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Predicting Particle Number Concentrations Near a Highway Based on Vertical Concentration Profile Yifang Zhuã, William C ARTICLE IN PRESS Atmospheric Environment 39 (2005) 1557–1566 www.elsevier.com/locate/atmosenv Predicting particle number concentrations near a highway based on vertical concentration profile Yifang ZhuÃ, William C. Hinds Department of Environmental Health Sciences, University of California Los Angeles, Southern California Particle Center and Supersite, Center for Occupational and Environmental Health, 650 Charles E. Young Drive South, Los Angeles, CA 90095, USA Received 6 October 2004; received in revised form 22 November 2004 Abstract This paper presents vertical profiles of particle number concentration and size distribution near the Interstate 405 Freeway. Based on vertical concentration data, averaged emission factor for total particle number concentration was determined to be 8.3e14 particleÀ1 vehicleÀ1 mileÀ1. A simple analytical solution was obtained by solving the atmospheric diffusion equation and used to predict horizontal profiles of total particle number concentrations. Good agreement was achieved when model results were compared with previously published experimental data (J.Air Waste Manage. Assoc. 52 (2002b) 174–185). Model inputs are traffic, wind, and locations. With the particle emission factors, traffic compositions, and meteorological data, particle number concentrations near freeways can be quantitatively determined using the simple equation achieved in this study. r 2004 Elsevier Ltd. All rights reserved. Keywords: Ultrafine particles; Atmospheric dispersion; Emission factor; Vertical concentration profile 1. Introduction o100 nm) (Penttinen et al., 2001; Peters et al., 1997; Wichmann et al., 2000a, b). These studies suggested that Epidemiological studies all over the world have health effects might be more associated with the number consistently linked increases in particulate matter (PM) of ultrafine particles than with the mass of the fine exposure to increases in mortality and morbidity particles. (Dockery et al., 1993; Vedal, 1997). Improvement in Health effects have been related to traffic volume. particle measurement technologies has allowed exposure Children living near high traffic streets are more likely to to smaller particle sizes to be evaluated. The link have more medical care visits per year for asthma between PM and adverse health effects has been (English et al., 1999) and a higher prevalence of most suggested to be stronger as smaller particle sizes are respiratory symptoms (Ciccone, 1998; Oosterlee et al., considered (Wichmann and Peters, 2000). Recent 1996) than those living near lower traffic streets. epidemiological studies, dealing with short-term effects Although it is not clear which traffic related pollutant in adults and children with asthma and daily mortality, contributes to the observed adverse health effect, traffic have addressed the role of ultrafine particles (diameter is the major source of ultrafine particles in an urban environment (Hitchins et al., 2000; Schauer et al., 1996; ÃCorresponding author. Tel.: +1 310 794 7565; Shi et al., 1999; Zhu et al., 2002a, b, 2004). Both diesel fax: +1 310 794 2106. and gasoline engines generate a significant number of E-mail address: [email protected] (Y. Zhu). particles in the ultrafine size range (Morawska et al., 1352-2310/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2004.11.015 ARTICLE IN PRESS 1558 Y. Zhu, W.C. Hinds / Atmospheric Environment 39 (2005) 1557–1566 1998; Ristovski et al., 1998). Increases in particle predict particle number concentration at any specified number concentration have been observed during the distance from a freeway based on traffic and atmo- rush hours (Wichmann and Peters, 2000). With increas- spheric conditions. ing concern about ultrafine particle exposure near traffic sources, Zhu and co-workers conducted measurements of particle number concentration and size distribution near two major freeways in Los Angeles, CA, in two 2. Experiment seasons (Zhu et al., 2002a, b, 2004). Measurements showed ultrafine particle concentration immediately Previously, we have reported horizontal concentration downwind near both freeways were approximately 25 profiles of ultrafine particles and co-pollutants near the to 30 times greater than the concentrations upwind and Interstate 405 Freeway in summer 2001 (Zhu et al., decreased rapidly with increasing distance downwind 2002b). During our 2001 summer field campaign, from the freeway. vertical concentration profiles were obtained on 17th Although a number of line source models have been and 18th July 2001. These data were used to determine developed by US Environmental Protection Agency and average particle number emission factors from the many other research institutes for estimating