Meteorological background and diurnal cycles of pollutants
Two types of episodes that will be discussed in the following sections were
identified concerning the meteorological patterns and the O3
concentrations recorded.
Type A episode. Under “usual summer conditions”, with the Azores High
located west of Iberia, and a ridge of high pressures extending into southern
France, air masses in the western Mediterranean Basin rotate clockwise
(anticyclonic) during the day, following the combined sea breezes and upslope
flows at eastern Iberia and a simultaneous generalised compensatory sinking
is observed in the basin. During nighttime, drainage flows into the sea
develop at the coastal strip, subsidence over the basin weakens and the wind
over the sea is observed moving southward, transporting the coastal emissions
almost parallel to the shoreline (Gangoiti et al., 2001). At the same time,
Atlantic gap winds (through the Ebro and Carcassonne valleys), weaken during
daytime due to inland sea breezes and become strengthened during nighttime
(Millán et al., 1997; Gangoiti et al., 2001, 2006; Millán, 2014). In
such conditions, the air layers over the sea in front of Barcelona tend to
move within the southwesterlies during the day, following the clockwise
rotation, i.e. towards southern France and the Gulf of Genoa, and within the
northerlies (towards Valencia) during the night. Thus, direct transport of
O3 and precursors from the Fos–Berre–Marseille–Piombino (Livorno) area
towards the BMA is weak or null. However, indirect transport is more likely,
first into the sea during nighttime conditions, and then following the
daytime southwesterlies for the combined coastal sea-breeze and anticyclonic
gyre at the coastal strip of Catalonia, which could bring a fraction of the
referred O3 and precursors originated in southern France, together with
those emitted at the eastern coast of Iberia.
Type B episode. When the anticyclone establishes over central Europe with
relative low pressure to the west over the Atlantic, the flow pattern over
the western Mediterranean changes: southerly winds blow at height over
eastern Iberia, while at ground level, gap winds may weaken or stop the
Mistral and the tramontana winds in the Gulf of Lion, and Barcelona could
then be directly affected by O3 and precursors, coming with the
easterlies blowing at the marine boundary layer (emissions from Corsica,
Sardinia and Italy). However, under these atmospheric conditions O3
levels did not reach the observed values found during type A episodes, and
the O3 daily records did not show the classical pattern of accumulation
from one day to the next, characteristic of the highest O3 episodes in
the western Mediterranean (Millán et al., 1997, 2000; Castell et al.,
2008a).
Under the above “usual summer conditions”, Millán et al. (1991, 1996a,
b, c, 1997, 2000, 2002), Gangoiti et al. (2001) and Castell et al. (2008a)
demonstrated the vertical recirculation of O3-rich masses in the western
Mediterranean, with O3 being formed from precursors transported inland
by the combined upslope and sea-breeze winds. O3 loaded air masses,
elevated by topography and sea-mountain breezes will be transported back to
the coastal area at a certain altitude during the day and accumulates in
elevated stably stratified layers at the coastal areas during the late
evening and night. During nighttime and at ground level O3 depletion
dominates mainly in urban and industrial centres, driven by reaction with new
emissions, which at the coastal area are transported offshore within the
stable surface drainage flows.
The synoptic atmospheric situation in July 2015 was characterised by an
intense high-pressure system over central and southern Europe during almost
the whole month (Fig. 2). Type A and B scenarios alternated, transporting
warm air masses from North Africa towards higher latitudes by the
anticyclonic dynamic and reaching extremely high temperatures in Europe. The
stagnation of air masses induced a regional meteorological scenario in the
area under study, characterised by local/regional recirculations and
sea–land breezes, both channelled by the complex topography. The flow
pattern, together with the observed stably stratified layers developed up to
a height of 2000–2500 m a.s.l. (Fig. 3) associated with subsidence,
enhanced the accumulation of pollutants and caused several pollution episodes
in the northeastern Iberian Peninsula. Coastal and pre-coastal locations
(Barcelona) were mainly affected by daily sea breezes, starting blowing from
the east (around 08:00 UTC) and turning progressively to the south and
southwest. The sea breezes were channelled through the valleys, which are
mainly located following a north-to-south axis, and arrived at the monitoring
stations predominantly from a southerly direction. However, during the night
atmospheric conditions were much more stable with flow patterns dominated by
land breezes from NNW.
Data from the non-tethered balloon measurements (at VIC) of
temperature, relative humidity and particle number concentrations performed
from 10:00 to 11:30 UTC on 16 July 2015. Red lines identify the limit
between different atmospheric layers.
