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Origin of high summer PM10 and TSP concentrations at rural sites in Eastern Spain
Atmospheric Environment 36 (2002) 3101–3112
Origin of high summer PM10 and TSP concentrations at rural sites in Eastern Spain Sergio Rodr!ıgueza,*, Xavier Querola, Andre! s Alastueya, Enrique Mantillab b
a Institute of Earth Sciences ‘‘Jaume Almera’’, CSIC, C/Lluis Sol!e i Sabar!ıs, s/n, 08028, Barcelona, Spain ! ! Centro de Estudios Ambientales del Mediterraneo, CEAM. Parque tecnologico, C-4, sector oeste, 46980, Paterna, Valencia, Spain
Received 18 February 2002; received in revised form 18 March 2002; accepted 22 March 2002
Abstract Concentrations of airborne particulates undergo a seasonal evolution characterised by a summer maximum in rural areas in Eastern Spain. In the summer months the daily mean concentrations of PM10 and TSP (PM) experience wide variations. In 3-day periods, increases in the PM concentrations from 15 to 30–40 mg/m3 are frequently reported, and increases from 15 to 40–60 mg/m3 occur several times throughout the summer. These variations are simultaneously reported at rural stations throughout the flat Ebro basin (600 m a.s.l.) and at mountain sites located at high altitude (>1000 m a.s.l.). The origin of high and low PM episodes was investigated by correlating PM levels with the concentrations of gaseous pollutants, and making use of meteorological analysis and satellite observations. The highest PM events (daily concentrations in the range 40–60 mg/m3) were documented during outbreaks of African dust. The second highest PM events (daily concentrations in the range 20–45 mg/m3) were recorded during regional episodes associated with ozone events. These summer regional PM episodes were induced by the abrupt orography surrounding the Western Mediterranean and by the regional meteorology, which favour the ageing of polluted air masses into the basin. These regional events occur in a synoptic meteorological context characterised by a weak pressure horizontal gradient over the Western Mediterranean often associated with the development of the Iberian thermal low, when advection of air masses is not significant. In this meteorological context, the transport of particulate pollutants from urban/industrial to rural sites is brought about by the breeze circulation at the coastal (sea breeze) and mountain (mountain breeze) sites. The persistence of this breeze circulation for several days (periods of up to 2 weeks were reported) results in a low renovation of air masses leading to an accumulation of airborne particulates in the regional atmosphere. The lowest PM events (daily concentrations o20 mg/m3) were reported during abrupt entries of Atlantic air into the Mediterranean. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Aerosols; PM10; Saharan dust; Ozone; Regional pollution; Mediterranean; Spain
1. Introduction Regional levels of ambient airborne particles vary as a function of the dominant natural and anthropogenic emissions, the atmospheric conditions (e.g. radiation, temperature or humidity) and the levels of reactive gases and the regional meteorology.
*Corresponding author. E-mail address: [email protected] (S. Rodr!ıguez).
A number of studies have pointed out that the dynamic of polluted air masses in the Western Mediterranean is considerably influenced by local and mesoscale meteorological processes (Milla! n et al., 1997, 2000; Salvador et al., 1999; Soriano et al., 2001; Gangoiti et al., 2001). In Eastern Spain, transport of polluted air masses in summer is influenced by the breeze circulation and the Iberian thermal low (ITL) development (Mill!an et al., 1997). Under this scenario, the transport of pollutants undergoes marked daily cycles. Thus, at coastal sites, the sea breeze induces an inland
1352-2310/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 2 ) 0 0 2 5 6 - X
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transport during the day and a seaward return of pollutants at night. Moreover, the polluted air masses undergo vertical re-circulations over the Eastern coast of Spain due to the following factors: the inland up-slope winds over the coastal ranges, the westerly winds over the top of the mountains, the subsidence over the Mediterranean sea and the inland entry of the sea breeze. Thus, the breeze circulation is typically associated with high surface ozone concentrations (Mill!an et al., 1997). In addition to these regional circulations, synoptic scale meteorology induces frequent outbreaks of African dust in summer (Rodr!ıguez et al., 2001). A number of studies have reported that in rural environments in the Mediterranean, airborne particulate concentrations undergo a seasonal cycle characterised by a summer maximum (Bergametti et al., 1989; Kubilay and Saydam, 1995; Querol et al., 1998a, b; Rodr!ıguez et al., 2001, 2002). To the best of our knowledge, this seasonal cycle has not been reported at rural sites in Central and Northern Europe (e.g. Monn et al., 1995; . et al., 2001). In this Turnbull and Harrison, 2000; Ro. osli study the focus is on the influence of the typical summer meteorology on the levels of PM10 (particulate matter p10 mm) and total suspended particles (TSP) in rural areas in Eastern Spain. To this end, the temporal variations of PM10 and TSP concentrations at rural sites were studied on the basis of the meteorological analysis and the detection of African dust outbreaks using the TOMS-spectrometer satellite observations. Given that preliminary results based on interpretations of 1996–2000 PM10 and TSP time series showed that every summer the PM10 and TSP concentrations are influenced by the same meteorological processes, only data from the summer 2000 are presented.
2. Data and methods This study is based on PM (particulate matter: PM10 and TSP) measurements performed at rural stations in Eastern Spain. This region is characterised by an abrupt orography (Fig. 1), constituted by the Iberian range (NW–SE) and the Catalan coastal range (NE–SW). The
Atlantic ocean Ebro basin
Z
B T C
V
Balearic Islands
Iberian range
Mediterranean sea 0 - 100 100 - 200
200 - 500 500 - 800
800 - 1500 1500 - 3000
>3000
Fig. 1. Topography of the Western Mediterranean (in metres a.s.l.). The dotted square highlights the region where the rural monitoring stations referred to in Table 1 are located. The locations of the main urban and industrial settlements in Eastern Spain are also indicated. B: Barcelona, T: Tarragona, ! V: Valencia, Z: Zaragoza. C: Castello,
latter is crossed by deep valleys that reach the flat coastal area. Situated between these two ranges is the Ebro basin (Fig. 1), which is characterised by a semi-arid soil. The ranges are mainly covered by typical Mediterranean and coniferous forest. The main urban and industrial settlements in Eastern Spain are located along the coastal plain (Fig. 1). Levels of PM10, TSP and gaseous pollutants (NOx, SO2 and O3) are continuously measured at the rural stations belonging to the Valencia Autonomous Government and ENDESA (Empresa Nacional de Electricidad, S.A.) air quality networks. Levels of TSP are determined by means of automatic beta radiation attenuation monitors (Dasibi), whereas levels of PM10 are measured using automatic TEOM (Rupprecht and Patashnick) or GRIMM laser spectrometer monitors depending on the station (Table 1). Gaseous pollutants are monitored by standard methods. The rural stations (Table 1) are located in different micro-geographic settings covering the Ebro basin (MONAGREGA),
Table 1 TSP and PM10 rural monitoring stations Station MONAGREGA SORITA MORELLA CORATXAR VILAFRANCA
Parameter PM10 PM10 TSP TSP TSP
Equip. TEOM GRIMM Beta Beta Beta
AM 3
17 mg/m 12 mg/m3 21 mg/m3 15 mg/m3 20 mg/m3
%R
Location
Altitude
Observations
99% 77% 92% 92% 98%
40.51N,0.21W 40.71N, 0.21W 40.61N, 0.11W 40.41N,0.01 39.71N,0.11W
600 m 640 m 1153 m 1235 m 1125 m
Flat Ebro basin Into a valley Top of the range Top of the range Top of the range
See location in Fig. 1. AM=annual mean in 2000; %R=annual percentage of available data in 2000.
a.s.l. a.s.l. a.s.l. a.s.l a.s.l
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the top of the range close to the coast (VILAFRANCA, MORELLA and CORATXAR) and the bottom of a valley (SORITA). It should also be pointed out that some of these sites are located at high altitude (>1000 m a.s.l.), and most of them can be considered as representing the regional background environment. The temporal variation of PM concentrations was interpreted on the basis of the meteorological synoptic chart classification, air Hysplit back-trajectory analysis (Draxler, 1994), local meteorological variables (continuously measured at the monitoring stations), and TOMS maps of UV-radiation absorbing aerosol index (Herman et al., 1997) in order to detect the outbreaks of African dust events. TOMS maps are a very useful tools for the detection of African dust episodes but it cannot detect the dust load if the transport occurs at altitudes o1 km (Herman et al., 1997). Furthermore, as TOMS maps represents the vertical integrated dust load, aerosols measured by TOMS might not be observed at ground level if the transport occurs at high altitudes.
