Abstract
Abstract. A fitting method using a semi-empirical Gaussian dispersion model solution was successfully applied to obtain both dispersion coefficients and a particle number emission factor (PNEF) directly from ultrafine particle (UFP; particles smaller than <0.1 μm in diameter) concentration profiles observed downwind of major roadways in California's South Coast Air Basin (SoCAB). The effective Briggs' formulation for the vertical dispersion parameter σz was adopted in this study due to its better performance in describing the observed profiles compared to other formulations examined. The two dispersion coefficients in Briggs' formulation, α and β, ranged from 0.02 to 0.07 and from −0.5 × 10−3 to 2.8 × 10−3, respectively, for the four freeway transects studied and are significantly different for freeways passing over vs. under the street on which measurements of the freeway plume were made. These ranges are wider than literature values for α and β under stable conditions. The dispersion coefficients derived from observations showed strong correlations with both surface meteorology (wind speed/direction, temperature, and air stability) and differences in concentrations between the background and plume peak. The relationships were applied to predict freeway plume transport using a multivariate regression, and produced excellent agreement with observed UFP concentration profiles. The mean PNEF for a mixed vehicle fleet on the four freeways was estimated as 7.5 × 1013 particles km−1 vehicle−1, which is about 15% of the value estimated in 2001 for the I-405 freeway, implying significant reductions in UFP emissions over the past decade in the SoCAB.
Highlights
Ultrafine particles (UFP) are generally defined as particles smaller than 0.1 μm in diameter (Morawska et al, 2008)
We explore the values for α and β derived by fitting Eqs. (2) and (3) to the daily average profiles for the pre-sunrise sampling periods, in order to quantitatively investigate the effects of both meteorology and traffic density on the magnitude of peak concentrations and decay rates of freeway plumes (Table 2)
The fits do not explain slightly elevated UFP concentrations immediately upwind of the freeways. These elevations likely result from a combination of wind variability on a short timescale and eddy diffusion in the direction opposite to the prevailing winds, neither of which is captured in the model
Summary
Ultrafine particles (UFP) are generally defined as particles smaller than 0.1 μm in diameter (Morawska et al, 2008). While the dominant factor causing differences in dispersion/dilution rates between nocturnal (stable) and daytime (well mixed) conditions is clearly atmospheric stability combined with different boundary layer heights (Kerminen et al, 2007; Hu et al, 2009; Hussein et al, 2006; Zhu et al, 2006), quantitative and systematic meteorological dependencies of the decay of primary pollutants with distance downwind of major roads have yet to be developed, for stable atmospheres This gap prevents the reliable prediction of the extent and magnitude of roadway plumes under stable conditions when their greater downwind extent potentially impacts large populations. The objectives are to investigate how routinely measured variables affect UFP plume magnitude, transport, and concentration decay rates, and to evaluate the areal impact of traffic plumes from major roadways For this reason, the effectiveness of the analytical solution for Gaussian dispersion to fit observed UFP concentration profiles is examined, and both dispersion coefficients and emission factors are obtained directly from the observations in this study. Appropriate parameterization of dispersion coefficients and emission factors based on observable variables can provide predictive capability for the extent of freeway plumes under stable conditions
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