Prevalence of wide area impacts downwind of freeways under pre-sunrise stable atmospheric conditions (original) (raw)
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A wide area of air pollutant impact downwind of a freeway during pre-sunrise hours
Atmospheric Environment, 2009
We have observed a wide area of air pollutant impact downwind of a freeway during pre-sunrise hours in both winter and summer seasons. In contrast, previous studies have shown much sharper air pollutant gradients downwind of freeways, with levels above background concentrations extending only 300 m downwind of roadways during the day and up to 500 m at night. In this study, real-time air pollutant concentrations were measured along a 3600 m transect normal to an elevated freeway 1-2 h before sunrise using an electric vehicle mobile platform equipped with fast-response instruments. In winter pre-sunrise hours, the peak ultrafine particle (UFP) concentration (w95 000 cm À3 ) occurred immediately downwind of the freeway. However, downwind UFP concentrations as high as w40 000 cm À3 extended at least 1200 m from the freeway, and did not reach background levels (w15 000 cm À3 ) until a distance of about 2600 m. UFP concentrations were also elevated over background levels up to 600 m upwind of the freeway. Other pollutants, such as NO and particle-bound polycyclic aromatic hydrocarbons, exhibited similar long-distance downwind concentration gradients. In contrast, air pollutant concentrations measured on the same route after sunrise, in the morning and afternoon, exhibited the typical daytime downwind decrease to background levels within w300 m as found in earlier studies. Although pre-sunrise traffic volumes on the freeway were much lower than daytime congestion peaks, downwind UFP concentrations were significantly higher during pre-sunrise hours than during the daytime. UFP and NO concentrations were also strongly correlated with traffic counts on the freeway. We associate these elevated pre-sunrise concentrations over a wide area with a nocturnal surface temperature inversion, low wind speeds, and high relative humidity. Observation of such wide air pollutant impact area downwind of a major roadway prior to sunrise has important exposure assessment implications since it demonstrates extensive roadway impacts on residential areas during pre-sunrise hours, when most people are at home.
Atmospheric Chemistry and Physics Discussions, 2013
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 σ z 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 × 10 13 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.
Atmospheric Environment, 2012
h i g h l i g h t s < UFP size distributions were measured in real-time with a mobile platform. < Wind direction shown critical to ultrafine number and size downwind of freeways. < Particle number decreases up to half as winds deviate from perpendicular. < Particle size also increases, further reducing lung deposition and dose by w15%. a b s t r a c t High ambient ultrafine particle (UFP) concentrations may play an important role in the adverse health effects associated with living near busy roadways. However, UFP size distributions change rapidly as vehicle emissions dilute and age. These size changes can influence UFP lung deposition rates and dose because deposition in the respiratory system is a strong function of particle size. Few studies to date have measured and characterized changes in near-road UFP size distributions in real-time, thus missing transient variations in size distribution due to short-term fluctuations in wind speed, direction, or particle dynamics. In this study we measured important wind direction effects on near-freeway UFP size distributions and gradients using a mobile platform with 5-s time resolution. Compared to more commonly measured perpendicular (downwind) conditions, parallel wind conditions appeared to promote formation of broader and larger size distributions of roughly one-half the particle concentration. Particles during more parallel wind conditions also changed less in size with downwind distance and the fraction of lung-deposited particle number was calculated to be 15% lower than for downwind conditions, giving a combined decrease of about 60%. In addition, a multivariate analysis of several variables found meteorology, particularly wind direction and temperature, to be important in predicting UFP concentrations within 150 m of a freeway (R 2 ¼ 0.46, p ¼ 0.014).
Modeling the impact of roadway emissions in light wind, stable and transition conditions
Transportation Research Part D: Transport and Environment, 2013
This paper examines the processes that govern air pollution dispersion under light wind, stable and transition conditions by using a state-of-the-art dispersion model to interpret measurements from a tracer experiment conducted next to US highway 99 in Sacramento in 1981-82 during the early morning and late evening when winds were light and variable. We examine the roles of stability, wind meander, and boundary layer height on concentrations measured during this study. Our analysis suggests that currently used equations for vertical plume spread need modification when the winds are light. The shallow boundary layer associated with these conditions limits vertical mixing and hence reduces the rate at which concentrations fall off with distance from the road.
