This paper describes examples of modelling and of measurements of the dispersion and transport of exhaust gases from automobiles on a highway. Model runs were performed by a large-eddy-simulation model. The measurements were carried through by the DLR environmental research aircraft lee-side of the highway between München and Augsburg

Graf, J.

Paffrath, J.

Rösler, F. M.

Schumann, U.

Seiler, W.

Institute of Transport Research:Publications

The Science o[ the Total Environment, 93 (1990) 323-330 323 Elsevier EXPERIMENTAL AND NUMERICAL STUDY OF THE DISPERSION AND TRANSPORT OF AUTOMOBILE EXHAUST GASES FROM HIGHWAYS J. Graf I, D. Paffrath I, F.M. R6sler I, U. Schumann I and W. Seiler 2 IDLR, Institute of Atmospheric Physics, D-8031 Oberpfaffenhofen (F.R. Germany) 2FhG, Institute of Atmospheric Environmental Research, D-8100 Garmisch-Partenkirchen (F.R. Germany) SUMMARY This paper describes examples of modell ing and of measurements of the dispersion and transport of exhaust gases from automobiles on a highway. Model runs were performed by a large-eddy-simula- tion model. The measurements were carried through by the DLR en- vironmental research aircraft lee-side of the highway between M~nchen and Augsburg. INTRODUCTION Due to a strong increase in traffic density in the last 10 years) a considerable contribution of air pollution in the Federal Republic of Germany is produced by automobiles in cities and on roads. These emissions lead to serious problems of nitrogen oxi- des impact and of photochemically produced oxidants. Efforts should be made to answer the question of the distr ibution of the automobile emiss ions of a line source on a local and regional scale. One part of the emissions is distributed within the atmo- spheric boundary layer and transported on a regional scale, an- other part is handed over into the free troposphere, and the third part is deposited on the ground and vegetation. During the day one can assume that the boundary layer is well mixed, whereas during the night a stable boundary layer is pre- dominant. Since traffic density is higher during the day we are particularly interested in air pollution dispersion during the development of the convective boundary layer. For this purpose both numerical simulations and aircraft measurements have been carried out. 0048-9697/90/$03.50 1990 Elsevier Science Publishers B.V. 324 To resolve the turbulent eddies in the convect ive boundary layer a large-eddy-s imulat ion model is used. A short model descr ipt ion and obtained results are presented in the f irst sec- tion. First  results on the d ist r ibut ion of air po l lut ion compo- nents leeside of the highway Munich - Stuttgart  obta ined from airborne measurements  are presented in the second section. Numerical  S imulat ion The d ist r ibut ion of pol lutants in the convect ive boundary layer cannot be s imulated by convent ional  k - theory-models  (Sun, 1986 (ref. 4)). As the covar iance of temperature and concen- t rat ion is an important factor for produc ing the countergradient  f lux in a convect ive layer, models us ing second order c losure technique are essential.  A micro-  and mesoscale  model (MESOSCOP) has been developed (Schumann et al, 1987 (ref.3)) to study the turbulent  transport  in the strat i f ied boundary layer among other things by means of a large-eddy s imulat ion (LES) model. The turbulence parametr iza-  t ion of this LES-vers ion of the model is descr ibed in detai l  by Schmidt  1988 and Schmidt  and Schumann 1989 (refs. 1 - 2). The model makes use of the Monin-Obuchow relat ion for ca lcu lat ing the turbulent  momentum and heat flux at the surface as a funct ion of a prescr ibed surface heat f lux or a sur face- temperature  and roughness- length.  The model covers a domain of typ ica l ly  8 km in the two hor izontal  d i rect ions and 2.4 km in the vert ica l  direc- tion. To resolve the turbulent eddies a gr idspace of 50 to i00 m in all three direct ions is necessary. For this study an addit ional  prognost ic  equat ion for a passive and nonreact ive tracer gas has to be solved besides the basic model  equations. To s imulate the transport  and d i lut ion of a pol lutant  re leased from a cont inuous l ine source at the ground the concentrat ion flux w'c' was set to a constant  source rate Q at the lower boundary. We assumed an inf inite l ine source in the y -d i rect ion  (Fig. 1) and an environmental  background wind U of 4 -I m s t ransverse to the line source. 