The VIC site was characterised by stagnant conditions during the day,
reaching the maximum wind speed (4 m s-1 on average) at around
15:00 UTC, when sea-breeze intensity was at the highest (Fig. 4). During the
night very light winds blew from the north (Fig. 4). During the periods
14–20 July 2015 (episode type A) and 3–6 July 2016 (episode type B) the sea
breeze blew from 10:00 to 18:00 and 10:00 to 21:00 UTC, respectively, but in
the first episode the wind speed was higher (maximum of 2.7 m s-1 as an
average for the period) and maximal at 14:00 UTC, whereas in the second wind
speed was lower, with a maximum mean value for the period of 2.4 m s-1
at 17:00 UTC, but only 1.5 m s-1 at midday.
Averaged ground O3 concentrations during the type A episode recorded at VIC
were clearly influenced by these wind patterns, showing a typical midday
peak, followed by a higher peak at 13:00–14:00 UTC probably caused by the
transport of BMA air masses by the breeze (Fig. 4). Mean O3 levels
during this A episode reached 195 µg m-3 at 13:00 UTC.
During the type B episode average O3 levels were also very high
(142 µg m-3 at 14:00 UTC) but clearly lower than during the
A episode (Fig. 4).
Intensive surface measurements were only available for 10–17 July 2015 (when
the mobile laboratory was working at VIC). Average SO2 levels for this
period (included in the type A episode) showed a similar daily pattern to
that of O3 (Fig. 4) pointing to a probable Hewson type I fumigation
process (Hewson, 1964; Geiger et al., 1992); when midday convective flows that
abate the surface a SO2-rich layer accumulated in the limit of the
boundary layer. Ground-level concentrations of BC, NO2 and PMx
showed a similar daily pattern driven by stagnation and traffic rush hour,
with maximum concentrations around 06:00 UTC (08:00 local time, Fig. 4).
Finally, extremely high concentrations of NH3 (this is one of the most
intensive farming regions of Spain and mean values of the Vic Plain
dosimeters reached 30 µg m-3 NH3 for 1–31 July 2015)
followed the typical midday maximum due to evaporative emissions from
fertilisers, but the rapid increase of the wind speed and dilution by the
growth of the planetary boundary layer (PBL) thickness (see vertical profiles of temperature, aerosols
and O3 at VIC in following sections) probably account for a relative reduction
of ground-level NH3 concentrations during the central hours of the day
(Fig. 4).
Top: mean hourly (UTC) values for meteorological parameters and
O3 ambient air concentrations measured during the periods 3–6, 14–20 and
10–17 July 2015 recorded at the permanent VIC XVPCA station (O3) and at
the Gurb meteorological station (temperature, humidity and wind patterns,
Meteocat) located 1 km to the north of VIC. Bottom: mean hourly concentrations of
other gaseous and particulate pollutants measured at VIC with the laboratory
van (only during the period 10–17 July 2015) co-located with the XVPCA station.
The varying diurnal and nocturnal air mass patterns in the Vic Plain are also
shown by the PM2.5 chemical composition. Figure S5 shows the 8 h
concentration patterns of selected components during the week period of
10–17 July 2015, including several days (14 to 17 July 2016) of the type A
episode defined above, affected by polluted air masses from the BMA.
In addition to the regionally transported O3, concentrations of
elemental carbon (EC) and traffic and industry-related metals (including Zn,
Cu, Pb, Sn and Sb) were notably enhanced at the end of the week, and were
attributed to local sources. This enhancement was most obvious during the
00:00–08:00 UTC period (Fig. S5), under calm or northerly low wind
(drainage slope winds) carrying metallic pollutants from the Cu smelter
located 13 km to the north of Vic, and leading to high Cu, Zn, Sn, W, Pb and
Sb concentrations on the nights of 15 and 16 July 2015 (Fig. S5). The
increase in EC was related to local traffic emissions during the morning rush
hour as deduced from the peaking MAAP BC concentrations during
05:00–08:00 UTC (07:00–10:00 local time), up to 5 µg m-3
hourly BC, when compared to 3 µg m-3 recorded as maximum
traffic rush hour concentrations in the preceding days (data not shown). In
contrast, the rise of organic carbon (OC) concentrations observed during
08:00–16:00 UTC is attributed to the formation of secondary organic
aerosols.
