Atmospheric Environment 38 (2004) 1425–1436
ARTICLE IN PRESS
AE International – Europe
*Correspond
E-mail addr
1352-2310/$ - se
doi:10.1016/j.at
A development of ozone abatement strategies for the Grenoblearea using modeling and indicators
O. Couacha, F. Kirchnera,*, R. Jimeneza, I. Balina, S. Peregob, H. van den Bergha
aAir Pollution Laboratory (LPAS), Swiss Federal Institute of Technology (EPFL), CH-1015 Lausanne, Switzerlandb IBM Suisse, Altstetterstrasse 124, 8010 Zurich, Suisse (Switzerland)
Received 28 July 2003; received in revised form 20 November 2003; accepted 1 December 2003
Abstract
The Grenoble metropolitan area, located in the French Alps, regularly has periods of high ozone concentrations
during the summertime. Grenoble is located in a Y shaped convergence of three deep valleys, some 3000m above sea
level, with typical wind pattern. During the summer of 1999, a major field campaign GRENOble PHOTochemistry
(GRENOPHOT) was held in order to obtain measurements necessary for air quality model validation. The air quality
model METeorological PHOtochemistry MODel (METPHOMOD) is used to investigate the dynamic characteristics of
air pollution in the Grenoble area during the GRENOPHOT field campaign. The meteorological and atmospheric
chemistry simulations were validated using both ground and vertical profile measurements (e.g. lidar) performed during
the observation period (25–27 July) when the measured ozone concentrations reached 95 ppb. Both the spatial as well as
the temporal variability of the simulated ozone concentrations were in good agreement with the measured values. The
highest ozone values were found, in the southern zone, some 20 km downwind of the city center. It was found that
about 32 ppb of fresh ozone are generated in the Grenoble plume. For developing ozone abatement strategies it is
important to know whether in a specific area the ozone production is limited by VOC or NOx: Several indicators wereproposed for distinguishing between VOC and NOx limitation. The morning Y ¼ tVOC
HO =tNOx
HO (the ratio of the lifetimes
of OH against losses by reacting with VOC and NOx) was applied and the afternoon indicator was k¼ ½H2O2�=½HNO3�:The Y indicator gives information about the effect of changes in the emissions on the ozone formation in the air parcel
which passes there, whereas k provides information about the sensitivity of ozone formation in the aging plume. A
combination of both indicators can be used to obtain a comprehensive view of the ozone formation in the Grenoble
area. These results are confirmed by the investigation of the air mass regime with the time evolution of the ozone
isopleths at the maximums locations (south and north west valleys).
r 2003 Elsevier Ltd. All rights reserved.
Keywords: Indicators; Ozone production regimes; Mountains area; Grenoble
1. Introduction
In order to reduce the occurrence of ozone peaks in
urban regions, suitable combinations of NOx and VOC
emissions are necessary. The assessment of an urban
ozone strategy can be undertaken by application of a
validated photochemical model. The photochemical
ing author. Tel.: +41-22-693-61-38.
ess: [email protected] (F. Kirchner).
e front matter r 2003 Elsevier Ltd. All rights reserve
mosenv.2003.12.001
airshed model METPHOMOD (Perego, 1999) was
applied and validated over the city of Grenoble, France,
and the surrounding region during an Intensive Ob-
servation Period (IOP) from 25 to 27 July in the context
of the GRENOPHOT field campaign (Couach et al.,
2002; Quaglia et al., 2000). The difference between the
base case and simulations with reductions in NOx and
VOC emissions are presented and analyzed in this study.
Finally, a combination of the indicator Y ¼ tVOCOH =tNOx
OH
(the ratio of the lifetimes of OH against losses due to
d.
ARTICLE IN PRESS
Rural stations
y = 0.668x + 24.947R2 = 0.575
0
20
40
60
80
100
0 20 40 60 80 100
Ozone measured
Ozo
ne s
imul
ated
Suburb stations
y = 0.6594x + 17.478R2 = 0.4237
0
20
40
60
80
100
0 20 40 60 80 100
Ozone measured
Ozo
ne s
imul
ated
urban stations
y = 0.8582x + 1.9022R2 = 0.6398
0
20
40
60
80
100
0 20 40 60 80 100
Ozone measured
Ozo
ne s
imul
ated
Fig. 1. Comparison between ozone measured and simulated for the three classification types of measurements stations.
