- Email: [email protected]
Interaction of Planetary Boundary Layer and Free Troposphere
T.Schneideret aL (Editors),Atmospheric Ozone Research and its Policylmplications 0 1989 Elsevier SciencePublishersB.V., Amsterdam
-Printed in The Netherlande
605
INTERACTION OF PLANETARY BOUNDARY LAYER AND FREE TROPOSPHERE
P.J.H. Builtjes, MT-TNO, .Department of Fluid Dynamics, P.O. Box 342, 7300 AH Apeldoorn, the Netherlands
ABSTRACT Results from photochemical episodic dispersion model calculations and historical trends observed in the Los Angelos Air Basin, show that the relation between the precursor VOC- and NOx-emissions leading to hourly 03-concentrations in the atmospheric boundary layer is strongly non-proportional and that substantial emission reductions only lead to relatively small decreases in peak 03- concentrations. However, 03-concentrations during episodes are added upon a background 03-level of about 40-50 ppb which is, at least partly, also from antropogenic origin. Models capable of calculating this 03-background level, and the influence of precursor emissions on these levels, do need a description of the exchange between the free troposphere and the boundary layer, of which only some limited information is available uptill now.
1. INTRODUCTION
Much attention has been devoted in Europe as well as in the United States to the study of photochemical oxidant formation during episodes. These studies are motivated by the adverse effects that high hourly ozone concentrations have on human health. Levels of hourly ozone concentrations of 120 ppb (240 pg/m3), which serve often as an air quality guideline are regularly exceeded in the United States as well as in Europe. Uptill now only limited attention has been paid to more long term average ozone levels. However, there are at least two good reasons to adress long term average ozone levels: long term average ozone levels in the atmospheric boundary layer. for
-
-
example growing season daylight averages, have shown to have adverse effects on vegetation, including forests; episodic photochemical ozone levels are build upon an existing background level, which is closely linked to the long term average ozone level.
Considering the last point, several remarks can be made. Model studies carried out concerning photochemical oxidant formation during episodes clearly
show the non-proportional relationship between VOC- and NO,-precursor
emis-
sions and hourly maximum 03-levels. Recent applications of trajectory models and Eulerian grid models, both for one-day urban type situations as well as for multi-day long range transport situations all reveal similar trends: VOC-emission reduction brings down maximum 03-levels, but less than proportional (quite often calculations indicate for a XX VOC-emission reduction about an 0.5
x 9: reduction of 03-peak levels). NO,-emission
reductions also brings down
maximum 03-levels at locations where NO,-concentrations rural areas relatively far from large NO,-emission
are low, which means in
sources. However, in situa-
tions where NOx-concentrations are high, in and around industrial areas, NO,emission reductions result in an increase of maximum 03-levels (see for recent European calculations for example references 1, 2 , 3, 4 ) . Apart from this rather specific description of calculated effects of NO,and VOC-emission reductions a more general result is that calculations show a remarkable stiff behaviour of episodic 03-concentrations to considerable changes in NO,-and
VOC-emissions. In view of all the discussions devoted to an
evaluation of the historical trends in 03-concentrations in relation to emission trends for VOC and NO, as observed in the Los Angelos Air Basin (ref. 5 ) it can be stated that also reality shows a quite stiff behaviour, which is not in contradiction with model results. Next to this, it should be kept in mind that all these model studies assume a background ozone concentration of about 40-50
ppb which is kept unchanged
when NOx- and VOC-emissions are reduced in the calculations. A recent evaluation by Altshuller (ref. 6 ) pointed out that natural background 03-levels will be in the order of 10-20 ppb. This in in line with the observed increase in 03-concentrations in the (background) free troposphere of about 1% per year (ref. 7) over the last 15 years. Consequently, it is very likely that a substantial part of the background 03-leve1, either at groundlevel at remote places or in the free troposphere is of antropogenic origin. Two-dimensional global model calculations performed by Isaksen (ref. 8 ) do indicate the role of NO,-,
VOC- and CH4- and CO-emissions on the ozone forma-
tion in the free troposphere. Consequently, emission reductions of NO, and VOC will also have an effect on the background ozone level upon which the episodic ozone levels are 'added'. In this way, abatement of background ozone levels will also assist in bringing peak ozone levels down, and will obviously serve in decreasing long term average ozone levels. It is clear that the background ozone levels will be influenced by precursor-emissions over a very large area. However, apart from a first attempt by De Leeuw e.a. (ref. 9 ) , no models have been developed and applied to calculate more long term average background ozone levels in the boundary layer and the influence of precursor emissions.
