Atmospheric Environment (v.39, #31)

BAB II: a project to evaluate the accuracy of real-world traffic emissions for a motorway by U. Corsmeier; M. Kohler; B. Vogel; H. Vogel; F. Fiedler (5627-5641).
To ensure the efficiency of strategies to reduce air pollutants caused by road traffic, it is essential to evaluate the emission data to be used in emission calculation models. This basic approach of the BAB II project (BundesAutoBahn––federal highway) has been reached by simultaneous measurements of gaseous and particulate emissions on the windward and lee side perpendicular to a motorway. The differences between the measurements allows for the calculation of the emissions caused by traffic on the motorway. Measurements of CO, NO, NO x , CO2, O3, VOC, and particulates of high horizontal and vertical resolution were made. The experimental setup was made symmetric to the motorway, with 52 m high towers on each side. The field phase took place from 1 to 25 May 2001 at the motorway A656 near Heidelberg, Germany. In cases of wind direction perpendicular to the motorway, the height of the plume caused by traffic emissions is detectable and the emissions released on the motorway can be calculated. With traffic census carried out simultaneously, the emission data can be estimated using emission factors given in the literature. Comparison between real-world traffic emissions and calculated emissions allows for an evaluation of the emissions calculated by the models.This paper gives an overview of BAB II, its measuring concept, the experimental setup, and the quality assurance and control program. It is shown in detail that the method of emission estimation by measurements of concentration differences between both sides of a motorway works quite well if the meteorological assumptions for determining real-world traffic emissions have been fulfilled. Detailed results in terms of measured and model-calculated emissions of gaseous and particulate species will be reported in a number of subsequent papers.
Keywords: Real-world traffic emissions; Emission calculation model; Traffic census; Evaluation strategy; Particulate matter;

Quality assurance of air pollutant measurements during the BAB II field experiment by U. Vogt; A. Dreiseidler; G. Baumbach; R. Kurtenbach; M. Petrea; M. Kohler; U. Corsmeier (5642-5664).
The main objective of the field project BAB II (Bundesautobahn—Federal motorway) was to calculate the emissions of a motorway from ambient air measurements upwind and downwind of the lanes. The project was accompanied by a series of quality assurance measures, because all results have to be comparable to each other. Consequently, NO/NO x , CO, and VOC analysers participated in an inter-calibration with calibration gases. Subsequent results of ambient air measurements with all analysers for all measured components—NO/NO x , CO, O3, VOCs, elemental carbon (EC), and particulate matter (mass and number concentration)—were compared to each other.The quality assurance program had two main objectives: On the one hand, the data were harmonised after the detection of systematic deviations between the readings of the different analysers by applying individual correction equations or correction factors. On the other hand, the harmonised data were used for the determination of measurement uncertainty.In general, the measurement uncertainties determined were about 3–9%, except for VOCs and the particle number concentration. For all components, the upwind–downwind concentration differences in the actual experiment were much higher than the measurement uncertainties determined, thus bending confidence to the concentration differences being due to the traffic emissions rather than to measurement uncertainties.
Keywords: Quality assurance; Data harmonisation; Measurement uncertainty; NO/NO x ; CO; VOC; Elemental carbon; Particulate matter;