O. Couach et al. / Atmospheric Environment 38 (2004) 1425–14361426
reactions with VOC and NOx) (Kirchner et al., 2001)
and the afternoon values of the indicator
k¼ cH2O2=cHNO3
(Sillman, 1995) are used to obtain a
comprehensive view of the ozone formation regimes
over the Grenoble area.
2. Modeling period and results
In the horizontal direction, the large simulation
domain is a square with 240 km sides with 40� 40 cells
of 6� 6 km. This computational domain covers the
whole Rh #one-Alpes region including the principal cities.
The higher resolution simulation covers a rectangle of
68� 78 km, centered on the city of Grenoble and
directed north–south with 34� 39 cells of 2� 2 km. In
the vertical direction, we employ a system of Cartesian
coordinates that is identical for the large and the small
fields of 26 levels with increasing spatial interval in the
vertical from 50m up to 8000m above the surface. The
emission inventories are space and time-resolved and
include the emissions of NOx; CO, CH4, and 23 non-
methane hydrocarbon species (NMHC). Biogenic
emissions from forests are included. These emissions
are lumped into 19 classes of VOC as required for
the Regional Atmospheric Chemistry Mechanism
(RACM) (Stockwell et al., 1997). The meteorological
data inputs include near surface wind direction and
speed, temperature, air humidity and the upper
layer pressure. These data, provided and calculated
by the Swiss Institute of Meteorology (Majewski,
1991) with their synoptic scale model, enable us to
initialize the boundary conditions of the nesting
model.
ARTICLE IN PRESS
(A) (B)
(C)
Fig. 2. Difference in the ozone mixing ratios between the base case and the different emission reduction scenarios at 17:00 LT. (A) 50%
NOx reduction emission scenario, (B) 50% VOC reduction emission scenario and (C) 50% (NOx-VOC) reduction emission. The dotted
lines indicate the places where the difference in the ozone mixing ratios is equal to 0.
O. Couach et al. / Atmospheric Environment 38 (2004) 1425–1436 1427
Three-dimensional photochemical model simulations
have been performed for a 3-day high ozone episode
(25–27 July 1999) in the topographically complex area of
Grenoble. The primary focus was the region in the south
of Grenoble where the ozone levels reached the highest
concentrations and in the northwest valley (Gresivau-
dan). The simulation of this high ozone episode yielded
very satisfactory results. Both the spatial as well as the
temporal variability of the simulated ozone concentra-
tions correspond reasonably well to the measured values
(Couach et al., 2002, 2003). Fig. 1 shows the correlations
between the measured and simulated ozone concentra-
tions for specific stations.
The best results are obtained in the rural ground
stations. In the city and in the suburbs, the results
are more variable. These results are normal since
outside of the city the air is less influenced by local
emissions.
3. Reduction scenarios
In order to understand the impact of the city on the
ozone production regime, two simulations were per-
formed with the validated model. The first involves a
reduction in NOx emissions of 50% and the second
comprises a reduction in VOC emissions of 50%. The
difference in the ozone concentrations between the base
case (real) and the two simulations with reduced
emissions are shown in Fig. 2.
ARTICLE IN PRESS
Table 1
Predicted peak ozone mixing ratios (ppb): maximum values
represent the base case and minimum values represent the
minimum from a reduction in NOx emissions of 50% and a
reduction in VOC emission of 50% at six ground stations of
GRENPOPHOT 1999
Stations Max
value
(ppb)
Min
value
(ppb)
Decrease
Absolute (ppb) Relative
St-Barth!el!emy 102 87 16 15.3%
Vif 87 77 10 11.5%
St-Niziler 93 78 16 16.8%
Voreppe 96 83 13 13.1%
Versoud 82 72 10 12.0%
Villeneuve 71 73 �2 �2.3%
Table 2
Predicted peak ozone mixing ratios (ppb): maximum values
represent the base case and minimum values represent a total
reduction of traffic emissions over the Grenoble area
Stations Max
value
(ppb)
Min
value
(ppb)
Decrease
Absolute (ppb) Relative
St-Barth!el!emy 102 88 14 13.7%
Vif 87 76 11 12.4%
St-Niziler 93 81 13 13.8%
Voreppe 96 86 10 10.4%
Versoud 82 75 7 8.5%
Villeneuve 71 71 0 0%
O. Couach et al. / Atmospheric Environment 38 (2004) 1425–14361428
Locally, there are variations in the magnitude of the
ozone reduction and we see that there are two regions
more affected by these reductions. In the south and
northwest regions, a 50% NOx emission reduction in
Grenoble decreases the ozone peak by 14 ppb whereas
the same VOC reduction decreases the ozone peak by
5 ppb (Fig. 2(A)). In addition a 50% NOx emission
reduction results in an increase in the ozone concentra-
tions in the center of the city. In Saint-Barth!el!emy, a
rural ground station south of Grenoble, a 50% NOx
emission reduction in Grenoble decreases the ozone
peak by 15% whereas the same VOC reduction clearly
does not lower the peak value (Fig. 2(B)). Fig. 2(C)
shows that the 50% NOx emission reduction is really
more effective than the same reduction for the VOC in
terms of the ozone peak. The response of the three
dimensional model to traffic emission controls are
shown in Tables 1 and 2. For the GRENOPHOT
ground stations the domain-wide peak ozone concentra-
tions similarly demonstrates a clear sensitivity to NOx
emissions.
