We analyzed 2005–2017 data sets on ozone (O3)
concentrations in an area (the Vic Plain) frequently affected by the
atmospheric plume northward transport of the Barcelona metropolitan area
(BMA), the atmospheric basin of Spain recording the highest number of
exceedances of the hourly O3 information threshold
(180 µg m-3). We aimed at evaluating the potential benefits
of implementing local-BMA short-term measures to abate emissions of
precursors. To this end, we analyzed in detail spatial and time variations of
concentration of O3 and nitrogen oxides (NO and NO2,
including OMI remote sensing data for the latter). Subsequently, a
sensitivity analysis is done with the air quality (AQ) data to evaluate
potential O3 reductions in the north of the BMA on Sundays compared
with weekdays as a consequence of the reduction in regional emissions of
precursors.
The results showed a generalized decreasing trend for regional background
O3 as well as the well-known increase in urban O3 and higher
urban NO decreasing slopes compared with those of NO2. The most
intensive O3 episodes in the Vic Plain are caused by (i) a relatively
high regional background O3 (due to a mix of continental,
hemispheric–tropospheric and stratospheric contributions); by (ii) intensive
surface fumigation from mid-troposphere high O3 upper layers arising
from the concatenation of the vertical recirculation of air masses; but also
by (iii) an important O3 contribution from the northward
transport/channeling of the pollution plume from the BMA. The high relevance
of the local-daily O3 contribution during the most intense pollution
episodes is clearly supported by the O3 (surface concentration) and
NO2 (OMI data) data analysis.
A maximum decrease potential (by applying short-term measures to abate
emissions of O3 precursors) of 49 µg O3 m-3
(32 %) of the average diurnal concentrations was determined. Structurally
implemented measures, instead of episodically, could result in important
additional O3 decreases because not only the local O3 coming
from the BMA plume would be reduced, but also the recirculated O3 and
thus the intensity of O3 fumigation in the plain. Therefore, it is
highly probable that both structural and episodic measures to abate
NOx and volatile organic compound (VOC) emissions in the BMA
would result in evident reductions of O3 in the Vic Plain.
Introduction
Tropospheric ozone (O3) is a secondary atmospheric pollutant produced
by the photooxidation of volatile organic compounds (VOCs) in the presence of
nitrogen oxides (NOx=NO+NO2). Its generation is
enhanced under high temperature and solar radiation (Monks et al., 2015, and
references therein). Thus, O3 maxima occur generally in the
afternoon, with the highest levels typically registered in summer, when
exceedances of regulatory thresholds are most frequent.
O3 is one of the key air pollutants affecting human health and the
environment (WHO, 2006, 2013a, b; GBD, 2016; Fowler et al., 2009; IPCC,
2013). According to EEA (2018), in the period 2013–2015, more than 95 %
of the urban population in the EU-28 was exposed to O3 levels
exceeding the WHO guidelines set for the protection of the human health
(maximum daily 8 h average concentration of 100 µg m-3).
On a global scale, approximately 90 % of the tropospheric O3 is
produced photochemically within the troposphere (Stevenson et al., 2006;
Young et al., 2013), the remaining part being transported from the
stratosphere (McLinden et al., 2000; Olson et al., 2001). The main global
sink of tropospheric O3 is photolysis in the presence of water vapor.
Dry deposition, mainly by vegetation, is also an important sink in the
continental planetary boundary layer (PBL) (Jacob and Winner, 2009).
On a regional scale, O3 levels vary substantially depending on the
different chemical environments within the troposphere. O3 chemical
destruction is largest where water vapor concentrations are high, mainly in
the lower troposphere, and in polluted areas where there is direct O3
destruction by titration. Thus, the hourly, daily and annual variations in
O3 levels at a given location are determined by several factors,
including the geographical characteristics, the predominant meteorological
conditions and the proximity to large sources of O3 precursors (Logan,
1985).
Southern Europe, especially the Mediterranean basin, is the most exposed to
O3 pollution in Europe (EEA, 2018) due to the specific prevailing
meteorological conditions during warm seasons, regional pollutant emissions,
high biogenic VOCs' (BVOCs) emissions in spring and summer and the vertical
recirculation of air masses due to the particular orographic features that
help stagnation–recirculation episodes (Millán et al., 2000; EC, 2002,
2004; Millán, 2009; Diéguez et al., 2009, 2014; Valverde et al.,
2016). Periods with high O3 concentrations often last for several
days and can be detected simultaneously in several countries. Lelieveld et
al. (2002) reported that during summer, O3 concentrations are 2.5–3
times higher than in the hemispheric background troposphere. High O3
levels are common in the area, not only at the surface, but also throughout
the PBL (Millán et al., 1997; Gangoiti et al., 2001; Kalabokas et al.,
2007). Photochemical O3 production is favored due to frequent
anticyclonic conditions with clear skies during summer, causing high
insolation and temperatures and low rainfall. Besides, the emissions from the
sources located around the basin, which is highly populated and
industrialized, and the long-range transport of O3 contribute to the
high concentrations (Millán et al., 2000; Lelieveld et al., 2002;
Gerasopoulos, 2005; Safieddine et al., 2014).
In this context, the design of efficient O3 abatement policies is
difficult due to the following circumstances.
The meteorology driving O3 dynamics is highly influenced by the
complex topography surrounding the basin (see the above references for
vertical recirculation of air masses and Mantilla et al., 1997, Salvador et
al., 1997, Jiménez and Baldasano, 2004, and Stein et al., 2004)
The complex nonlinear chemical reactions between NOx and
VOCs (Finlayson-Pitts and Pitts, 1993; Pusede et al., 2015), in addition to
the vast variety of the VOC precursors involved and the involvement of BVOCs
in O3 formation and destruction (Hewitt et al., 2011)
The transboundary transport of air masses containing significant
concentrations of O3 and its precursors, which contribute to
increased O3 levels, mainly background concentrations (UNECE, 2010)
The contribution from stratospheric intrusions (Kalabokas et al., 2007)
The fact that O3 concentrations tend to be higher in rural areas (EEA,
2018), where local mitigation plans are frequently inefficient, because the
emission of precursors takes place mostly in distant urban and industrial
agglomerations
Sicard et al. (2013) analyzed O3 time trends during 2000–2010 in the
Mediterranean and observed a slight decrease in annual O3 averages
(-0.4 % yr-1) at rural sites and an increase at urban and
suburban stations (+0.6 % and +0.4 %, respectively). They
attributed the reduction at rural sites to the abatement of
NOx and VOC emissions in the EU. Paradoxically, this led to
an increase in O3 at urban sites due to a reduction in the titration
by NO. Their results also suggested a tendency to converge at remote and
urban sites. Paoletti et al. (2014) also reported convergence in the EU and
the US in the period 1990–2010, but found increasing annual averages at both
rural and urban sites, with a faster increase in urban areas. Querol et
al. (2016) determined that O3 levels in Spain remained constant at
rural sites and increased at urban sites in the period 2000–2015. This was
suggested to be a result of the preferential reduction of NO versus
NO2, supported by the lack of a clear trend in Ox
(O3+NO2). They also found that the target value was constantly
exceeded in large areas of the Spanish territory, while most of the
exceedances of the information threshold took place in July, mainly downwind
of urban areas and industrial sites, and were highly influenced by summer
heatwaves. The Vic Plain (located north of Barcelona) was the area
registering the most annual exceedances of the information threshold in
Spain, with an average of 15 exceedances per year per site.