3. Results and discussions In the study area the seasonal evolution of PM is characterised by the typical summer maximum of the rural Mediterranean environments (Fig. 2). Fig. 3 shows the daily PM concentrations from early summer to early autumn 2000 (July–October). Note the high degree of correlation between the PM concentrations recorded at the different rural sites. Moreover, the range of PM10 and TSP concentrations is very narrow (Fig. 3). Consequently, these summer TSP concentrations are considered representative of the regional PM10 levels in the rural environment in Eastern Spain. These regional PM10 concentrations measured at rural sites in 2000 (12–17 mg/m3 PM10 annual mean, Table 1) are relatively
PM10-MONAGREGA
TSP-MORELLA
TSP-CORATXAR
TSP-VILAFRANCA
g/m
3
40 30 20 10 0 J F M A M J
J A S O N D
Fig. 2. Monthly mean concentrations (1996–2000) of PM10 and TSP at rural sites in Eastern Spain.
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high when compared with the 2010 EU limit values for PM10 (annual mean p20 mg/m3 and do not exceed the daily concentrations of 50 mg/m3 on more than 7 days/ yr, EU Directive 1999/30/CE). Note that in the period from July to September, daily PM10 concentrations exceeding 40 and 50 mg/m3 were recorded for 11 and 3 days, respectively, at the MONAGREGA rural site. The above seasonal pattern (Fig. 2) contrasts with the seasonal evolution of PM described for rural sites from Central and Northern Europe. In the Swiss Alps, Monn et al. (1995) reported an autumn–winter maximum (weekly PM levels from 20 to 60 mg/m3 in autumn– winter and from 10 to 30 mg/m3 in summer). In the study area, during the year 2000 the weekly PM levels ranged from 5 to 15 mg/m3 in autumn and winter (March is not included) and from 20 to 40 mg/m3 in summer months and March. At rural sites in Switzerland, the monthly PM10 concentrations only exceeded the level of 20 mg/ . et al., 2001) during the period m3 in November (Ro. osli April 1997–May 1998. In contrast, in Eastern Spain the PM10 monthly concentration of 20 mg/m3 is exceeded throughout the summer (Fig. 2). Thus, this level was surpassed during 4 months in 2000 at the MONAGREGA rural site. Turnbull and Harrison (2000) did not report a significant PM10 seasonal cycle at rural sites in the United Kingdom. 3.1. Daily PM concentrations and synoptic meteorological scenarios The different periods of high and low daily PM concentrations detected from early summer to early autumn 2000 were classified as a function of the synoptic meteorological scenarios (Fig. 3 and Table 2) and origin of the air masses. 3.1.1. July 2000 Weak gradient conditions over the Western Mediterranean and the ITL development constituted the prevailing scenario from 26 June to 3 July (Fig. 5a). On 4 July an Atlantic cold front crossed Eastern Spain, causing a drop in temperature (T) and in the PM and O3 concentrations. On 6 and 7 July a peak in the PM concentrations was reported. The TOMS satellite observations showed a plume of African dust (Fig. 4) over Eastern Spain and the Western Mediterranean. Subsequently, abrupt entries of Atlantic air masses in the Western Mediterranean basin persisted up to 17 July (Fig. 5b), leading to a fall in T and in the PM and O3 concentrations. Owing to the fact that several cold fronts reached the Western Mediterranean during this period (on 11, 15 and 16 July), the temperature decreased reaching the minimum values for this month (B121C). The ozone and PM concentrations also fell reaching the minimum values for this month: O3o60 mg/ m3 and PMo15 mg/m3. These are very low PM and O3
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PM10-MONAGREGA TSP-MORELLA
70
African
60
PM10-SORITA
TSP-CORATXAR
TSP-VILAFRANCA
African Regional
African
Regional
African
Regional
Regional
50 Atlan .
g/m3
40
Atlan .
Atlan.
Atlan. Atlan.
30 20 10
(a)
0 35
T-MORELLA T-VILAFRANCA
30
T-CORATXAR T-MONAGREGA
25 T °C
T-SORITA
20 15 10
(b)
5
175
O3-CORATXAR O3-MONAGREGA
O3- MORELLA O3-VILAFRANCA
150
g/m3
125 100 75 50 25 1 (c)
20
10
July
1
10
20
August
1
10
20
September
1
10
20
October
Fig. 3. Daily mean values of (a) PM10 and TSP concentrations, (b) temperature and (c) O3 concentrations in selected rural stations from July to October 2000. The arrows highlight the periods in which the main African (grey arrow), regional (white arrow) and Atlantic (black dotted arrow) episodes occurred.
(Mill!an et al., 2000) levels when compared with those that are typical of this region in summer (Fig. 3). From 18 to 23 July the ‘‘typical summer’’ scenario prevailed. This was characterised (Fig. 5c) by weak gradient
conditions over the Western Mediterranean, the ITL and the North Atlantic anticyclone affecting northwest Europe. In this period an increase in T and in the PM and O3 concentrations was again reported. These weak
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Table 2 Dates of the PM episodes recorded in summer 2000 Dates
Pressure pattern
Local wind
Air mass origin
PM10
1–3 July 4–5 July 6–7 July 8–17 July 18–23 July 24 July–1 August
WGC(WM)+ITL L(A) H(NAF) H(A)+L(A) WGC(WM)+ITL+H(A+WE) WGC(WM)+ITL+H(A+WE)/L(A)
Breeze S-advection S-advection NW-advection Breeze/SE Breeze/NW
Regional (SE-IB) W-Atlantic Africa NW-Atlantic Regional (WM) Regional/Atlantic
22 19 43 15 31 24
2–7 August 8–23 August 24–26 August 27 August–1 September
H(A+WE) WGC(WM)+ITL+H(A+WE) L(P)+H(NAF) H(A+WE)/L(A)
N/NW-advection Breeze/SE S-advection NW/S-advection
NW-Atlantic Regional (WM+SE-IB) Africa NW/W-Atlantic
20 28 50 22
2–10 September 12–17 September 18–24 September 25–27 September 28–30 September
H(A+WE)/L(A) WGC+ITL L(A) H(NAF) L(A)
NW/N-advection Breeze S-advection S-advection NW-advection
NW-Atlantic+Western Europe Regional (WM) W/SW-Atlantic Africa NW-Atlantic
22 42 16 43 14
1–6 October 7–16 October 17–20 October 21–24 October 25–26 October 26–30 October
H(A+WE)+L(A) H(A)/L(A) WGC+H(IB) L(G) L(G) H(A+WE)
NW-advection NW-advection Breeze/SE S-advection S- advection N/S
NW-Atlantic NW-Atlantic Regional (WM/W-IB) Africa+intensive rains Africa Regional
19 10 21 11 ND 14
Meteorological pressure patterns, local wind direction, air mass origin deduced from the back-trajectory analysis, and mean PM10 (in mg/m3) concentrations at the MONAGREGA station during the episodes (ND: No data). Pressure patterns: L indicates a depression located over the Atlantic at mid-latitudes, L(A), in front of Portugal, L(P) or over the straits of Gibraltar, L(G). H indicates anticyclone over North Africa, H(NAF), the Atlantic ocean, H(A), or the Atlantic and Western Europe, H(A+WE). ITL: Iberian thermal low, WGC: Weak gradients conditions, IB: Iberian Peninsula, WM: Western Mediterranean. Local winds (direction): S: South, SE: Southeast, N: North, NW: Northwest.
gradient conditions over the Western Mediterranean favour the intense breeze circulation (Gangoiti et al., 2001). In a 3-day period (17–19 July), the PM and O3 concentrations increased from 14 to 32 mg/m3 and from 69 to 91 mg/m3, respectively (as daily values at the rural monitoring stations). From 24 July to 1 August the typical summer scenario (Fig. 5d) alternated with smooth advections of Atlantic air masses brought about by the displacement of an Atlantic depression towards the Iberian Peninsula. 3.1.2. August 2000 From 2 to 7 August cool Atlantic air masses entered the Western Mediterranean basin via the Gulf of Lyon (Fig. 5e) and reached Eastern Spain, decreasing the T and the PM and O3 concentrations and attaining values o151C for T; o15 mg/m3 for PM and o80 mg/m3 for O3 at most sites. From 8 to 23 August the summer meteorological scenario favouring the breeze circulation was again dominant (Fig. 5f), with an increase in the T and the PM and O3 levels. During this period the PM and O3 concentrations ranged between 20 and 40 mg/m3
and between 100 and 150 mg/m3, respectively. These ozone levels fall within the range of the high ozone episodes typical of this region in summer (Mill!an et al., 2000). From 24 to 26 August an African dust outbreak over the Western Mediterranean and Eastern Spain (Fig. 4) resulted in a peak PM event which coincided with a decrease in the O3 levels. The PM10 concentrations during the African event exceeded the forthcoming EU PM10 daily limit value of 50 mg/m3 at the MONAGREGA station. Subsequent abrupt entries of Atlantic air masses (Fig. 5g) brought about a marked decrease in the PM10 levels from 64 to 11 mg/m3 at the MONAGREGA station in a 3-day period (from 25 to 27 August). A similar decrease was reported at the other stations (Fig. 3). 3.1.3. September 2000 From 2 to 10 September several Atlantic cold fronts crossed Eastern Spain giving rise to low PM and O3 concentrations. From 12 to 17 September a high PM, O3 and T episode was reported under conditions favouring the breeze circulation: weak gradient conditions over the
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Fig. 4. Above: TOMS UV-radiation absorbing aerosol index maps during the African dust outbreaks between July and October 2000. Below: synoptic charts (altitude in metres a.s.l. of the pressure level of 850, or 700 hPa for the event at the end of September) showing the meteorological patterns causing the African transport of air masses.