4. Indicators to distinguish regimes ozone formation
Indicators have been developed for distinguishing
between NOx saturated and NOx limited ozone forma-
tion (Kirchner et al., 2001; Sillman and He, 1997). They
can be applied to determine the limiting factor of ozone
formation from measurements. The afternoon values of
NOy (Milford et al., 1994), O3/NOz(NOz=NOy–NOx),
and H2O2/HNO3 (Sillman, 1995), and the morning
values of Y ¼ tVOCOH =tNOx
OH (which describes the ratio of
the lifetimes of OH against the losses by reacting with
VOC and NOx) (Kirchner et al., 2001; Palacios et al.,
2002) are proposed as indicators. These indicators have
been tested in many modeling studies for many different
places. The indicator Y which can be measured by a
pump and probe approach (Jeanneret et al., 2001) is
different from the other indicators: it describes the
instantaneous regime of the air parcel whereas the other
indicators act to integrate the past of the particular air
parcel. The two kinds of indicators provide different
information. If one finds high ozone values at a certain
place and is interested in the question whether VOC or
NOx reduction may lower them, one may apply
integrating indicators. If one wants to know at which
location the emissions should effectively be reduced, one
can apply Y . The effectiveness of emission reduction
according to ozone formation depends therefore on the
location where this reduction is made.
In this work we apply both types of indicators and test
them against the simulation results. For the integrating
indicators we choose k=HNO3/H2O2 because this
indicator was found to be the most robust integrating
indicator (Kirchner et al., 2001; Tonnesen and Dennis,
2000a, b). Then, we combine the modeling results with
the results from the indicator studies to obtain a more
comprehensive understanding of the ozone formation in
the Grenoble area.
4.1. The indicator k
The indicator k mainly depends on the two following
reactions:
(1)
2HO2 - H2O2 + O2,(2)
OH + NO2 +[M] - HNO3+[M].The H2O2 formation is favorable under NOx limited
conditions while the HNO3 production is prefered under
NOx saturated conditions. A value of 0.4 for k is
suggested by Sillman (Sillman, 1995). This value
represents the intermediate border between a regime
more sensitive to VOC and one more sensitive to NOx
ARTICLE IN PRESSO. Couach et al. / Atmospheric Environment 38 (2004) 1425–1436 1429
and it is valid for moderately polluted conditions with
80–150 ppb O3 (Sillman and He, 2002).
4.2. The indicator Y
Three-dimensional simulations were performed to test
the behavior of the indicator Y for different times 8:00,
10:00 and 12:00 LT. The indicator was tested by
increasing the NOx emissions in the whole area. Between
8:00 and 9:00 LT the mean value of NOx emissions over
the whole modeling domain was 12mol km�2 h�1. For
test case 1 in the first 2min after 8:00 LT the
same amount of 12mol km�2 NOx emissions was
additionally emitted from each surface grid cell.
Together with the additional NOx in each grid cell
either Tracer A or Tracer B was emitted, depending
on the value of the indicator Y ¼ tVOCOH =tNOx
OH at 8:00 LT
in that grid cell. Tracer A was emitted from those
grid cells where Y was below 0.2, tracer B from the
other grid cells. The emission rate was 12mol km�2 h�1
and therefore identical with the one of the
additional emitted NOx. Both tracers are chemi-
cally inert. During the following hours of the simula-
tion the tracers provide two important pieces of
information:
* The kind of the tracer provides information on the
origin of the air parcel.* The sum of all tracers provides information on the
amount of NOx that has been additionally emitted to
this air parcel.