In this study, we analyze NO, NO2 and O3 surface data around
the Barcelona metropolitan area (BMA) and the Vic Plain, as well as
NO2 satellite observations, in the period 2005–2017, with the aim of
better understanding the occurrence of high O3 episodes in the area
on a long-term basis. Previous studies in this region focused on specific
episodes, whereas we aim at assessing the spatial distribution, time trends
and temporal patterns of O3 and its precursors, and exceedances of
the information threshold on a long time series. After better understanding
the 2005–2017 O3 episodes, we aim to evaluate, as a first
approximation using air quality monitoring and OMI remote sensing data, the
effect that episodic mitigation measures of O3 precursors would have
on the Ox concentrations in the Vic Plain.
We recognize that the O3 problem has to be studied with executable
models with dispersion and photochemical modules, which allow one to perform
sensitivity analyses. It is also well recognized that there is a complex
O3 phenomenology in the study area and that although models have
greatly improved in the last 10 years, there are still problems in
reproducing some of the processes in detail, such as the channeling of
O3 plumes in narrow valleys or the vertical recirculation patterns.
Our study intends to obtain a sensitivity analysis for O3
concentrations using air quality data. Ongoing collaboration is being
established with modelers to try to validate model outputs with this
experimental sensitivity analysis and then to implement a prediction system
for efficiently abating O3 precursors to reduce O3
concentrations, for which executable models are the sole tool available.
MethodologyThe area of study
The study is set in central Catalonia (Spain), in the northeastern corner of
the Iberian Peninsula (Fig. 1). Characterized by a Mediterranean climate,
summers are hot and dry with clear skies. In the 21st century, heatwaves have
occurred frequently in the area, often associated with high O3 levels
(Vautard et al., 2007; Guerova and Jones, 2007; Querol et al., 2016; Guo et
al., 2017).
Location and main topographic features of the area of study.
The capital city, Barcelona, is located on the shoreline of the Mediterranean
Sea. Two sets of mountain chains lie parallel to the coastline (SW–NE
orientation) and enclose the Pre-coastal Depression: the Coastal (250–500 m
above sea level (a.s.l.)) and Pre-Coastal (1000–1500 m a.s.l.) mountain
ranges. The Vic Plain, situated 45–70 km north of Barcelona
(500 m a.s.l.), is a 230 km2 plateau that stretches along a S–N
direction and is surrounded by high mountains (over 1000 m a.s.l.). The
complex topography of the area protects it from Atlantic advections and
continental air masses, but also hinders the dispersion of pollutants
(Baldasano et al., 1994). The two main rivers in the area (Llobregat and
Besòs) flow perpendicularly to the sea and frame the city of Barcelona.
Both rivers' valleys play an important role in the creation of air-flow
patterns. The Congost River is a tributary to the Besòs River and its
valley connects the Vic Plain with the Pre-coastal Depression.
The BMA stretches across the Pre-Coastal and Coastal depressions and is a
densely populated (> 1500 people per km2, MFom, 2017) and highly
industrialized area with large emissions originating from road traffic,
aircraft, shipping, industries, biomass burning, power generation and
livestock.
During summer, the coupling of daily upslope winds and sea breezes may cause
the penetration of polluted air masses up to 160 km inland, channeled from
the BMA northward by the complex orography of the area. These air masses are
injected at high altitudes (2000–3000 m a.s.l.) by the Pyrenean mountain
ranges. At night time, the land breeze prevails, and winds flow toward the
sea followed by subsidence sinking of the air mass, which can be transported
again by the sea breeze of the following day (Millán et al., 1997, 2000,
2002; Toll and Baldasano, 2000; Gangoiti, 2001; Gonçalves et al., 2009;
Millán, 2014; Valverde et al., 2016). Under conditions of a lack of
large-scale forcing and the development of a thermal low over the Iberian
Peninsula that forces the confluence of surface winds from coastal areas
toward the central plateau, this vertical recirculation of the air masses
results in regional summer O3 episodes in the western Mediterranean.
In addition, there might be external O3 contributions, such as
hemispheric transport or stratospheric intrusions (Kalabokas et al., 2007,
2008, 2017; Querol et al., 2017, 2018).
Air quality and meteorological and remote sensing data
We evaluated O3 and NOx AQ data together with
meteorological variables and satellite observations of background
NO2.
The regional government of Catalonia (Generalitat de Catalunya, GC) has a
monitoring network of stations that provides average hourly data of air
pollutants (XVPCA, GC, 2017a, b). We selected a total of 25 stations (see
Fig. 2). To study the O3 phenomenology in the Vic Plain, we selected
the eight stations marked in green which met the following constraints:
(i) location along the S–N axis (Barcelona–Vic Plain–Pre-Pyrenean range);
(ii) availability of O3 measurements; and (iii) availability of at
least 9 years of data in the period 2005–2017, with at least 75 % data
coverage from April to September. The remaining selected stations (used only
as reference ones for interpreting data from the main Vic–BMA axis stations)
met the following criteria: (i) location across the Catalan territory and
(ii) availability of a minimum of 5 years of valid O3 data in the
period 2005–2017. We chose this period due to the poor data coverage of most
of the AQ sites in the regional network of AQ monitoring stations before
2005.
In addition, we selected wind and temperature data from five meteorological
stations from the Network of Automatic Meteorological Stations (XEMA,
Meteocat, 2017) closely located to the previously selected AQ stations, as
well as solar radiation data from two solar radiation sites from the Catalan
Network of Solar Radiation Measurement Stations (ICAEN-UPC, 2018) located in
the cities of Girona and Barcelona.
We also used daily tropospheric NO2 column satellite measurements
using the Ozone Monitoring Instrument (OMI) spectrometer aboard NASA's Earth
Observing System (EOS) Aura satellite (see OMI, 2012; Krotkov and Veefkind,
2016). The measurements are suitable for all atmospheric conditions and for
sky conditions where cloud fraction is less than 30 % binned and averaged
into 0.25∘×0.25∘ global grids.
Location (a) and main characteristics (b) of the
selected air quality monitoring sites (S–N axis: green squares on the map
and shaded gray on the table; rest of the stations: white squares) and
meteorological/solar radiation stations (red circles) selected for this
study. Types of air quality monitoring sites are urban (traffic or
background: UT, UB), suburban (traffic, industrial or background: SUT, SUI,
SUB) and rural (background or industrial: RB, RI). PLR (Palau Reial air
quality monitoring site) and BCN (Barcelona) meteorological and solar
radiation sites are closely located.
Data analysisOx calculations
We calculated Ox concentrations to better interpret
O3 dynamics. Kley and Gleiss (1994) proposed the concept of
Ox to improve the spatial and temporal variability analysis
by decreasing the effect of titration of O3 by NO with the subsequent
consumption of O3 in areas where NO concentrations are high.
Concentrations were transformed to ppb units using the conversion factors at
20 ∘C and 1 atm (DEFRA, 2014).
Ox concentrations were only calculated if there were at
least six simultaneous hourly recordings of O3 and NO2 from
12:00 to 19:00 local time (LT; see Sect. 2.3.6), June–August, in the period
2005–2017. The stations used for these calculations were those located along
the S–N axis (Barcelona–Vic Plain–Pre-Pyrenean Range).
Variability of concentrations across the air quality monitoring
network
To study the variability of concentrations of NO, NO2, O3 and
Ox across the air quality monitoring network, we calculated
June–August averages (months recording the highest concentrations of
O3 in the area) from hourly concentrations provided by all the
selected AQ sites. For each of them, we calculated daily averages and daytime
high averages (12:00 to 19:00 LT).