Fig. 5. UK Meteorological Office synoptic charts of pressure (mb) at sea level.
Iberian Peninsula and Western Mediterranean together with high pressures over the Mediterranean. Even the ITL was developed on some days (Fig. 5h). During this event high PM and O3 concentrations were again reported in the range between 25 and 45 mg/m3 for PM and between 100 and 150 mg/m3 for O3. In the 18–24
September period, Atlantic cold fronts reached the Western Mediterranean (Fig. 5i), with a fall in the T and the PM and O3 concentrations. Subsequently, an African event induced a PM peak during 25–27 September (Fig. 4). Owing to the high mineral dust load, the daily PM10 levels at the MONAGREGA
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station reached 54 mg/m3, exceeding the forthcoming EU daily limit value for PM10 (50 mg/m3). Low O3 concentrations were reported owing to the influence of the relatively ‘‘clean’’ Saharan air. Subsequent Atlantic episodes again diminished the PM levels. 3.1.4. October 2000 In the 1–16 October period, several Atlantic episodes caused frequent low PM and O3 events (Fig. 5j). From 17 to 20 October weak gradient conditions and a slight anticyclonic situation prevailed (Fig. 5k), and a high T; PM and O3 episode was again reported. From 21 to 24 October intensive rains depleted the PM concentrations. An African dust event (Fig. 4) on 25 and 26 October produced a PM peak. On 31 October a cold front reached the Western Mediterranean (Fig. 5l) and led to a decrease in the T; PM and O3 concentrations. The highest PM events (daily concentrations between 40 and 60 mg/m3 in the July—October 2000 period) were reported during the African dust episodes in late August, late September and late October (Figs. 3 and 4). The transport of African dust to Eastern Spain occurs when the North African anticyclone (usually located between Algeria and Egypt at the 850 hPa pressure level) shifts to the southeast of Spain and/or when depressions develop to the West or South of Portugal (Fig. 4). The occurrence of the African dust outbreaks is corroborated by the TOMS absorbing aerosol index maps (Fig. 4). High PM and low ozone concentrations are recorded during the African episodes as reported by studies carried out in Atlantic regions (Savoie et al., 1992; Rodr!ıguez and Guerra, 2001). The second highest PM events (daily concentrations between 20 and 45 mg/m3 in the July—October 2000 period) are recorded during periods characterised by a weak pressure horizontal gradient over the Western Mediterranean, which is often associated with ITL development. Advection of air masses is not significant in this scenario. In this meteorological context, the dynamics of the air masses over Eastern Spain is highly influenced by atmospheric circulations activated by the heating of the ground during daylight (Milla! n et al., 1997). These circulations such as the breeze at the coastal and mountain (anabatic winds) sites favour the inland transport of pollutants from the urban/industrial coastal sites to the rural sites (Fig. 1) during daylight. The prevalence of these breeze circulation (up to 2 weeks) results in increased PM concentrations at the rural sites owing to the accumulation of airborne particulates in the regional atmosphere caused by the scarce renovation of the air masses. Details on the features of these ‘‘regional’’ PM episodes associated with ozone event increases in the temperature (Fig. 3) are provided in the following section. The lowest PM episodes are recorded during periods characterised by intense advections of Atlantic air
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masses toward the Western Mediterranean. These events occur in some meteorological scenarios, such as Atlantic depressions developing off the western or northern coasts of the Iberian Peninsula (Fig. 5i), the North Atlantic anticyclone bringing strong northern winds over Spain (Fig. 5b) or the North Atlantic anticyclone inducing entries of Atlantic air masses via the Gulf of Lyon (Fig. 5e). Most of these Atlantic events are associated with rainfall and with a cold front crossing the Iberian Peninsula. Since the Western Mediterranean is surrounded by high coastal ranges (Fig. 1), these Atlantic events represent entries of cool air which ‘‘clean’’ the Mediterranean atmosphere, leading to a decrease in the temperature and PM and ozone concentrations (Fig. 3 and Table 2). In the period July—October the lowest PM events were recorded during intense Atlantic episodes, e.g. daily PM concentrations o20 mg/m3 in the periods 9–18 July, 3–6 August or 4–5 September (Fig. 3). The successive occurrence of African, Atlantic and regional episodes accounts for the main variations in the PM concentrations (Fig. 3 and Table 2). 3.2. Study of the regional PM episodes The African dust outbreaks, which account for the highest PM events at the rural sites, have already been studied in detail by Rodr!ıguez et al. (2001). Thus, this section focuses on the study of the regional PM episodes given that they account for the second highest PM events at the rural sites in summer. Wind direction and velocity did not undergo daily cycles during periods dominated by synoptic scale meteorological processes. As expected, southern winds prevailed throughout the day during advection of African dusty air masses. Northern winds prevailed throughout the day during the intense Atlantic episodes caused by the North Atlantic anticyclone. However, where these are caused by Atlantic depressions, the prevalence of northern or southern winds depends on the latitude of the depression. In periods of weak gradient conditions over the Western Mediterranean, often associated with the ITL, the wind speed shows marked daily cycles with maximum values during daylight owing to the activation of up-slope winds over the mountain slopes (mountain breeze) and of the inland sea breeze. In most of these events, the wind direction is characterised by SE winds during daylight and NW winds during the night, but in some periods of intensive ITL development light SE winds may blow even during the night. In this typical summer scenario, the back-trajectory analysis shows that the air masses entering the mainland from the Mediterranean Sea during daylight have a regional origin. These trajectories show a short route (Figs. 6d, 7c and 8d) owing to the weak gradient conditions over the
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Fig. 6. Hourly values of: (a) wind velocity and direction at MORELLA and (b) TSP at MORELLA and CORATXAR. Isentropic air back-trajectories (calculated at 12:00 UTC and 3 days backward for 1000 m a.s.l.) in the period 7–20 September 2000 (c, d and e).
Western Mediterranean. An anticyclonic curvature caused by the compensatory subsidence (which typically occurs between Eastern Spain and the Balearic Islands, Fig. 1) associated with the thermal buoyancy over the warmer terrain is also observed in the trajectories (examples in Figs. 5a and c). Examples of these types of trajectories are shown in Figs. 6d, 7c and 8d, discussed below. Both the breeze circulation and the lack of significant advection of air masses during these events provide evidence of the regional origin of the airborne particulates. Fig. 6 shows the hourly values of wind velocity and direction recorded at the MORELLA station for the regional PM episode recorded in mid September 2000 (Fig. 3 and Table 2). From 7 to 10 September the wind direction and speed did not show significant variations during the day (except on 8 September), and the
trajectory analysis showed transport patterns from the North Atlantic and Western Europe (Fig. 6c). From 13 to 17 September weak gradient conditions prevailed over the Iberian Peninsula, and even the ITL was developed for some days (Fig. 5h), and an increase in the PM concentrations was reported (Figs. 3 and 6). During this period the wind speed and direction showed marked daily cycles due to the up-slope winds and the sea breeze circulation. Moreover, the trajectory analysis showed that the air masses entering the mainland during daylight had a regional origin. The scarce renovation of the air masses, due to the development of the breeze circulation and the lack of significant advection of air masses, accounts for the accumulation of airborne particulates in the regional atmosphere. From 18 to 20 September a significant fall in the PM concentrations at the rural sites was brought about by the entry of cool
20
360
15
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10
180
NW SW SE 5
90
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0
Wind direction, degrees
Wind speed, m/s
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NE (a)
18-20
60
TSP-MORELLA
g/m3
50
PM10-MONAGREGA
40 30
(c)
20 10 0
(b)
15
16
17 18 July 2000
Northern advections
19
20
Regional episode
Fig. 7. (a) Hourly values of wind velocity and direction at MORELLA; (b) hourly values of TSP and PM10 at MORELLA and MONAGREGA; (c) isentropic air back-trajectories (calculated at 12:00 UTC and 3 days backward for 1000 m a.s.l.) during the period 15–20 July 2000.
