A 3-D regional photochemical tracer/transport model for Europe and the Eastern Atlantic has been developed based on the NASA/GISS CTM. The model resolution is 4x5 degrees latitude and longitude with 9 layers in the vertical (7 in the troposphere). Advective winds, convection statistics and other meteorological data from the NASA/GISS GCM are used. An extensive gas-phase chemical scheme based on the scheme used in our global 2D model has been incorporated in the 3D model. In this work ozone formation in the troposphere is studied with the 3D model during a 5 day period starting June 30. Extensive local ozone production is found and the relationship between the source regions and the downwind areas are discussed. Variations in local ozone formation as a function of total emission rate, as well as the composition of the emissions (HC/NO(x)) ratio and isoprene emissions) are elucidated. An important vertical transport process in the troposphere is by convective clouds. The 3D model includes an explicit parameterization of this process. It is shown that this process has significant influence on the calculated surface ozone concentrations

Berntsen, Terje

Isaksen, Ivar S. A.

English

NASA Technical Reports Server

N95- 10604
"3©31 / i
OZONE FORMATION DURING AN EPISODE OVER EUROPE :
A 3-D CHEMICAL/TRANSPORT MODEL SIMULATION
Terje Berntsen and Ivar S. A. [sa'ksen
Institute of Geophysics,
University of Oslo,
P.O.Box 1022, Blindern,
N-0315 Oslo, Norway
ABSTRACT
A 3D regional photochemical tracer/transport model for Eu-
rope and the Eastern North Atlantic has been developed based
on the NASA/GISS CTM. The model resolution is 4x5 degrees
latitude and longitude with 9 layers in the vertical (7 in the
troposphere). Advective winds, convection statistics and other
meteorological data from the NASA/GISS GCM are used. An
extensive gas-phase chemical scheme based on the scheme used
in our global 2D model has been incorporated in the 3D model.
In this work ozone formation in the troposphere is studied with
the 3D model during a 5 day periode starting June 30. Exten-
sive local ozone production is found and the relationship be-
tween the source regions and the downwind areas are discussed.
Variations in local ozone formation as a function of total emis-
sion rate, as well as the composition of the emissions (HC/NO_
ratio and isoprene emissions) are elucidated. An important ver-
tical transport process in the tropophere is by convective clouds.
The 3D model includes an exphcit parameterization of this pro-
cess. It is shown that this process has significant influence on
the calculated surface ozone concentrations.
1 Introduction
Ozone formation in the troposphere over Europe and other
industrialized regions leading to ambient concentrations well
above health standards during periodes with high pressure weat-
her situations, is now a well estabhshed fact (Guicherit; 1988).
The increase is observed at urban as well as rural locations.
Measurements (Kley et al.; 88) show that typical ozone values
have increased from 5-15 ppbv at turn of the century, to 30-
40 ppbv at present most probably due to increased man made
emissions of NO_ and hydrocarbons.
Several model studies have been performed to study this ef-
fect. This include box models, 1D and 2D eulerian models. Up
to recently, lack of computer resources has put severe restric-
tions on the chemical scheme as well as the area and resolution
of the models. The approach has been either to do regional stud-
ies with rather comprehensive chemical schemes (Chang et al.;
87), or global studies with simpler schemes (Crutzen and Zim-
mermann ; 91). This paper presents results from a 3D model
with an extensive chemical scheme covering an area from 25°W
to 55°E and 24°N to 76°N. The model is intended to be ex-
tended to a global scale after development and testing on less
computer costly scales.
2 Model description
2.1 Transport
The model was originally developed by Prather et al.(87) to
simulate global distribution and temporal variabihty of CFCs.
As consistent global data sets with real-time meteorological data
from numerical weather prediction models are not available, the
model is set up to use data from the free running NASA/GISS
general circulation model (GCM).
In this study calculations are done for a European "win-
dow" (25°W to 55°E and 24°N to 76°N) with a resolution of
50 longitude and 40 latitude. In the vertical there are 9 layers
.(a-coordinates , cr = (P - PT)/(P, -- PT)) between the surface
and 10 hPa. Table 1 gives the vertical resolution.
a 1.0 .95 .87 .73 .55
P (hPa) 984 934 854 720 550
Z (km) 0.0 0.5 1.2 2.7 4.9
a 0.39 0.25 0.14 0.061 0.0
P (hPa) 390 255 150 70 10
Z (kin) 7.4 10.3 13.7 18.5 31.2
Table 1: Horizontal boundaries of the 9 layers in the model.
P., =98_ hPa is the mean surface pressure in the GCM.
The meteorological data consist of 4 hour integrated values
of horizontal mass fluxes and surface pressure, total optical
depth, surface precipitation and number of convective events
(see Prather et al.;87). Temperature, humidity, precipitation
(from cumulus convection and large scale precipitation) are given
as 5 day averages from the GCM. Tracer transport due to con-
vection is calculated by a mass flux scheme which divide the con-
vection into three classes (dry, moist/shallow and moist/deep).