Time trends
By means of the Mann–Kendall method, we analyzed time trends for NO,
NO2 and O3 for the period 2005–2017. In addition, we used
the Theil–Sen statistical estimator (Theil, 1950a–c; Sen, 1968) implemented
in R package Openair (Carslaw and Ropkins, 2012) to obtain the regression
parameters of the trends (slope, uncertainty and p value) estimated via
bootstrap resampling. We examined the annual time trends of seasonal averages
(April–September) for each pollutant. Data used for these calculations were
selected according to the recommendations in EMEP-CCC (2016): the stations
considered have at least 10 years of data (75 % of the total period
considered, 2005–2017), and at least 75 % of the data are available
within each season. In addition, we analyzed annual time trends of
tropospheric NO2 measured by satellite along the S–N axis and of
greenhouse gases (GHGs) emitted in Catalonia and the average number of
vehicles entering the city of Barcelona.
Assessment of O3 objectives according to air quality
standards
We identified the maximum daily 8 h average concentrations by examining 8 h
running averages using hourly data in the period 2005–2017. Each 8 h
average was assigned to the day on which it ended (i.e., the first average of
one day starts at 17:00 LT on the previous day), as determined by EC (2008).
To assess the time trends and patterns of the Exceedances of Hourly
Information Thresholds (EHITs) established by EC (2008) (hourly mean of
O3 concentration greater than 180 µg m-3), we used
all the data independently of the percentage of data availability.
Tropospheric NO2 column
We analyzed daily average tropospheric column NO2 measurements from
2005 to 2017 aiming at two different goals: on the one hand, to quantify the
tropospheric NO2 in the area along the S–N axis and obtain annual
time trends and monthly/weekly patterns; and on the other hand, to assess
qualitatively the tropospheric NO2 across a regional scale (western
Mediterranean Europe) in two different scenarios, by means of visually
finding patterns that might provide a better understanding of O3
dynamics in our area of study. The scenarios were days with the maximum 8 h
O3 average above the 75th percentile at the Vic Plain stations and
days with the maximum below the 25th percentile. See selected regions for
retrieval of NO2 satellite measurements in Fig. S1.
Time conventions
When expressing average concentrations, the times shown indicate the start
time of the average. For example, 12:00–19:00 LT averages take into account
data registered from 12:00 to 19:59 LT. All times are expressed as local
time (UTC+1 during winter and UTC+2 during summer) and the 24 h time clock
convention is used.
Results and discussionVariability of concentration of pollutants across the air quality
monitoring network
We analyzed the mean NO, NO2, O3 and Ox
concentrations (June to August) in the study area in the period 2005–2017.
As expected, the highest NO and NO2 concentrations are registered in
urban/suburban (U/SU) traffic sites in and around Barcelona (MON, GRA, MNR
and CTL, 7–10 µg NO m-3, and CTL and MON,
30–36 µg NO2 m-3). Also, as expected, the remote
high-altitude rural background (RB) sites (MSY and MSC) register the lowest
NO (< 1 µg m-3) and NO2
(2–4 µg m-3) concentrations; see Fig. S2.
Spatial variability of mean June–August O3(a) and
Ox(b) concentrations from 12:00 to 19:00 LT
observed in selected air quality monitoring sites. Data from Ciutadella
(CTL), Palau Reial (PLR), Montcada (MON), Granollers (GRA), Montseny (MSY),
Tona (TON), Vic (VIC), Manlleu (MAN), Pardines (PAR), Montsec (MSC), Begur
(BEG), Bellver de Cerdanya (BdC), Berga (BER), Agullana (AGU), Santa Pau
(STP), Mataró (MAT), Manresa (MNR), Ponts (PON), Sort (SOR), Juneda
(JUN), La Sénia (LSE), Constantí (CON), Gandesa (GAN), Vilanova i la
Geltrú (VGe) and Alcover (ALC) air quality monitoring stations.
The lowest June–August average O3 concentrations
(45–60 µg m-3) are recorded in the same U/SU traffic sites
(MON, GRA, MNR and CTL) where titration by NO is notable, while the highest
ones (> 85 µg m-3) are recorded at the RB sites, MSC being
the station recording the highest June–August O3 levels
(102 µg m-3). These spatial patterns are significantly
different when we consider the 8 h daily averages of O3
concentrations for June–August 12:00–19:00 LT (Fig. 3a). Thus, these
concentrations are repeatedly high (85–115 µg m-3) in the
whole area of study. The highest O3 concentrations
(> 107 µg m-3) were recorded at the four sites located
downwind of the BMA along the S–N corridor (MSY, TON, VIC and MAN), and
downwind of Tarragona (PON, RB station). Figure 3a also shows a positive
O3 gradient along the S–N axis (O3 levels increase farther
north) following the BMA plume transport and probably an increase in the
mixing layer height (MLH). The higher O3 production and/or fumigation
in the northern areas are further supported by the parallel northward
increasing Ox gradient (δOx,
Fig. 3b). Time series show that in 85 % of the valid data in June–August
(849 out of 1001 days in 2005–2017), this positive gradient is evident
between CTL and TON (δOxTON-CTL>0). The
average Ox increase between CTL in Barcelona and TON is
15 ppb. Taking into account the low NO2 concentrations registered at
this station, this is equivalent to approximately 29 µg m-3
of O3 (+30 % Ox in TON compared with CTL).
Thus, TON in the Vic Plain records the highest 12:00–19:00 LT, June–August
Ox and O3 concentrations in the study area. The MNR
site also exhibits very high Ox levels (Fig. 3b), but these
are mainly caused by primary NO2 associated with traffic emissions.
Time patternsAnnual trends
Figure 4 shows the results of the trend analysis of NO, NO2,
O3 and Ox averages (April to September, the
O3 season according to the European AQ Directive) by means of the
Mann–Kendall test.
NOx levels exhibit a generalized and progressive decrease
during the time period across Catalonia. In particular, NO2 tended to
decrease along the S–N axis during the period (U/SU sites CTL, MON and MAN
registered -1.6 % yr-1, -2.0 % yr-1 and
-1.3 % yr-1, respectively, with statistical significance in all
cases). A similar trend was found for NO in these stations, with higher
negative slopes (-2.2 % yr-1, -4.3 % yr-1 and
-1.1 % yr-1, the latter without statistical significance).
Results of the time trend assessment carried out for annual season
averages (April–September) of NO (a), NO2(b),
O3(c, d) and Ox(e) levels using
the Theil–Sen statistical estimator shown graphically. Only the trends with
statistical significance are shown. (d) Numerical results; the
symbols shown for the p values relate to how statistically significant the
trend estimate is: p<0.001***= (highest statistical significance), p<0.01=** (mid), p<0.05=* (moderate), and p<0.1=+ (low).
No symbol means lack of a significant trend. Units are µg m-3.
Shaded air quality monitoring sites belong to the S–N axis. Types of air
quality monitoring sites are urban (traffic or background: UT, UB), suburban
(traffic, industrial or background: SUT, SUI, SUB) and rural (background:
RB). Data from AQ stations with at least 10 years of valid data within the
period.