Atlantic air masses into the Western Mediterranean basin (Figs. 3, 5i and 6). These regional PM episodes were also reported in July (18–23; Figs. 3, 5c and 7) and August (8–23; Figs. 3, 5f and 8). Fig. 8 shows hourly data of wind speed and direction and TSP concentrations at the MORELLA mountain site from 4 to 28 August. From 4 to 7 August advections of Atlantic air masses produced strong northern winds resulting in low PM concentrations (Figs. 5e and 8c). From 8 to 23 August weak gradient conditions over the Western Mediterranean and the ITL prevailed (Fig. 5f), resulting in a PM event of regional origin (Figs. 3 and 8d). Southern winds associated with breeze circulation prevailed in this period and backtrajectories showed the anticyclonic curvature over the Western Mediterranean and Eastern Spain. From early 24 to early 26 August southern winds prevailed during the day, and hourly PM concentrations >100 mg/m3 were reported owing to an African dust outbreak (Figs. 4 and 8e). From 26 mid-August, an abrupt entry of northern cool Atlantic air masses in the Mediterranean basin led to reductions in the PM levels (Figs. 5g and 8e). In an attempt to elucidate the origin of the high ozone episodes in this region, Mill!an et al. (1997, 2000) proposed a conceptual model in which the polluted air masses undergo vertical re-circulations over the Eastern
coast of Spain owing to the inland up-slope winds, the westerly winds over the top of the mountains, the subsidence over the Mediterranean sea and the subsequent inland entry of the sea breeze. In this scenario, Mill!an et al. (1997) propose that polluted air mass trajectories tend to describe a helicoidal curve along the eastern coast of Spain in a southward direction. The subsidence (relative high pressures) over the Western Mediterranean sea is attributed to the compensatory sinking associated with the thermal buoyancy over the warmer terrain together with the large scale anticyclonic subsidence. This subsidence accounts for the backtrajectories showing the anticyclonic curvature described above for the regional PM episodes (e.g. Figs. 6d, 7c and 8d). Salvador et al. (1999) have argued that given the abrupt orography of the region and given that the movement of air masses is influenced by the heating of mountain and valleys slopes, meteorological models able to simulate the atmospheric dynamics need a very high resolution, at least 2 km 2 km to account for 95% of terrain variance. Using high-resolution trajectory analysis, Gangoiti et al. (2001) have demonstrated the recirculations of polluted air masses over the Western Mediterranean and the Eastern coast of Spain. Moreover, back-scattering Lidar measurements have documented the transport of aerosols towards a high altitude by up-slope winds on the coastal ranges in Eastern Spain
Wind speed, m/s
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5
90
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0
NW SW SE NE
(a)
Wind direction, degrees
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3110
150
TSP-MORELLA
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PM10-MONAGREGA
g/m3
100 75 50 25 0
(b)
4
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6
7
8
9
10
11
12
13
14
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18
19
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25
26
27
August 2000 Northern advections 4
5
6
7
Regional episode
c
African episode
d
e 27
21
8 -2 3
26
24 25
Fig. 8. (a) Hourly values of wind velocity and direction at MORELLA; (b) hourly levels of TSP at MORELLA and of PM10 at MONAGREGA; (c, d and e) isentropic air back-trajectories (calculated at 12:00 UTC and 3 days backward for 1000 m a.s.l.) during the period 8–27 August 2000.
(Soriano et al., 2001). Thus, the back-trajectories associated with the ‘‘regional’’ PM episodes (Figs. 6d, 7c and 8d) probably do not represent the actual and detailed movement of the air masses, but they may be used as tracers of the regional air mass and particulate sources. Although these PM regional episodes are more frequently observed, and have a longer duration in summer, they are also reported in the semi-warm periods (late spring and early autumn). A typical example, which is also associated with an ozone episode, occurred in the period 17–20 October 2000 (Fig. 3 and Table 2). This event was brought about by an anticyclonic situation together with a weak pressure horizontal gradient over the Iberian Peninsula and the Western Mediterranean (Fig. 5k). The temperature increase (Fig. 3) was not as high as during the summer event, and consequently the ITL was not present.
The PM events attributed to regional sources are associated with ozone episodes. When interpreting the occurrence of the simultaneous high PM and O3 events both the role of meteorology and that of photochemistry should be taken into account. From the point of view of meteorology, the increase in the PM and O3 concentrations during the regional episodes and the decrease during the Atlantic events account for the correlated daily O3 and PM time series. In the photochemical context, the formation of secondary particles by photochemical processes is expected to be significant in these aged polluted air masses transported to the rural environment. Under strong summer insolation, photochemical reactions of NOx, organic vapours and SO2 give rise to acid compounds, secondary particles, O3 and other oxidants. Moreover, O3 is involved directly (oxidation of gases by O3) and/or indirectly (e.g. oxidation of gases by the OH radicals formed by O3
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photolysis) in the formation of secondary particles (e.g. sulphate, nitrate, organic particulate matter). The crustal particulate matter is also a key component of PM10 in Eastern Spain during regional episodes. This is due to the temperature increases that favour the turbulence dynamics in arid areas. Querol et al. (1998a) have shown that the load of crustal particulate matter in the Ebro basin increases during summer because of soil re-suspension. In addition, Rodr!ıguez et al. (2002) found 58% and 21% loads for secondary and crustal particulate matter in PM10 sampled in summertime in the study area.
4. Conclusions At rural sites in Eastern Spain in the flat Ebro basin (600 m a.s.l.) and in the Iberian and Catalan ranges (altitudes>1000 m a.s.l.), the concentrations of PM10 and TSP (PM) undergo a seasonal evolution that is characterised by a summer maximum. In the summer months, the highest PM events are reported during African dust outbreak events (daily concentrations 40– 60 mg/m3). The second highest PM events (daily concentrations 20–45 mg/m3) are reported during regional pollution episodes associated with ozone events. These regional events are induced by the specific Mediterranean context (such as orography, meteorology, and high degree of insolation). The lowest PM10 and TSP events (daily concentrations o20 mg/m3) are reported during abrupt entries of Atlantic air masses into the Western Mediterranean basin. The frequent occurrence of African dust outbreak and regional PM episodes has implications on air pollution regulation strategies, and accounts for the marked difference in the features of the airborne particulates between Southern and Northern Europe. The African dust events are more frequently observed in Mediterranean countries than in Central–Northern Europe. In Eastern Spain, around 10 African dust events are reported every year (Rodr!ıguez et al., 2001). The high load of mineral dust brought by the African air masses interferes with the monitoring of the anthropogenic fraction of PM10 in Mediterranean countries, and could result in exceedances of the forthcoming EU daily limit values for PM10 (50 mg/m3). Mineral dust concentrations >20 mg/m3 in PM10 and TSP during African dust outbreaks over Southern Europe have been reported by several authors (Molinaroli et al., 1993; Dulac et al., 1992; Kubilay et al., 2000; Rodr!ıguez et al., 2001, 2002). Regional PM episodes reach maximum intensity in summer. In this season, westerly air flow related to the mid-latitude general circulation (typically associated with intensive westerly winds, eastward moving depressions and Atlantic cold front passages and rain) shift northward to Central and Northern Europe. This
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westerly air flow favours the dilution of pollutants and the frequent renovation of air masses over the flat terrain, which characterises these European regions. In contrast, the Western Mediterranean remains under weak gradient conditions in summer, and the abrupt orography surrounding the Western basin induces a regional meteorology that favours the ageing and recirculation of polluted air masses. The variations in the meteorology and orography from the Western Mediterranean to Central–Northern Europe result in a different behaviour of the airborne particulate pollutants (such as seasonal evolution), and influence on the background levels of suspended particles. At rural sites in Central Europe, high PM events due to regional pollution sources are mostly recorded in winter during stagnant episodes caused by cold temperature inversions and low wind speed (Monn . et al., 2001). However, at rural sites et al., 1995; Ro. osli in the Western Mediterranean high PM events are reported throughout the summer owing to the higher frequency of the above-described regional pollution episodes. Moreover, when comparing two regions with the same degree of urban/industrial development (similar rate emissions of air pollutants) from the Western Mediterranean and from the Central–Northern Europe, higher background levels of airborne particulate pollutants are expected in the Mediterranean region given the smaller capacity of ‘‘self-cleaning’’ of the Mediterranean atmosphere.
Acknowledgements The authors wish to thank the Spanish Interministerial Commission for Science and Technology (REN2001-0659-C03-03), Ministry of the Environment, the Department of Environment of the Government of Valencia and ENDESA for supporting this study and for supplying the PM10, TSP and gaseous pollutant measurements. We are also indebted to Jay Herman (Goddard Space Flight Center, NASA) for kindly supplying TOMS maps over the Mediterranean.