Estimation of gaseous real-world traffic emissions downstream a motorway by M. Kohler; U. Corsmeier; U. Vogt; B. Vogel (5665-5684).
The consequences of air pollution scenarios caused by road traffic or the impact of exhaust gas reduction techniques are estimated by emission models. To ensure the quality of model results, it is necessary to evaluate the used emission factors under real-world conditions. Therefore, the Institut für Meteorologie und Klimaforschung (IMK) of the Forschungszentrum Karlsruhe initiated the field campaign BAB II (BundesAutoBahn, Federal motorway). The campaign was conducted in May 2001 with the objective of measuring the traffic emissions at a motorway section and to compare them to modelled emissions. Based on experiences during a precursor campaign (BAB I, 1997), a symmetric experimental set-up was installed which allowed measurements up- and downwind of a motorway nearby Heidelberg, Germany. This paper focuses on the determination of source intensities and emission factors for CO and NO x , whereas other papers in this issue handle VOC and particulate matter. First the basic approach of BAB II measurements up- and downwind of a motorway was approved, showing that it is possible to detect the plume originating from traffic emissions. A case study during a traffic jam illustrates that driving patterns have a strong impact on the emissions and therefore a detailed traffic census is required to obtain reliable emission calculations. Two different strategies were used: (i) long-time measurements during the whole campaign to obtain vertical profiles each 30 min and (ii) measurements during eight special operation periods (SOP) in a higher temporal resolution of 5 min, using instrumentation in elevators. It could be shown that even at a distance of 60–80 m from the motorway the structure of the plume is still inhomogeneous, and concentration changes within short times. The inhomogeneity of the plume not only affects the temporal scale, the spatial scale is also influenced and frequently concentration maxima in higher altitudes are observed. Mean source intensities of 9.5 kg km−1  h−1 CO and 4.4 kg km−1  h−1 NO x on working days have been calculated. The ratio of CO/NO x was found to be 2.1 on working days and 4.5 on weekends and holidays. Emission factors of 2.62 g km−1  veh−1 CO and 1.08 g km−1  veh−1 NO x have been determined, which agrees well with results derived from similar studies.
Keywords: Source intensity; CO; NO x ; Traffic emissions; Emission calculation; Emission model; Plume structure;

NMHC measurements of motorway emissions during the BAB II field campaign by Monica Petrea; Ralf Kurtenbach; Peter Wiesen; Ulrich Vogt; Günter Baumbach (5685-5695).
Non-methane hydrocarbons (NMHC) were measured vertically resolved up-wind and down-wind of the motorway BAB A656 Heidelberg–Mannheim, Germany, for 4 weeks in April and May 2001. The horizontal distribution of C2–C9 aliphatic and aromatic hydrocarbons was monitored by using two compact quasi on-line gas chromatography (GC) instruments. The vertical distribution of NMHC was determined by using adsorption tubes, which were analysed off-line by GC in the laboratory. More than 50 NMHC were detected and 26 of them were quantified. From the measured data, the emission source strength for a variety of NMHC were calculated and discussed.
Keywords: Road traffic emission; Non-methane hydrocarbons (NMHC); Emission source strength; Emission model; Volatile organic compounds (VOC); Gas chromatography (GC); Flux measurements;

Particulate size distributions and mass measured at a motorway during the BAB II campaign by E. Rosenbohm; R. Vogt; V. Scheer; O.J. Nielsen; A. Dreiseidler; G. Baumbach; D. Imhof; U. Baltensperger; J. Fuchs; W. Jaeschke (5696-5709).
From 1 May to 25 May 2001, the BAB II campaign was carried out at the motorway BAB (656) near Heidelberg. Atmospheric concentrations of particulate matter and gases were measured together with the meteorological conditions. This paper is focused on the particulate matter measured upwind and downwind from the motorway at ground level. In order to determine the source contribution from the motorway traffic, it was necessary to measure upwind and downwind simultaneously due to variations in background concentrations. The particle number contribution from the motorway was found to be 35,000 particles cm−3 for particles with diameters close to 20 nm and 5000 particles cm−3 for particles with diameters close to 70 nm. Bimodal size distributions were observed on the downwind side, whereas the upwind side showed unimodal size distributions. For particulate mass, it can be estimated that the contribution from the motorway to the PM1 concentrations is in a range 0.6–1.3 μg m−3 for the chosen measurement sites approximately 60 m from the road at a height of 6 m. The soot measurements showed diurnal variation; however, the upwind downwind difference was not measured. Correlation factors showed good correlation between total particle number and number of particles with diameters below 80 nm, CO and NO. There was no correlation between particle number and PM10, which is due to the observation that particle number was dominated by the 20 nm particles.
Keywords: Particle number concentration; SMPS; Motorway; PM10 and PM1; Soot;