The annual averages of tropospheric NO2 across the S–N axis
decreased by 35 % from 2005 to 2017 (-3.4 % yr-1 with
statistical significance). The marked drop in NO2 from 2007 to 2008
can be attributed to the reduction in emissions associated with the financial
crisis starting in 2008. The time trends of average traffic (number of
vehicles) entering Barcelona on working days from 2005 to 2016 (Ajuntament de
Barcelona, 2010, 2017) and the GHGs emitted in Catalonia attributed to
industry and power generation sectors calculated from the Emissions
Inventories published by the Regional Government of Catalonia from 2005 to
2016 (GC, 2017c) (Fig. 5a) support this hypothesis. We found both decreasing
trends to be statistically significant, but the GHG emissions decreasing
trend is significantly higher (-3.8 % yr-1) than the traffic
(-1.2 % yr-1), which suggests that the crisis had a more severe
effect on industry and power generation than on road traffic. This is also
supported by a larger decrease in GHG emissions and OMI-NO2 from
2005–2007 (pre-crisis) to 2008 (start of the crisis) than BMA traffic
counting and urban NOx levels (without a 2007–2008 steep
change and a more progressive decrease, Fig. 5b). Thus, in the BMA, the
financial crisis caused a more progressive decrease (without a 2007–2008
steep change) in the circulating vehicles and therefore its associated
emissions.
(a) Annual average traffic entering Barcelona during
weekdays (weekends not considered) during 2005–2016 versus GHG emissions
(attributed to industry and power generation sectors) in Catalonia during
2005–2016. (b) Annual NOx measured at CTL
(Ciutadella) and MON (Montcada) air quality monitoring sites versus annual
OMI-NASA measured background NO2 during 2005–2017.
April–September O3 and Ox mean concentration trends
are shown in Fig. 4. The data show that seven out of the eight RB sites
registered slight decreases in O3 concentrations during the period
(BdC, AGU and STP; -1.6 % yr-1, -1.1 % yr-1 and
-1.4 % yr-1, respectively, in all cases with statistical
significance), while in BEG, PON, LSE and GAN the trends were not significant
(not shown). As in several regions of Spain and Europe (Sicard et al., 2013;
Paoletti et al., 2014; EEA, 2016; Querol et al., 2016; EMEP, 2016), the
opposite trends are found for U/SU sites, with increases in O3
concentrations during the period at some stations (CTL, MON, MAN, MAT, MNR
and ALC; +0.4 % yr-1 to +3.2 % yr-1, all with
statistical significance). When considering Ox, the
increasing trends in U/SU sites are neutralized in some cases (CTL, MON, MAN,
MAT and ALC). This, and the higher NO decreasing slopes compared with those
of NO2, support the hypothesis that the U/SU O3 increasing
trends are probably caused by less O3 titration (due to decrements in
NO levels) instead of a higher O3 generation. The marked decrease in
the vehicle diesel emissions of NO/NO2 time trends (Carslaw et al.,
2016) might have caused these differential NO and NO2 trends,
although other causes cannot be rejected.
Monthly hourly average concentrations of O3(a) and
Ox(b) along the S–N axis during 2005–2017. Data
from Ciutadella (CTL), Montcada (MON), Granollers (GRA), Montseny (MSY), Tona
(TON), Vic (VIC), Manlleu (MAN) and Pardines (PAR) air quality monitoring
stations.
Monthly weekday average concentrations of O3 concentrations
calculated between 12:00 and 19:00 LT along the S–N axis during 2005–2017.
Data from Ciutadella (CTL), Montcada (MON), Granollers (GRA), Montseny (MSY),
Tona (TON), Vic (VIC), Manlleu (MAN) and Pardines (PAR) air quality
monitoring stations.
Monthly and daily patterns
Figure 6a shows 2005–2017 monthly average hourly O3 concentrations
measured at sites along the S–N axis, showing the occurrence of chronic-type
episodes with repeated high O3 concentrations
(90–135 µg m-3) in the afternoons of April–September days
at the Vic Plain sites (TON, VIC, MAN) and the remote RB sites (MSY and PAR).
Typically, at the remote RB stations, O3 concentrations are high
during the whole day throughout the year and daily O3 variations are
narrower than at the other stations, with high average levels even during
October–February (MSY: 50–70 µg m-3 and PAR:
50–80 µg m-3). During the night these mountain sites are
less affected by NO titration, leading to high daily O3 average
concentrations. However, in summer, midday–afternoon concentrations are
relatively lower than at the stations located in the S–N valley (TON, VIC,
MAN).
Regarding monthly average daily Ox (Fig. 6b), the profiles
of RB sites TON and MSY are very similar to the respective O3
profiles. In the case of the BMA U/SU sites (CTL, MON, GRA), the nocturnal
Ox concentrations increase with respect to O3 due to
the addition of secondary NO2 from titration. Midday–afternoon
Ox levels are much lower at the BMA U/SU stations than those
in the S–N valley (MAN, TON), similarly to O3 levels, supporting the
contribution of local–regional O3 from the BMA plume and/or from the
fumigation of high-altitude reserve strata as MLH grows (Millán et al.,
1997, 2000; Gangoiti et al., 2001; Querol et al., 2017) as well as production
of new O3.
Weekly patterns
Accordingly, Fig. 7 shows the O3 weekly patterns for these O3
average concentrations. As expected, the variation of intra-annual
concentration values is pronounced in the Vic Plain sites (TON, VIC, MAN;
20–45 µg m-3 in December–January versus
110–125 µg m-3 in July), due to the higher summer
photochemistry, the more frequent summer BMA plume transport (due to intense
sea breezing) and fumigation from upper atmospheric reservoirs across the
S–N axis, and the high O3 titration in the populated valleys in
winter. However, at the remote mountain sites of MSY and PAR, the
intra-annual variability is much reduced (70–80 µg m-3 in
December versus 100–120 µg m-3 in July), probably due to the
reduced effect of NO titration at these higher-altitude sites and the
influence of high-altitude O3 regional reservoirs.
During the year, CTL, MON and GRA (U/SU sites around the BMA) register very
similar weekly patterns of the 8 h maxima, with a marked and typical high
O3 weekend effect, i.e., higher O3 levels than during the
week due to lower NO concentrations. From April to September, CTL O3
8 h concentrations are lower than MON's and GRA's (the latter located north
of the BMA following the sea breeze air mass transport), despite being very
similar from October to March (when sea breezes are weaker). An O3
weekend effect is also clearly evident during the winter months in the Vic
Plain sites (TON, VIC, MAN) and MSY. However, from June to August, a marked
inverse weekend effect is clearly evident at these same sites, with higher
O3 levels during weekdays. This points again to the clear influence
of the emission of precursors from the BMA on the O3 concentrations
recorded at these inland sites.
Weekday (W) (Monday to Friday in the BMA and Tuesday to Friday in
the Vic Plain) to weekend (WE) pollutant concentrations (O3, NO and
NO2) measured at AQ sites and background NO2 (remote sensing
OMI) for June to August, per year along the period 2005–2017. O3
concentrations (a) are averaged from 12:00 to 19:00 LT hourly
concentrations, and NO and NO2 concentrations are calculated from
daily averages, including OMI-NO2. Each short line depicts the
increasing or decreasing tendency of weekday concentrations (left side of
each short line) with respect to weekend levels (right side of the short
lines). Thus, a horizontal line would represent same pollutant levels along
the week (concentration in W = concentration in WE). We consider BMA AQ
sites CTL, MON and GRA, and Vic Plain AQ sites TON and MAN. The continuous
lines show the percentage of variation of pollutant levels during weekends
with respect to weekdays: increasing (> 0) or decreasing (< 0), i.e., a
quantification of the inclination of each short line.