607 To be able to do this global 2-dimensional models as developed by Isaksen (ref. 8) have to be coupled with boundary layer models. Critical in this is the description of the exchange between the boundary layer and the free troposphere. Some remarks on these exchange processes will be made. First in chapter 2 some observations will be presented. Chapter 3 contains a overview of exchange processes and some descriptions. The paper ends with chapter 4, conclusions and recommandations. 2. OBSERVATIONS OF BACKGROUND 03-LEVELS The atmosphere can be devided in the atmospheric, planetary boundary layer or mixed layer with a height of upto about 2 km, the free troposphere with a height from above the mixed layer upto the tropopause at about 10-15 km and above that the stratosphere. In all three layers ozone is present and photochemical activity occurs. On a yearly averaged basis the 03-concentration increases from groundlevel to reach a maximum at about 1-2 km; further upward a steady decrease is found to about 10 km, after which an 03 increase into the stratosphere is found to levels
reaching 10 ppm
(see
for example ref. 7).
Our interest here are the ozone levels in the background free troposphere and at remote sites far from antropogenic emission sources. The ozone budget in the troposphere (free troposphere + atmospheric boundary layer) consists of four components: transport from the stratosphere, photochemical production, deposition at the ground and photochemical destruction. It is now generally accepted that the photochemical production term is significant and often even dominant (ref. 10, 11).
Some of the background ozone observations will be
described here. At moderate latitudes the seasonal pattern of observed 03-concentrations at remote sites shows a maximum in spring (aprillmay). This maximum has a value of about 40 ppb; the winter minimum is 20 ppb (ref. 11).
As has been stated
earlier, the observations show an increase in this value of about 1-2% per year at the moment. In areas with more industry and traffic the ozone pattern shows a maximum in the summer, a direct consequence of emissions of NOx and VOC from antropogenic origin. Analysis of Dutch ozone-stations for situations where the mean wind velocity was high also showed a seasonal pattern with a maximum in spring of 40-50 ppb (ref. 13, 14). In the situation with high windspeed the mixing is vigorous and the groundlevel 03 values can be considered to be the level of the free troposphere.
608 The question about the origin of the ozone in the free troposphere, where it
has a life time of several weeks is still open. Stratopheric intrusions can play a role, the observed spring 03-maximum is an indication. Tropopause folding at the occurance of a surface cold front associated with a so-called jet-streak can produce strong stratospheric intrusions, see for example Reiter, ref. 15. However, in general these intrusions do not reach ground level but spread out in horizontal direction at a height of 1-2 km. High ozone concentrations observed at ground level are an order of magnitude more often due to photochemical production in the atmospheric boundary layer than to stratospheric intrusions, ref. 16. Obviously the long term average 03 concentration at ground-level will have a stratospheric contribution. It is tempting to compare the rather uncertain estimate of this contribution of 12-15 ppb made by Reiter, ref. 15, with the estimate of the natural ozone level of 10-20 ppb made by Altshuller, ref. 6. However, the contribution of stratospheric intrusions to the ozone in the free troposphere could be larger than this 12-15 ppb and in this way could be of comparable magnitude to the contribution by transport of antropogenic ozone and precursors from the atmospheric boundary layer into the free troposphere. The exact antropogenic part of the ozone levels in the free troposphere is still unknown. 3 . EXCHANGE PROCESSES BETWEEN THE FREE TROPOSPHERE AND THE BOUNDARY LAYER
Most models, Eulerian grid as well as trajectory models which are used to calculate ground level 03-concentrations during episodic conditions have a maximum vertical extent to upto about 2 km, which means that at the upper boundzry the free troposphere starts. Only the 'super'-models which are Eulerian grid models used for the calculation of acidifying pollutants and photochemistry during episodes reach upto about 10 km, see for example ref. 17. These models need this vertical extent to incorporate the convective clouds which play an important role in the formation and transport of acidifying pollutants. Photochemical models in principle do not need this vertical extent explicitly. However, to calculate long term average ozone levels boundary layer models need to be 'coupled' with a model for the free troposphere. Before discussing the exchange processes which have to be described between the boundary layer and free troposphere model some remarks should be made concerning the 'free tropospheric' 2-dimensional model developed by
Isaksen, ref. 8 .