Vertical distribution of aerosol particles and NO x close to a motorway by D. Imhof; E. Weingartner; U. Vogt; A. Dreiseidler; E. Rosenbohm; V. Scheer; R. Vogt; O.J. Nielsen; R. Kurtenbach; U. Corsmeier; M. Kohler; U. Baltensperger (5710-5721).
In May 2001, the large-scale field project BAB II was performed at the highly frequented motorway BAB (Bundesautobahn) A656 with two traffic lanes in each direction between the German cities Heidelberg and Mannheim. Extensive measurements of air pollutants were carried out on both sides of the motorway. In a distance of 60 m (north side) and 84 m (south side) to the traffic lanes, two 52-m-high towers were installed, at which electrically powered elevators were fixed. In these elevators, two NO x analysers, an Electrical Low Pressure Impactor (ELPI; measurement of the particle number size distribution in the diameter range D between 30 nm and 10 μm) and a Diffusion Charger (DC; measuring the particle surface area concentration), were operated to record continuous vertical profiles from 5 to 50 m above the earth's surface. On the upwind side, particle number and surface area concentration as well as NO x values were constant over the entire height profile. On the downwind side, increased concentrations appeared in the near-ground range: in the forenoon, a monotonous decrease in pollutant concentrations with increasing height was found, while around noon the concentration maximum of the particles was slightly shifted to 10 m above ground. This height dependence was found for two different size ranges, i.e., for particles with D < 300 nm (consisting of soot particles and nucleation mode particles formed by condensation as a result of cooling of the exhaust gas after emission), and for coarse particles ( D > 1 μ m , abrasion and resuspension products). In the size range between 300 and 700 nm, no height dependence was found, corroborating the fact that motor traffic emits only few particles in this size range. On the downwind side of the motorway, only background concentrations were measured above 25 m. The results of the profile measurements were confirmed by stationary measurements of particle size distributions with Scanning Mobility Particle Sizers (SMPS) at various heights. A good correlation between particle surface area and NO x concentration was observed. Vehicle emission factors were determined for the particle surface area, number and volume of several size ranges.
Keywords: Vertical profile; Particle number size distribution; Surface area; Traffic emissions; Emission factor; LDV/HDV;

Traffic measurements and high-performance modelling of motorway emission rates by Jörg Kühlwein; Rainer Friedrich (5722-5736).
Within the framework of an extensive emission data validation experiment, the emission rates of the German motorway A 656 near Heidelberg have been determined using a new sophisticated emission model in combination with local traffic measurements. For this purpose, traffic parameters that are essential for the vehicles’ emission behaviour have been quantified by a measuring program that has been adapted to the special needs of a microscopic emission model. Traffic flows were recorded by automatic and manual counts. Fleet compositions were analysed by registration number evaluations. Speeds and accelerations were measured at different characteristic times and different road cross-sections. The road gradient was determined from precise altitude data.All experimental traffic data were used as input of a high-performance emission model and replaced traffic data from general databases step by step. The inclusion of all traffic and road data measured resulted in hourly daytime emission rates of 6000–11000 g km−1  h−1 for CO, 2500–4500 g km−1  h−1 for NO x , 350–600 g km−1  h−1 for NMHC and 150–280 g km−1  h−1 for PM. Uncertainty analyses show that the experimentally derived traffic data significantly reduce the errors of the modelled emission rates. The detection of traffic congestion situations by suitable monitoring methods is very important to the modelling of high-quality emission data for individual road sections.
Keywords: Traffic emissions; Traffic data; Emission model; Uncertainties; Validation;

Vehicle-induced turbulence near a motorway by N. Kalthoff; D. Bäumer; U. Corsmeier; M. Kohler; B. Vogel (5737-5749).
In spring 2001, turbulence measurements were carried out adjacent to the motorway from Mannheim to Heidelberg in order to determine the intensity and range of vehicle-induced turbulence compared to the natural atmospheric turbulence.It was found out that the location of the motorway on a 1 m high dam leads to mean vertical winds in the vicinity of the dam (distance of ∼3 m). The windward/lee difference of the mean vertical velocities, w windw lee, normalized to velocity perpendicular to the motorway, u, amounts to about (w windw lee)/u≈0.1. Due to the different surface properties, kinematic heat flux at 4.75 m height in the lee vicinity of the motorway (distance of ∼3 m) is higher than on the windward side by up to 100%. However, comparison of the resulting thermally induced production with mechanical production of turbulent kinetic energy (TKE) shows that the higher kinematic heat flux is not responsible for the lee increase of standard deviation of the vertical wind speed ( σ w ) or TKE. The significant increase in the turbulence parameter σ w and TKE in the lee is restricted to an area close to the ground less than 50 m away from the motorway. Near the motorway (distance of ∼3 m), lee values exceed the windward values by about 70% for typical TKE values of e≈1 m2  s−2 and by about 50% for typical σ w values of σw ≈0.5 m s−1. Analysis of the energy spectrum reveals that this additional energy occurs in the frequency range starting from f>0.1 Hz. The lee/windward difference of the TKE values exhibits a distinct functional relationship with commercial cars ( R = 0.77 ) and a moderate relationship with passenger cars ( R = 0.58 ) such that the increase of TKE can be attributed to vehicle traffic.
Keywords: Vehicle-induced turbulence; Motorway-induced turbulence; Turbulent kinetic energy; Turbulence spectra;