References Bergametti, G., Dutot, A.L., Buat-Menard, P., Losno, R., Remoudaki, E., 1989. Seasonal variability of the elemental composition of atmospheric aerosols particles over the Northwest Mediterranean. Tellus 41B, 353–361. Draxler, R.R., 1994. Hybrid single-particles Lagrangian integrated trajectories. Version 3.2, NOAA-ARL. Dulac, F., Tanr!e, D., Bergametti, G., Buat-M!enard, P., Desbois, M., Sutton, D., 1992. Assessment of African airborne dust mass over the Western Mediterranean sea using meteosat data. Journal of Geophysical Research 97, 2489–2506.
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Gangoiti, G., Mill!an, M.M., Salvador, R., Mantilla, E., 2001. Long-range transport and re-circulation of pollutants in the western Mediterranean during the project regional cycles of air pollution in the west–central Mediterranean area. Atmospheric Environment 35, 6267–6276. Herman, J.R., Bhartia, P.K., Torres, O., Hsu, C., Seftor, C., Celarier, E., 1997. Global distribution of UV-absorbing aerosols from Nimbus7/TOMS data. Journal of Geophysical Research 201, 16911–16922. Kubilay, N., Saydam, A.C., 1995. Trace elements in atmospheric particulates over the Eastern Mediterranean: concentrations, sources and variability. Atmospheric Environment 29, 2289–2300. Kubilay, N., Nickovic, S., Moulin, C., Dulac, F., 2000. An illustration of the transport and deposition of mineral dust onto the Eastern Mediterranean. Atmospheric Environment 34, 1293–1303. Mill!an, M.M., Salvador, R., Mantilla, E., Kallos, G., 1997. Photo-oxidant dynamics in the Mediterranean basin in summer: results from European research projects. Journal of Geophysical Research 102, 8811–8823. Mill!an, M.M., Mantilla, E., Salvador, R., Carratal!a, A., Sanz, M.J., Alonso, L., Gangoiti, G., Navazo, M., 2000. Ozone cycles in the Western Mediterranean basin: interpretation of monitoring data in complex coastal terrain. Journal of Applied Meteorology 39, 487–508. Molinaroli, E., Gerzoni, S., Giacarlo, R., 1993. Contribution of Saharan dust to the Central Mediterranean Basin. In: Johnson, N.J., Basu, A. (Eds.), Processes Controlling the Composition of the Clastic Sediments. Geological Society of America, Boulder, CO, Special Paper, Vol. 284, pp. 303– 312. Monn, C.H., Braendly, G., Schaeppi, G., Schindler, C.H., Ackermann-Liebrich, U., Leuenberger, P.H., Team, S., 1995. Particulate matter o10 mm (PM10) and total suspended particles (TSP) in urban, rural and alpine air in Switzerland. Atmospheric Environment 29, 2565–2573. Querol, X., Alastuey, A., Puicercus, J.A., Mantilla, E., Ruiz, C.R., Lopez-Soler, A., Plana, F., Juan, R., 1998a. Seasonal evolution of suspended particles around a large coal-fired
power station: chemical characterisation. Atmospheric Environment 32 (11), 719–731. Querol, X., Alastuey, A., Puicercus, J.A., Mantilla, E., Miros, * J.V., Lopez-Soler, A., Plana, F., Art!ınano, B., 1998b. Seasonal evolution of suspended particles around a large coal-fired power station: particles levels and sources. Atmospheric Environment 32 (11), 1963–1978. Rodr!ıguez, S., Guerra, J.C., 2001. Monitoring of ozone in a marine environment in Tenerife (Canary Islands). Atmospheric Environment 35, 1829–1841. Rodr!ıguez, S., Querol, X., Alastuey, A., Kallos, G., Kakaliagou, O., 2001. Saharan dust contributions to PM10 and TSP levels in Southern and Eastern Spain. Atmospheric Environment 35, 2433–2447. Rodr!ıguez, S., Querol, X., Alastuey, A., Plana, F., 2002. Sources and processes affecting levels and composition of atmospheric aerosol in the Western Mediterranean. Journal of Geophysical Research, in press. . Ro. osli, M., Theis, G., Kunzli, . N., Staehelin, J., Mathys, P., Oglesby, L., Camenzind, M., Braun-Fahrl.ander, C., 2001. Temporal and spatial variation of the chemical composition of PM10 at urban and rural sites in the Basel area, Switzerland. Atmospheric Environment 35, 3701–3713. ! J., Mill!an, M.M., 1999. Horizontal grid Salvador, R., Calbo, size selection and its influence on mesoscale model simulations. Journal of Applied Meteorology 38, 1311–1329. Savoie, D.L., Prospero, J.M., Oltmans, S.J., Graustein, W.C., Turekian, K.K., Merrill, J.T., Levy II, H., 1992. Sources of nitrate and ozone in the marine boundary layer of the Tropical North Atlantic. Journal of Geophysical Research 97 (11), 575–589. Soriano, C., Baldasano, J.M., Buttler, W.T., Moore, K.R., 2001. Circulatory patterns of air pollutants within the Barcelona air basin in summertime situation: Lidar and numerical approaches. Boundary-Layer Meteorology 98, 33–55. Turnbull, A.B., Harrison, R.M., 2000. Major components contributions to PM10 composition in the UK atmosphere. Atmospheric Environment 34, 3129–3137.
Origin of high summer PM10 and TSP concentrations at rural sites in Eastern Spain Sergio Rodr!ıgueza,*, Xavier Querola, Andre! s Alastueya, Enrique Mantillab b
a Institute of Earth Sciences ‘‘Jaume Almera’’, CSIC, C/Lluis Sol!e i Sabar!ıs, s/n, 08028, Barcelona, Spain ! ! Centro de Estudios Ambientales del Mediterraneo, CEAM. Parque tecnologico, C-4, sector oeste, 46980, Paterna, Valencia, Spain
Received 18 February 2002; received in revised form 18 March 2002; accepted 22 March 2002
Abstract Concentrations of airborne particulates undergo a seasonal evolution characterised by a summer maximum in rural areas in Eastern Spain. In the summer months the daily mean concentrations of PM10 and TSP (PM) experience wide variations. In 3-day periods, increases in the PM concentrations from 15 to 30–40 mg/m3 are frequently reported, and increases from 15 to 40–60 mg/m3 occur several times throughout the summer. These variations are simultaneously reported at rural stations throughout the flat Ebro basin (600 m a.s.l.) and at mountain sites located at high altitude (>1000 m a.s.l.). The origin of high and low PM episodes was investigated by correlating PM levels with the concentrations of gaseous pollutants, and making use of meteorological analysis and satellite observations. The highest PM events (daily concentrations in the range 40–60 mg/m3) were documented during outbreaks of African dust. The second highest PM events (daily concentrations in the range 20–45 mg/m3) were recorded during regional episodes associated with ozone events. These summer regional PM episodes were induced by the abrupt orography surrounding the Western Mediterranean and by the regional meteorology, which favour the ageing of polluted air masses into the basin. These regional events occur in a synoptic meteorological context characterised by a weak pressure horizontal gradient over the Western Mediterranean often associated with the development of the Iberian thermal low, when advection of air masses is not significant. In this meteorological context, the transport of particulate pollutants from urban/industrial to rural sites is brought about by the breeze circulation at the coastal (sea breeze) and mountain (mountain breeze) sites. The persistence of this breeze circulation for several days (periods of up to 2 weeks were reported) results in a low renovation of air masses leading to an accumulation of airborne particulates in the regional atmosphere. The lowest PM events (daily concentrations o20 mg/m3) were reported during abrupt entries of Atlantic air into the Mediterranean. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Aerosols; PM10; Saharan dust; Ozone; Regional pollution; Mediterranean; Spain
1. Introduction Regional levels of ambient airborne particles vary as a function of the dominant natural and anthropogenic emissions, the atmospheric conditions (e.g. radiation, temperature or humidity) and the levels of reactive gases and the regional meteorology.
*Corresponding author. E-mail address: [email protected] (S. Rodr!ıguez).