This
zonal
averaged global model has several vertical layers from the ground level upto a height of 17 km. The fluxes at the upper boundary layer were determined by the stratospheric transport into the troposphere as a function of season and latitude. This exchange process as well as the vertical distribution in the model domain itself are determined by using a modification of the mean velocity
609 and diffussion field derived by Plumb and Mahlman (ref. 18) which are a function of the season and month. So in this approach the exchange processes between the boundary layer and the free troposphere are given by the vertical mean velocity and the vertical turbulent diffusivity as prescribed in an averaged way by Plumb and Mahlman, ref. 18. However, to calculate the ground level ozone concentrations for longer time periods using a specific boundary layer model coupled with the for example by the Isaksen-model calculated values at a height of 2 km (the free tropospheric values) the calculations should be performed for the actual meteorological conditions in a brute-force approach (see ref. 9 ) . This requires an explicit description of the exchange processes. The following exchange processes occur between the free troposphere and the atmospheric boundary layer:
-
cumulus convective clouds stratus clouds high and low pressure systems diurnal growth of the mixed layer rain scavenging and wet chemistry frontal systems landlsea breeze and heat island phenomena topographic effects. First, some general remarks should be made. These eight exchange processes
-
can - in a parametrized way most convenient be described by a mean vertical velocity which is either upwind or downwind. Although also real turbulent transport takes place which can be described by gradient type transport the use of a mean vertical velocity is more convenient to avoid 'counter-gradient'-
transport and arbitrary splits between the two descriptions, which are to a large extent splits based on averaging time. In principle, complex weather forecast models as used for example at the European Centre in Reading, United Kingdom. have implicitly incorporated all these exchange processes. However, a calculation with such a forecast model with build-in complex non-lineair chemistry can not be foreseen for the near future, and in addition the grid resolution of those models is quite large which will average out local effects. In recent literature concerning exchange processes most attention is given to cumulus convective clouds, see ref. 19, 20. 21, 22. A cumulus nimbus cloud can reach upto 10 km, with a cloud base at about 0.5-2 km. Vertical upward velocities inside and close to the cloud reach from 1-10 m/s, or even upto 40
m l s . Downward velocities occur in the cloud itself, and also further away,
610 covering a total area around the cloud of about 20 x 20 km. So the cloud causes mixing over a box of about 20 x 20 km upto a height of 10 km, although the effectivity of the mixing process is only about 70% (ref. 2 2 ) . How large the exchange process over an area is depends on the cloud cover. On a long term average basis the exchange depends on the occurence of cumulus clouds. In Europe on a yearly basis this is only 4-8%. In addition cumulus clouds will hardly play a role during photochemical episodes. Stratus clouds are associated with much smaller upward velocities of about 0.05-0.10 m/s. Although these mean vertical velocities are small, stratus
clouds occur frequently in Europe and
so
will play a role in the long term
average exchange process.