325 11- ~ / / / / / v  • L,X ~'67 Fig. i. L ine source in the model domain and environmental  wind U. The init ial  value of the concentrat ion c is set equal zero. A per iod ic  boundary condi t ion is appl ied at the lateral  boundary. At the outflow, upper and surface boundary Neumann condit ions are prescribed. At the inf low boundary c is set equal zero. The numerical  s imulated cross-wind integrated concentrat ion is shown in Fig. 2, after  30 min and 60 min. integrat ion time. The plume is carr ied upward by the wel l  organized updraft  of convec- tion. There is a local maximum at the height  of about 1 000 m, near the top of the mixed layer. This result  is in agreement with laboratory exper iment  of Deardorf f  1978 and Wi l l is  and Deardorff  1983 (refs. 5 - 6), where a local maximum was also found in the upper mixed layer. T= 30 rnin. 3000.00 , [m] 6000. 0.00 0.00 x [m] Fig. 2. Cross-wind integrated concentrat ion after  30 min and 60 min integrat ion-t ime. 326 Aircraf t  measurements of automobi le exhaust qases Usual ly  automobi le exhaust gases from roads or highways are measured by fixed or mobi le ground stations and by instrumented towers (e.g. Chock, 1977, Cadle et al. 1977, Rao et al. 1979, Sist la et al. 1979, Rodes and Holland, 1981, Esser, 1982, Bau- mann, K. 1987, Kuhler et al. 1988 (refs. 7 - 14)). However, know- ledge on the three-dimensional  d ispers ion in the atmospher ic  boundary layer from measurements is poor, because the instal la- t ion and operat ion of instrumented towers is expensive and, there- fore, l imited within the close range of the highway (max. 1 km). Thus l i tt le is known about the behaviour  of exhaust gases at height ranges above 150 m and at larger distances. In order to obtain data on pol lut ion d istr ibut ion and meteoro- logical condit ions within extended height and distance ranges from the highway, part icu lar ly  for va l idat ion and ver i f icat ion pur poses of d ispers ion models, measurements should be performed by instrumented aircraft. We made measurements of NO, NOx, SO2, 03, A i tken part ic les and v is ib i l i ty  and of meteorological  parameters l ike air temperature, humidity, and air pressure by means of a Queen Air  Be-65 airplane at the highway between Munich and Augs- burg in Bavaria (Paffrath, D. and Peters, W., 1983 (ref. 15)). An example of the results of concentrat ion and meteorological  parameters measurements along traverses across the highway is shown in Fig. 3. Mean f l ight a l t i tude was 50 m above ground. Crossing points are denoted by arrows (i), (2), (3) and (4). Fl ight paths (i) - (2) and (3) - (4) are downwind, f l ightpath (2) - (3) is upwind of the highway. Maximum values of NO x between 50 and 90 ppb and of NO between 30 and 50 ppb were measured above the crossing points (2), (3), (4). There we also find peak values of Aitken part ic le concentra-  tion. These part ic les are emitted by automobi le engines too. On the other hand ozone concentrat ions are reduced for about 20 ppb below background ozone near these points which is due to the oxi- dat ion of NO to NO 2 thereby reducing the ozone to molecular  oxy- gen. No corre lat ion can be detected between SO 2 and nitrogen oxi- 327 ~ NE-PA Oct. 21. 82 11:O8 local time 50m gnd. 90 18 ~ ... . .  mp.slutct .91--'~" i . . . . . . . . . . .  ,i .......... • ,it',,t 1 ,s[, ,-~ ..... : : : :~  .... | 2 3 ~,t,.c, 0~1 & ,•m'• ~-~ Oct. 21. 82 11:08 local time 50 m grid. i,ot,o~ . -----=, ,0,0 " '~ i \ so ,o ,,-" ',~ ;, ,..., / ""..*',,, ..: t o ~ .  . ^ 1 2 3 ~ a'°t"= A i so'2s ' ' I o ~ " ' s'o 1 2 3 ~,st,.c, ~ml 4 Fig. 3. Example of plots of meteorological parameters and concen- trations of pollutants along a flight path crossing the highway des. SO 2 concentrations are rather low (5 ppb) and nearly con- stant, which demonstrates that the S02-emission by traffic is small although diesel fuel contains considerable sulfur amounts. No significant correlation can be seen between scattering coeffi- cient and nitrogen oxides because the nephelometer responds to large particles and the production of large particles in auto- mobile engines is rather small compared with the background par- ticle content of the air. The flights were performed at different heights 50 m, 110 m, 180 m and 270 m above ground. From the results x a two-dimensional NOx-isoline distribution downwind of the highway was obtained by an interpolation technique (Fig. 4). 328 f l i ght  leve ls  300  - 250- 200 '  180 . . . . . .  150 • 110 . . . . . .  100 .  50 - -  50  " 0 NOx (ppb} . . . . . . . . . . . .  