynamic diameter, wind speed, friction height, and surface roughness (5, 12, 42, 45). To successfully collect size-segregated airborne particles and to conduct chemical analysis of sorbed organic contaminants, the methods utilized must minimize sampling artifacts that may alter contaminant size distributions. Previous field sampling methods have been affected by many artifacts, including high limits of detection, long sampling times, poor aerosol collection efficiencies, and contaminant blowoff from or sorption to the impaction substrate during collection (19, 46, 47). Recently, Poster et al. (19) verified through field and laboratory tests a sensitive and unbiased method for collection of PAHs. The use of a Berner-type impactor allows for collection of samples while minimizing sampling artifacts and overcoming poor analytical detection limits. Sufficiently large samples are collected during 12-h periods for analysis of semivolatile organic contaminants by standard chromatographic methods. The objective of this research was to quantify the aerosol mass size distributions of individual PAHs in urban and overwater atmospheres. Concentrations of individual PAHs and particle mass in size-segregated airborne particulate matter were measured in Chicago, IL, and over southern Lake Michigan during summer and winter conditions. These concentrations are used to calculate mass size distributions, which then are correlated to the PAH subcooled liquid vapor pressures.
Methods Sampling. As part of the Atmospheric Exchange over Lakes and Oceans (AEOLOS) project (38, 48-58), gradients in atmospheric concentrations of numerous species were quantified over southern Lake Michigan adjacent to Chicago, IL. One mobile and two stationary sampling platforms were 10.1021/es990089c CCC: $18.00
1999 American Chemical Society Published on Web 08/27/1999
established covering a trajectory from Chicago out over southern Lake Michigan to southwestern Michigan. During July 1994 and January 1995, ambient aerosol samples were collected at these stations to assess the impact of the urban industrial complex on neighboring Lake Michigan. Air samples were collected for 12-h periods during sampling intensives. The sampler was deployed atop the roof of a 10-m building in Chicago, IL (41°50′04′′ N by 87°39′29′′ W), and on a 1-m wood platform in South Haven, MI (42°27′52′′ N by 86°10′09′′ W). Over-water samples were collected aboard the U.S. EPA’s R/V Lake Guardian at two water quality monitoring stations in southern Lake Michigan, LM5 and LM1 (42°00′00′′ N by 87°25′00′′ W and 41°46′00′′ N by 87°20′00′′ W, respectively). Shipboard sampling was performed by suspending the impactor from a yard arm extending over the starboard bow rail of the ship, placing the instrument approximately 5 m above the water surface. The vessel was held on station by a bow anchor, thereby allowing the ship to remain oriented directly into the wind. Wind speed, direction, and air temperature were recorded at all stations with commercially available sensors. Airborne particles were collected using a Berner-type impactor (Hauke LPI 150/0,15 ln3; Hauke GmbH, Gmunden, Austria; 19, 59-62). Flow through the stainless steel impactor is controlled by a critical orifice downstream of the final stage, thereby maintaining consistent cut sizes throughout sampling. This impactor collects particles ranging in size from 0.15 to ∼38 µm on five impaction plates. Uncoated aluminum foil substrates were used as impaction substrates, thereby preventing the complication of partitioning of gases into the grease, and further avoiding cleanup and removal of grease before chemical analysis. Berner and Lurzer (59) demonstrated that for particles less than ∼5 µm aerodynamic equivalent diameter (aed), the measured size distribution was identical to that collected with greased plates. A polyurethane foam (PUF) plug was mounted immediately downstream of the Berner impactor in a glass sleeve to trap the vapor-phase PAHs. This sampling instrument was chosen due to the large volume of air sampled (∼100 m3) during the 12-h period, and the available literature detailing a thorough laboratory and field investigation of sampling artifacts for PAHs (19). After sample collection, foils were resealed in aluminum foil pouches, PUF plugs were returned to glass jars, and all samples were stored in the dark at -4 °C until chemical analysis. Analysis. Prior to sampling, impaction substrates were weighed to 0.001 mg (Mettler UMT-2) and were reweighed upon returning to the laboratory to determine the particulate matter (PM) concentration for each size fraction of every sample. Samples were brought to room temperature while sealed within their foil pouches prior to weighing at ∼20 °C and ambient RH (∼50%). Approximately half of each foil (determined gravimetrically) was extracted by sonication at 30 °C for 30 min in a sealed test tube containing 30 mL of dichloromethane (63). PAHs were quantified relative to perdeuterated internal standards (acenaphthene-d10, phenanthrene-d10, benz[a]anthracene-d12, benzo[a]pyrene-d12, and benzo[g,h,i]perylene-d12) by gas chromatography/mass selective detection using a Hewlett-Packard 5890 series II GC equipped with a 30-m HP5-MS capillary column coupled to a HP 5972 series II mass spectrometer operated in the selective ion monitoring mode. Forty-one individual PAHs were identified based on both retention time relative to known standards and the mass of the molecular ion. Chrysene + triphenylene and dibenz[a,h]anthracene + dibenz[a,c]anthracene are reported as the sum of the two respective compounds due to chromatographic coelution. To quantify the accuracy of the analysis method, six samples (0.96-9.21 µg) of NIST Standard Reference Material 1649a (Urban Dust/ Organics) were analyzed for PAHs.