-
High and low pressure systems which have a scale of about 1000 x 1000 km are apart from a frontal zone
-
always present. Although the associated vertical
velocity is only about 0.01 m/s, it occurs very frequently. These vertical velocities can easily follow from the divergence and convergence of the horizontal windfield, or simply from the anomaly of the yearly average pressure (which is 1013 Mbar at sea level). The diurnal growth of the mixed layer is of course an important entrainment/ detrainment process which has to be taken into account, but is often already incorporated in the boundary layer models. Rain scavenging and wet chemistry are of less importance for ozone but are essential for a number of other pollutants. Frontal systemslstructures and conveor belts occur in line shaped compact areas and are also associated with vertical upward and downward velocities. Finally, land-sea breeze, heat island phenomena and orography are local effects which can produce vertical velocities. An assessment which of the processes is of more importance can not be given.
This depends heavily on whether episodes or only long term averages are considered, and on the finest horizontal grid resolution desired. It should also be noted that often more than one process will be in operation, for example frontal structures with clouds. Simple parametrizations
of
the
different exchange
processes
are
not
available, but are required in any calculation of ozone in the atmospheric boundary layer where the influence of the free troposphere has to be taken into account. 4 . CONCLUSIONS AND RECOHMANDATIONS
-
Long term average ozone levels should be evaluated because of their adverse
effects on vegetation and their role as background ozone level upon which high episodic ozone levels are 'added'. In view of model results and historic NOxand VOC-emission and ozone trends the effect of emission reductions on peak
611 ozone levels is less than proportional. Abating long term average ozone levels would be an additional way to bring peak ozone levels down.
-
The observed increase of 1-2'6 per year in the background ozone levels in the free troposphere and at remote sites show the influence of antropogenic emissions on these levels. At the moment an accurate quantative value for this contribution to the background ozone levels and the contribution due to stratospheric intrusions can not yet be given.
-
To determine long term average ozone levels in the atmospheric boundary
layer the common type boundary layer models should be coupled with free tropospheric models, for example 2-dimensional global models. A description of the exchange processes between the free troposphere and the boundary layer is required for an adequate coupling, but parametrized descriptions are to a large extent still lacking.
REFERENCES Hov, 0. e.a., Evaluation of the photo-oxidants precursor relationship in Europe, CEC Air Poll. Res. Rep. 1, 1986. Dement, R.G. and Hough, A.M., The impact of emission reduction scenarios on photochemical ozone and other pollutants formed downwind of London, Bamberg Workshop, FRG, October 1987. Builtjes, P.J.H. e.a., PHOXA, the use of a photochemical dispersion model for several episodes in north-western Europe, 16th Int. Tech. Meeting on Air Poll. Modell. and its Appl., Lindau, FRG, April 1987. Selby, K., A modelling study of atmospheric transport and photochemistry in the mixed layer during anticyclonic episodes in Europe, Part 11: Calculations of photo-oxidant levels along air trajectories, J. of Climate and Appl. Met. 26, 1317-1338, October 1987. Kuntasal, G. and Chung, T.Y., Trends and relationships of 03, NOx and HC in the south coast air basin of California, JAPCA 37, 1158-1163, 1987. Altshuller, A.P., Estimation of the natural background of ozone present at surface rural locations, JAPCA 37, 1409-1417, December 1987. Hartmannsgruber, R. e.a., Opposite behaviour of ozone measurements at Hohenpeissenberg 1967-1983. In: Atmospheric Ozone Symp. Halkidiki 1984. Isaksen, I. and Hov, O., Calculation of trends in the tropospheric concentration of 03, OH, CO, CH4 and NO,, Tellus, 1986. Leeuw, F.A.A.M. de, e.a., Long term averaged ozone calculations, Symp. Atm. Ozone Research and its policy implications, Nijmegen, the Netherlands, May 1988. 10 Verkovich, F.M. e.a., The reservoir of ozone in the boundary layer of the
Eastern United States and its potential impact on the global tropospheric ozone budget, J. of Geoph. Res. 90, no. D 3 , pg. 5687-5698, June 1985. 1 1 Logan, J.A., Tropospheric ozone: seasonal behaviour, trends and antropogenic influence, J. of Geoph. Res. 90, pg. 10463, 1985. 12 Bojkov, R.D., Surface ozone during the second half of the nineteenth century, J. of Climate, Appl. Met. 25, 343, 1986. 13 Aalst, R.M. van, Emissions, chemical processes and deposition. In: Dutch Ozone-Symposium, Ede, November 1986 (in Dutch).