C.Oo . . . .  ' - -  2~ 50 gO . .h lghwoy ~ d is tance  f rom source  ~ m / ~  , i • i , i . i ' D , ~ , ~ . , , i , J 1 2 3 4 5 6 7 8 9 10  km Fig. 4. NO x concentrat ion isol ine plot downwind of the h ighway It can be seen that the exhaust  gases are d ispersed upwards up to about 300 m above ground at a d istance between 1 and 2 km from the source. This is due to low wind ve loc i tes  assoc iated with convect ive processes. At 180 m a second maximum is found at a d istance of about 5 km. From the pos i t ion  of the maximum concentrat ion  wi th  reference to the pos i t ion  of the source and from hor izontal  wind speeds one can est imate mean vert ica l  ve loc i t ies  of NO x dispers ion:  at 50 m gnd: 8 cm/s at ii0 m gnd: 40 cm/s at 180 m gnd: 24 cm/s at 270 m gnd: 50 cm/s The fo l lowing mass f luxes could be est imated from the concen- t rat ions and wind velocit ies:  mean conc. upwind mean conc. downwind mass f luxes ppb ppb g/s /km NO i0 35 i0 NO 13 18 3 NO 2 23 53 13 SO~ 5 5 0 L CONCLUSIONS: 329 It could be shown that the concentration field of automobile exhaust gases downwind of a line source (highway) can be simula- ted quite well by a large-eddy simulation model which has been developed at the DLR Institute of Atmospheric Physics. In addi- tion the concentration field of a highway in Bavaria was de- termined by means of an instrumented aircraft. With the measurements one can validate the simulation models and obtain initial values for these models. This shall be done by combined ground and aircraft measurements of exhaust gas compo- nents, of meteorological and turbulence parameters and of deposi- tion processes during different weather situations up- and down- wind of a selected line source. The aim of a planned project is to quantify the fractions of exhaust gases which undergo deposition on the surface, longway transport in the boundary layer on mesoscales and handover to the free troposphere. REFERENCES 1 H. Sohmidt, Grobstruktur-Simulation konvektiver Grenzschich- ten. DFVLR-FB 88-30 (1988). 2 H. Schmidt and U. Schumann, Coherent structure of the convec- tive boundary layer deduced from large-eddy simulations. Sub- mitted to J. Fluid. Mech. 1988. 3 U. Schumann, T. Haul, H. H611er and H. Schmidt, A mesoscale model for the simulation of turbulence, clouds and flow over mountains: Formulation and validation examples. Contr. Phys. Atmos. 60 (1987) 413-446. 4 W.-Y. Sun, Air pollution in a convective boundary layer. Atmos. Environ. 20 (1986) 1877-1886. 5 J.W. Deardorff, Progress in understanding entrainment at the top of the mixed layer. Workshop on the Planetary Boundary Layer, 14-18 Aug. 1978, Boulder, Color., American Meteorologi- cal Soc., Boston, Mass., ISBN 0-933876-51-3, 36-66. Bound. Layer Met. 7, 81-106. 6 G.E. Wil l is and J.W. Deardorff, On plume rise within a convec- tive boundary layer. Atmosph. Environment 17 (1983) 2935-2447. 7 D.P. Chock, General motors sulfate dispersion experiment - an overview of the wind, temperature, and concentration fields, Atmos. Environ. ii (1977) 553-559. 8 S.H. Cadle, D.P. Chock, P.R. Monson and J.M. Heuss, General motors sulfate dispersion experiment: Experimental procedure and results, Air Pollution Control Association Journal 27, No.l (1977) 33-38. 330 9 S.T. Rao, S., L. Sedefian and L. Czapski, Characteristics of turbulence and dispersion of pollutants near major highways, J. Appl. Meteorol. 18 (1979) 282-293. i0 G. Sistla, P. Samson, M. Keenan and S.T. Rao, A study of pollutant dispersion near highways, Atmos. Environ. 13 (1979) 669-685. ii Ch.E. Rodes and D.M. Holland, Variations of NO, NO^ and 03 concentrations downwind of a Los Angeles freeway, ~tmos. Environ. 15 (1981) 243-250. 12 J. Esser, Schadstoffkonzentrationen im Nahbereich von Autobah- nen in Abh&ngigkeit von Verkehr und Meteorologie, Umwelt 3 (1982) 158-164. 13 K. Baumann, Dispersion of exhaust gases perpendicular to a heavily used motorway, Rep. IVD Institut f~r Verfahrenstechnik und Dampfkesselwesen (1987), uni Stuttgart. 14 M. Kuhler, J. Kraft, W. Koch and H. Windt, Dispersion of car emissions in the vicinity of a highway, In: Environ. Met. ed. by K. Grefen, K. and J. L6bel, Kluwer Academic Publishers (1988). 15 D. Paffrath and W. Peters, Aircraft measurements of various pollution components and of meteorological parameters. VIth World congress on air quality, 16-20 May 1983, Paris. 

Experimental and Numerical Study of the Dispersion and Transport of Automobile Exhaust Gases from Highways

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