Model Calculations of Ozone in the Atmospheric Boundary Layer over Europe

T. Schneider et al. (Editors),Atmospheric Ozone Research and its Policy Znrplicatio~ 0 1989 Elsevier Science Publishers B.V.,Amsterdam -Printed in The Netherlande

MODEL CAL(XILATI0NS

657

OF OZONE IN THE ATUXPHWIC BWNDARY LAYER OVER EUROPE

0. HOV Norwegian Institute for Air Research, P.0.Box 64, N-2001 LillestrQm (Nomay1 ABSTRACT

Ozone episodes in the atmospheric baundary layer over Europe in the sunner are superimposed on a background level which has an early aumner maximum and which increases 1-3% annually. The Norwegian long-range transport model based on trajectory calculations with chemistry, is used to calculate the concentrations of ozone at 14 rural sites in Europe during the ozone episode 28 May-3 June 1982. The effects of changing physical parameters and emissions of nitrogen oxides and volatile organic canpounds, are discussed.

INTRODUCTION There are canprehensive networks of rural ozone measuring instnnnents in many European Countries and in North America. It is well established that the ozone concentrations can be elevated in episodes during the sumner half year in anticyclonic weather situations. On a different time scale, ozone near the ground Over Europe has an annual cycle with a late spring to early sumner maximum and a mid winter minimum. The monthly mean concentration in the spring maximum was about 45 ppb at R6rvi.k south of Gothenburg in Sweden for the 1980-1983 period and about 25 ppb in the winter minimum (ref. l), while the maximum hourly ozone concentration at t h i s site in 1985 was 107 ppb (ref. 2). The annual variation of ozone at Relxvik for clean air oompared to polluted air situations, judged fran the particle counts, showed that in polluted air the spring maxirmrm is higher and is delayed by more than one month, while in clean air the spring maximum canes earlier and is laver than the average for all measurements (ref. 1). This indicates that there is an anthropogenic influence on the ozone concentrations measured at t h i s rural site throughout the year. Historical records of ozone measurements in Europe and North America indicate that in the last part of the nineteenth century the values were anly about half of the mean of surface ozone measurements taken in the same geographical regions during the last 10-15 years (refs. 3, 4 ) , while measurements Over the last decsdes in Europe support a linear increase in ozone by 1-3%/a (refs. 5-7). Ozone episodes in the atmospheric boundary layer are therefore superimposed on a background level which is slowly

658 increasing. Tne change i n the backgraud concentration is probably con( r e f . 8). t r o l l e d by changes i n the emissions of nitrogen oxides (-) Both changes i n the emissions of volatile organic canpounds (Voc) and Nox are important for changes i n episodic ozone, as w i l l be further discussed i n t h i s paper. MODEL DESCRIPTION Model calculations using the Norwegian lagrangian long-range transport model with CM6-X chemistry (ref. 9) for the time period 28 May 1982, 1200 W , to 3 June 1982, 1200 W, were carried out to 14 receptor points within the grid area, see Figure 1. Calculations were carried o u t every 6 h M, i . e . at arrival times oo00, 0600, 1200 and 1800 @TF a t each site.

Fig. 1. Map of PlIEP grid and 96 h back trajectories for 28 May, 1 and 3 June 1982, 1200 W,t0 (1) Illmitz, A u s t r i a , (2) Langenbrlgge, (3) Schauinsland and (4)Deuselbach, a l l FRG, ( 5 ) Ri&, Denmark, ( 6 ) Rl)rvik, Sweden, (7) Langesund and (8) Jelm, Norway, (9) Sappe-r and (10) Waarde, The N e t h e r l a n d s , (11)Colders, France, (12) Bottesford, (13) Sibton and (14) Stoddey, UK.

659 The model has been described in sane detail previously (refs. 10, 11). The pollutants are assumed to be cunpletely vertically mixed throughout the boundary layer which has a variable depth along the 96 h long 850 mb trajectories. No mass transport takes place through the top of the wellmixed layer. Lateral diffusion is not treated explicitly, but the emission data are given in a 150 km grid where finer details than 150 km in the concentration fields are smoothed out. During transport, pollutants are emitted into the air parcel according to the emission maps for NOx and VOC. Instantaneous concentrations are predicted upon arrival of a trajectory. The horizontal resolution of the concentration fields is determined by the choice of emission grid and density of trajectory arrival points. The canbined effects of vertical wind shear and diffusion due to heat exchange is difficult to handle in lagrangian models. Radiosonde observations are used to estimate the mixing height field over Europe at 1200 C X F every day. Objective analysis of temperature, relative humidity and absolute humidity are carried out at OOOO and 1200 in the 150 )an grid, as vertical averages between the surface and the 850 mb level. The temperature is used to evaluate temperature-dependent reaction rate coefficients. The relative humidity is used as a rough indication of cloud cover, which influences the photodissociation rates. Dry deposition velocites for 1 m above the ground were taken for ozone as 0.5 m / s for daytime over land, 0.05 m / s for nighttime over land and 0 over sea, for NO2 0.5 m / s over land, 0 over sea, for HN03 1.0 cm/s, PAN 0 . 2 m/s. To arrive at a model where average boundary layer concentrations are calculated rather than the concentration at 1 m, the deposition velocities at 1 m for 03,NOz and PAN were simply reduced by 50%. Detailed calculations for June 1985 using meteorological data fran the Numerical Weather Prediction Model at The Norwegian Meteorological Institute for surface pressure, surface stress, sensible heat flux density and temperature at 2 m height together with data for the surface roughness length

