Environ. Sci. Technol. 1998, 32, 1522-1529
Use of an Iridium Tracer To Determine the Size Distribution of Aerosol Emitted from a Fleet of Diesel Sanitation Trucks A. E. SUAREZ, P. F. CAFFREY, P. V. BORGOUL, AND J. M. ONDOV* University of Maryland, Department of Chemistry and Biochemistry, College Park, Maryland 20742 F. DIVITA, JR.† Desert Research Institute, Reno, Nevada, 89506
To determine the size distributions of soot from a fleet of heavy-duty diesels, size-segregated ambient outdoor aerosol was collected with 9-stage micro-orifice impactors (MOI) during two 30 day periods when 800 diesel sanitation vehicles (SV) operated by the City of Baltimore burned fuel tagged with iridium(III) 2,4-pentanedionate. Ambient background samples were collected between and after release periods. Additionally, several tagged emission samples were collected with an eight-stage MOI mounted down stream of a radial diluter installed on-board a diesel SV in normal operation. Ambient, background, and emissions samples were analyzed gravimetrically for particulate mass, for iridium by instrumental neutron activation analysis, and for organic and elemental carbon (OC and EC) by a thermal/optical reflectance method. Size distributions of freshly emitted Ir-containing particles contained a major accumulation aerosol peak at a modal aerodynamic diameter of 0.12 µm; however, 43 ( 1% of the Ir mass was contained in particles with diameters between 0.22 and 1.8 µm. Fractions of particulate mass, OC and EC contained in this interval were similar. Tagged soot aerosol collected in Baltimore contained modes at 0.4 and, sometimes, 2.4 µm, in addition to the primary particle mode at 0.12 µm. Submicrometer modes are attributed to fresh SV emissions, whereas the 2.5 µm mode is attributed to resuspension. The results suggest that inferences on the evolution of urban soot aerosol made from size distribution data may be invalid unless a unique tracer is employed.
Introduction Soot is an important component of urban fine-particle aerosol. As a product of incomplete combustion of virtually all carbonaceous fuels, soot contains elemental (graphitic) and organic forms of carbon. The latter includes polycyclic aromatic hydrocarbons (PAH) thought to be responsible for mutagenicity and carcinogenicity of urban aerosol particles (1). Motor vehicles are thought to be the largest single source of atmospheric PAHs in the United States, accounting for approximately 36% of the annual anthropogenic emission * Corresponding author: phone: 301-405-1857; fax: 301-314-9121; e-mail:
[email protected]. † Current address: E. H. Pechan & Assoc., Inc., 5528-B Hempstead Way, Springfield, VA 22152. 1522
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(2); emissions from diesel-powered vehicles are 50-fold greater than those from gasoline engines (3). Atmospheric contributions of soot-borne PAH to water bodies are thought to be substantial, especially, for those which lie adjacent to populous urban areas where ambient atmospheric concentrations often exceed those in rural areas by factors of 10100. Dry deposition accounts for nearly half of the total atmospheric PAH fluxes (4). Furthermore, Poster et al. (5) argue that scavenging of submicrometer particles containing PAH is the principle source of PAH in wet deposition. Dryparticle deposition rates increase rapidly for particles larger than 0.4 µm (6), roughly, as the square of the diameter. Holsen and Noll (7) have suggested that PAH fluxes could be dominated by even very small amounts of PAH on large settleable particles. The efficiency of precipitation and incloud scavenging is also size dependent, as are respiratory deposition and light absorption. Thus, knowledge of the size distribution of sources of urban soot is of paramount importance in estimating its environmental effects. Despite their importance, studies of the size distribution of diesel soot and its chemical constituents are few. Often, size distributions are determined during operation on dynamometers (8-10), which limits the number of vehicles measured and may not adequately represent the range of conditions of actual use, e.g., vehicle load and acceleration profiles. Furthermore, changes that may occur after atmospheric discharge (e.g., by condensation/loss of volatile organic compounds) may not be reflected. Size distributions of particulate emissions from motor vehicles in actual operation have been obtained from tunnel studies (11, 12). However, these encompass a limited range of driving conditions. Furthermore, emissions from heavy-duty diesel (HDD), light-duty diesel (LDD) vehicles, and gasoline-fueled vehicles have not been resolved in tunnel studies. Herein, we used Ir as an intentional tracer to measure the ambient size distribution of soot emitted from a fleet of HDD sanitation vehicles (SV) operated by the City of Baltimore, MD. Iridium was chosen as a tracer for this study as it is truly rare in the earth’s crust, has no industrial sources contributing to its atmospheric concentration, can be determined in our laboratory instrumentally with a detection limit of 500 fg by neutron activation analysis (NAA), and is commercially available as the 2,4-pentanedionate (38.07% Ir) for $25.00 g-1 (Colonial Metals, Elkton, MD). Furthermore, Ir(III) forms a highly stable d6 octahedral pentanedionate complex (IrPD), which is readily soluble in organic solvents (i.e., toluene) miscible in diesel fuel. The work was conducted as part of the EPA-sponsored AEOLOS project (Atmospheric Exchange Over Lakes and Oceans) designed to determine the influence of Baltimore’s urban and industrial sources on water quality in the Chesapeake Bay, which lies 2.5 µm in diameter and a polycarbonate housing for 47 mm Gelman “Teflo” filters through which air is sampled at 80 L min-1. Eight MOI/UMFPS sets were collected during the 1995 campaign. Two of the sets were collected on a Sunday, i.e., when SVs do not operate. In addition, a 47 mm sampling head was loaded and mounted on the sampling mast to VOL. 32, NO. 10, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Sampling Periods and Pertinant Data date
site
sample duration (h)
days
Aug 1995 ambient
fire station
12-14, 16, 20, 21, 24, 30
12
Sept 1995 source
1995 Mack/Heil sanitation truck fire station
1, 19, 21, 27, 28
5
Nov/Dec ambient 14, 28, 29, 30, 4-7, 11, 12 1995 background July 1996 ambient fire station & Marine 6,7 background Police Station Aug 1996 ambienta fire station 6-9, 12-16 and 19-23, 26-30 Oct 1996 a
ambient Lead Training background Center
29-30, 30-31
time of day
sampler
24
6 a.m. to 6 p.m. and MOI, UMFPS 6 p.m. to 6 a.m. 7:30 a.m. to 12:30 p.m. MOI/radial diluter 12 p.m. to 12 p.m. UMFPS
24
12 p.m. to 12 p.m.