A number of studies have pointed out that the dynamic of polluted air masses in the Western Mediterranean is considerably influenced by local and mesoscale meteorological processes (Milla! n et al., 1997, 2000; Salvador et al., 1999; Soriano et al., 2001; Gangoiti et al., 2001). In Eastern Spain, transport of polluted air masses in summer is influenced by the breeze circulation and the Iberian thermal low (ITL) development (Mill!an et al., 1997). Under this scenario, the transport of pollutants undergoes marked daily cycles. Thus, at coastal sites, the sea breeze induces an inland
1352-2310/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 2 ) 0 0 2 5 6 - X
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transport during the day and a seaward return of pollutants at night. Moreover, the polluted air masses undergo vertical re-circulations over the Eastern coast of Spain due to the following factors: the inland up-slope winds over the coastal ranges, the westerly winds over the top of the mountains, the subsidence over the Mediterranean sea and the inland entry of the sea breeze. Thus, the breeze circulation is typically associated with high surface ozone concentrations (Mill!an et al., 1997). In addition to these regional circulations, synoptic scale meteorology induces frequent outbreaks of African dust in summer (Rodr!ıguez et al., 2001). A number of studies have reported that in rural environments in the Mediterranean, airborne particulate concentrations undergo a seasonal cycle characterised by a summer maximum (Bergametti et al., 1989; Kubilay and Saydam, 1995; Querol et al., 1998a, b; Rodr!ıguez et al., 2001, 2002). To the best of our knowledge, this seasonal cycle has not been reported at rural sites in Central and Northern Europe (e.g. Monn et al., 1995; . et al., 2001). In this Turnbull and Harrison, 2000; Ro. osli study the focus is on the influence of the typical summer meteorology on the levels of PM10 (particulate matter p10 mm) and total suspended particles (TSP) in rural areas in Eastern Spain. To this end, the temporal variations of PM10 and TSP concentrations at rural sites were studied on the basis of the meteorological analysis and the detection of African dust outbreaks using the TOMS-spectrometer satellite observations. Given that preliminary results based on interpretations of 1996–2000 PM10 and TSP time series showed that every summer the PM10 and TSP concentrations are influenced by the same meteorological processes, only data from the summer 2000 are presented.
2. Data and methods This study is based on PM (particulate matter: PM10 and TSP) measurements performed at rural stations in Eastern Spain. This region is characterised by an abrupt orography (Fig. 1), constituted by the Iberian range (NW–SE) and the Catalan coastal range (NE–SW). The
Atlantic ocean Ebro basin
Z
B T C
V
Balearic Islands
Iberian range
Mediterranean sea 0 - 100 100 - 200
200 - 500 500 - 800
800 - 1500 1500 - 3000
>3000
Fig. 1. Topography of the Western Mediterranean (in metres a.s.l.). The dotted square highlights the region where the rural monitoring stations referred to in Table 1 are located. The locations of the main urban and industrial settlements in Eastern Spain are also indicated. B: Barcelona, T: Tarragona, ! V: Valencia, Z: Zaragoza. C: Castello,
latter is crossed by deep valleys that reach the flat coastal area. Situated between these two ranges is the Ebro basin (Fig. 1), which is characterised by a semi-arid soil. The ranges are mainly covered by typical Mediterranean and coniferous forest. The main urban and industrial settlements in Eastern Spain are located along the coastal plain (Fig. 1). Levels of PM10, TSP and gaseous pollutants (NOx, SO2 and O3) are continuously measured at the rural stations belonging to the Valencia Autonomous Government and ENDESA (Empresa Nacional de Electricidad, S.A.) air quality networks. Levels of TSP are determined by means of automatic beta radiation attenuation monitors (Dasibi), whereas levels of PM10 are measured using automatic TEOM (Rupprecht and Patashnick) or GRIMM laser spectrometer monitors depending on the station (Table 1). Gaseous pollutants are monitored by standard methods. The rural stations (Table 1) are located in different micro-geographic settings covering the Ebro basin (MONAGREGA),
Table 1 TSP and PM10 rural monitoring stations Station MONAGREGA SORITA MORELLA CORATXAR VILAFRANCA
Parameter PM10 PM10 TSP TSP TSP
Equip. TEOM GRIMM Beta Beta Beta
AM 3
17 mg/m 12 mg/m3 21 mg/m3 15 mg/m3 20 mg/m3
%R
Location
Altitude
Observations
99% 77% 92% 92% 98%
40.51N,0.21W 40.71N, 0.21W 40.61N, 0.11W 40.41N,0.01 39.71N,0.11W
600 m 640 m 1153 m 1235 m 1125 m
Flat Ebro basin Into a valley Top of the range Top of the range Top of the range
See location in Fig. 1. AM=annual mean in 2000; %R=annual percentage of available data in 2000.
a.s.l. a.s.l. a.s.l. a.s.l a.s.l
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the top of the range close to the coast (VILAFRANCA, MORELLA and CORATXAR) and the bottom of a valley (SORITA). It should also be pointed out that some of these sites are located at high altitude (>1000 m a.s.l.), and most of them can be considered as representing the regional background environment. The temporal variation of PM concentrations was interpreted on the basis of the meteorological synoptic chart classification, air Hysplit back-trajectory analysis (Draxler, 1994), local meteorological variables (continuously measured at the monitoring stations), and TOMS maps of UV-radiation absorbing aerosol index (Herman et al., 1997) in order to detect the outbreaks of African dust events. TOMS maps are a very useful tools for the detection of African dust episodes but it cannot detect the dust load if the transport occurs at altitudes o1 km (Herman et al., 1997). Furthermore, as TOMS maps represents the vertical integrated dust load, aerosols measured by TOMS might not be observed at ground level if the transport occurs at high altitudes.
3. Results and discussions In the study area the seasonal evolution of PM is characterised by the typical summer maximum of the rural Mediterranean environments (Fig. 2). Fig. 3 shows the daily PM concentrations from early summer to early autumn 2000 (July–October). Note the high degree of correlation between the PM concentrations recorded at the different rural sites. Moreover, the range of PM10 and TSP concentrations is very narrow (Fig. 3). Consequently, these summer TSP concentrations are considered representative of the regional PM10 levels in the rural environment in Eastern Spain. These regional PM10 concentrations measured at rural sites in 2000 (12–17 mg/m3 PM10 annual mean, Table 1) are relatively
PM10-MONAGREGA
TSP-MORELLA
TSP-CORATXAR
TSP-VILAFRANCA
g/m
3
40 30 20 10 0 J F M A M J
J A S O N D
Fig. 2. Monthly mean concentrations (1996–2000) of PM10 and TSP at rural sites in Eastern Spain.
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high when compared with the 2010 EU limit values for PM10 (annual mean p20 mg/m3 and do not exceed the daily concentrations of 50 mg/m3 on more than 7 days/ yr, EU Directive 1999/30/CE). Note that in the period from July to September, daily PM10 concentrations exceeding 40 and 50 mg/m3 were recorded for 11 and 3 days, respectively, at the MONAGREGA rural site. The above seasonal pattern (Fig. 2) contrasts with the seasonal evolution of PM described for rural sites from Central and Northern Europe. In the Swiss Alps, Monn et al. (1995) reported an autumn–winter maximum (weekly PM levels from 20 to 60 mg/m3 in autumn– winter and from 10 to 30 mg/m3 in summer). In the study area, during the year 2000 the weekly PM levels ranged from 5 to 15 mg/m3 in autumn and winter (March is not included) and from 20 to 40 mg/m3 in summer months and March. At rural sites in Switzerland, the monthly PM10 concentrations only exceeded the level of 20 mg/ . et al., 2001) during the period m3 in November (Ro. osli April 1997–May 1998. In contrast, in Eastern Spain the PM10 monthly concentration of 20 mg/m3 is exceeded throughout the summer (Fig. 2). Thus, this level was surpassed during 4 months in 2000 at the MONAGREGA rural site. Turnbull and Harrison (2000) did not report a significant PM10 seasonal cycle at rural sites in the United Kingdom. 3.1. Daily PM concentrations and synoptic meteorological scenarios The different periods of high and low daily PM concentrations detected from early summer to early autumn 2000 were classified as a function of the synoptic meteorological scenarios (Fig. 3 and Table 2) and origin of the air masses. 3.1.1. July 2000 Weak gradient conditions over the Western Mediterranean and the ITL development constituted the prevailing scenario from 26 June to 3 July (Fig. 5a). On 4 July an Atlantic cold front crossed Eastern Spain, causing a drop in temperature (T) and in the PM and O3 concentrations. On 6 and 7 July a peak in the PM concentrations was reported. The TOMS satellite observations showed a plume of African dust (Fig. 4) over Eastern Spain and the Western Mediterranean. Subsequently, abrupt entries of Atlantic air masses in the Western Mediterranean basin persisted up to 17 July (Fig. 5b), leading to a fall in T and in the PM and O3 concentrations. Owing to the fact that several cold fronts reached the Western Mediterranean during this period (on 11, 15 and 16 July), the temperature decreased reaching the minimum values for this month (B121C). The ozone and PM concentrations also fell reaching the minimum values for this month: O3o60 mg/ m3 and PMo15 mg/m3. These are very low PM and O3
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PM10-MONAGREGA TSP-MORELLA
70
African
60
PM10-SORITA
TSP-CORATXAR
TSP-VILAFRANCA
African Regional
African
Regional
African
Regional
Regional
50 Atlan .
g/m3
40
Atlan .