We carried out a trend analysis of NO, NO2 and O3 levels
measured at AQ sites and background NO2 from remote sensing (OMI) for
week (W) and weekend (WE) days independently. To this end we averaged the
concentrations for three sites in the BMA (CTL, MON and GRA) and three
receptor sites at the Vic Plain (TON, VIC and MAN), and considering WE to be
Saturday, Sunday and Monday for the Vic AQ sites data (adding Mondays to
account for the “clean Sunday effect”) and Saturday and Sunday for the BMA
sites data.
We estimated time trends of W and WE concentrations separately by the
Mann–Kendall method along the study period. For O3 (12:00 to
19:00 LT) we found statistically significant increases in both the BMA and
the Vic Plain. Increases in O3 in the BMA double the ones in the Vic
Plain, and trends of W and WE are very similar per area (O3 BMA W:
+2.0 % yr-1, O3 BMA WE: +2.2 % yr-1,
O3 Vic Plain W: +0.8 % yr-1, O3 Vic Plain WE:
+1.0 % yr-1). As seen before, both NO and NO2 levels
(daily averages) in the BMA decrease in a statistically significant way where
NO decrements are larger than NO2. We found that the decrease in W NO
levels is higher than the WE ones (NO BMA W: -3.4 % yr-1, NO BMA
WE: -2.7 % yr-1) because emissions are higher during W days, and
these decreased along the period. Regarding NO2, W and WE decreases
remain similar (NO2 BMA W: -1.9 % yr-1, NO2 BMA
WE: -1.7 % yr-1) but lower than NO in both cases, thus reducing
the O3 titration effects and increasing O3 levels on both WE
and W days. Regarding NO2-OMI levels, only W levels show a
statistically significant decreasing trend (-3.4 % yr-1), and not
the WE levels.
We then assessed the variations of WE concentrations with respect to Ws per
year and plotted them by short tilted lines in Fig. 8, where the left- and
right-hand sides of each tilted line represent W and WE concentrations,
respectively. These W to WE variations are then plotted in percentage by
continuous lines (> 0 depicts increasing and < 0 decreasing W to WE).
Panel (a) shows O3 data
averaged from 12:00 to 19:00 LT from the BMA and the Vic Plain, panel (b) daily averages of NO and NO2 concentrations in the BMA, and panel (c) daily NO2-OMI levels along the S–N axis. The results
show again a constant drop in W to WE NOx levels in the BMA
along the period (negative percentages in panel b), with the
subsequent O3 weekend effect in the BMA (positive percentages in panel a). In the Vic Plain sites, O3 concentrations remain
constantly high along the study period, showing an inverse weekend effect
almost during the whole period (negative percentages in the plot, except for
2005 to 2007 and 2017). Using the Mann–Kendall test to estimate trends for
the W to WE variations, we found a clear statistically significant decreasing
trend along the period (reduction of the difference between W to WE levels:
from -38 % in 2005 to -17 % in 2017, Fig. 8c). We attribute this to the
decrease in W–NOx levels described before for the annual
averages.
Furthermore we found a pattern of nearly parallel O3 W to WE
variation cycles between the Vic Plain and the BMA sites (Fig. 8a). Due to the inverse W to WE
O3 at Vic and in the BMA, this parallel trend means in fact that
maximum W to WE variations in the Vic Plain and the BMA tend to follow a
reverse behavior; i.e., maximum W to WE variations in the BMA tend to occur
when W to WE variations in the Vic Plain are minimum (for example 2007, 2010,
or 2014). NOx W to WE variations tend to follow a similar
behavior to O3 W to WE variations in the Vic Plain sites (mostly from
2008 to 2016) where years with high W to WE variations of
NOx in the BMA tend to correspond to years with maximum
O3 W to WE variations in the Vic Plain (2009 and 2015). This behavior
is probably associated with differences in air mass circulation patterns
along the period (such as higher or lower breeze development). In those years
with lower breeze development, the transport of the BMA plume is weaker; then
NOx would tend to accumulate at the BMA (low W to WE
NOx variation), which would generate more O3; thus,
W to WE variation would be higher in the BMA and lower in the Vic Plain. By
contrast, in years with stronger breeze development and thus increased
transport of the BMA plume, W to WE variations of NOx in the
BMA are higher, W to WE variations of O3 in the BMA are lower (less
O3 is generated during WE) and higher W to WE O3 variations
are recorded in the Vic Plain sites.
(a) July O3 and (b)Ox
daily cycles plotted from mean hourly concentrations measured in air quality
monitoring sites located along the S–N axis during 2005–2017. The black
arrows point to the O3 and Ox maxima time of the
day. Data from Ciutadella (CTL), Montcada (MON), Granollers (GRA), Montseny
(MSY), Tona (TON), Vic (VIC), Manlleu (MAN) and Pardines (PAR) air quality
monitoring stations.
Peak O3 concentration patterns along the S–N axis
July is the month of the year when most of the annual exceedances of the
O3 EHITs are recorded in Spain (Querol et al., 2016), including our
area of study. Figure 9 shows the average O3 and Ox
July hourly concentrations along the S–N axis during 2005–2017. A
progressive time shift and a marked positive northward gradient of O3
and Ox maxima are shown, pointing again to the gradual
increase in O3 and Ox due to the plume transport,
new O3 formation and fumigation from upper reservoirs as MLH grows.
Figure 10a shows the 2005–2017 trends of the EHITs from the European AQ
Directive (> 180 µg m-3 h-1 mean; EC, 2008)
registered at the selected sites in the S–N valley, as well as the average
temperatures measured during July in the early afternoon near Vic (at Gurb
meteorological site), the background NO2 measured by OMI
(June–August) and the average solar radiation measured in Girona and
Barcelona (June–August). In 2005, 2006, 2010, 2013, 2015 and 2017, the
highest EHITs at almost all the sites were recorded. Temperature and
insolation seem to have a major role in the occurrence of EHITs in 2006,
2010, 2015 and 2017. The effect of heatwaves on O3 episodes is widely
known (Solberg et al., 2008; Meehl et al., 2018; Pyrgou et al., 2018).
However, because the emissions of precursors have clearly decreased
(-30 % decrease in June to August OMI-NO2 levels across the
S–N axis from 2005 to 2017; -2.7 % yr-1 with statistical
significance), the number of EHITs recorded in the warmest years has probably
decreased with respect to a scenario where emissions would have been
maintained. In any case, some years (for example 2009 and 2016) seem to be
out of line for temperature and insolation being the driving forces, and
other major causes also have to be relevant, with further research needed to
interpret fully interannual trends. Otero et al. (2016) found that
temperature is not the main driver of O3 in the southwestern
Mediterranean, as it is in central Europe, but the O3 levels recorded
the day before (a statistical proxy for the occurrence of Millán et
al. (1997)'s vertical recirculation of air masses). Again, the Vic Plain
sites (TON, VIC, MAN) recorded most (75 %) of the EHITs reported by the
AQ monitoring stations in Catalonia (25 %, 34 % and 16 %,
respectively). The higher urban pattern of MAN, as shown by the higher NO
concentrations, with respect to TON, might account for both the lower
exceedances and the different interannual patterns.