Sulfate concentrations did not show any trend, as expected from secondary
inorganic components present in relatively homogeneous concentrations on a
regional scale, whereas nitrate (and partially of ammonium)
concentrations increased during the 00:00–08:00 UTC periods as a result of
gas to particle partitioning (Fig. S5) due to the thermal instability of
ammonium nitrate (Hertel et al., 2012), under typical high daytime
temperatures reached in July 2015. Interestingly, the stronger southerly
winds during the daytime in the second part of the week (see below) appear to
have brought polluted air from the BMA as signalled by slightly higher V
concentrations (tracer of fuel oil combustion), but the fumigation from
high strata (polluted air masses that were injected the previous day or days)
might also account for these SO2 and V increases.
The concentrations of mineral matter and all its components (Al, Fe, Mg, Li,
Ti, Rb, Sr, Ti) were constant during the week, with relatively higher
concentrations in the 08:00–16:00 UTC samples (Fig. S5), indicating a
higher resuspension caused by stronger afternoon winds. The increment on the
15 July 2015 (08:00–16:00 UTC) was attributed to resuspension of local
dust, given that the occurrence of African dust outbreaks was not observed
during this period.
O3 hourly concentrations recorded at the coastal (BEG, blue)
and remote inland western pre-Pyrenean (MSC, clear green, 1570 m a.s.l.)
sites, two urban background sites of Barcelona (PLR, CTL, grey and black), two
urban sites in the northern periphery of Barcelona metropolitan area
(GRA, MON, orange and yellow), the inner Vic Plain sites (TON, VIC and MAN,
red, pink and violet) and the remote eastern pre-Pyrenean site of PAR
(brown), during July 2015.
The free sounding measurements carried out at 11:00 UTC on 16 July 2015
revealed stratified air masses up to 3000 m a.g.l. (Fig. 3). The vertical
profiles of potential temperature, water vapour and aerosol concentration
distributions can be used for the identification of atmospheric layers
presenting different properties: a lower layer up to about 1100 m a.g.l.
characterised by a relatively high aerosol concentration, well mixed and with a
relatively high and uniform water vapour content. A clear discontinuity between
1100 and 1500 m a.g.l. limits the mentioned lower layer and a series of
stably stratified layers up to a height of 3000 m a.g.l. This layering of
pollutants is probably related to the development of regional and mesoscale
convective cells driven by the combined upslope and sea-breeze flows
developed the day before (Millán et al., 1997).
O3 and Ox episodes
Figure S6 shows the average O3 and Ox ground-level concentrations
recorded in July 2015 in the study area at the XVPCA air quality monitoring
network and with the passive dosimeters. Ox maximum concentrations were
recorded at the Vic Plain area and in the coastal sites northeast of
Barcelona. This may be due to the high O3 concentrations in Vic and to a
higher proportion of primary NO2 (emitted mainly from diesel engines,
and not formed in the atmosphere from NO titration by O3) in the coastal
cities, respectively.
In July 2015, the O3 hourly information threshold was exceeded a total
of 74 times at the XVPCA stations of Catalonia, 57 taking place in the Vic
Plain stations (TON, VIC and MAN), and 69 in the surrounding areas
(pre-Pyrenees, High Llobregat River and Montseny).
Figure 5 shows hourly O3 concentrations for the study period from
selected monitoring sites. O3 concentrations recorded at a coastal
(Begur, BEG; blue, 200 m a.s.l.) and a remote inland western pre-Pyrenean
site (MSC, light green, 1570 m a.s.l.) (Fig. 5a) show relatively narrow
diurnal variations and multiday episodes, with low or enhanced
concentrations, according to meteorological fluctuations (accumulation and
air mass renovation cycles of 3 to 12 days cause a wider O3 and Ox
concentrations range than the typical daily cycles evidenced in most of the
other sites). O3 variations at the coastal BEG are opposed, in periods
such as 1–3, 10–12 and 26 July 2015 and several periods from
14–20 July 2015, to those at the inland MSC. As shown by the polar plots
from Fig. 6, relatively low O3 concentrations (but still high in
absolute terms) were recorded at the BEG coastal site (easternmost site in
this figure) when the wind blows from the sea, whereas polluted air masses
are transported towards the inland remote MSC (westernmost location in the
figure) site under the same meteorological conditions. Conversely, when
westerly winds blow, the inland remote MSC site received relatively clean air
masses with low O3 (Fig. 6), which are progressively loaded with
regional pollution as these are transported towards the coastal BEG site.
Polar plots of hourly O3 concentrations at the real-time
measurement sites.
O3 and Ox (O3 + NO2) hourly concentrations
recorded at the coastal (BEG, blue, at this site only O3 is available
due to the lack of NO2 measurements), an urban background site of
Barcelona (CTL, black), an urban site in the northern periphery of
Barcelona metropolitan area (GRA, orange), the intermediate inland rural
site of MSY (720 m a.s.l., green), and the inner Vic Plain site (TON, red)
during July 2015. The pink and blue squares mark the A and B O3 and
Ox episodes distinguished in this study, respectively.