-10
-5
0
5
10
150 2 4
H2O2
ozo
ne
red
uct
ion
(pp
b)
Fig. 3. Predicted reduction in peak ozone (ppb) at 17:00 LT resultin
VOC (black) and from a 50% reduction in the emission rate for NOx
To account for the amount of tracers in the different
grid cells of the surface layer a value Vtr is defined as
Vtr;x;y ¼ctr;x;yP
tr
Pxi¼1
Pyj¼1ðctr;i;jÞ
� �� �=N
; ð1Þ
where ctr,x,y is the concentration of the tracer tr in the
surface layer grid cell xy, i and j are coordinates in x and
y direction, respectively, and N is the total number of
grid cells in the surface layer of the model. Immediately
after starting the release of the tracer, Vtr in each surface
layer grid cell is 1 for the tracer which is emitted in this
grid cell and 0 for the other tracer. Dilution occurs by
vertical mixing and by transport across the borders of
the modeling area. Because Vtr is a relative value, the
sum of VA and VB is greater than 1 for grid cells where
the air is less diluted than the average and below 1 for
dilution above the average. Grid cells with S(Vtr)o0.5
have not been considered for the evaluation because
their air parcels had been too far from the locations of
NOx emissions at 8 a.m., maybe in high altitudes or
outside the modeling area.
5. Results
5.1. Indicator k
Fig. 3 illustrates the NOx-VOC sensitivity for the
simulations over Grenoble. Following the method of
Sillman and He (1997), the figure shows the change in
ozone concentrations associated with either reduced
6 8 10
/HNO3
NOx controls
VOC controls
g from a 50% reduction in the emission rate for anthropogenic
(gray) plotted against H2O2/HNO3 in simulations for Grenoble.
ARTICLE IN PRESS
noon
0% 20% 40% 60% 80% 100%
-18 to -10
-10 to -3
-3 to +3
+3 to +10
+10 to +18
Tracer B (NOx saturated)
Tracer A (NOx limited)
> 0.2
< 0.2
Number ofgrid cells
Number ofgrid cellsconsidered
Mean tracermixing ratioppb
33
476
589
201
26
33
365
284
201
26
4.03
3.97
4.83
7.65
11.89
noon
0% 20% 40% 60% 80% 100%
-18 to -10
-10 to -3
-3 to +3
+3 to +10
+10 to +18
Tracer B (NOx saturated)
Tracer A (NOx limited)
> 0.2
< 0.2
Number ofgrid cells
Number ofgrid cellsconsidered
Mean tracermixing ratioppb
33
476
589
201
26
33
365
284
201
26
4.03
3.97
4.83
7.65
11.89
Number ofgrid cells
Number ofgrid cellsconsidered
Mean tracermixing ratioppb
33
476
589
201
26
33
365
284
201
26
4.03
3.97
4.83
7.65
11.89
5 p.m.
0% 20% 40% 60% 80% 100%
0 to +3
+3 to +5
+5 to +8
+8 to +15
+15 to +20
Tracer
Number ofgrid cells
Number ofgrid cells considered
Mean tracermixing ratioppb
13
254
533
216
290
13
254
533
156
3
4.58
2.85
1.72
1.25
1.12
5 p.m.
0% 20% 40% 60% 80% 100%
0 to +3
+3 to +5
+5 to +8
+8 to +15
+15 to +20
Tracer
Number ofgrid cells
Number ofgrid cells considered
Mean tracermixing ratioppb
13
254
533
216
290
13
254
533
156
3
4.58
2.85
1.72
1.25
1.12
Number ofgrid cells
Number ofgrid cells considered
Mean tracermixing ratioppb
13
254
533
216
290
13
254
533
156
3
4.58
2.85
1.72
1.25
1.12
3 p.m.
0% 20% 40% 60% 80% 100%
-5 to -2
-2 to 2
+2 to +8
+8 to +15
+15 to +24
Number ofgrid cells
Number ofgrid cells considered
Mean tracermixing ratioppb
12
320
645
348
1
12
320
564
50
1
3.91
2.84
2.03
3.05
6.44
3 p.m.