Atlan.
Atlan. Atlan.
30 20 10
(a)
0 35
T-MORELLA T-VILAFRANCA
30
T-CORATXAR T-MONAGREGA
25 T °C
T-SORITA
20 15 10
(b)
5
175
O3-CORATXAR O3-MONAGREGA
O3- MORELLA O3-VILAFRANCA
150
g/m3
125 100 75 50 25 1 (c)
20
10
July
1
10
20
August
1
10
20
September
1
10
20
October
Fig. 3. Daily mean values of (a) PM10 and TSP concentrations, (b) temperature and (c) O3 concentrations in selected rural stations from July to October 2000. The arrows highlight the periods in which the main African (grey arrow), regional (white arrow) and Atlantic (black dotted arrow) episodes occurred.
(Mill!an et al., 2000) levels when compared with those that are typical of this region in summer (Fig. 3). From 18 to 23 July the ‘‘typical summer’’ scenario prevailed. This was characterised (Fig. 5c) by weak gradient
conditions over the Western Mediterranean, the ITL and the North Atlantic anticyclone affecting northwest Europe. In this period an increase in T and in the PM and O3 concentrations was again reported. These weak
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Table 2 Dates of the PM episodes recorded in summer 2000 Dates
Pressure pattern
Local wind
Air mass origin
PM10
1–3 July 4–5 July 6–7 July 8–17 July 18–23 July 24 July–1 August
WGC(WM)+ITL L(A) H(NAF) H(A)+L(A) WGC(WM)+ITL+H(A+WE) WGC(WM)+ITL+H(A+WE)/L(A)
Breeze S-advection S-advection NW-advection Breeze/SE Breeze/NW
Regional (SE-IB) W-Atlantic Africa NW-Atlantic Regional (WM) Regional/Atlantic
22 19 43 15 31 24
2–7 August 8–23 August 24–26 August 27 August–1 September
H(A+WE) WGC(WM)+ITL+H(A+WE) L(P)+H(NAF) H(A+WE)/L(A)
N/NW-advection Breeze/SE S-advection NW/S-advection
NW-Atlantic Regional (WM+SE-IB) Africa NW/W-Atlantic
20 28 50 22
2–10 September 12–17 September 18–24 September 25–27 September 28–30 September
H(A+WE)/L(A) WGC+ITL L(A) H(NAF) L(A)
NW/N-advection Breeze S-advection S-advection NW-advection
NW-Atlantic+Western Europe Regional (WM) W/SW-Atlantic Africa NW-Atlantic
22 42 16 43 14
1–6 October 7–16 October 17–20 October 21–24 October 25–26 October 26–30 October
H(A+WE)+L(A) H(A)/L(A) WGC+H(IB) L(G) L(G) H(A+WE)
NW-advection NW-advection Breeze/SE S-advection S- advection N/S
NW-Atlantic NW-Atlantic Regional (WM/W-IB) Africa+intensive rains Africa Regional
19 10 21 11 ND 14
Meteorological pressure patterns, local wind direction, air mass origin deduced from the back-trajectory analysis, and mean PM10 (in mg/m3) concentrations at the MONAGREGA station during the episodes (ND: No data). Pressure patterns: L indicates a depression located over the Atlantic at mid-latitudes, L(A), in front of Portugal, L(P) or over the straits of Gibraltar, L(G). H indicates anticyclone over North Africa, H(NAF), the Atlantic ocean, H(A), or the Atlantic and Western Europe, H(A+WE). ITL: Iberian thermal low, WGC: Weak gradients conditions, IB: Iberian Peninsula, WM: Western Mediterranean. Local winds (direction): S: South, SE: Southeast, N: North, NW: Northwest.
gradient conditions over the Western Mediterranean favour the intense breeze circulation (Gangoiti et al., 2001). In a 3-day period (17–19 July), the PM and O3 concentrations increased from 14 to 32 mg/m3 and from 69 to 91 mg/m3, respectively (as daily values at the rural monitoring stations). From 24 July to 1 August the typical summer scenario (Fig. 5d) alternated with smooth advections of Atlantic air masses brought about by the displacement of an Atlantic depression towards the Iberian Peninsula. 3.1.2. August 2000 From 2 to 7 August cool Atlantic air masses entered the Western Mediterranean basin via the Gulf of Lyon (Fig. 5e) and reached Eastern Spain, decreasing the T and the PM and O3 concentrations and attaining values o151C for T; o15 mg/m3 for PM and o80 mg/m3 for O3 at most sites. From 8 to 23 August the summer meteorological scenario favouring the breeze circulation was again dominant (Fig. 5f), with an increase in the T and the PM and O3 levels. During this period the PM and O3 concentrations ranged between 20 and 40 mg/m3
and between 100 and 150 mg/m3, respectively. These ozone levels fall within the range of the high ozone episodes typical of this region in summer (Mill!an et al., 2000). From 24 to 26 August an African dust outbreak over the Western Mediterranean and Eastern Spain (Fig. 4) resulted in a peak PM event which coincided with a decrease in the O3 levels. The PM10 concentrations during the African event exceeded the forthcoming EU PM10 daily limit value of 50 mg/m3 at the MONAGREGA station. Subsequent abrupt entries of Atlantic air masses (Fig. 5g) brought about a marked decrease in the PM10 levels from 64 to 11 mg/m3 at the MONAGREGA station in a 3-day period (from 25 to 27 August). A similar decrease was reported at the other stations (Fig. 3). 3.1.3. September 2000 From 2 to 10 September several Atlantic cold fronts crossed Eastern Spain giving rise to low PM and O3 concentrations. From 12 to 17 September a high PM, O3 and T episode was reported under conditions favouring the breeze circulation: weak gradient conditions over the
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Fig. 4. Above: TOMS UV-radiation absorbing aerosol index maps during the African dust outbreaks between July and October 2000. Below: synoptic charts (altitude in metres a.s.l. of the pressure level of 850, or 700 hPa for the event at the end of September) showing the meteorological patterns causing the African transport of air masses.
Fig. 5. UK Meteorological Office synoptic charts of pressure (mb) at sea level.
Iberian Peninsula and Western Mediterranean together with high pressures over the Mediterranean. Even the ITL was developed on some days (Fig. 5h). During this event high PM and O3 concentrations were again reported in the range between 25 and 45 mg/m3 for PM and between 100 and 150 mg/m3 for O3. In the 18–24
September period, Atlantic cold fronts reached the Western Mediterranean (Fig. 5i), with a fall in the T and the PM and O3 concentrations. Subsequently, an African event induced a PM peak during 25–27 September (Fig. 4). Owing to the high mineral dust load, the daily PM10 levels at the MONAGREGA
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station reached 54 mg/m3, exceeding the forthcoming EU daily limit value for PM10 (50 mg/m3). Low O3 concentrations were reported owing to the influence of the relatively ‘‘clean’’ Saharan air. Subsequent Atlantic episodes again diminished the PM levels. 3.1.4. October 2000 In the 1–16 October period, several Atlantic episodes caused frequent low PM and O3 events (Fig. 5j). From 17 to 20 October weak gradient conditions and a slight anticyclonic situation prevailed (Fig. 5k), and a high T; PM and O3 episode was again reported. From 21 to 24 October intensive rains depleted the PM concentrations. An African dust event (Fig. 4) on 25 and 26 October produced a PM peak. On 31 October a cold front reached the Western Mediterranean (Fig. 5l) and led to a decrease in the T; PM and O3 concentrations. The highest PM events (daily concentrations between 40 and 60 mg/m3 in the July—October 2000 period) were reported during the African dust episodes in late August, late September and late October (Figs. 3 and 4). The transport of African dust to Eastern Spain occurs when the North African anticyclone (usually located between Algeria and Egypt at the 850 hPa pressure level) shifts to the southeast of Spain and/or when depressions develop to the West or South of Portugal (Fig. 4). The occurrence of the African dust outbreaks is corroborated by the TOMS absorbing aerosol index maps (Fig. 4). High PM and low ozone concentrations are recorded during the African episodes as reported by studies carried out in Atlantic regions (Savoie et al., 1992; Rodr!ıguez and Guerra, 2001). The second highest PM events (daily concentrations between 20 and 45 mg/m3 in the July—October 2000 period) are recorded during periods characterised by a weak pressure horizontal gradient over the Western Mediterranean, which is often associated with ITL development. Advection of air masses is not significant in this scenario. In this meteorological context, the dynamics of the air masses over Eastern Spain is highly influenced by atmospheric circulations activated by the heating of the ground during daylight (Milla! n et al., 1997). These circulations such as the breeze at the coastal and mountain (anabatic winds) sites favour the inland transport of pollutants from the urban/industrial coastal sites to the rural sites (Fig. 1) during daylight. The prevalence of these breeze circulation (up to 2 weeks) results in increased PM concentrations at the rural sites owing to the accumulation of airborne particulates in the regional atmosphere caused by the scarce renovation of the air masses. Details on the features of these ‘‘regional’’ PM episodes associated with ozone event increases in the temperature (Fig. 3) are provided in the following section. The lowest PM episodes are recorded during periods characterised by intense advections of Atlantic air