Data from two urban background sites of Barcelona (PLR and CTL, 81 and
5 m a.s.l., grey and black in Fig. 5b) show evidence of a high nocturnal
O3 consumption, with differences due to local NOx traffic
emissions. Following the transport of air masses by combined breezes, the two
sites located in the northern periphery of the BMA, along the Besòs River
valley (GRA and MON, 140 and 33 m a.s.l., orange and yellow in Fig. 5c; 20
and 6 km from BMA in NE an NNE directions, respectively) show local O3
production, with higher midday concentrations, while very low nocturnal
levels reflect again the intensive O3 consumption (in a densely
populated basin). O3 concentrations were closer between GRA and MON than
between the two Barcelona urban sites (PLR and CTL).
Relevant O3 net production and fumigation can be readily seen in the
inner Vic Plain (TON, VIC and MAN; 620, 498, 460 m a.s.l.; red, pink and
violet in Fig. 5d; 45, 55 and 62 km from BMA in a NNE direction,
respectively) as well as at the remote eastern pre-Pyrenean site of Pardines
(PAR, brown, 1226 m a.s.l., 102 km from BMA in a NNE direction), where
O3 formation and fumigation seems to have already reached its maximum,
and similar O3 concentrations were recorded at all sites during the
midday increase. This suggests that the intensity of O3 formation and
fumigation was clearly reduced in the Vic Plain–Pyrenees transect with
respect to the Barcelona–Vic Plain (an intermediate production place would be
MSY (720 m a.s.l.; green in Fig. 7, 39 km from BMA in a NE direction).
Polar plots of GRA, TON, MSY, VIC, MAN show clearly that the highest O3
levels were recorded with wind blowing from the direction where BMA is
located (Fig. 6).
As can be observed in Figs. 5 and 7, during two periods (1–2 and
7–20 July 2015) O3 concentrations increased progressively from
Barcelona city towards the northern BMA (GRA and MON), the intermediate MSY
regional background area and towards the northern Vic Plain sites; and from
there it slightly decreased towards the eastern pre-Pyrenees (PAR) following
the midday–afternoon combined breeze transport (Figs. 5 and 7). During these
days, no exceedance of the information threshold was produced in the urban
environment; only sporadic peak concentrations above the human protection target
value were recorded in the close surroundings of Barcelona. However, frequent
exceedances of both thresholds were recorded in a regional transport context
towards the north of the BMA.
While differences in O3 concentrations between TON, GRA, MSY, BEG and
CTL were observed during the period 3–6 July 2015 (type B episode), Ox
concentrations show a very similar behaviour along the Vic Plain, both
qualitatively and quantitatively (Fig. 7, Ox is not reported at BEG due to
the lack of NO2 measurements). Conversely, in the period 7–20 July 2015
(which includes the type A episode), characterised by a change in the synoptic
conditions, differences in daily maximum Ox values resemble the same
behaviour of O3 alone, with a positive and marked inland gradient.
O3 concentrations at BEG, a coastal site far in the northeast, were
higher during the former period and showed low intra-day variation,
indicating probable long-range transport of polluted air masses (Fig. 7).
O3 and Ox concentrations at the regional background site (MSY,
720 m a.s.l., green in Fig. 7) depict also the meteorologically influenced
patterns (in the sense previously described for BEG and MSC), but with a
clear overlapped and pronounced daily fluctuation, with marked higher
concentrations indicating O3 generation from a regional origin and fumigation from upper layers,,
especially on 1–2 and 7–20 July 2015 (Fig. 7).
Diurnal O3 concentrations in the Vic Plain (around 460–620 m a.s.l.)
were markedly higher than at the coastal (CTL, PLR) sites, and slightly
higher than at mountain sites (MSY, PAR and MSC, from 720, 1226 and
1570 m a.s.l.) during the 1–2 and 7–20 July 2015 periods. The O3
hourly information threshold of 180 µg m-3 was exceeded
55 times in the Vic Plain (three sites), with 50 of these exceedances taking
place during 1 July 2015 and 14–20 July 2015. For these exceedances, an
hourly contribution of up to 150 µg m-3 of Ox (mostly
O3) both from fumigation of recirculated return layers (injected at an
altitude of 1500–3000 m a.g.l. in the prior day(s)), and from transport
and photochemical generation of O3 of the BMA plume, might be estimated
based on the differences of the Ox early afternoon maxima recorded at
the coastal BMA sites (CTL, PLR) and the ones in the Vic Plain (TON, MON,
VIC). Thus, as shown in Fig. 7, the 14–18 July 2016 midday maxima recorded at
CTL (into BMA) range between 38 and 62 ppb Ox, on an hourly basis, whereas
at TON (in the Vic Plain) they reach 102–115 ppb. Accordingly,
differences of 50–73 ppb Ox (close to 100–150 µg m-3
Ox) between CTL and TON can be estimated for these days. Furthermore, different intensity and duration of fumigation from upper high O3 layers might also contribute to the higher inland O3 surface concentrations since the PBL depth is much higher at the inland Vic Plain than at the coastal site, where sea breeze flows favour a lower growth of the PBL.