0% 20% 40% 60% 80% 100%
-5 to -2
-2 to 2
+2 to +8
+8 to +15
+15 to +24
Number ofgrid cells
Number ofgrid cells considered
Mean tracermixing ratioppb
12
320
645
348
1
12
320
564
50
1
3.91
2.84
2.03
3.05
6.44
Number ofgrid cells
Number ofgrid cells considered
Mean tracermixing ratioppb
12
320
645
348
1
12
320
564
50
1
3.91
2.84
2.03
3.05
6.44
∆ ∆ O
3 (pp
b)∆ ∆
O3 (
ppb)
∆ ∆ O
3 (pp
b)
Tracer
Tracer
Fig. 4. Comparison between the tracer (tracer A in black Yo0.2 (NOx limited) and tracer B in white Y>0.2 (NOx saturated)) mixing
ratios and the changes in the ozone formation at noon and 15:00 LT due to additional NOx and tracer emissions emitted at 08:00 LT in
the Grenoble simulation.
O. Couach et al. / Atmospheric Environment 38 (2004) 1425–14361430
ARTICLE IN PRESS
noon
0% 20% 40% 60% 80% 100%
-18 to -14
-14 to -2
-2 to +3
+3 to +10
+10 to +18
< 0.2
Number ofgrid cells
Number ofgrid cellsconsidered
Mean tracermixing ratioppb
110
344
477
391
4
103
258
266
391
4
10.63
11.45
12.94
18.97
18.06
noon
0% 20% 40% 60% 80% 100%
-18 to -14
-14 to -2
-2 to +3
+3 to +10
+10 to +18
Number ofgrid cells
Number ofgrid cellsconsidered
Mean tracermixing ratioppb
110
344
477
391
4
103
258
266
391
4
10.63
11.45
12.94
18.97
18.06
Number ofgrid cells
Number ofgrid cellsconsidered
Mean tracermixing ratioppb
110
344
477
391
4
103
258
26
1
4
10.6
11.45
12.94
18.97
18.06
3 p.m.
0% 20% 40% 60% 80% 100%
-8 to -2
-2 to 2
+2 to +8
+8 to +15
+15 to +24
Tracer
Number ofgrid cells
Number ofgrid cellsconsidered
Mean tracermixing ratioppb
17 17 9.66
3 p.m.
0% 20% 40% 60% 80% 100%
-8 to -2
-2 to 2
+2 to +8
+8 to +15
+15 to +24
Tracer
Number ofgrid cells
Number ofgrid cellsconsidered
Mean tracermixing ratioppb
17 17 9.66
Number ofgrid cells
Number ofgrid cellsconsidered
Mean tracermixing ratioppb
17
403
584
320
2
17
403
501
87
2
9.66
6.89
4.88
7.72
12.48
5 p.m.
0% 20% 40% 60% 80% 100%
-2 to +2
+2 to +4
+4 to +8
+8 to +15
+15 to +24
Tracer
Number ofgrid cells
Number ofgrid cellsconsidered
Mean tracermixing ratioppb
21
194
21
3
7.64
5.99
5 p.m.
0% 20% 40% 60% 80% 100%
-2 to +2
+2 to +4
+4 to +8
+8 to +15
+15 to +24
Tracer
Number ofgrid cells
Number ofgrid cellsconsidered
Mean tracermixing ratioppb
21
194
21
3
7.64
5.99
Number ofgrid cells
Number ofgrid cellsconsidered
Mean tracermixing ratioppb
21
38
58
13
194
21
388
578
32
3
7.64
6.34
4.16
2.68
5.99
∆ ∆ O
3 (pp
b)∆ ∆
O3 (
ppb)
∆ ∆ O
3 (pp
b)
Tracer
Fig. 5. Comparison between the tracer (tracer A in black Yo0.2 (NOx limited) and tracer B in white Y>0.2 (NOx saturated)) mixing
ratios and the changes in the ozone formation at noon, 15:00 and 17:00 LT due to additional NOx and tracer emissions emitted at 10:00
LT in the Grenoble simulation.
O. Couach et al. / Atmospheric Environment 38 (2004) 1425–1436 1431
ARTICLE IN PRESS
3 p.m.
0% 20% 40% 60% 80% 100%
-8 to -2
-2 to +2
+2 to +8
+8 to +15
+15 to +20
Number ofgrid cells
Number ofgrid cellsconsidered
Mean tracermixing ratioppb
23
389
620
260
34
23
389
34
9.59
6.58
4.17
5.26
7.02
3 p.m.