612 14 Guicherit, R., Ozone on an urban and regional scale with special reference to the situation in the Netherlands, MT-TNO Rep. no. P 871030, May 1987. 15 Reiter, E.R., Stratospheric-Tropospheric Exchange Processes, Rev. Geoph. and Space Physics 13, 4, 459-474, 1975. 16 Derwent. R.G. editor, Ozone in the United Kingdom, Dep. of Environment Rep., February 1987. 17 Stern, R.M. e.a., Application of a regional model for the transport and 18 19 20
21 22
deposition of acidifying pollutants to Central Europe, 16th Int. Tech. Meeting on Air. Poll. Modell. and its Appl. Lindau, FRG, April 1987. Plumb, R.A. and I.D. Mahlman, The zonally-averaged transport characteristics of the GFDL general circulationltransport model, J. Atm. Scien.. 1986. Ching, J.K.S., Evidence for cloud venting of mixed layer ozone and aerosols, Atm. Env. 22, 2, 225-242, 1988. Isaac, G.A. e.a., The role of cloud dynamics in redistributing pollutants and the implications for scavenging studies, p. 1-13, In: Precipitation Scavenging, Dry Deposition and Resuspension, Pruppacher e.a. editors, Elsevier Science Publish, Co. Inc. 1985. Dickerson, R.R., Thunderstorms: An important mechanism in the transport of air pollutants, Science, vol. 235, 460-464, January 1987. Ching, J.K.S., Modelling non-precipitating cumulus clouds as flow-throughreactor transformer and venting transporter of mixed layer pollutants, Int. Conf. on Energy Transf. and Interactions with small and meso-scale atm. processes, Lausanne, Switzerland, March 1987.
-Printed in The Netherlande
605
INTERACTION OF PLANETARY BOUNDARY LAYER AND FREE TROPOSPHERE
P.J.H. Builtjes, MT-TNO, .Department of Fluid Dynamics, P.O. Box 342, 7300 AH Apeldoorn, the Netherlands
ABSTRACT Results from photochemical episodic dispersion model calculations and historical trends observed in the Los Angelos Air Basin, show that the relation between the precursor VOC- and NOx-emissions leading to hourly 03-concentrations in the atmospheric boundary layer is strongly non-proportional and that substantial emission reductions only lead to relatively small decreases in peak 03- concentrations. However, 03-concentrations during episodes are added upon a background 03-level of about 40-50 ppb which is, at least partly, also from antropogenic origin. Models capable of calculating this 03-background level, and the influence of precursor emissions on these levels, do need a description of the exchange between the free troposphere and the boundary layer, of which only some limited information is available uptill now.
1. INTRODUCTION
Much attention has been devoted in Europe as well as in the United States to the study of photochemical oxidant formation during episodes. These studies are motivated by the adverse effects that high hourly ozone concentrations have on human health. Levels of hourly ozone concentrations of 120 ppb (240 pg/m3), which serve often as an air quality guideline are regularly exceeded in the United States as well as in Europe. Uptill now only limited attention has been paid to more long term average ozone levels. However, there are at least two good reasons to adress long term average ozone levels: long term average ozone levels in the atmospheric boundary layer. for
-
-
example growing season daylight averages, have shown to have adverse effects on vegetation, including forests; episodic photochemical ozone levels are build upon an existing background level, which is closely linked to the long term average ozone level.