Mean hourly levels of O3 and Ox (O3 + NO2)
for sites in a south (coast) to north (inland) transect (CTL, GRA and BEG,
and MSY, TON, VOIC, MAN and PAR, respectively) following the inland transport
of pollutants from the coast, and maxima time shift according to the sea-breeze transport (right) for the periods 3–6 July 2015 (B type episode,
left) and 14–20 July 2015 (type A episode, right). Time is UTC.
Type A episode (14–20 July 2015)
During this episode, a progressive time shift of the daily hourly O3 and
Ox maxima was observed from the Barcelona area (10:00 UTC, at CTL into
the BMA) towards the metropolitan periphery (11:00, at GRA), the intermediate
mountain sites (13:00, MSY, 39 km from BMA), the Vic Plain (12:00, 13:00 and
14:00, TON, VIC and MAN, 45, 55 and 62 km from the BMA, respectively) and
the northern pre-Pyrenean site (16:00, PAR, 102 km from BMA) (Fig. 8). As
described above, this variation points to the process of O3 and Ox
formation with a mean Ox difference between the urban-coastal sites and
the Vic Plain hourly maxima of up to 73 ppb Ox (around
150 µg m-3) for the TON site when Ox hourly
maxima from CTL are subtracted (Fig. 7), with maximum average O3 hourly levels of
around 200 µg m-3. These Ox differences are mostly due
to O3 differences (Fig. 8). Accordingly, during these intense O3
pollution episodes, more than 50 % of the Ox and O3 hourly
maxima concentrations are attributable to (i) O3 contributions from the
previously mentioned surface fumigation of recirculated strata (over the
VIC–MAN–TON area) containing the polluted air masses injected the day before
by complex topographically induced circulations, and to (ii) the local O3
generation and surface transport of the BMA plume into inland valleys.
Attributing these O3 exceedances to local/regional causes is also
supported by the spatial distribution of the hourly O3 maxima, the
number of hourly exceedances of the information threshold, the time shift of
the exceedances at the different sites (as moving towards the north)
(Fig. 9), and the polar plots of hourly O3 concentrations pointing
towards BMA as the main source region (Fig. 6). It is important to note
that, as previously stated,
in the coastal sites the PBL height is markedly reduced when compared with
the inland regions and then the capture of these high-altitude O3-rich
layers by the PBL growth and the consequent fumigation on the surface is less
probable in the coastal areas than in the inland ones.
Top: hourly O3 maxima (and number of hours exceeding
180 µg m-3) in the study sites with real-time O3
measurements (shadowed areas indicate two different degrees of exceedances,
1–3 and 13–23 h). Bottom: frequency of occurrence of hourly (UTC)
O3 exceedances of 180 µg m-3 along the day; both for
July 2015.
Thus, during the A episode, O3 has mostly a major local/regional origin
(with Ox maximum hourly levels progressively increasing from 166 to
246 µg m-3 from the BMA to the Vic Plain). The concatenation
of daily cycles of regional/long-range recirculation of air masses and
regional/local O3 production in the A episode accounted for the
accumulation of Ox and the consequent exceedance of the hourly
information threshold. Castell et al. (2008a) have already reported a
correlation between their “recirculation factor” (a meteorological
parameter devised to increase with the concatenation of days with regional
vertical recirculation of air masses) with the occurrence of O3 episodes
in 2003. The relevance of these recirculations in originating these high
O3 episodes in southern Europe has been highlighted already, not only by
scientific papers by the CEAM team but also assumed by the European
Commission (EC, 2004).
Figures 10 and 11 show results for the vertical profiles (0–1100 m a.g.l.)
of O3 concentrations, particle number concentrations for particles
> 3 nm (red), 0.3–0.5 µm (blue), 0.5–1.0 µm
(brown), ambient temperature, relative humidity and wind direction, obtained
at the beginning of the type A episode (from 14 to 17 July 2015).