0% 20% 40% 60% 80% 100%
-8 to -2
-2 to +2
+2 to +8
+8 to +15
+15 to +20
Number ofgrid cells
Number ofgrid cellsconsidered
Mean tracermixing ratioppb
23
389
620
260
34
23
389
34
9.59
6.58
4.17
5.26
7.02
Number ofgrid cells
Number ofgrid cellsconsidered
Mean tracermixing ratioppb
23
389
620
260
34
23
389
486
158
34
9.59
6.58
4.17
5.26
7.02
5 p.m.
0% 20% 40% 60% 80% 100%
-2 to +2
+2 to +4
+4 to +8
+8 to +15
+15 to +22
Number ofgrid cells
Number ofgrid cellsconsidered
Mean tracermixing ratioppb
61
383
528
238
116
61
383
508
1
6.91
5.16
3.62
3.29
2.16
5 p.m.
0% 20% 40% 60% 80% 100%
-2 to +2
+2 to +4
+4 to +8
+8 to +15
+15 to +22
Number ofgrid cells
Number ofgrid cellsconsidered
Mean tracermixing ratioppb
61
383
528
238
116
61
383
1
6.91
5.16
3.62
3.29
2.16
Number ofgrid cells
Number ofgrid cellsconsidered
Mean tracermixing ratioppb
61
383
528
238
116
61
383
109
1
6.91
5.16
3.62
3.29
2.16
∆ ∆ O
3 (pp
b)∆ ∆
O3 (
ppb)
Tracer
Tracer
Fig. 6. Comparison between the tracer (tracer A in black Yo0.2 (NOx limited) and tracer B in white Y>0.2 (NOx saturated)) mixing
ratios and the changes in the ozone formation at 15:00 and 17:00 LT due to additional NOx and tracer emissions emitted at noon in the
Grenoble simulation.
O. Couach et al. / Atmospheric Environment 38 (2004) 1425–14361432
VOC or reduced NOx relative to the base case for each
location in the model domain. The positive values
represent locations where, by decreasing the emissions, a
reduction in ozone is obtained while negative values
result from locations where reduced emissions cause
more ozone. According to the results for indicator k the
ozone production is NOx limited at the time of the
maximum ozone. In fact the value of the indicator k is
always >0.4. The points representing ozone increase
due to NOx reduction result from grid cells in the center
of Grenoble and are caused by the lower ozone titration
by NO.
5.2. Indicator Y
At noon the ozone concentrations in test case 1 at the
ground level of the modeling area differ between �18
and +18ppbV from those of the base case result. At
3 p.m. the changes in ozone are in the range between �5
and +24 ppbV (see Fig. 4). The grid cells of the surface
layer were classified by their amount of ozone change.
Only those grid cells with S(Vtr)X0.5 were considered.
The tables on the right side of the Figs. 4–6 show for
each class the number of cells, the number of considered
cells (S(Vtr)X0.5), and the average mixing ratio of
tracers (corresponding to the additional NOx) in the
considered grid cells. It can be seen that those grid cells
with only minor changes exhibit the highest dilution
(highest number of not considered cells) and the smallest
amount of total tracer (or smallest amount of added
NOx).
The shading of the bars in Fig. 4 denotes the
partitioning of the two tracers within each class of cells.
At noon those cells with a decrease in ozone contain
ARTICLE IN PRESSO. Couach et al. / Atmospheric Environment 38 (2004) 1425–1436 1433
high amounts of the tracer B and therefore originate
from grid cells for which the indicator Y indicates NOx
saturation at 8:00 LT. In contrast the air parcels with the
largest ozone increases contain only small amounts of
tracer B but high amounts of A. In the following hours
transport and mixing lowers the contrast between the
different air parcels but nevertheless also at 15:00 LT the
grid cells with the highest ozone increase contain at least
80% of tracer A.
In test case 2 we emitted the additional NOx and the
tracers at 10:00 LT, instead of 8:00 LT. The tracers A
and B were emitted according to the values of Y at 10:00
LT. Everything else was as in test case 1. Fig. 5 shows
Fig. 7. Ozone mixing ratios isopleths [ppb] (grayscale) and Ox [ppb] (l
LT for the maximum location in south of the city.
that at noon the grid cells with more than 3 ppbV
increase of ozone contain nearly 100% A. Grid cells
with ozone decrease contain about 70% tracer B. Also at
15:00 LT ozone decrease is related to high tracer B
values whereas high ozone increase (8 ppb and more) is
related to more than 70% tracer A. Due to mixing the
differences become smaller at 17:00 LT but there is still
the tendency that ozone increase is related to larger
tracer A values.