vehicular traffic, which was later used as an input parameter for pollutant concentrations, these models have focused the atmospheric dispersion model developed in this primarily on gas-phase pollutants. Some of them have study. the capacity to predict PM10 and sometimes PM2.5. Near the sampling site, Freeway 405 runs generally However, little attention has been given to particle north and south. Average traffic density was number concentrations. Nagendra and Khare presented 230 vehicles minÀ1 passing the sampling site in both a thorough review on line source models (Nagendra and directions with approximately 5% being heavy-duty Khare, 2002). More recently, Jamriska and Morawska diesel trucks. During the sampling period, a consistent developed a box model to assess the traffic-related sea breeze (eastward from the ocean) developed in mid- emission rates of fine and ultrafine particles to local morning, reached its maximum early to mid-afternoon, areas in Queensland, Australia (Jamriska and Moraws- and died out in the early evening. The region upwind of ka, 2001). Johnson and Ferreira developed a statistical the freeway is a residential area without any industrial or model to predict submicrometer particle concentration other obvious PM sources. A 12 m scissors lift with a 6 m (diameter less than 1 mm) contributed by traffic flows in extension mast was used to sample carbon monoxide Brisbane, Australia (Johnson and Ferreira, 2001). CA- (CO) and particle number concentrations at different LINE4 software package was recently adapted to heights at a fixed location, 50 m downwind from the determine the average emission factors for ultrafine center of Freeway 405. A detailed description of the particles based on their number concentration (Gra- sampling site, can be found in Zhu et al. (2002a). motnev et al., 2003). A video recorder (camcorder) was located on top of a Predicting particle size distributions as a function of 10-m tower near the sampling site and operated distance from an emission source is apparently much continuously throughout the measurement. The cam- more challenging and controversial. Based on data corder was high enough to capture all nine lanes on published in the series of papers from Zhu et al. the 405 Freeway. After sampling session, the video- (2002a, b, 2004), Jacobson and Seinfeld argued that tapes were replayed and traffic density, defined as enhanced Brownian coagulation due to van der Waals number of vehicles passing per minute, was determined forces and fractal geometry may account for observed manually. particle size evolution (Jacobson and Seinfeld, 2004); Total particle number concentration was measured while Zhang et al. (2004) hypothesized that condensa- every minute by a condensation particle counter (CPC tion/evaporation was the dominant mechanisms in 3022A; TSI Inc., St. Paul, MN). Data reduction and altering particle size distributions. Although discrepan- analysis were done by the Aerosol Instrument Manager cies exist in the literature, they seem to agree that software (version 4.0, TSI Inc., St. Paul, MN). In atmospheric dispersion/dilution is the most important addition to total particle number concentration, CO process in decreasing particle number concentrations concentration was measured every minute by a near- near the source. continuous CO monitor (Dasibi Model 3008, Environ- The present research focuses on determining particle mental Corporation, Glendale, CA). The CO monitor number emission factors based on vertical concentration was calibrated by standard CO gas (RAE systems Inc., profiles near the Interstate 405 freeway and developing a Sunnyvale, CA) in the laboratory and automatically simple atmospheric dispersion models that predict zeroed each time the power was turned on. Before each ultrafine particle number concentration as these particles measurement, all instruments were synchronized. Ver- are transported away from a major emission source—a tical concentration profiles for CO, and total particle freeway. The goal of this research is to quantitatively number concentrations measured by the CPC were ARTICLE IN PRESS Y. Zhu, W.C. Hinds / Atmospheric Environment 39 (2005) 1557–1566 1559 obtained near the freeway. Measurements were taken at well the pollutant concentration distribution under 50 m downwind from the center of Freeway 405 at 0.6, strong convective condition, under ground-level release, 3.0, 5.5, 8.0, 10.4, 12.8, 15.3, and 17.7 m above the and near sources (Seinfeld and Pandis, 1998; Sharan et ground. al., 1996a; Turner, 1994). Instead, the atmospheric Meteorological parameters including ambient tem- diffusion equation was found to provide a more general perature, wind speed and directions were measured on approach than the Gaussian models. Assuming incom- site by a computerized weather station (Wizard III, pressible
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