Vertical profiles of particle number concentrations for particles
> 3 nm (red, N3), 0.3–0.5 µm (blue, PM0.3-0.5),
0.5–1.0 µm (maroon, PM0.5-1) and wind direction obtained with
the tethered balloon measurements on 14 and 17 July 2015.
Vertical profiles of O3, temperature and relative humidity
obtained with the tethered balloon measurements on 14 and 17 July 2015.
In the profiles from 07:06 to 08:21 UTC on 14 July 2015, a boundary
layer (150 to 250 m thick) with relatively high levels of N3 (0.8 to
2.0 × 104 cm-3) was differentiated from the free troposphere (0.2 to
0.8 × 104 cm-3) (Fig. 10). However, in the profile
obtained from 09:42 to 10:52 UTC on 17 July 2015, the growth by convective
turbulence accounts for a homogeneous boundary layer and profile of
N0.3-0.5 below 1000 m a.g.l. (Fig. 10). Inside the PBL
nucleation occurred (yellow to red areas in Fig. 12 for 16 July 2015)
regionally driven by photochemical processes. Minguillón et al. (2015)
showed the occurrence of these nucleation events into the PBL as
convective transport elevates and dilutes air masses from polluted areas
under high insolation in Barcelona. During the period 14–16 July 2015 nucleation
episodes occurred occasionally, but only inside the PBL. On
17 July 2015 at 09:42–10:52 UTC new particle formation occurred probably at
relatively high altitudes, also inside the PBL, as deduced from
the high N3 levels measured from 400 to 1000 m a.g.l., with
concentrations reaching 1 × 104 cm-3, while
simultaneously low concentrations
(< 0.3 × 104 cm-3) were measured at ground level
(Fig. 10). This vertical gradient is not observed for the coarser particles
(N0.3-0.5 and N0.5-1) and O3 (Figs. 10 and 11, for which
relatively constant levels were measured inside the PBL),
suggesting new particle formation.
Time variation of altitude, temperature, relative humidity, N3,
particle number size distributions and O3 concentrations during the
tethered balloon measurements on 16 July 2015. 1–3 illustrate the
nucleation episode recorded at surface level with particle number size
distributions, and 4 the typical regional background N size distribution at
about 300 m above ground.
On 14 July 2015 at 07:06–08:21 UTC a well-stratified atmosphere (Fig. 11) with
both thermal and O3 layers is observed, with a general upward increasing
trend for O3 from 40 µg m-3 at ground level to much
higher levels in different strata, one reaching
150 µg m-3 in strata at 500 m a.g.l. and others of 140, 100 and
40 µg m-3, at 300, 800–1000 or
400 m a.g.l. respectively, reflecting, in addition to stratification of
O3 concentrations in altitude, the effect of surface depletion by NO
titration and by deposition during the night (see in Fig. 11 the progressive
O3 depletion from 150 µg m-3 at 500 m a.g.l. to
40 µg m-3 at surface levels). From 13:49 to 15:03 UTC on
14 July 2015 (Fig. 11) the vertical profile changed substantially, with an
already unstable atmosphere near the ground, showing very high surface
O3 concentrations of 217 µg m-3 that increase up to
330 µg m-3 in a layer around 100 m a.g.l., decreasing again
through an upper layer with values of 240 µg m-3 until
300 m a.g.l. (where measurements were not available due to instrumental
problems). This 100–200 m a.g.l. high O3 layer agrees with the
modelled O3 concentrations for the study area (Toll and Baldasano, 2000;
Barros et al., 2003; Gonçalves et al., 2009) and reflects elevated
O3 concentrations due to local production and transport of O3, that
decrease from 100 m a.g.l. to the surface due to its titration, consumption
and deposition. On 15 and 16 July 2015, a similar upward increasing O3
gradient was observed in the early morning (Fig. 12). On 17 July 2015 at
07:39–08:40 UTC O3 concentrations were relatively constant, but also
showed a strongly stratified profile, in the range of
100–165 µg m-3 in the lower 500 m. In the last profile, from
09:42 to 10:52 UTC, O3 concentrations increased from 140 to
200 µg m-3 for 200 to 1000 m a.g.l., but again a maximum
close to 200 µg m-3 was observed at the same height around
100 m a.g.l. (Fig. 11).
Vertical profiles of BC (5 min time resolution) and O3 (10 s
time resolution) at VIC.
Thus, vertical profiles of the type A episode are characterised in the early
morning by a strong stratification, showing low ground-level O3
concentrations, due to low production (low insolation) and/or consumption
(titration and deposition), and increasing concentrations with altitude. This
variation is related to prevailing meteorological conditions enhancing local
recirculation or larger-scale transport with high O3 masses injected
(the day before) at certain altitudes by vertical recirculations into the
residual layer, above the nocturnal surface stably stratified boundary layer.