Quality Assurance. Average recoveries of analytical surrogates spiked into each impactor sample prior to analysis indicated that no correction for handling errors or biases was necessary (100 ( 23%, 67 ( 26%, 93 ( 15%, and 100 ( 23%; average ( standard deviation; naphthalene-d8, fluorened10, fluoranthene-d10, perylene-d12, respectively). Furthermore, there was no systematic change in recovery with the mass loadings on the foil. Results of the SRM analysis showed recovery of 102.3 ( 21.9% (average ( SD) of the 14 certified PAH concentrations (64) measured with no dependence of the measured concentration on the mass of SRM extracted. Analyte breakthrough on the PUF sorbent was determined by analysis of 11 backup PUF collected at both the urban and over-water locations during summer conditions, when the atmospheric PAH concentrations and ambient temperatures were highest and breakthrough was most likely to occur. Percent breakthrough during summer sampling averaged 4.9% for phenanthrene and less than 5% for all other compounds. Adsorption of gas-phase PAHs to the foils in the Berner impactor was found to be a minor artifact even when allowing the clean foil to be coated with atmospheric organic matter (19). Furthermore, Poster et al. (19) suggest that the equations derived by Zhang and McMurray (47) are appropriate to assess possible losses of PAHs. The efficiency of the final stage (0.15-0.45 µm) of the Hauke 150/0,15 ln3 is calculated to be 88% for compounds with large gas/particle partition ratios (19, 47). The sampling efficiencies on the other four stages are higher. The collection efficiency drops to below 90% on the final stage for compounds more volatile than fluorene. Therefore, only concentrations of compounds with subcooled liquid vapor pressures equal to or less than that of fluorene are presented and discussed here. Procedural impactor substrate blanks and PUF blanks were processed concurrently with field samples to quantify operational detection limits. A blank based limit of detection (LOD), calculated for every compound as three times the respective field blank, are approximately 0.74 and 0.04 ng/ m3 for the sum of all PAHs in the gas and particle-bound phases, respectively. Compounds not present above the LOD are not reported in the following results. Data Analysis. To succinctly describe contaminant mass size distributions, geometric mean diameters (GMD) were calculated as
log GMD )
∑m log Dp ∑m i
i
(1)
i
where mi is the mass of compound in size class i and Dpi is the geometric mean particle diameter collected on stage i (43). Geometric standard deviations (σg) were calculated as
∑m (log Dp - log GMD) ∑m
2
(log σg)2 )
i
i
(2)
i
using GMD as defined above. These summary statistics were calculated for compounds for which the PAH concentrations in four or more of the five impactor stages were above the limits of detection. To further simplify the summary of all data collected, only those compounds that were present above the LOD in more than 70% of the samples collected are presented and discussed here. To facilitate comparisons of the impactor data, mass concentration size spectra were plotted by normalizing the compound concentrations determined for each stage to the logarithmic bin width. Thus, the data are represented as dC/ [d log Dp] in units of ng/m3. For this Berner-type impactor, the value of [d log Dp] remains constant at approximately 0.5. Further data analysis included the relation of the size VOL. 33, NO. 19, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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distribution summary statistics to compound specific, subcooled liquid vapor pressures. The vapor pressures were calculated from the average ambient air temperature according to the temperature-dependent relationship found in the Supporting Information.