For the period 2005–2017, trends of the EHITs measured by air
quality monitoring stations along the S–N axis. (a) Annual trends
of the EHITs, average temperatures measured in Vic (Gurb) (July during 13:00
to 16:00 LT), background NO2 measured by OMI-NASA (June to August)
and average solar radiation measured at Girona and Barcelona (June to
August). (b) Monthly patterns of the EHITs, average temperatures
measured in Vic, background NO2 measured by OMI and solar radiation
measured at Girona and Barcelona. (c) Weekly patterns of the EHITs
and background NO2 measured by OMI. (d) Hourly patterns of
the EHITs. Despite the incomplete data availability in MAN 2005, almost 20
EHITs were recorded. AQ data from Ciutadella (CTL), Montcada (MON),
Granollers (GRA), Montseny (MSY), Tona (TON), Vic (VIC), Manlleu (MAN) and
Pardines (PAR) monitoring stations.
Average hourly O3 concentrations for all days with EHIT
records and those without for Tona (TON), Vic (VIC), Manlleu (MAN) and
Pardines (PAR) air quality monitoring stations, (b) as well as for
the NO2 levels at TON (c). Average hourly increments of
O3 concentrations for all days with and without EHIT
records (d); in all cases for June–August 2005–2017.
Figure 10b shows that most EHITs occurred in June and July (30 % and
57 %, respectively), with much less frequency in May, August and
September (6 %, 8 % and <1 %, respectively). Although
temperatures are higher in August than in June, the latter registers
significantly more EHITs, probably due to both the stronger solar radiation
and the higher concentrations of precursors (such as NO2; see
OMI-NO2 and solar radiation in Fig. 10b).
Figure 10c shows that EHITs occurred mainly between Tuesday and Friday
(average of 19 % of occurrences per day). On weekends and Mondays, EHITs
were clearly lower (average of 9 % of occurrences per day) than during
the rest of the week, probably due to (i) the lower emissions of
anthropogenic O3 precursors (such as NOx; see
OMI-NO2) during weekends and (ii) the effect of the lower Sunday
emissions in the case of the lower exceedances recorded during Mondays.
During weekends and in August, OMI-NO2 along the S–N axis is
relatively lower (-29 % weekday average and -43 % in August with
respect to March) following the emissions patterns associated with industrial
and traffic activity that drop during vacations and weekends (Fig. 10).
NOx data from AQ monitoring sites follow similar patterns
(not shown here).
Figure 10d shows that the frequency of occurrence of the EHITs at MSY (45 km
north of Barcelona) is lower and earlier (maxima at 14:00 LT) than at Vic
Plain sites (TON, VIC, MAN). The EHITs occurred mostly at 15:00, 16:00, 16:00
and 19:00 LT at TON, VIC, MAN and PAR (53, 63, 72 and 105 km north of
Barcelona), respectively. PAR registered not only much later EHITs, but a
much lower number than TON–VIC–MAN sites, again confirming the progressive
O3 maxima time shift northward of Barcelona.
The results in Fig. 11b clearly show that during non-EHIT days, the daily
O3 patterns are governed by the morning–midday concentration growth
driven to fumigation and photochemical production, while on EHIT days there
is a later abrupt increase, with maxima being delayed as we increase the
distance from Barcelona along the S–N axis. This maximal second increase in
O3 is clearly attributable to the influence of the transport of the
plume of the BMA (horizontal transport), as the secondary NO2 peak at
15:00 LT (Fig. 11c) and the wind patterns (see Fig. S3) seem to
confirm. The differences in the late hourly O3 concentration
increases in EHIT versus non-EHIT days are even more evident when calculating
hourly O3 slopes (hourly increments or decrements of concentrations);
see Fig. 11d. The first increment (fumigation and photochemistry) makes O3
levels scale up to 120 µg m-3 during EHIT episodes and to
nearly 100 µg m-3 during non-EHIT days. On EHIT days, the
later peak (transport from the BMA and causing most of the
180 µg m-3 exceedances) in the O3 slope occurs again
between 14:00 and 20:00 LT, depending on the distance to BMA, but this
feature is not observed on non-EHIT days.
Idealized two-dimensional section of O3 circulations in the
coastal region of Barcelona to the Pre-Pyrenees on a typical summer
day (a) and night (b). The gray shaded shape represents a
topographic profile in the south-to-north direction from the Mediterranean
Sea to the southern slopes of the Pre-Pyrenean ranges (i.e., along the S–N
axis). The colored dots and abbreviations depict the air quality monitoring
stations located along the S–N axis: Ciutadella (CTL), Montcada (MON),
Granollers (GRA), Montseny (MSY), Tona (TON), Vic (VIC), Manlleu (MAN) and
Pardines (PAR). Modified and adapted to the S–N axis from Millán et
al. (1997, 2000) and Querol et al. (2017, 2018).
Relevance of local/regional pollution plumes in high O3
episodes in northeastern Spain
Figure 12 depicts the basic atmospheric dynamics in the study area during a
typical summer day, when the atmospheric conditions are dominated by
mesoscale circulations. According to the previous references, indicated in
Fig. 12 with enclosed numbering (coinciding with the numbering below), the
following O3 contributions to surface concentrations in the study
area can be differentiated.
Vertical recirculation of O3-rich air masses, which create
reservoir layers of aged pollutants.
Vertical fumigation of O3 from the above reservoirs and the
following sources aloft if the MLH growth is large enough.
Regional external O3 layers (from other regions of southern
Europe, such as southern France, Italy, Portugal and Tarragona)
High free tropospheric O3 background due to hemispheric long-range
transport
High free tropospheric O3 background due to stratospheric
intrusions
Horizontal transport of O3. Diurnal BMA plume northward
transported and channeled into the Besòs–Congost valleys.
Local production of O3 from precursors.
During summer, the intense land heating due to strong solar radiation begins
early in the morning. The associated convective activity produces morning
fumigation processes (letter b in Fig. 12) that bring down O3 from
the reservoir layers aloft, creating sharp increases in O3
concentrations in the morning (see Figs. 11 and S3). The breeze transports
air masses from the sea inland and creates a compensatory subsidence of aged
pollutants (including O3) previously retained in reservoir and
external layers and high free troposphere background aloft (Millán et
al., 1997, 2000; Gangoiti et al., 2001). This subsided O3 then
affects the marine boundary layer and reaches the city the following day with
the sea breeze, producing nearly constant O3 concentrations in the
city during the day (Figs. S3 and 9). As the breeze develops, coastal
emissions and their photochemical products are transported inland, generating
the BMA plume (letter c in Fig. 12) that, in addition to the daily generated
O3, also contains recirculated O3 from the marine air masses.
Furthermore, during the transport to the Vic Plain, new O3 is
produced (letter d in Fig. 12) by the intense solar radiation and the
O3 precursors emitted along the way (e.g., BVOCs from vegetation,
NOx from industrial and urban areas and highways).
This new O3 gets mixed with the BMA plume and channeled northward to
the S–N valleys until it reaches the Vic Plain and the southern slopes of
the Pre-Pyrenees. As the BMA plume (loaded with O3 and precursors)
travels northward, a second increase in O3 concentrations can be
observed in the daily cycles of O3 at these sites (see Figs. 11
and S3). This was described as the second O3 peak by Millán et
al. (2000).
The marked MLH increase in the Vic Plain compared with the BMA (Soriano et
al., 2001; Querol et al., 2017) may produce a preferential and intensive
top-down O3 transport (letter b in Fig. 12) from upper O3
layers (letters a, b.1, b.2 and b.3 in Fig. 12), contributing to high
O3 surface concentrations. During the sea's/mountain breezes'
development, some air masses are injected upward to the N and NW return flows
(controlled by the synoptic circulations dominated by the high-pressure
system over the Azores) aloft helped by the orography (e.g., southern slopes
of mountains) and again transported back to the coastal areas where at late
evening/night it can accumulate at certain altitudes in stably stratified
layers.