Atmospheric Environment, 2009
Ultrafine particles (UFPs, diameter < 100 nm) and co-emitted pollutants from traffic are a potential health threat to nearby populations. During summertime in Raleigh, North Carolina, UFPs were simultaneously measured upwind and downwind of a major roadway using a spatial matrix of five portable industrial hygiene samplers (measuring total counts of 20-1000 nm particles). While the upper sampling range of the portable samplers extends past the defined ''ultrafine'' upper limit (100 nm), the 20-1000 nm number counts had high correlation (Pearson R ¼ 0.7-0.9) with UFPs (10-70 nm) measured by a co-located research-grade analyzer and thus appear to be driven by the ultrafine range. Highest UFP concentrations were observed during weekday morning work commutes, with levels at 20 m downwind from the road nearly fivefold higher than at an upwind station. A strong downwind spatial gradient was observed, linearly approximated over the first 100 m as an 8% drop in UFP counts per 10 m distance. This result agreed well with UFP spatial gradients estimated from past studies (ranging 5-12% drop per 10 m). Linear regression of other vehicle-related air pollutants measured in near real-time (10-min averages) against UFPs yielded moderate to high correlation with benzene (R 2 ¼ 0.76), toluene (R 2 ¼ 0.49), carbon monoxide (R 2 ¼ 0.74), nitric oxide (R 2 ¼ 0.80), and black carbon (R 2 ¼ 0.65). Overall, these results support the notion that near-road levels of UFPs are heavily influenced by traffic emissions and correlate with other vehicle-produced pollutants, including certain air toxics.
2010
Vehicular emissions from arterials may present a risk to public health considering the type of surrounding built environments that can trap pollutants. In order to study the influence of urban morphometry on flow and dispersion of vehicular emissions, field measurements were performed in major arterials in 5 Southern Californian cities with different building geometries. Local mean wind, turbulence, virtual temperature, roadside fine particulate matter (PM2.5) concentration, and traffic flow data were collected in summer 2008. In each city, data were collected for three days, covering two hours during the morning and evening commute and lighter mid-day traffic. First, the observation shows the influence of building geometry on street level concentration of particulates. Tall buildings cause a strong downdraft which upon impinging the street level flushes street canyon from pollutants. Second, field experiments help us understand the influence of local meteorological variables and their interaction with urban canopy to particle concentration. Concentrations at the windward side of buildings within urban canopy are extremely sensitive to wind direction. In addition to wind direction, turbulent flux, sensible heat flux and turbulent velocity are also affecting concentrations by enhancing vertical transport.
Real-Time Prediction of Size-Resolved Ultrafine Particulate Matter on Freeways
Environmental Science & Technology, 2012
Ultrafine particulate matter (UFP; diameter <0.1 μm) concentrations are relatively high on the freeway, and time spent on freeways can contribute a significant fraction of total daily UFP exposure. We model real-time size-resolved UFP concentrations in summer time on-freeway air. Particle concentrations (32 bins, 5.5 to 600 nm) were measured on Minnesota freeways during summer 2006 and 2007 (Johnson, J. P.; Kittelson, D. B.; Watts, W. F. Environ. Sci. Technol. 2009, 43, 5358−5364). Here, we develop and apply two-way stratified multilinear regressions, using an approach analogous to mobilemonitoring land-use regression but using real-time meteorological and traffic data. Our models offer the strongest predictions in the 10−100 nm size range (adj-R 2 : 0.79−0.89, average adj-R 2 : 0.85) and acceptable but weaker predictions in the 130−200 nm range (adj-R 2 : 0.41−0.62, average adj-R 2 : 0.52). The aggregate model for total particle counts performs well (adj-R 2 = 0.77). Bootstrap resampling (n = 1000) indicates that the proposed models are robust to minor perturbations in input data. The proposed models are based on readily available real-time information (traffic and meteorological parameters) and can thus be exploited to offer spatiotemporally resolved prediction of UFPs on freeways within similar geographic and meteorological environments. The approach developed here provides an important step toward modeling population exposure to UFP.
Air pollution near arterial roads: An experimental and modelling study
Atmospheric Chemistry and Physics Discussions, 2017
Aiming to advance in the understanding of pollutant dispersion near arterial roads, we measured, simultaneously, meteorological parameters, emission mass rates and TSP, PM10 and PM2.5 concentrations at several locations downwind two roads, located on a flat region without any other source of pollutants. We also implemented on a state of the art commercial CFD software, an air quality model to simulate the dispersion of solid and gas-phase pollutants emitted from arterial roads. Numerical results of long-term averages and daily measurements of particle concentration showed high correlation with experimental measurements (R2>0.76). We found that the plots of pollutants concentration vs distance to the road edge describe a unique curve when expressed in terms of non-dimensional numbers and that this curve is well described by a beta function. Profiles of vertical concentration sketch an exponential function at the road edge, an S shape downwind and a flat shape far from the road. Particles exhibit a Rosin Rambler size distribution with average diameter of ~ 7 µm. This distribution remains unaltered downwind from the road, which implies that at any location, PM10 and PM2.5 concentrations are a constant fraction of TPS concentration. Experimental data confirmed this observation. Previous results can be used to determine the size of the area impacted by roads, identify mitigating and adaptive countermeasures, and to improve the accuracy of vehicular emission factors.