UMFPS
9, 9, 18
6 a.m. to 6 a.m. 6 a.m. to 4 p.m. 6 a.m. to 4 p.m. 6 p.m. to 11:30 p.m. 12:30 p.m. to 6 a.m.
MOI in sector UMFPS out of sector MOI
18
Samplers were operated only when winds direction between 210 and 330°.
provide a “field blank” for each of the sampling periods. Ten additional aerosol and 10 field-blank samples were collected for 24 h with the UMFPS in November and December of 1995 (i.e., 10 weeks after the city-wide release and 6 weeks after the last SV release of 1995) to monitor the postrelease Ir background. During the August 1996 city-wide release, MOI samplers were interfaced with a data logger (Campbell Scientific, model CR10) and relay module programmed to collect samples at the fire station only when the wind speed > 0.5 m s-1 and wind direction lay between 210 and 330°. The UMFPSs were operated under out-of-sector conditions. This strategy excluded aerosol advected from the east and, thus, less influenced by city SV emissions, in an effort to maximize the amount of Ir-tagged aerosol sampled. As shown in Table 1, the latter two samples were collected at the fire station only between 6 a.m. and 4 p.m. to better correspond to the operating schedule of the SVs. A set of MOI and 47 mm field-blank substrates were also collected. Background aerosol samples were collected at the fire station and Marine Police Station (MPS) with a UMFPS in July 1996, i.e., 1 month before the 1996 city-wide release. In addition, two MOI samples were consecutively collected at the Lead Training Institute (LTI) at the end of October 1996, i.e., 1 month after the end of the 1996 release (see Table 1). Single Vehicle Studies. Iridium tracer was added to the 151 L fuel tank of a 1995-Mack/Heil model MS300P diesel SV powered with a 376 in.3, 6 cylinder engine on five occasions in September 1995. On those occasions, 8.2 mL aliquots of solutions containing 100 g of IrPD in 7.58 L of toluene were distributed to the driver for addition to the truck’s tank before refueling. An eight-stage MOI (D50s 0.09 to 15 µm) fitted with a radial diluter was used to collect five sets of size-segregated aerosol emitted from the truck’s exhaust. Exhausted aerosol was aspirated through a 1 m, 9.5 mm-ID probe using a 12-V DC (Thomas Instruments, Inc., model 907BDC22A) vacuum pump wired into the truck’s ignition system and only operated while the vehicle was making collections in Baltimore, typically 5 h day-1. The sampling rate, 6.9 L min-1, was determined from the pressure drop across a calibrated orifice installed above the diluter. At this rate, the aerosol residence time in the probe before dilution was 0.6 s. Coated and uncoated Gelman “Teflo” filters were used for three sets of the on-board measurements (September 21, 27, and 28th). On the 1st and 19th of September, 37 mm aluminum foils (MSP Corporation, Minneapolis) were used to collect samples for mass and carbon analysis, respectively. Analyses. All of the samples collected on Teflon filters were analyzed for Ir by NAA. Samples were irradiated for either 4 h in the Brookhaven National Laboratory 30 MW 1524
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High-Flux Beam Reactor (7.5 × 1013 n cm-2 s-1 thermal neutron flux) or for 6 h in the National Institute of Standards and Technology (NIST) 20 MW Research Reactor (1 × 1014 n cm-2 s-1 thermal neutron flux). Iridium was determined by observing the 316.51 keV γ-ray from 74 day 192Ir on 2640% efficient high-purity Ge solid-state detectors, from 30 to 60 days after irradiation. Quantification of Ir was achieved by irradiating 20 ng aliquots of Ir in the Ir(III) chloride form. Iron was determined similarly from the 1291.61 keV γ-ray from 44.4 day 59Fe. Cobalt flux monitors (Al wire containing 0.5% Co) were used to adjust for spatial flux gradients across the irradiation container. The detection limit for Ir by this method was typically 500 fg. Iridium could not be detected in any of the unused 37 mm filters and was detected in only one of the MOI field blanks. Its mass in the latter corresponds to an ambient concentration of