Later, at night, land breezes returning to the coastal areas develop.
Depending on the orography, these drainage flows of colder air traveling to
the coastal areas can accumulate on the surface or keep flowing to the sea.
The transported O3 is consumed along the course of the drainage flows
by deposition and titration. Next day, the cycle starts anew, producing
almost closed loops enhancing O3 concentrations throughout the days in
the area. When the loop is active for several days, multiple O3 EHITs
occur over the Vic Plain.
The main complexity of this system arises from the fact that all these
vertical/horizontal and local/regional/hemispheric/stratospheric
contributions are mixed and that all contribute to surface O3
concentrations with different proportions that may largely vary with time and
space across the study area. However, for the most intense O3
episodes, the local–regional contribution might be very relevant to cause
EHITs in the region. Furthermore, the intensity and frequency of O3
episodes are partially driven by the occurrence of heatwaves in summer and
spring (Vautard et al., 2007; Gerova et al., 2007; Querol et al., 2016; Guo
et al., 2017). If local and regional emissions of precursors are high, the
intensity of the episodes will also be high. Thus, even though heatwave
occurrences increase the severity of O3 episodes, an effort to reduce
precursors should be undertaken to decrease their intensity.
The generation of the O3 episodes in 2005–2017 for the S–N corridor
BMA–Vic Plain–Pre-Pyrenees occurs in atmospheric scenarios described in
detail by Millán et al. (1997, 2000, 2002), Gangoiti et al. (2001),
Kalabokas et al. (2007, 2008, 2017), Millán (2014) and Querol et
al. (2018) for other regions of the Mediterranean basin, including Spain, or
described in the same area for specific episodes (Toll and Baldasano, 2000;
Gonçalves et al., 2009; Valverde et al., 2016; Querol et al., 2017).
However, results from our study show a higher role of the local–regional
emissions in the occurrence of O3 EHITs. Thus, our results
demonstrate an increase in the EHITs northward from Barcelona to around
70 km and a decrease from there to 100 km from Barcelona following the same
direction. There is also a higher frequency of occurrence of these in July
(and June) and from Tuesday to Friday and a time shift in the frequency of
occurrence of EHITs from 45 to 100 km. The mountain site of MSY (located at
700 m a.s.l.) registered many fewer EHITs than the sites in the valleys
(TON–VIC–MAN, 460–600 m a.s.l.) during the period, showing the key role
of the valley in channeling the high O3 and precursor BMA plume in
July (when sea breeze and insolation are more intense). Furthermore, in the
Vic Plain, we detected an inverse O3 weekend effect, suggesting that
local–regional anthropogenic emissions of precursors play a key role in
increasing the number of EHITs on working days, with a Friday/Sunday rate of
5 for VIC for 2005–2017. Despite this clear influence of the BMA plume on
EHIT occurrence, Querol et al. (2017) demonstrated that at high atmospheric
altitudes (2000–3000 m a.s.l.) high O3 concentrations are
recorded, in many cases reaching 150 µg m-3 due to the
frequent occurrence of reservoir strata. As also described above, the higher
growth of the MLH in TON–VIC–MAN as compared with the coastal area accounts
also for higher top-down O3 contributions. On the other side, close
to the Pyrenees (PAR station), large forested and more humid areas give rise
to a thinner MLH, hindering O3 fumigation too. Furthermore, in these
more distant northern regions O3 consumption by ozonolysis of BVOCs
might prevail over production due to weaker solar radiation during the later
afternoon.
Daily average background NO2 levels in western
Europe (a, b) and Catalonia (c, d), July 2005–2017, in two
different scenarios. (a, c) P25: days when the maximum daily 8 h
mean O3 concentrations in the Vic Plain are below percentile 25
(< 105 µg m-3) and (b, d) P75: same but with
concentrations above percentile 75 (> 139.5 µg m-3).
Figure 13 shows the distribution of average background OMI-NO2 levels
across the Western Mediterranean Basin in two different scenarios: when the
O3 levels in the Vic Plain are low (left) or high (right). To this
end, we averaged the values from VIC and TON (in the Vic Plain) from all the
maximum daily 8 h mean O3 concentrations calculated for all the days
in July within 2005–2017, and we calculated the 25th (93 out of 370 days,
105 µg m-3) and 75th (93 days, 139.5 µg m-3)
percentiles of all the data (P25 and P75, respectively). For both scenarios,
NO2 concentrations are highest around large urban and industrial
areas, including Madrid, Porto, Lisbon, Barcelona, Valencia, Paris,
Frankfurt, Marseille and especially the Po Valley. The shipping routes toward
the Gibraltar Strait and around the Mediterranean can be observed, as well as
important highways such as those connecting Barcelona to France and Lyon to
Marseille. As expected, the mountain regions (the Pyrenees and the Alps) are
the areas with lower NO2. Regional levels of background
OMI-NO2 in the P75 scenario are markedly higher, with hotspots
intensified and spanning over broader areas. Over Spain, new hotspots (marked
in yellow), such as the coal-fired power plants in Asturias (a), ceramic
industries in Castelló (c) and the coal-fired power plant in Andorra,
Teruel (b), appear, in the latter case, with the pollution plume being
channeled along the Ebro Valley with a NW transport. Furthermore, it is
important to highlight that the maxima background NO2 along the
eastern coastline in Spain, including the BMA, tends to exhibit some
northerly–northwesterly displacement, when compared with the P25 scenario,
thus pointing to the relevance of the local emissions in causing inland
O3 episodes.
These qualitative results suggest in general less synoptic forcing in western
Europe in the P75 scenario; hence, in these conditions NO2 is
accumulated across the region, and especially around its sources. On the
eastern coast of the Iberian Peninsula, mesoscale circulations tend to
dominate, hence the northwestern displacement (taking the coastal regions as
a reference) of the background NO2. The bottom part of Fig. 13 zooms
our study area and shows the maximum daily 8 h mean O3
concentrations in all the selected AQ sites averaged for both scenarios. As
shown in the P75 scenario, NO2 is significantly intensified across
Catalonia, especially north of the BMA spreading to the Vic Plain. Comparing
O3 in both scenarios, in the P75 the O3 levels are much
higher (mostly > 105 µg m-3) across the region except for
the urban sites in Barcelona (due to NO titration), reaching up to
154 µg m-3 in the Vic Plain.
Conversely, in the P25 scenario, background NO2 concentrations are
lower, and the BMA NO2 spot is significantly smaller and spreads
along the coastline rather than being displaced to the north–northwest. In
this case, synoptic flows seem to weaken sea breeze circulations and vertical
recirculation, thus reducing the amount of background NO2 and the
inland transport from the coast. In these conditions, O3 levels are
markedly lower across the territory, the RB PON site (downwind of the
city/industrial area of Tarragona) being the one recording the maximum daily
8 h mean O3 concentration (99 µg m-3).
Sensitivity analysis for Ox using air quality
monitoring data
We demonstrated above that the lower anthropogenic emissions of O3
precursors in the BMA during weekends cause lower O3 and
Ox levels in the Vic Plain than during working days (inverse
O3 weekend effect). To apply a sensitivity analysis using air quality
monitoring data for the O3 levels in the Vic Plain if the BMA's
emissions were reduced, we compared weekend O3 and
Ox patterns with weekdays considering only data from June
and July (August OMI-NO2 levels are markedly lower, Fig. 10b;
therefore, this month was not included).
