An assessment of the sea/valley breeze
and its impact on ozone behaviour
R. Cocci Grifoni, G. Passerini & S. Tascini
Dipartimento di Energetica, Università Politecnica delle Marche, Italy
Abstract
Ozone (O3) is the secondary pollutant formed in the atmosphere through
photochemical reactions involving nitrogen oxides and volatile organic
compounds in the presence of sunlight. In addition to the chemical and emission
variables, a variety of meteorological variables also influence ozone
concentrations. Understanding the meteorological parameters (namely
temperature, vertical temperature structure, surface winds, aloft winds, local flow
patterns, etc) that influence ozone levels is important for selecting variables to
help ozone prediction.
The selected study area is a typical coastal area located in the middle of Italy.
Topography in this area is fairly complex and the proximity to the coast implies
a significant interaction between the sea and valley breezes. The climate in this
area, in fact, is classified under sub-coastal where there is an all year round sea
breeze.
The sea/land breezes play a significant role in the distribution of ozone and
transport of ozone from the coast to the sea and valley areas. The sea breeze is a
weak system, extending vertically to a height of less than 1 km with the low
wind speed. The land breeze can transport the photochemically produced ozone
and its precursors over the sea. The accumulated ozone on the sea can return to
the land in the daytime with the sea breeze. This kind of transport tends to
contribute significantly to high-ozone episodes in coastal areas.
For this purpose RAMS simulations have been implemented by assimilating
data from provincial monitoring networks and from the European Center for
Medium-range Weather Forecast elaborations. Four regular nested grids
represent the simulation domain: the first grid is an 18x18 mesh with a 50000m
side width; the finest grid is a 37x37; the four nesting ratios are respectively
1,5,5,5. Simulations show that the sea/valley breezes in complex coastal areas
play a significant role in the distribution and transport of ozone.
Coastal Environment V, incorporating Oil Spill Studies, C. A. Brebbia, J. M. Saval Perez
& L. Garcia Andion (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-710-8
258 Coastal Environment V, incorporating Oil Spill Studies
1
Introduction
During summer 2000 the Ancona province monitoring network recorded a
severe ozone episode in Falconara Marittima, a coastal town on the Adriatic Sea.
The event was so strong that drastic measures were taken in order to reduce
the pollutant concentration, including a forced reduction of road traffic and a
forced block of plants and productive activities.
The selected study area is a typical coastal area located in the middle of Italy.
Topography in this area is fairly complex and the proximity to the coast implies
a significant interaction between the sea and valley breezes. The sea-breeze/landbreeze circuit is a meso-scale circulation of air caused by the differential heating
and cooling of the land-and sea-surfaces in the coastal zone. The climate in this
area, in fact, is classified under sub-coastal where there is an all year round sea
breeze. During the summer, the sea breezes combine with upslope winds to
create re-circulations along the coast with permanence time of the order of days.
This work analyzes air pollution dynamics and associated meteorological
processes in this area. Our aim is to show the meteorological aspect and
complementary modelling results (using the Regional Atmospheric Modelling
System, RAMS) to help in the interpretation of observed ozone cycles [1].
2
The study case
The Falconara Marittima municipality area, although located on a shoreline,
involve an area of about 25 Km2 presenting a rather complex orography. It
extents on an almost flat land on the north and on a hilly area southward. The
Esino River along with a very important national route (SS16) and a highway
(A14) passes through this territory; moreover a big refinery and an airport are all
located in this area.
The region is subjected to the presence of breezes that are dominant during
the warmest period of the year. Due to the moderate steepness of the Esino
valley, valley breezes are very slight although they have a synergic effect with
nocturnal land breezes (as well as the mountain breezes for the sea breezes).
Air quality monitoring is carried on through the integrated monitoring
network of the Province Authority and a mobile lab is available in the event of
severe episodes. At present, there are three monitoring stations within the
Falconara municipality area, identified as:
“Falconara Scuola” – located nearby the refinery and the sea
“Falconara Acquedotto”- close to the refinery but at a higher altitude
“Falconara Alta” – located in the historical town centre, several
kilometres from the refinery and the see
All three monitoring station, along with air quality values, collect meteorological
data.
In fig.1 ozone trends from 'Falconara Scuola' and 'Falconara Acquedotto'
monitoring stations are shown. It is easily noticeable how high are the peaks
Coastal Environment V, incorporating Oil Spill Studies, C. A. Brebbia, J. M. Saval Perez
& L. Garcia Andion (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-710-8
Coastal Environment V, incorporating Oil Spill Studies
259
reached by ozone during that summer (200 ug/m3 is the law limit for hourly
averaged observation). The solar activity was high (see fig.2) but not exceptional
for that area.