In test case 3 the time of additional NOx and tracer
emissions was noon LT. The tracers A and B were
emitted according to the values of Y at noon LT. At
15:00 LT ozone increase and decrease are again well
ine) in the Grenoble plume the 27 July at noon, 15:00 and 17:00
ARTICLE IN PRESS
Fig. 8. Ozone mixing ratios isopleths [ppb] (grayscale) and Ox [ppb] (line) in the northwest valley (Gresivaudan) the 27 July at noon,
15:00 and 17:00 LT.
O. Couach et al. / Atmospheric Environment 38 (2004) 1425–14361434
correlated to the tracer concentrations. Due to mixing
the correlation is weaker at 17:00 LT.
6. Discussion
Depending on the chosen indicator one may arrive at
different conclusions concerning ozone abatement stra-
tegies in the Grenoble area. Whereas k tells us that the
ozone production is completely NOx limited, Y tells us
that there are NOx saturated regions in this area. Upon
closer inspection to the results one can see that at the
time of the ozone maximum the ozone formation is
really limited by NOx all over the area. But at noon and
in the early afternoon our studies reveals regions of NOx
saturation.
For investigating this effect in more detail we
calculated additional emission reduction scenarios where
we reduced the NOx and VOC emissions independently
by 0%, 25%, 50%, 75%, and 100% calculating all
possible combinations of these reductions (for example a
reduction of 25% VOC and 100% NOx). Fig. 7 shows
the isopleths for ozone and for Ox(=O3 +NO2) for the
location of maximum ozone in the south of the city of
Grenoble. It shows clearly that the ozone formation at
this place is NOx saturated at noon and at 3 p.m. and
NOx limited at 5 p.m., Ox (a species which excludes
possible ozone titration effects) shows the same
ARTICLE IN PRESS
Fig. 9. Spatial control ozone production regimes calculated with the Y indicator (bold line) over the Grenoble area the 27 July at
08:00, 10:00 and noon LT. Tick line show the highway and thin lines show the topography.
O. Couach et al. / Atmospheric Environment 38 (2004) 1425–1436 1435
dependence. Fig. 8 shows that the situation at the place
of the ozone maximum in the North of the city of
Grenoble is completely different. Here we find NOx
limitation all the time.
Ozone abatement strategies have to consider all these
spatial and temporal changes in the limitation of
maximum ozone values. A combination of the results
of the indicator studies and of the emission reduction
scenarios gives us a comprehensive view of the situation.
The indicator k tells us that the NOx emissions in that
region must be lowered in order to decrease the
maximum ozone concentrations. But from the reduction
scenarios we know that reducing only NOx will lead to
increasing ozone values at some locations at noon and in
the early afternoon. For finding the places where the
NOx reduction must be accompanied by VOC reduc-
tions in order to avoid increasing ozone concentrations
in the early afternoon we can use the results from the
indicatorY. The study shows that the indicatorY works
well and that there is a strong correlation between the Y
values at the place and the time of emission and the
change in the ozone several hours later.
Fig. 9 presents the distribution of NOx sensitive and
NOx saturated areas according to the indicator values of
Y. At the NOx saturated locations the NOx reduction
must be accompanied by VOC reduction. Fig. 9 shows
that the areas with indicator values below or above 0.2
do not considerably change between 8 a.m. and noon.
That means that by this method for each location a
specific regime can be attributed and therefore location
specific emission reductions can be developed.
7. Conclusion
Three-dimensional photochemical model simulations
have been performed for a 3 day episode in the
topographically complex area of Grenoble and base
case was validated. Based on the model scenarios over
Grenoble and on the combination of the indicators Y
ARTICLE IN PRESSO. Couach et al. / Atmospheric Environment 38 (2004) 1425–14361436
and k one can say that the region of the maximum ozone
is NOx limited. Nevertheless this study shows also that
some areas are NOx saturated before the time of the
ozone maximum. Applying the indicator Y allowed the
identification of the locations where NOx reduction
must be accompanied by VOC reduction in order to
avoid increasing ozone concentrations at noon and in
the early afternoon.
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