and Businger’s equations which relate the deposition velocity at the top of the surface layer (50 m height) to the deposition velocity at 1 m above the ground, show that the deposition velocity for SO2 at 50 m typically was 50-75% of the value at 1 m (ref. 12). The initial concentrations assigned at the starting point of the 96 h long trajectories can be important for the developnent along the trajectory. The integration was started with a set of wncentrations corresponding to a slightly polluted atmosphere, with the removal processes in equilibrium with Nihc and VOC emissions at 10% of the average emissions for Western Europe.

660 The

emissions of

Nox

estimated by the PHOXA-project f o r the ozone

episode in late May 1982 for the part of the g r i d RTM-I11

model

area

covered by

the

13) were found to be about the double of the emis-

(ref.

sions estimated in PlIEP f o r the PHOxA-arsa and for that time of t h e year ( r e f . 12), while the VUC-emissions estimated by PHOXA w e r e canparable with the emissions estimated in connection with model work i n (ref. 10). The doubling of Nox emissions estimated by the PHOXA-project f o r t h e l a t e May 1982 episode wmpared to the emissions estimate f o r t h a t time of the year, w a s applied throughout the PZEp grid, whlle t h e voc emissions were kept approximately as in r e f . 10. RESULTS AND DISCUSSION I n the t i m e period

28 May-3

June 1982, there w a s a high pressure

system located Over north Europe w i t h its center Over Denmark on 30 May 1982, moving eastward and w i t h its center Over E a s t Europe on 2 June. The wind speeds were law, and the maximum hourly ozone concentration recorded w a s about 160 ppb, i n the N e t h e r l a n d s on 1 June. I n Figure 1 is shown 4 day back t r a j e c t o r i e s to the 14 receptor sites for 1200 GWT on 28 May, 1 and 3 June 1982, while i n Figure 2 is shown the 1200 GWT mixing height f i e l d f o r 31 May 1982.

MIXING HEIGHT

24 6 8101214161~0eTzli2~~~~~~8

height f i e l d i n metres f o r 1200 m, 31 May (Contours f r a n 204 to 2492 m i n i n t e r v a l s of 458 m. )

Fig. 2.

Mixing

1982.

661 The measurements of ozone are made near the ground surface, usually only one or a few metres above the pund. This means that the measured concentrations usually are significantly reduced at night through ground removal below the noctural inversion and by local emissions of NOx becoming trapped in the shallow ncctural mixed layer. On the other hand, in the model a concentration representative of a layer with height canparable to the M o n mixing height the day before, is calculated at night. This concentration is only weakly influenced by ground removal at

night, and therefore the calculated diurnal variation of O3 is usually smaller than the measured. It should be kept in m i n d that for measured and calculated ozone concentrations, only the day time values when the atmospheric boundary layer is well mixed, are really canparable. In Figure 3 is shown the caparison of calculation and measurement for 4 sites where the agreement was satisfactory. Sane sites (in particular Colaniers and Illmitz) showed a poor agreemant between measured and calculated ozone concentrations. The calculations with the choice of physical and chemical parameters giving the results shcun in F i g . 3, were canpared with the results of calculations where sane of the most important parameters were altered. In Table 1 is given an indication about the changes calculated in the ozone concentrations when the temperature, mixing height, cloud cover, ground deposition velocities or initial conditions were changed. It can be seen that the parameter changes all influenced the ozone ooncentrations significantly. The degree of change canpared to the reference case can be evaluated fran the b o t h 3 lines of Table 1; increasing T by 2% increases the 03-levels in general less than 10 ppb, 50% reduction in mixing height reduces O3 by typically 10 ppb, a clear sky assumption causes O3 to go up nearly 10 ppb, deposition reduced a factor of 2 increases O3 le8S than 10 ppb, reducing the initial concentfations significantly reduces O3 between 10 and 20 ppb (canpared to an initial concentration of 32 ppb in the reference case). Calculations were carried out to see haw the concentrations of O3 at the 14 receptor sites changed during the 28 May-3 Jue 1982 perid with changes in the emissions of NOx and Mc. Uniform emission changes were carried out throughout the grid.

662

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DAY NUMBER [START ING U ITH 28 W.Y 1982I

BOTTESFORD

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2 3 4 5 6 7 DAY NUMBER ISTARTING UlTH 28 HAY 1982)

1

2 3 4 5 . 6 1 DAY NUKBER ISTARTlFtG U l T A 28 I?!Y I98?)

DEUSELBACH

DAY NUHBER ISTART!NG UlTH 28 MAY (982)

Fig. 3. Measured (full line) hourly ozone concentrations and calculated values every 6 h (stars) for 28 May-3 June 1982 for Langesund (south w a s t of Norway), S a(The Netherlands), Bottesford (UK) and Deuselbach (FRG).