Scuola
O3 - Falconara
August 2000
Acquedotto
400
350
300
ug/m3
250
200
150
100
50
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
09
08
07
06
05
04
03
02
01
0
da y
Figure 1: Ozone trend in Falconara during summer 2000.
Solar Radiation
1400
1200
w/m2
1000
800
600
400
200
0
01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
day
Figure 2: Solar radiation trend in Falconara during summer 2000.
3
RAMS modelling for August 2000
In order to model local scale meteorology the RAMS model has been used,
running on a parallel machine to shorten calculation time.
The whole period has been fragmented in 36 or 48 hours simulation. Each
simulation was carried on starting from results of previous one and introducing
further ground observations related to the period.
Coastal Environment V, incorporating Oil Spill Studies, C. A. Brebbia, J. M. Saval Perez
& L. Garcia Andion (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-710-8
260 Coastal Environment V, incorporating Oil Spill Studies
3.1 Simulation set-up
The simulation involved basic RAMS microphysics in order to evaluate clouds
formation. Four nested square grids have been necessary in order to reach the
desired resolution: 400 m horizontally. The computational domain had the
following characteristics:
•
number of nodes in X direction per grid: 18, 17, 17, 17, 37
•
number of nodes in Y direction per grid: 18, 17, 17, 17, 37
•
number of nodes in Z direction per grid: 30, 30, 30, 30, 30
•
number of soil layers: 11
•
coarsest grid specification:
adopted projection: polar stereographic
cell width in X direction: 50000m
•
cell width in Y direction: 50000m
spatial nesting ratio for each grid level in both (X and Y) directions:
1:5:5:5
height of first cell layer: 50m
•
vertical stretching ratio: 1.15
•
max vertical height: 1500m
•
time nesting ratio for each grid level: 1:5:5:5
•
Upper air data: from European Centre for Medium-range Weather
forecast (ECMWF) with 0.5 degree horizontal resolution
•
Surface data: from Ancona province authority monitoring network
•
Orography data: from the regional cartography service with 100 m
horizontal resolution
•
Land use and sea surface temperature: from United States Geological
Survey (USGS) with 1 km horizontal resolution.
4
Simulations result
We expected to find slight breezes in the early morning and in the late evening
and their total absence during the night when earth and sea temperatures are
closer. All simulation confirmed the theoretical considerations.
The first consideration that must be done is about the conditions that foster
ozone formation. Literature clearly states as they are influenced by three of
mayor atmospheric quantities: pressure, insolation (also indirectly as surface
temperature) and horizontal wind velocity [2], [3].
Coastal Environment V, incorporating Oil Spill Studies, C. A. Brebbia, J. M. Saval Perez
& L. Garcia Andion (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-710-8
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4.1 High pressure
Observations lead to consider that every ozone episode occurs in conditions of
high atmospheric stability that implies synoptic high pressure. Looking at the
Mediterranean basin, this situation is verified typically along the whole summer
season that is characterised by the presence of an anti-cyclonic system involving
the whole Mediterranean Sea. On the other hand, strong reduction of ozone
dynamics has been recorded at the first stage of development of Lows or during
the transition between two different Highs within the same season [4].
The simulations confirmed the persistence of a high-pressure system all over
the area during August 2000.
4.2 Surface temperature and solar radiation
It is well known from the literature the strong correlation between the daily
maxima of surface temperature and the ozone episodes; on the contrary there is
no specific correlation with the daily mean value. In fact, the mayor forcing is
the irradiation and a correct statistical parameters could be the duration of
radiation period or the surface radiation daily maximum. Unfortunately all these
data were unavailable.
In compliance with season values, solar radiation was constantly high al the
month.
Figure 3:
Extended convective cell of a sea breeze.
Coastal Environment V, incorporating Oil Spill Studies, C. A. Brebbia, J. M. Saval Perez
& L. Garcia Andion (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-710-8
262 Coastal Environment V, incorporating Oil Spill Studies
4.3 Horizontal wind field
Ozone episodes are almost always recognisable in conditions of slight horizontal
wind fields that, normally, reduce the pollutant dispersion. Furthermore, a scarce
contribution of synoptic circulation allows local meteorological regimes (e.g.
breezes) to develop.
Figure 4 shows the stable sea breeze developed on August 12th and it well
represents the general trend all over the month.
Moreover, simulations showed how the breezes system was persistent and
well formed performing a complete rotation. In the morning, breezes become
intense around 9.00 (local time, LT) or 10.00 LT and come from East but almost
parallel to the coastline. At later hours they will tend to rotate clockwise being
ever stronger until 14.00 LT, when they are perpendicular to the coast. As the
night approaches, breezes become slighter in a direction opposite to that of the
morning while they completely cease around 21.00 LT. During the 12 sunny
hours, we have a rotation of 180° that will be completed by further 180° during
the nocturnal land breeze phenomena. A complete rotation of 360 has been found
out in a lapse of 24 hours in all critical days. Fig.2 shows breeze directions at
different hours.
