Evaluation of 1047-nm Photoacoustic Instruments and Photoelectric Aerosol Sensors in Source-Sampling of Black Carbon Aerosol and Particle-Bound PAHs from Gasoline and Diesel Powered Vehicles

A series of measurements have been performed at Hill Air Force Base to evaluate real-time instruments for measurements of black carbon aerosol and particlebound PAHs emitted from spark and ignition compression vehicles. Vehicles were operated at idle or fast idle in one set of measurements and were placed under load on a dynamometer during the second series. Photoacoustic instruments were developed that operated at a wavelength of 1047 nm where gaseous interference is negligible, although sensitivity to black carbon is good. Compact, efficient, solid-state lasers with direct electronic modulation capabilities are used in these instruments. Black carbon measurements are compared with samples collected on quartz fiber filters that were evaluated using the thermal optical reflectance method. A measure of total particle-bound PAH was provided by photoelectric aerosol sensors (PAS) and is evaluated against a sum of PAH mass concentrations obtained with a filter-denuder combination. The PAS had to be operated with a dilution system held at approximately 150 °C for most of the source sampling to prevent spurious behavior, thus perhaps compromising detection of lighter PAHs. PA and PAS measurements were found to have a high degree of correlation, perhaps suggesting that the PAS can respond to the polycyclic nature of the black carbon aerosol. The PAS to PA ratio for ambient air in Fresno, CA is 3.7 times as large in winter than in summer months, suggesting that the PAS clearly does respond to compounds other than BC when the instrument is used without the heated inlet. Introduction Quantification of black carbon aerosol is important for both health and the Earth’s radiation balance (1, 2), so the accurate, rapid assessment of these aerosols is needed, in both the direct analysis of source emissions as well as in cities or remote locations. Black carbon aerosol is produced by the combustion of fossil fuels and biomass with regional emissions of sufficient magnitude to cause disruptions of atmospheric heating profiles and possibly large-scale perturbations of the Earth’s hydrological cycles (3). Historical trends point toward increasing levels of BC production (4). It is unhealthy to breathe the polycyclic aromatic hydrocarbons that may exist in the vapor phase, may be absorbed in liquid-phase aerosol, or may be adsorbed to the surface of black carbon aerosol (5-9). PAHs as a group are reasonably anticipated to be carcinogenic (10), and a few individual PAHs, such as benz[a]anthracene and benzo[a]pyrene, are probably carcinogenic (11). Diesel exhaust has also been linked to allergic inflammation (12) and is considered a probable carcinogen. This paper provides an assessment of real-time instruments for measurement of BC and particle-bound PAHs for use in sampling combustion sources. The source measurements are also contrasted against ambient measurements made at the Fresno, CA Supersite. The methods for quantifying black carbon mass concentration can be divided into two categories based on whether they require a filter substrate. Examples of filterbased techniques include thermal and optical methods, and laboratory and real-time instruments exist for each. Thermal methods start with the collection of aerosol on a filter, followed by measurement of the carbon atoms evolved when the filter is heated, with elemental carbon being defined as the carbon mass observed in some temperature range. Filterbased optical methods measure the optical attenuation across a filter as aerosols are deposited and exploit the observation that most particulate light absorption in the atmosphere is due to black carbon aerosol. Accurate mass flow meters are vital for quantifying the volume of air sampled to obtain the observed aerosol accumulation on a filter. Filter methods have limitations with respect to dynamic range, lack of a common definition of elemental carbon, and scattering aerosol interferences in the black carbon measurement (13, 14). Photoacoustic instruments are also used to measure BC and they overcome the deficiencies of the filter-based sampling methods (15). The photoelectric aerosol sensor (PAS) utilizes an ultraviolet lamp to ionize particles containing PAHs. The charged particles are collected on a filter, and the charge is measured with an electrometer. The PAS signal was originally correlated to 17 individual PAHs from several sources, with a universal conversion factor of 1-3 ng m-3 femtoamp-1 (16-18). Because PAHs are such a complex mix of vapor and particlephase compounds, the PAS responds differently to each type of source, and the sensitivity varies in comparison to extractive methods (19). Furthermore, independent evaluations of the PAS are limited, and investigators report widely varying conversion factors (16, 20, 21). A chronicle is provided in this paper of the efforts employed to successfully use the PAS in source sampling, as well as a comparison of results from the PAS with measurements from a filter and denuder sampler followed by extraction and laboratory quantification of individual PAHs. Generally, good correlation is found between BC and PAS measurements raising the following issue: does the PAS respond to the polycyclic nature of the black carbon aerosol? This question takes on particular relevance because the laboratory extractions of PAHs showed * Corresponding author phone: (775)674-7023; fax: (775)674-7060; e-mail: [email protected].