Results and Discussion Particle Mass. Aggregate (sum of all size fractions) concentrations of particulate matter (Σ-PM) ranged from 8.2 to 23.3 µg/m3 in the urban atmosphere and from 8.2 to 36.6 µg/m3 in the over-water atmosphere. Aerosol concentrations collected by the Berner-type impactor (Σ-PM) were lower than those collected simultaneously by dichotomous samplers (10 µm inlet cut size; 53, 65). On average, the sum of the five impactor stages represented 59 ( 17% and 45.7 ( 7% (average ( SD) of the aerosol mass measured by the dichot over Lake Michigan and in Chicago, respectively. This is likely due to incomplete collection by the Berner-type impactor of large aerosols that often contain a significant portion of the particle mass, especially in urban atmospheres (66). Comparison of impactor results between locations is not possible due to nonconcurrent sampling. However, for all samples collected during the summer intensive, the urban atmospheric aerosol concentration measured by the dichot was significantly elevated relative to paired over-water samples and to the respective rural samples (27.7 ( 3.1 µg/m3 Chicago, n ) 19; 17.4 ( 1.8 µg/m3 over-water, n ) 17; 20.8 ( 3.8 µg/m3 rural, n ) 8; average ( standard error; 53, 65). There was no significant difference between the 19 urban aerosol concentrations measured in the summer and the three measured during winter (30.9 ( 1.6 µg/m3). Geometric mean diameters (GMD) of Σ-PM averaged 1.71 µm and ranged from 0.72 µm on July 19, 1994 over-water to 2.39 µm on January 21, 1995, in Chicago. Despite this large range of values, there were no significant differences in Σ-PM GMDs between any of the sampling locations or periods. Σ-PM geometric standard deviations (σg) averaged 3.3 and did not vary significantly with location or sampling period. However, Σ-PM GMDs positively correlate with measured Σ-PM σg values, such that 57% of the variability in the observed GMD can be explained by changes in the σg of particulate matter. This indicates that increases in Σ-PM are due to additions of large aerosols that greatly increase σg. Resuspension of urban dust and soils is a likely source of the increased large aerosols fortifying the Σ-PM concentration and associated Σ-PM GMD. Accordingly, decreases in Σ-PM GMD correspond to decreases in Σ-PM σg and are likely due to decreases in the concentration of largest particle concentrations. Furthermore, this suggests that the dry deposition of large particles decreases the breadth of the Σ-PM particle size distribution as the air mass is transported away from aerosols sources. PAH Concentrations. Aggregate particle-bound concentrations (sum of all size fractions) of individual PAHs ranged from 0.85 to 10.9 ng/m3 in the urban atmosphere and from 1.0 to 14.2 ng/m3 in the over-water atmosphere. The apparent similarity between over-water and urban PAH concentrations is in part due to summer sampling in the urban area during northerly winds, which contained relatively low PAH concentrations. Summer over-water sampling was during a period when southwesterly winds transported urban air out over the lake, increasing total PAH concentrations approximately 12-fold over regional background (north wind) concentrations (38). Total (vapor + particle) PAH concentrations measured with the impactor/PUF sampler agree well with those collected with a modified high volume sampler operated with a glass fiber filter and polyurethane foam adsorbent plug (GFF + PUF) and analyzed independently (38). An average of 97.6 ( 38.3% (average ( SD) of the highvolume concentration (GFF + PUF) was measured using the 3326
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FIGURE 1. Comparison of the concentrations of particle-bound individual PAHs (ng/µg) measured simultaneously by filter method (high-volume w/GFF) and impactor method (Berner-type impactor) over Chicago and southern Lake Michigan during July 1994 and January 1995. impactor/PUF sampler. Additionally, the aggregate (sum of all size fractions) particle-bound PAH concentration (ng/g) measured in the impactor was 90.3 ( 36.9 %(average ( SD) of the concentrations measured on only the filter from the high-volume collection method (Figure 1), indicating that minimal “blowoff” or adsorption artifacts occur within the impactor. Furthermore, the discrepancies between the parallel samples for PAHs are far less than for collection of particle mass. This suggests that the impactor and the highvolume air sampler are collecting approximately the same particles. Furthermore, the difference between the total particle mass collected by the impactor and the dichot are not important with regard to the analysis for PAHs. However, the mass of particles collected on the high-volume filter was not determined (53), and as such no comparisons of mass collected by this sampler can be made. Finally, there appear to be relatively small amounts of PAHs in the largest particle collected by the impactor, as described below. This may further account for the presumable lack of importance of the apparent incomplete collection of large particle mass by the impactor. PAH Size Distributions. Polycyclic aromatic hydrocarbon geometric mean diameters ranged from 0.3 µm for benzo[ghi]perylene on July 18, 1994, at the over-water station to 9.8 µm for anthracene on January 21, 1995, in Chicago. Overwater and rural samples were typically nearly uniformly unimodal with modal peaks in the 0.45-1.4-µm size class (Figure 2). Urban samples often exhibited a smaller secondary mode in the >12.2-µm size class; however, with the limited number of size cuts in this impactor, a detailed analysis of this secondary mode is not possible. Furthermore, urban GMDs were significantly larger than those collected overwater for compounds with vapor pressures lower than pyrene only during the summer period (Table 1). This difference is likely due to sampling the urban atmosphere during northerly winds in the summer, which delivers to the sampling location air masses from differing source regions and conditions. Additionally, geometric standard deviations (σg) about these GMDs are significantly larger (p < 0.05) in the city for benz[a]fluorene, chrysene + triphenylene, benzo[b]fluoranthene, benzo[e]pyrene, and benzo[a]pyrene during both summer and winter, indicating that the mass size distribution is broader for these contaminants in the urban atmosphere
FIGURE 2. Typical particle size distributions of five PAHs and total particle mass measured with the Berner-type impactor: (a) LM 5 over southern Lake Michigan on January 16, 1995, and (b) Chicago, IL, on January 20-21, 1995. than at over-water locations. The relative difference between urban and over-water σg values increases from higher to lower molecular weight PAHs, meaning that the increase in σg is more pronounced for higher vapor pressure compounds. No statistically significant seasonal or diurnal variations were found in the PAH GMDs or σg values at any of the locations. Several studies have reported the concentrations of atmospheric PAHs as a function of particle diameter. In a series of field experiments, benzo[a]pyrene was found to be associated with atmospheric particles