A parameterisation of motorway-induced turbulence has been developed and has been integrated in a diffusion model to simulate the transport and the chemical reactions of air pollutants in the vicinity of a motorway. Turbulence and therefore diffusion is influenced by the motorway in two different ways, namely traffic-induced turbulence and production of turbulence due to the flow over the dam on which the motorway is located. Both effects are parameterised. By comparing measured vertical profiles of CO of the BAB II project with simulated vertical profiles of CO the parameterisation is tested. Using the new parameterisation, the plume height and the maximum concentration near the surface are modelled correctly. With the extended model concentration distributions of reactive species in the vicinity of the motorway are simulated. The strong influence of the motorway-induced turbulence on the concentrations of nitrogen oxides is shown. With increasing the distance to the motorway, concentration of NO decreases much faster than the NO2 concentration. When the motorway-induced turbulence is taken into account the plume height increases up to 200% at a distance of 60 m on the leeside of the motorway. At the same distance motorway-induced turbulence reduces the concentration of CO by 50–70%, the concentration of NO by about 50%, and the concentration of NO2 by about 40%, respectively. The percentage changes depend on the atmospheric stability. Since the motorway is also a dominant source of coarse mode particles produced by abrasion processes of tyres, brake linings and street surface and by resuspension, concentration distributions of coarse mode particles are simulated. The maximum range of particles decreases with increasing particle diameter and is clearly increased by the motorway-induced turbulence.
Keywords: Traffic-induced turbulence; Highway diffusion modelling; Vehicle wake; Chemical reactions; Particle resuspension;

Comparison of measured and model-calculated real-world traffic emissions by U. Corsmeier; D. Imhof; M. Kohler; J. Kühlwein; R. Kurtenbach; M. Petrea; E. Rosenbohm; B. Vogel; U. Vogt (5760-5775).
The quality of an emission calculation model based on emission factors measured on roller test stands and statistical traffic data was evaluated using source strengths and emission factors calculated from real-world exhaust gas concentration differences measured upwind and downwind of a motorway in southwest Germany. Gaseous and particulate emissions were taken into account. Detailed traffic census data were taken during the measurements. The results were compared with findings of similar studies.The main conclusion is the underestimation of CO and NO x source strengths by the model. On the average, it amounts to 23% in case of CO and 17% for NO x . The latter underestimation results from an undervaluation by 22% of NO x emission factors of heavy-duty vehicles (HDVs). There are significant differences between source strengths on working days and weekends because of the different traffic split between light-duty vehicles (LDVs) and HDVs. The mean emission factors of all vehicles from measurements are 1.08 g km−1  veh−1 for NO x and 2.62 g km−1  veh−1 for CO. The model calculations give 0.92 g km−1  veh−1 for NO x and 2.14 g km−1  veh−1 for CO.The source strengths of 21 non-methane hydrocarbon (NMHC) compounds quantified are underestimated by the model. The ratio between the measured and model-calculated emissions ranges from 1.3 to 2.1 for BTX and up to 21 for 16 other NMHCs. The reason for the differences is the insufficient knowledge of NMHC emissions of road traffic.Particulate matter emissions are dominated by ultra-fine particles in the 10–40 nm range. As far as aerosols larger than 29 nm are concerned, 1.80×1014  particles km−1  veh−1 are determined for all vehicles, 1.22×1014  particles km−1  veh−1 and an aerosol volume of 0.03 cm3  km−1  veh−1 are measured for LDVs, and for HDVs 7.79×1014  particles km−1  veh−1 and 0.41 cm3  km−1  veh−1 are calculated. Traffic-induced turbulence has been identified to have a decisive influence on exhaust gas dispersion near the source.
Keywords: Real-world emissions; Emission model; Traffic census; Emission measurements; Gaseous emissions; Particulate matter emissions;