Box plots of Ox measured in TON (a) and MAN (b) (12:00 to
19:00 LT) per weekday June and July 2005–2017 for those days with
δOxTON-CTL>0 (n=545 for TON and n=479
for MAN of valid data). Each box represents the central half of the data
between the lower quartile (P25) and the upper quartile (P75). The line
across the box displays the median values. The whiskers that extend from the
bottom and the top of the box represent the extent of the main body of data.
The outliers are represented by black points.
Figure 14 shows the average Ox concentrations (12:00 to
19:00 LT) in TON (Fig. 14a) and MAN (Fig. 14b) (both AQ sites in the Vic Plain) according to the
day of the week for the period considered. Data in VIC cannot be used for
Ox calculations due to the lack of NO2 measurements.
Despite the large variability in extreme values (i.e., maximum values with
respect to minimum values, represented by whiskers), the interquartile range
is quite constant on all the weekdays (between 13.6 and 17.3 ppb in TON and
between 12.7 and 19.1 in MAN). The average Ox decrease
between the days with the highest Ox levels (Wednesday in
TON and Friday in MAN) and the days with the lowest Ox
levels (Sunday in TON and Monday in MAN) is between 6.5 (TON) and 7.7 ppb
(MAN), approximately 13 and 15 µg O3 m-3, a
10 %–12 % decrease. The observed decrements on Ox
levels downwind of the BMA due to the reduction in O3 precursors'
emissions in the BMA during weekends can give us a first approximation of the
effect that episodic mitigation measures could have on the
Ox or O3 levels in the Vic Plain. Thus, we
considered feasible a scenario with a maximum potential of
Ox reduction of 24.5 ppb (approximately
49 µg O3 m-3, 32 % decrease) when applying
episodic mitigation measures (lasting 1–2 days equivalent to a weekend when,
on average, NO and NO2 are reduced by 51 % and 21 %,
respectively, compared with week days in the BMA monitoring sites). This was
calculated as the difference between the P75 of Ox values
observed on Wednesdays minus the P25 of Ox values on
Sundays. Obviously, if these mitigation measures would be implemented
structurally, instead of episodically, Ox and O3
decreases would probably be larger because not only would the local
O3 coming from the BMA plume be reduced, but also the recirculated
O3 and thus the intensity of O3 fumigation in the plain.
Therefore, it is probable that both structural and episodic measures to abate
VOCs and NOx emissions in the BMA would result in evident
reductions of O3 in the Vic Plain, as evidenced by modeling tools by
Valverde et al. (2016).
Conclusions
We analyzed 2005–2017 data sets on ozone (O3) concentrations in an
area frequently affected by the northward atmospheric plume transport of the
Barcelona metropolitan area (BMA) to the Vic Plain, the area of Spain
recording the highest number of exceedances of the hourly O3
information threshold (EHIT, 180 µg m-3). We aimed at
evaluating the potential benefits of implementing local short-term measures
to abate emissions of precursors. To this end, we analyzed in detail spatial
and time (interannual, weekly, daily and hourly) variations of the
concentration of O3 and nitrogen oxides (including remote sensing
data for the latter) in April–September and built a conceptual model for the
occurrence of high O3 episodes. Finally, a sensitivity analysis is
done with the AQ data to evaluate potential O3 reductions in the
north of the BMA on Sundays, compared with weekdays, as a consequence of the
reduction of emissions of precursors.
Results showed a generalized decrease trend for regional background
O3 ranging from -1.1 % yr-1 to -1.6 % yr-1,
as well as the well-known increase in urban O3
(+0.4 % yr-1 to +3.2 % yr-1) and higher urban NO
decreasing slopes than those of NO2 (-2.2 % yr-1 to
-4.3 % yr-1 and -1.3 % yr-1 to
-2.0 % yr-1, respectively), which might account in part for the
urban O3 increase.
The most intensive O3 episodes in the north of the BMA have
O3 contributions from relatively high regional background O3
(due to a mix of continental, hemispheric–tropospheric and stratospheric
contributions) as well as O3 surface fumigation from the
mid-troposphere high-O3 upper layers arising from the concatenation
of the vertical recirculation of air masses (as a result of the interaction
of a complex topography with intensive spring–summer sea and mountain breeze
circulations (Millán et al., 1997, 2000; Gangoiti et al., 2001; Valverde
et al., 2016; Querol et al., 2017). However, we noticed that for most EHIT
days in the Vic Plain, the exceedance occurs when an additional contribution
is added to the previous two: O3 supply by the channeling of the BMA
pollution plume along the S–N valley connecting the BMA and Vic. Thus,
despite the large external O3 contributions, structural and
short-time local measures to abate emissions of precursors might clearly
influence spring–summer O3 in the Vic Plain. This is supported by
(i) the reduced hourly exceedances of the O3 information threshold
recorded on Sundays at the Vic AQ monitoring site (9 in 2005–2017) compared
with those on Fridays (47), as well as by (ii) the occurrence of a typical
and marked Sunday O3 pattern at the BMA AQ monitoring sites and an
also marked but opposite one in the sites of the Vic Plain, and by (iii) a
marked increase in remote sensing OMI-NO2 concentrations over the BMA
and northern regions during days of the P75 diurnal O3 concentrations
compared with those of the P25.
Finally, we calculated the difference between the P75 of Ox
diurnal concentrations recorded at two of the Vic Plain AQ monitoring
stations for Wednesdays minus those of the P25 percentile of
Ox for Sundays, equivalent to 1–2 days of emissions
reductions in the BMA. A maximum decrease potential by applying short-term
measures of 24.5 ppb (approximately 49 µg O3 m-3,
32 % decrease) of the diurnal concentrations was calculated. Obviously,
structurally implemented measures, instead of episodic ones, would result
probably in important additional Ox and O3
abatements because not only the local O3 coming from the BMA plume
would be reduced, but also the recirculated O3 and thus the intensity
of O3 fumigation on the plain. Therefore, it is highly probable that
both structural and episodic measures to abate NOx and VOC
emissions in the BMA would result in evident reductions in O3 in the
Vic Plain.
Data availability
All data used in this study can be accessed here: 10.17632/7bjyfwzh7z.3 (Massagué, 2019).
The supplement related to this article is available online at: https://doi.org/10.5194/acp-19-7445-2019-supplement.
Author contributions
JM performed the data compilation, treatment and analysis with the aid of
XQ, CC and ME. JM, CC, ME, JB, AA and XQ contributed to the discussion and
interpretation of the results. JM and XQ wrote the manuscript. JM, CC, ME,
JB, AA and XQ commented on the manuscript.
Competing interests
The authors declare that they have no conflict of
interest.
Acknowledgements
We would like to thank the
Department of Territory and Sustainability of the Generalitat de Catalunya
for providing us with air quality data, and the Met Office from Catalonia
(Meteocat) for providing meteorological data, as well as to NASA for
providing OMI-NO2 data and the ICAEN-UPC for providing solar
radiation measurements.
Financial support
This research has been supported by the Agencia Estatal de Investigación
from the Spanish Ministry of Science, Innovation and Universities and FEDER
funds under the HOUSE project (CGL2016-78594-R), by the Spanish Ministerio
para la Transición Ecológica (17CAES010/Encargo), by the
Generalitat de Catalunya (AGAUR 2017 SGR41) and by the Agencia Estatal de Investigación for the PhD grant (FPI BES-2017-080027) awarded to Cristina Carnerero.
Review statement
This paper was edited by Delphine Farmer and reviewed by two anonymous referees.
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