The persistence of breezes (only few days are characterised by absent or
unstable breezes) was a first confirmation to our hypotheses about ozone
episodes in August 2000 event.
Several authors, such as Liu et al. [5], [6] confirmed the generally accepted
conclusion that breezes, as well as other PBL-related phenomena, lead always to
a worsening of air quality conditions [7], [8]. In fact, these phenomena can
trigger recirculation or stagnation of pollutants due to whirling motions of smallscale motions thermally induced by temperature gradients, or mechanically
induced by terrain roughness and natural obstacles. Examples of situation
potentially dangerous for pollutant accumulation are katabatic winds along
mountain versants or whirls formed alee an obstacle (e.g. a ridge).
Figure 4:
Breeze directions in Falconara Marittima.
Coastal Environment V, incorporating Oil Spill Studies, C. A. Brebbia, J. M. Saval Perez
& L. Garcia Andion (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-710-8
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263
Figure 5: Relative humidity and temperature profiles a few kilometres offshore
at 15:00 am, 08/12/2000.
Figure 3 shows a vertical profile of humidity for day 12 at 6:00 am in
correspondence of Falconara and some kilometres off the coastline. It can be
immediately noticed as relative humidity is higher near the sea surface while
simulation showed that upper profiles remain substantially unvaried during the
night.
Figure 6: Relative humidity and temperature profiles a few kilometres offshore
at 6:00 am, 08/12/2000.
Observed wind vectors at locations in the coastal area obviously show a
diurnal variation with changeover between the land breeze and the sea breeze.
During the day, the photochemical processes are very active and result in very
Coastal Environment V, incorporating Oil Spill Studies, C. A. Brebbia, J. M. Saval Perez
& L. Garcia Andion (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-710-8
264 Coastal Environment V, incorporating Oil Spill Studies
high ozone concentrations. The NOx concentrations decrease and reach a low
level at noon due to the photochemical loss and the increase of mixing layer. At
night and in the early morning, the land breeze can transport the precursors and
photochemical produced ozone over the sea. In the daytime the accumulated
ozone over the sea can return to the land with the sea breeze and can contribute
significantly to high-ozone episodes in clean coastal areas.
Moreover, light land breeze developed during night-times (generally weaker
than the diurnal breeze) moves ozone [9],[10] over marine surface where high
nocturnal humidity inhibits dry deposition. The observations also show a similar
trend. The dangerous re-circulation phenomena persisted for 2/3 days and several
times dramatically affecting, according to our hypotheses, ozone concentration
values.
Figure 7:
Convective cell of sea breeze compressed by inland turbulent
boundary layer. Terrain colour scale indicates surface temperature.
The acute episode of August 12th has an explanation in the establishment of a
breeze front as can be seen in Fig.5. In that situation the breeze convective cell is
compressed by the air of the inland mixed layer more heated than the coastal
one. This scenario is particularly dangerous since a strong convergence motion is
produced and coupled to re-circulation dynamics.
5
Discussion and conclusions
We carried out several high-resolution simulations in the Falconara area and all
confirmed the hypotheses we tried to explore. In fact, we found out the
persistence of a breeze regime often able to trigger advection phenomena. In
these conditions, ordinary ozone depletion dynamics may be affected by the
“bouncing” of air masses between land and sea.
Coastal Environment V, incorporating Oil Spill Studies, C. A. Brebbia, J. M. Saval Perez
& L. Garcia Andion (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-710-8
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It is also very important to consider the dispersion effect that breeze-driven
“air bouncing" may have on ozone-precursors concentrations ratio in certain
hours.
We missed upper air sounding in order to have a complete and clearer idea of
boundary layer dynamics, first of all mixed layer height and its evolution. These
data would be very advantageous for this kind of study
Understanding breeze cycle in our study area is an inalienable starting point
for any meteorological study. However, the most significant result comes from
the comparison between the shown simulations with those related to some
particular periods. In fact, we could find out two days highly characterized by
weak insolation (probably cloudy days) and by no breeze development. In
correspondence to these two periods we could clearly recognize a consequent
strong ozone depletion. Another interesting aspect is related to experimental
evidence: an about 12-hours-delay between the breeze ceasing and the ozone
abatement.
These last observations and consideration strongly encouraged us to sustain
the hypothesis of considering breezes as mayor forcing during coastal ozone
episodes.
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& L. Garcia Andion (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-710-8
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Coastal Environment V, incorporating Oil Spill Studies, C. A. Brebbia, J. M. Saval Perez
& L. Garcia Andion (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-710-8