The advection of tracers are solved numerically by the method
of conservation of second order moments (Prather; 86). A
timestep of one hour is applied in calculation of the advective
transport.
2.2 Chemistry
The model has been expanded to include an extended ozone
chemistry scheme. It includes a gas-phase chemistry with first
order scavenging of water soluble species. The scheme follows
the basic pattern of the scheme developed for use in our global
2D model (Isaksen and Hov; 87), with recent updates of reaction
mechanisms and rates according to IUPAC (89),
62
https://ntrs.nasa.gov/search.jsp?R=19950004192 2020-06-16T11:46:32+00:00Z
Figure 1: Relative distribution of NO emissions. Numbers give
percentage pr. grid square of the total NO-emission which is
9.35 Tg/y as NO.
NASA/JPL (90) and Atkinson (90). The scheme includes 45
components, 16 photolytical reactions and 91 thermal reactions.
The quasi steady state approximation (QSSA) of Hesstvedt et
al. (78) is applied to solve the scheme, with a timestep of one
hour.
Photolysis rates are calculated by the two-stream approxi-
mation method of Isaksen et ah (77) with corrections for dif-
fusive scattering (Jonson and Isaksen ; 91) for a clear sky sit-
uation. Modifications of photolysis rates due to clouds are cal-
culated according to the parameterization given by Chang et
al.(87). To run a 5 day simulation on a DEC-station 5000/200
takes about 45 minutes CPU-time.
2.3 Emissions
Reasonable emission rates of primary pollutants are crucial if
the model shall be able to simulate the real atmosphere. Table
2 gives the emission rates of the primary pollutants (except
isoprene). Emission rates and geographical distribution of NO_
and hydrocarbons are derived on the basis of data from EMEP
(D.Simpson,private com.) and EPA (Watson et at.;91). In the
EMEP data hydrocarbon emissions are given as total man made
VOC, while the EPA data divide the VOC emissions into groups
such as paraffins, olefins, aromats, etc., but on a much courser
grid (100 x 10°). The total VOC emissions were therefore taken
from EMEP, while the spieciation was done according to the
EPA data. However there was considerable differences between
the total emission rates in the two sets of emission data, which
means that this is a very significant source to uncertainties in
the results. Fig. 1 and 2 show the relative distribution of NO
and C2H6 emissions applied in the model. In this study no daily
variation in the emissions are included (except for isoprene), nor
any emissions from sources above the ground (airplanes and
lightning).
Isoprene emissions are parameterized as a function of sunlight,
surface temperature and deciduous forest cover according to
Lubkert and Schopp (1989) and Veldt (1988):
E(kg/h *km 2) = 10 (° _'T'-215)
E is the hourly emission rate of isoprene in kg km -2 h l during
NO NO._ CO C2H6 C4Hlo
9.4 0.44 124.0 10.9 6.5
C6Hx4 C_.H4 C3H6 m-Xylene HCHO
4.3 4.4 1.9 4.5 0.67
Table 2: Total emission rates applied in the model (except for
isoprene). All values are in Tg/y of the emitted species.
"4
o
Figure 2: Relative distribution of C_.H6 emissions. Numbers give
percentage pr. grid square of the total C2H6-emission which is
10.9 Tg/y.
daytime, and T, is the ambient temperature in deg. C.
The data from the NASA/GISS GCM only include 5 day av-
eraged temperatures, so a daily variation in the surface temper-
ature (T,) as a function of the local time (It), with an amplitude
of 10 deg. C has been included
T, = 5.0. SIN(MOD((lt + 24 - 10),24)* 2_r/24.0) + T(GCM)
Fig. 3 shows the calculated relative distribution of isoprene
emissions at 15 GMT. The very pronounced maximum results
from a combination of very dense forest cover and high surface
temperature during the 5 day period. As isoprene emissions are
highly uncertain this maximum can at least serve as sensitivity
test of the impact of biogenic emissions.
2.4 Boundary conditions and initialization
With such a limited.model area, the results will be very sensitive
to the fluxes across the vertical boundaries of the model.
The boundary concentration of the species are obtained from
results from a 2D channel model (Solberg et al.; 89) covering
a zonal band belween 30°N and 60°N, and a 2D zonally aver-
aged global model (Isaksen and tier; 87). The results from the
channel model are distributed latitudinally according to the rel-
ative latitudinal distribution in the 2D global model to obtain
a 3D distribution wich can be used to initiate the model, and
as boundary conditions. To account for the possibility of a pol-
luted air mass being transported out of the model area and back
63
Figure 3: Relative distribution (_ ) at 15 GMT of the surface
fluz of isoprene. Numbers are on logaritmic scale relative to the Figure 4: Calculated surface concentration (ppbv) of ozone at
grid cell with largest f_z (2.9.10 n molec/(cm _"s )) 13 GMT on 5th day of the simulation.
again due to a sudden shift in the wind direction, the boundary
concentrations approach the upwind concentrations during out-
flow and return to the original concentrations during a 12 hour
relaxation period.
3 Results
The model has been run for a 5 day periode starting at midnight
GMT June 30. Fig. 4 shows the calculated surface concentra-
tion of ozone at 13 GMT the 5th day of the simulation. There is
a broad maximum of about 70 ppbv over Northwestern Europe