Considering the last point, several remarks can be made. Model studies carried out concerning photochemical oxidant formation during episodes clearly
show the non-proportional relationship between VOC- and NO,-precursor
emis-
sions and hourly maximum 03-levels. Recent applications of trajectory models and Eulerian grid models, both for one-day urban type situations as well as for multi-day long range transport situations all reveal similar trends: VOC-emission reduction brings down maximum 03-levels, but less than proportional (quite often calculations indicate for a XX VOC-emission reduction about an 0.5
x 9: reduction of 03-peak levels). NO,-emission
reductions also brings down
maximum 03-levels at locations where NO,-concentrations rural areas relatively far from large NO,-emission
are low, which means in
sources. However, in situa-
tions where NOx-concentrations are high, in and around industrial areas, NO,emission reductions result in an increase of maximum 03-levels (see for recent European calculations for example references 1, 2 , 3, 4 ) . Apart from this rather specific description of calculated effects of NO,and VOC-emission reductions a more general result is that calculations show a remarkable stiff behaviour of episodic 03-concentrations to considerable changes in NO,-and
VOC-emissions. In view of all the discussions devoted to an
evaluation of the historical trends in 03-concentrations in relation to emission trends for VOC and NO, as observed in the Los Angelos Air Basin (ref. 5 ) it can be stated that also reality shows a quite stiff behaviour, which is not in contradiction with model results. Next to this, it should be kept in mind that all these model studies assume a background ozone concentration of about 40-50
ppb which is kept unchanged
when NOx- and VOC-emissions are reduced in the calculations. A recent evaluation by Altshuller (ref. 6 ) pointed out that natural background 03-levels will be in the order of 10-20 ppb. This in in line with the observed increase in 03-concentrations in the (background) free troposphere of about 1% per year (ref. 7) over the last 15 years. Consequently, it is very likely that a substantial part of the background 03-leve1, either at groundlevel at remote places or in the free troposphere is of antropogenic origin. Two-dimensional global model calculations performed by Isaksen (ref. 8 ) do indicate the role of NO,-,
VOC- and CH4- and CO-emissions on the ozone forma-
tion in the free troposphere. Consequently, emission reductions of NO, and VOC will also have an effect on the background ozone level upon which the episodic ozone levels are 'added'. In this way, abatement of background ozone levels will also assist in bringing peak ozone levels down, and will obviously serve in decreasing long term average ozone levels. It is clear that the background ozone levels will be influenced by precursor-emissions over a very large area. However, apart from a first attempt by De Leeuw e.a. (ref. 9 ) , no models have been developed and applied to calculate more long term average background ozone levels in the boundary layer and the influence of precursor emissions.
607 To be able to do this global 2-dimensional models as developed by Isaksen (ref. 8) have to be coupled with boundary layer models. Critical in this is the description of the exchange between the boundary layer and the free troposphere. Some remarks on these exchange processes will be made. First in chapter 2 some observations will be presented. Chapter 3 contains a overview of exchange processes and some descriptions. The paper ends with chapter 4, conclusions and recommandations. 2. OBSERVATIONS OF BACKGROUND 03-LEVELS The atmosphere can be devided in the atmospheric, planetary boundary layer or mixed layer with a height of upto about 2 km, the free troposphere with a height from above the mixed layer upto the tropopause at about 10-15 km and above that the stratosphere. In all three layers ozone is present and photochemical activity occurs. On a yearly averaged basis the 03-concentration increases from groundlevel to reach a maximum at about 1-2 km; further upward a steady decrease is found to about 10 km, after which an 03 increase into the stratosphere is found to levels
reaching 10 ppm
(see
for example ref. 7).
Our interest here are the ozone levels in the background free troposphere and at remote sites far from antropogenic emission sources. The ozone budget in the troposphere (free troposphere + atmospheric boundary layer) consists of four components: transport from the stratosphere, photochemical production, deposition at the ground and photochemical destruction. It is now generally accepted that the photochemical production term is significant and often even dominant (ref. 10, 11).