Nevertheless, during specific days, homogeneous O3 vertical profiles up
to 1000 m a.g.l. (the maximum height reached with captive sounding) were
also evidenced, but probably not maintained at higher levels (where we were
not able to measure with our system). Thus, as shown by the 4500 m profile
measured with the free sounding on 16 July 2016 (Fig. 3), high PM (and
probably O3) strata are present between 1500 and 3000 m a.g.l., these
being probably the polluted air masses injected the day before in the northern
mountain ranges and recirculated to the coast at certain altitudes (see
modelling outputs below). On the other hand, with constant southerly winds
(from the coastal area to the Vic Plain) usually associated with the combined
sea-breeze and upslope flows, O3 was enriched in the lower 100–200 m
atmospheric layer, generated by the intensive local photochemical production.
O3 concentrations reached maximal values (up to
330 µg m-3) on the top of this layer, while they decreased at
lower heights by titration and deposition, although hourly levels of
225 µg m-3 were still recorded. These results are consistent
with the gradient of O3 concentrations between the Vic Plain (around
500 m a.s.l.) and the MSY mountain site (720 m a.s.l., and closer to
the sea) during the episodes (Figs. 7 to 9). At higher altitude, O3
concentrations slightly decreased but were still high
(150–240 µg m-3) due to the O3 formation in air masses
constantly transported from the coastal area, which also incorporates O3
and precursors recirculated the day before, as is shown in what follows.
Interesting results are also obtained by comparing the vertical profiles of
BC and O3 (Fig. 13). BC is a tracer of local primary pollution at ground
level, and of the potential transport and stratification of regional/local
primary pollutants (together or not with regional O3) when present at
high altitude. On 14 July 2015 07:06–08:21 UTC, at 350 m a.g.l. (and
similarly for 15–17 July 2015 but at varying heights, 100–350 m a.g.l.) a
clear discontinuity is evidenced with sudden and simultaneous decreases of BC
and O3 above these heights. The relatively high BC levels within the
lower layer suggest nocturnal accumulation, while O3 appears in
strata (with low values near the ground due to titration and deposition) and
with a high concentration just above that level (350 m), now with low BC
concentrations. There is a further upward decrease of BC and an increase of
O3 up to the limit of the sounding (870 m).
The occurrence of an O3 maximum layer around 100–200 m a.g.l., on top
of the nocturnal stably stratified PBL, reinforces the idea of an
important local production contributing to an upward increase of O3
inside the layer. Finally, at the highest altitudes reached in this study
(900–1000 m a.g.l.), BC and O3 concentrations were often
anti-correlated or unrelated, possibly more related with aged air masses
recirculated within the whole region and with a mixed origin: including
local-to-regional sources, more distant over the W-Mediterranean or even from
hemispheric transport of air masses as reported by UNECE (2010).
Figure 14 shows mean O3 hourly concentrations recorded at VIC for the
episodes A and B, as well as mean wind speed and direction. Mean hourly
concentrations are characterised by an increase until 10:00–11:00 UTC,
followed by an inflexion point and a more marked increase, with a maximum
between 13:00 and 14:00 UTC, and then a progressive decrease, more marked in
episode A. As stated above, processes contributing to increased levels
were attributable to fumigation, photochemical production and transport of
high O3 air masses, all controlled by insolation. Millán et
al. (2000) described this characteristic diurnal O3 pattern typically
for inland valley stations (as in our case around 75 km from the coast),
where the first O3 increase is attributed to O3 contributions from
surface fumigation of high recirculated return strata as well as from the
arrival of higher O3 air masses transported by sea breeze and the local
photochemical production from precursors. On the other hand, the second
O3 concentration “hump” is coincident with maximum wind speed and
probably corresponds to a more intensified sea-breeze transport compared with
local photochemical formation and fumigation. Figure 14 shows that the two
O3 increases (and consequently the contributions from the three above
processes) are more pronounced in the type A compared to the type B episode,
and that the second maximum (more associated with inland surface transport by
sea breeze) is wider, coinciding with a shift of the maximum wind speed
towards the late afternoon, in the B episode.
Mean hourly O3 concentrations, and wind speed and wind
direction for episodes A and B, showing higher levels in the A episodes
for the two O3 maxima.
Maps of simulated NO2 and O3 concentrations at ground level with wind vectors at 10 m a.g.l. and at 1000 m a.g.l. with wind vectors at the same altitude, for selected hours on 3 July 2015.