and the British Isles, and a secondary maximum over Eastern
Europe. Earlier in the day the maximum is shifted towards the
east with maximum values at about the same level (table 3) Fig.
5 shows the calculated ozone concentrations (mean of the two
lowest layers in the model) at 4 different locations during the 5
day periode. In the main source regions ozone concentrations
show a very pronounced diurnal variation with a maximum con-
centration occuring usually before noon. The most important
reason for the night time reduction is surface deposition (_> 50%
of the loss) and reactions with NO. (R1 _ 20 % and R2 _ 25
in the regions with large NO_ emissions).
03 + NO --* O, + NO2 R1
03 "- NO, ---* 02 + NOn R2
The maximum surface concentration is very sensitive to the ven-
tilation of the boundary layer through convective transport. As
convective transport is only calculated every 4th hour, primary
pollutants and their products (ozone, PAN etc.) are allowed
to build up during the intervals between the convective events.
This effect might give rise to regular over and under predictions,
especially in the lowest layer of the model.
The suface ozone concentration and in particular the daily max-
imum is to a large degree determined by the composition of the
emissions. To analyse the sensitivity of the model to variations
in the emission rates and composition, several perturbations
have been performed. Table 3 shows the maximum ozone con-
centration during the 5th day of the simulation at 4 locations.
The locations were chosen to represent typical source regions.
Over Western Europe, represented in fig.5 and table 3 by
the grid cell located at 50°N and 5°E, NOx emissions are large
and the potential for further photochemical ozone production
if NO, emissions increase is low. In some cases it might even
be negative, because reaction R1 followed by R3 consttmes Oa
(and OH) faster than it it is produced by R4-R6. The limiting
factor for ozone production is then the abundance of RO2 rad-
icals ( organic peroxy radicals and HO2) which is produced by
oxidation of hydrocarbons and CO by the OH radical.
NO2+OH + M _ HNOs+ M R3
RO_ + NO _ RO + NO2 R4
NO2 + hv _ NO + O R5
02+0+M _03+ M R6
In a region with a larger fraction of hydrocarbons in the emis-
sions (42°N, 25°E) there is a large potential for further ozone
production if NO, emissions are increased. In this gridceU sur-
face NOx concentration sta5 around 0.5 ppbv during the periode
from 10 am to 8 pm, while at the more polluted area over the
Netherlands the corresponding NO. concentration is around 2.5
ppbv. Table 3 shows that an increase of 50 % in the NO, emis-
sions gives rise to an increase in the calculated surface ozone of
i_%.
Over Eastern Europe (50°N, 25°E) the ratio HC/NO. in the
emissions are higher than in the west. It is therefore a potential
for further ozone production if NOx emissions are increased. A
13 _ increase in daily maximum ozone at the surface is calcu-
lated for a 50 % increase in the NOx emissions.
At the Scandinavian site (62°N, 15°E) there are small diurnal
variations in the ozone concentration after the first day, and the
concentration is virtually insensitive to the various perturba-
tions of the emissions. Analysis show that there is net in situ
loss of ozone all the time during this periode, and a steady-state
situation between the local loss processes and transport is es-
tablished within a few days. However this probably points to
an underestimation of the emissions of precursors in this region
as observations show evidence of significant ozone production
at Scandinavian stations (Grennfeldt et al.;87).
64
03 (ppb) at (°N , °E)
Perturbation 50,5 42,25 50,35 62,15
Reference 65 79 56 38
CH4 conc.= 0 61 78 55 38
Isoprene em. = 0 64 61 55 38
NMHC em. * 2 74 83 66 39
NOx era. * 1.5 65 90 63 38
NO_ era. * 0.5 63 65 56 38
Table 3: Mazimum calculated concentration of ozone (ppbv) in
the lowest level during the 5 th day at _ locations.
50 _ N. 5.0 E, Z- 0-1 2 km
42 @ NI 25@ El Z: 0-_ 2 k,m
50 _ N 250 E Z- 0-1 .2 krn
62 l_ N, 15.0 E Z= 0 ! _2 km e--_-o
8_
£
x ,'; @
7
3.@
i
2g L ,--__ --
24 4 8 72 '4 t,
TIME I_l
12@
Figure 5: Calculated time development of ozone in the two low-
est layers (Z _ 0-1.2 km) at four locations during the 5 day
simulation.
4 Conclusion
We have developed a 3D chemical tracer model based on the
NASA/GISS CTM (Prather et al.; 87) for an extended Euro-
pean region with a comprehensive chemical scheme developed
to study formation of photochemical oxidants. The model has
been run for a 5 day periode in the beginning of July, and the
results are in general agreement with observations of ozone typ-
ical for this time of the year. Several perturbations have been
performed, and the sensitivity of the model is consistent with
other model studies. With some refinements of the model (es-
pecially with respect to convection) and improvement of the
emission database, the model should be well suited to study the
processes governing the formation of photochenfical oxidants on
regional scales.
Acknowledgement
This project was supported by BP-Norway Limited U.A.
Contract No. C-550088-1-AL.
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65


Ozone formation during an episode over Europe: A 3-D chemical/transport model simulation

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