Some of the background ozone observations will be
described here. At moderate latitudes the seasonal pattern of observed 03-concentrations at remote sites shows a maximum in spring (aprillmay). This maximum has a value of about 40 ppb; the winter minimum is 20 ppb (ref. 11).
As has been stated
earlier, the observations show an increase in this value of about 1-2% per year at the moment. In areas with more industry and traffic the ozone pattern shows a maximum in the summer, a direct consequence of emissions of NOx and VOC from antropogenic origin. Analysis of Dutch ozone-stations for situations where the mean wind velocity was high also showed a seasonal pattern with a maximum in spring of 40-50 ppb (ref. 13, 14). In the situation with high windspeed the mixing is vigorous and the groundlevel 03 values can be considered to be the level of the free troposphere.
608 The question about the origin of the ozone in the free troposphere, where it
has a life time of several weeks is still open. Stratopheric intrusions can play a role, the observed spring 03-maximum is an indication. Tropopause folding at the occurance of a surface cold front associated with a so-called jet-streak can produce strong stratospheric intrusions, see for example Reiter, ref. 15. However, in general these intrusions do not reach ground level but spread out in horizontal direction at a height of 1-2 km. High ozone concentrations observed at ground level are an order of magnitude more often due to photochemical production in the atmospheric boundary layer than to stratospheric intrusions, ref. 16. Obviously the long term average 03 concentration at ground-level will have a stratospheric contribution. It is tempting to compare the rather uncertain estimate of this contribution of 12-15 ppb made by Reiter, ref. 15, with the estimate of the natural ozone level of 10-20 ppb made by Altshuller, ref. 6. However, the contribution of stratospheric intrusions to the ozone in the free troposphere could be larger than this 12-15 ppb and in this way could be of comparable magnitude to the contribution by transport of antropogenic ozone and precursors from the atmospheric boundary layer into the free troposphere. The exact antropogenic part of the ozone levels in the free troposphere is still unknown. 3 . EXCHANGE PROCESSES BETWEEN THE FREE TROPOSPHERE AND THE BOUNDARY LAYER
Most models, Eulerian grid as well as trajectory models which are used to calculate ground level 03-concentrations during episodic conditions have a maximum vertical extent to upto about 2 km, which means that at the upper boundzry the free troposphere starts. Only the 'super'-models which are Eulerian grid models used for the calculation of acidifying pollutants and photochemistry during episodes reach upto about 10 km, see for example ref. 17. These models need this vertical extent to incorporate the convective clouds which play an important role in the formation and transport of acidifying pollutants. Photochemical models in principle do not need this vertical extent explicitly. However, to calculate long term average ozone levels boundary layer models need to be 'coupled' with a model for the free troposphere. Before discussing the exchange processes which have to be described between the boundary layer and free troposphere model some remarks should be made concerning the 'free tropospheric' 2-dimensional model developed by
Isaksen, ref. 8 .