Maps of simulated NO2 and O3 concentrations at ground level with wind vectors at 10 m a.g.l. and at 2000 m a.g.l. with wind vectors at the same altitude, for selected hours on 15 July 2015.
Modelling outputs for the A episode point to light winds from the south,
transporting pollutants from the BMA towards northern areas (including the
Vic Plain), and triggering the hourly O3 exceedances under the effect of
the sea- and land-breeze transport. Thus, Figs. 15 and 16 show the
horizontal wind vector at 10 m a.g.l. and NO2 and O3
concentrations both at ground level and at a height above the surface layer,
at different hours for two representative days of the type A and B episodes,
respectively. During the type A episode day (15 July 2015), the effect of the
land-breeze transport accumulates NO2 over the sea during the night,
starting intense O3 production when sun rises and sea breezes start the
inland transport. Maximum concentrations of O3, exceeding
180 µg m-3 were calculated by the model and measured at the
stations located in the Vic Plain (TON, VIC and MAN, Fig. 16), although the
model overestimated maximum O3 concentrations in TON and VIC and delayed
the hourly maximum value in all stations. The vertical distribution shows an
important accumulation of around 110 µg m-3 trapped in a
reservoir layer at around 1500 m a.s.l. during the night (Fig. 17), which
will fumigate downwards into the new developing mixing layer during the
following hours. Local O3 production from fresh precursors accumulated
during the night in the stably stratified surface layer and then progressed
inland along the midday hours. This results in an O3-enriched plume
within a layer of 1000–1500 m depth in the late afternoon, following the
model results (Fig. 17). This mixing layer also incorporates O3 from
upper reservoir layers after fumigation during the inland travel. The O3
located at upper levels can recirculate back into the sea and will be
potentially available to be transported inland (Millán et al., 1997,
2002), to start a new cycle the following day.
Spatial distributions of simulated O3 concentrations and wind
field vectors in the south–north vertical cross-section for different hours
on 3 and 15 July 2015.
Type B episode (3–6 July 2015)
As opposed to the type A episode, during the type B episode and on 22–31 July 2015,
despite the high O3 and Ox concentrations, the concentrations
were very similar in the urban and remote coastal sites and all along the
northern sites, including the Vic Plain. Hence, the averaged Ox hourly
concentrations of all the study sites were close to those at the coastal
urban site in Barcelona CTL (and in the case of the O3 close to the
remote coastal site of BEG) compared with the large differences reported for
the A episode (Fig. 8). The high Ox peak measured at the urban site
during the mornings of the B period (Fig. 8) and from 08:00 to 10:00 UTC in
the average hourly patterns (Fig. 9) is probably due to the contribution of primary
NO2. According to Carslaw et al. (2016) the Euro 1 to Euro 2 diesel
engines in Europe (early 1990s) emitted 5–10 % of primary NO2 and
90–95 % of NO, whereas the Euro 4 to Euro 5 equivalent engines (2004 and
2009 onwards) emit 16–29 % of primary NO2 and 71–84 % of NO.
Also as opposed to the A episode, during the B episode, Ox levels varied
in a very narrow range from east (coastal) to west (mountains, MSC site) and
from south (BMA) to north (Vic Plain) and at different heights (from
Barcelona and BEG at sea level to MSC at 1570 m a.s.l.). Following the
results of the model in Figs. 15 and 17, O3 does not recirculate
around the region in this period. There is no accumulation from one day to
the next in reservoir layers located over the region. Southerly winds blow at
height during the whole period and the combined sea-breeze and upslope winds
developed at lower layers during daytime, after coupling with the
southerlies, and vent out the O3 production and the rest of the pollutants to
the north. The circulation is open, as opposed to the type A episode, which
show a closed circulation (it is never completely closed) (Millán et al.,
1997, 2000). Unfortunately, vertical profiles of O3, UFP, PM and BC
profiles were not obtained for this episode.
Model outputs also evidence a net night and early morning transport of
O3 at lower layers from east and northeast during the B episode,
supporting the hypothesis of a regional transport from southern France,
advecting aged air masses to the whole region, while O3 and its
precursors from the BMA were transported during the morning to the southwest
regions (Fig. 15) giving rise to hourly O3 exceedances in some stations
situated in this area. Figure 15 also shows that during this episode
(3 July 2015) the combined sea breeze and upslope wind transported O3
and precursors to the western pre-Pyrenees area, and values lower than
180 µg m-3 were measured and modelled in all monitoring
stations. The vertical distribution of O3
also shows relatively low concentrations (as compared to 15 July) over most of the domain (Fig. 17).