G63

TABLE 1 Number of cases (out of 25) with ozone costcentrations calculated to exceed 60 ppb for each of the 14 sites and as a total for the trajectories arriving at the 14 sites e v e q 6 h fnm 28 May 1200 m to 3 June 1982, 1200 C3W (350 altogether), for the refcase, 2% increase i n absolute temperature, 50% reduction in mWng height, clear sky, deposition reduced by 50% and initial precursor axcentrations reduced by 90% (Et,o/lO). See Fig. 1 for explanation of site nmbex-8. Site m

1

Parameter change reference T*l .02 ynix/2 clear sky vg/2 Et=O/lO reference, 0 >50 ppb ference, 0 >70 ppb Aference, O3 >80 ppb

A

8 11 8 14 14 4 17 1 2 4

-0

TABLE 2 Percentage of trajectories with more than 60 ppb 0 at the arrival point for the sites in each of 4 geographical areas and fbr the sum of those sites. Time period 28 May 1200 M - 3 June 1982 1200 m (25 trajectories per site). See Fig. 1 for explanation of site nurnbers. Description A N O x ( % ) A=(%)

0 -25 -50 -62.5 -75 0

-50

-25

0 0 0 0 0 -25 -25 -25

FRG

sites vian sites sites ( 2+3+4) ( 5+6+7+8) 29.3 46.7 49.3 41.3 24.0 9.3 33.3 21.3

22.0 24.0 17.0

34.0 22.0

9.3 16.0 28.0 25.3 17.3 4.0 13.3 8.0

19.0 31.7 36.7 33.0 23.0 4.3 25.3 16.7

reduction in NOx emissions by 25% was calculated to increase O3 in all 4 geographical areas where receptor points were located. Reduction in NOx by 50% led to a further increase in O3 over the -25% case, but the increase w a s slight everywhere except in the UK. A further reduction in NOx emissions to -62.5% and then to -75% is 88en to decrease O3 mwhere. The CaSe w i t h ANDX = 50% end A W C = 0 o c Using the PlEP estimate of NDx and VOC emissions for May/June 1982, and starting A

664 f m m t h i s level of Nox emissions a further reduction in NOx

emissions

reduces O3 everywhere. The Nox emissions used in the reference case are so high that the O3 formation is suppressed (ref. 14). A reduction in VOC emissions by 25% is calculated to reduce O3 efficiently both relative to the reference case and to the case where A N O x = -50%. The decrease is dramatic in the A N o x = 0, A V O C = -25% case, which underlines the effect that very high NOx emissions has on episodic boundary layer ozone by prolonging its formation time. This leads to an increase in the probability that the boundary layer air is mixed into the free troposphere before the precursors are depleted. In the free troposphere the precursors are further diluted and take part in an efficient production of free tropospheric ozone. ACKNOWL-

This work is sponsored by the Cannission of the European Carmunities through two subcontracts with TNO, the Netherlands and by the Royal Norwegian Research Council for Science and Technology ("I?). The work has been carried out in co-operation with EMEP MSC-W at the Norwegian Meteorological Institute. REFERENCES

7 8 9 10 11 12 13 14

P. Grennfelt and J . Schjoldager, Ambio, 13 (1984) 61-67. P. Grennfelt and J . Schjoldager. Oxidant data collection in O E O Europe 1985-87 (OXIDATE). LillestrQm (NILU OR 22/87), 1987. R.D. Bojkw, J . Climate Appl. Meteor., 25 (1986) 343-352. A. Volz and D. Kley. Nature, 332 (1988) 240-242. U. Feister and W. Wannbt, J . A b s . chem, 5 (1987) 1-21. W. AttmaMspacher, R. Hartrnannsgruber and P. Lang, Meteomlo. Rdsch., 37 ( 1984) 193-199. 0. Hov, K.H. Becker, P. Builtjes, R.A. Cox and D. Kley, Air Pollution Research Report 1, CEC, Brussels, 1986. I.S.A. I~aksenand 0. H W , Tell-, 39B (1987) 271-285. G.Z. Whitten, J.P. Killus and R.G. Johnson, Modeling of auto exhaust snmg chamber data for EKMA developnent. SAI, California, 1984. A. Eliassen, 0. Hov, I.S.A. Isaksen, J . Saltbones and F. Stordal. J . Appl. Meteor., 21 (1982) 1645-1661. 0. Hov, F. Stordal and A. Eliassen. Photochemical oxidant control strategies in Europe: A 19 days case study using a Lagrangian model with chemistry. Lilles(NILU TR 5/85), 1985. 0 . Hov, A. Eliassen and D. Simpson, in I.S.A. Isaksen (Editor), -eric Ozone, Reidel, Dordrecht, 1988. P.J.H. Builtjes and E. Luken, Developnent of a strategy against photochemical oxidants, Phase VI Long-range transport. mE0, The Netherlands, 1987. S.C. Liu. M. Trainer. F.C. Fehsenfeld. D.D. Parrish. E.J. Williams. D.W. F&y, G. Htlbler and P.C. Murphy; J . Geophys R e s . , 92D (1987). 4191-4207.