This
zonal
averaged global model has several vertical layers from the ground level upto a height of 17 km. The fluxes at the upper boundary layer were determined by the stratospheric transport into the troposphere as a function of season and latitude. This exchange process as well as the vertical distribution in the model domain itself are determined by using a modification of the mean velocity
609 and diffussion field derived by Plumb and Mahlman (ref. 18) which are a function of the season and month. So in this approach the exchange processes between the boundary layer and the free troposphere are given by the vertical mean velocity and the vertical turbulent diffusivity as prescribed in an averaged way by Plumb and Mahlman, ref. 18. However, to calculate the ground level ozone concentrations for longer time periods using a specific boundary layer model coupled with the for example by the Isaksen-model calculated values at a height of 2 km (the free tropospheric values) the calculations should be performed for the actual meteorological conditions in a brute-force approach (see ref. 9 ) . This requires an explicit description of the exchange processes. The following exchange processes occur between the free troposphere and the atmospheric boundary layer:
-
cumulus convective clouds stratus clouds high and low pressure systems diurnal growth of the mixed layer rain scavenging and wet chemistry frontal systems landlsea breeze and heat island phenomena topographic effects. First, some general remarks should be made. These eight exchange processes
-
can - in a parametrized way most convenient be described by a mean vertical velocity which is either upwind or downwind. Although also real turbulent transport takes place which can be described by gradient type transport the use of a mean vertical velocity is more convenient to avoid 'counter-gradient'-
transport and arbitrary splits between the two descriptions, which are to a large extent splits based on averaging time. In principle, complex weather forecast models as used for example at the European Centre in Reading, United Kingdom. have implicitly incorporated all these exchange processes. However, a calculation with such a forecast model with build-in complex non-lineair chemistry can not be foreseen for the near future, and in addition the grid resolution of those models is quite large which will average out local effects. In recent literature concerning exchange processes most attention is given to cumulus convective clouds, see ref. 19, 20. 21, 22. A cumulus nimbus cloud can reach upto 10 km, with a cloud base at about 0.5-2 km. Vertical upward velocities inside and close to the cloud reach from 1-10 m/s, or even upto 40
m l s . Downward velocities occur in the cloud itself, and also further away,
610 covering a total area around the cloud of about 20 x 20 km. So the cloud causes mixing over a box of about 20 x 20 km upto a height of 10 km, although the effectivity of the mixing process is only about 70% (ref. 2 2 ) . How large the exchange process over an area is depends on the cloud cover. On a long term average basis the exchange depends on the occurence of cumulus clouds. In Europe on a yearly basis this is only 4-8%. In addition cumulus clouds will hardly play a role during photochemical episodes. Stratus clouds are associated with much smaller upward velocities of about 0.05-0.10 m/s. Although these mean vertical velocities are small, stratus
clouds occur frequently in Europe and
so
will play a role in the long term
average exchange process.
-
High and low pressure systems which have a scale of about 1000 x 1000 km are apart from a frontal zone
-
always present. Although the associated vertical
velocity is only about 0.01 m/s, it occurs very frequently. These vertical velocities can easily follow from the divergence and convergence of the horizontal windfield, or simply from the anomaly of the yearly average pressure (which is 1013 Mbar at sea level). The diurnal growth of the mixed layer is of course an important entrainment/ detrainment process which has to be taken into account, but is often already incorporated in the boundary layer models. Rain scavenging and wet chemistry are of less importance for ozone but are essential for a number of other pollutants. Frontal systemslstructures and conveor belts occur in line shaped compact areas and are also associated with vertical upward and downward velocities. Finally, land-sea breeze, heat island phenomena and orography are local effects which can produce vertical velocities. An assessment which of the processes is of more importance can not be given.
This depends heavily on whether episodes or only long term averages are considered, and on the finest horizontal grid resolution desired. It should also be noted that often more than one process will be in operation, for example frontal structures with clouds. Simple parametrizations
of
the
different exchange
processes
are
not
available, but are required in any calculation of ozone in the atmospheric boundary layer where the influence of the free troposphere has to be taken into account. 4 . CONCLUSIONS AND RECOHMANDATIONS
-
Long term average ozone levels should be evaluated because of their adverse
effects on vegetation and their role as background ozone level upon which high episodic ozone levels are 'added'. In view of model results and historic NOxand VOC-emission and ozone trends the effect of emission reductions on peak
611 ozone levels is less than proportional. Abating long term average ozone levels would be an additional way to bring peak ozone levels down.
-
The observed increase of 1-2'6 per year in the background ozone levels in the free troposphere and at remote sites show the influence of antropogenic emissions on these levels. At the moment an accurate quantative value for this contribution to the background ozone levels and the contribution due to stratospheric intrusions can not yet be given.
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To determine long term average ozone levels in the atmospheric boundary
layer the common type boundary layer models should be coupled with free tropospheric models, for example 2-dimensional global models. A description of the exchange processes between the free troposphere and the boundary layer is required for an adequate coupling, but parametrized descriptions are to a large extent still lacking.
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