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masses toward the Western Mediterranean. These events occur in some meteorological scenarios, such as Atlantic depressions developing off the western or northern coasts of the Iberian Peninsula (Fig. 5i), the North Atlantic anticyclone bringing strong northern winds over Spain (Fig. 5b) or the North Atlantic anticyclone inducing entries of Atlantic air masses via the Gulf of Lyon (Fig. 5e). Most of these Atlantic events are associated with rainfall and with a cold front crossing the Iberian Peninsula. Since the Western Mediterranean is surrounded by high coastal ranges (Fig. 1), these Atlantic events represent entries of cool air which ‘‘clean’’ the Mediterranean atmosphere, leading to a decrease in the temperature and PM and ozone concentrations (Fig. 3 and Table 2). In the period July—October the lowest PM events were recorded during intense Atlantic episodes, e.g. daily PM concentrations o20 mg/m3 in the periods 9–18 July, 3–6 August or 4–5 September (Fig. 3). The successive occurrence of African, Atlantic and regional episodes accounts for the main variations in the PM concentrations (Fig. 3 and Table 2). 3.2. Study of the regional PM episodes The African dust outbreaks, which account for the highest PM events at the rural sites, have already been studied in detail by Rodr!ıguez et al. (2001). Thus, this section focuses on the study of the regional PM episodes given that they account for the second highest PM events at the rural sites in summer. Wind direction and velocity did not undergo daily cycles during periods dominated by synoptic scale meteorological processes. As expected, southern winds prevailed throughout the day during advection of African dusty air masses. Northern winds prevailed throughout the day during the intense Atlantic episodes caused by the North Atlantic anticyclone. However, where these are caused by Atlantic depressions, the prevalence of northern or southern winds depends on the latitude of the depression. In periods of weak gradient conditions over the Western Mediterranean, often associated with the ITL, the wind speed shows marked daily cycles with maximum values during daylight owing to the activation of up-slope winds over the mountain slopes (mountain breeze) and of the inland sea breeze. In most of these events, the wind direction is characterised by SE winds during daylight and NW winds during the night, but in some periods of intensive ITL development light SE winds may blow even during the night. In this typical summer scenario, the back-trajectory analysis shows that the air masses entering the mainland from the Mediterranean Sea during daylight have a regional origin. These trajectories show a short route (Figs. 6d, 7c and 8d) owing to the weak gradient conditions over the
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Fig. 6. Hourly values of: (a) wind velocity and direction at MORELLA and (b) TSP at MORELLA and CORATXAR. Isentropic air back-trajectories (calculated at 12:00 UTC and 3 days backward for 1000 m a.s.l.) in the period 7–20 September 2000 (c, d and e).
Western Mediterranean. An anticyclonic curvature caused by the compensatory subsidence (which typically occurs between Eastern Spain and the Balearic Islands, Fig. 1) associated with the thermal buoyancy over the warmer terrain is also observed in the trajectories (examples in Figs. 5a and c). Examples of these types of trajectories are shown in Figs. 6d, 7c and 8d, discussed below. Both the breeze circulation and the lack of significant advection of air masses during these events provide evidence of the regional origin of the airborne particulates. Fig. 6 shows the hourly values of wind velocity and direction recorded at the MORELLA station for the regional PM episode recorded in mid September 2000 (Fig. 3 and Table 2). From 7 to 10 September the wind direction and speed did not show significant variations during the day (except on 8 September), and the
trajectory analysis showed transport patterns from the North Atlantic and Western Europe (Fig. 6c). From 13 to 17 September weak gradient conditions prevailed over the Iberian Peninsula, and even the ITL was developed for some days (Fig. 5h), and an increase in the PM concentrations was reported (Figs. 3 and 6). During this period the wind speed and direction showed marked daily cycles due to the up-slope winds and the sea breeze circulation. Moreover, the trajectory analysis showed that the air masses entering the mainland during daylight had a regional origin. The scarce renovation of the air masses, due to the development of the breeze circulation and the lack of significant advection of air masses, accounts for the accumulation of airborne particulates in the regional atmosphere. From 18 to 20 September a significant fall in the PM concentrations at the rural sites was brought about by the entry of cool
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Atlantic air masses into the Western Mediterranean basin (Figs. 3, 5i and 6). These regional PM episodes were also reported in July (18–23; Figs. 3, 5c and 7) and August (8–23; Figs. 3, 5f and 8). Fig. 8 shows hourly data of wind speed and direction and TSP concentrations at the MORELLA mountain site from 4 to 28 August. From 4 to 7 August advections of Atlantic air masses produced strong northern winds resulting in low PM concentrations (Figs. 5e and 8c). From 8 to 23 August weak gradient conditions over the Western Mediterranean and the ITL prevailed (Fig. 5f), resulting in a PM event of regional origin (Figs. 3 and 8d). Southern winds associated with breeze circulation prevailed in this period and backtrajectories showed the anticyclonic curvature over the Western Mediterranean and Eastern Spain. From early 24 to early 26 August southern winds prevailed during the day, and hourly PM concentrations >100 mg/m3 were reported owing to an African dust outbreak (Figs. 4 and 8e). From 26 mid-August, an abrupt entry of northern cool Atlantic air masses in the Mediterranean basin led to reductions in the PM levels (Figs. 5g and 8e). In an attempt to elucidate the origin of the high ozone episodes in this region, Mill!an et al. (1997, 2000) proposed a conceptual model in which the polluted air masses undergo vertical re-circulations over the Eastern
coast of Spain owing to the inland up-slope winds, the westerly winds over the top of the mountains, the subsidence over the Mediterranean sea and the subsequent inland entry of the sea breeze. In this scenario, Mill!an et al. (1997) propose that polluted air mass trajectories tend to describe a helicoidal curve along the eastern coast of Spain in a southward direction. The subsidence (relative high pressures) over the Western Mediterranean sea is attributed to the compensatory sinking associated with the thermal buoyancy over the warmer terrain together with the large scale anticyclonic subsidence. This subsidence accounts for the backtrajectories showing the anticyclonic curvature described above for the regional PM episodes (e.g. Figs. 6d, 7c and 8d). Salvador et al. (1999) have argued that given the abrupt orography of the region and given that the movement of air masses is influenced by the heating of mountain and valleys slopes, meteorological models able to simulate the atmospheric dynamics need a very high resolution, at least 2 km 2 km to account for 95% of terrain variance. Using high-resolution trajectory analysis, Gangoiti et al. (2001) have demonstrated the recirculations of polluted air masses over the Western Mediterranean and the Eastern coast of Spain. Moreover, back-scattering Lidar measurements have documented the transport of aerosols towards a high altitude by up-slope winds on the coastal ranges in Eastern Spain
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(Soriano et al., 2001). Thus, the back-trajectories associated with the ‘‘regional’’ PM episodes (Figs. 6d, 7c and 8d) probably do not represent the actual and detailed movement of the air masses, but they may be used as tracers of the regional air mass and particulate sources. Although these PM regional episodes are more frequently observed, and have a longer duration in summer, they are also reported in the semi-warm periods (late spring and early autumn). A typical example, which is also associated with an ozone episode, occurred in the period 17–20 October 2000 (Fig. 3 and Table 2). This event was brought about by an anticyclonic situation together with a weak pressure horizontal gradient over the Iberian Peninsula and the Western Mediterranean (Fig. 5k). The temperature increase (Fig. 3) was not as high as during the summer event, and consequently the ITL was not present.
The PM events attributed to regional sources are associated with ozone episodes. When interpreting the occurrence of the simultaneous high PM and O3 events both the role of meteorology and that of photochemistry should be taken into account. From the point of view of meteorology, the increase in the PM and O3 concentrations during the regional episodes and the decrease during the Atlantic events account for the correlated daily O3 and PM time series. In the photochemical context, the formation of secondary particles by photochemical processes is expected to be significant in these aged polluted air masses transported to the rural environment. Under strong summer insolation, photochemical reactions of NOx, organic vapours and SO2 give rise to acid compounds, secondary particles, O3 and other oxidants. Moreover, O3 is involved directly (oxidation of gases by O3) and/or indirectly (e.g. oxidation of gases by the OH radicals formed by O3
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photolysis) in the formation of secondary particles (e.g. sulphate, nitrate, organic particulate matter). The crustal particulate matter is also a key component of PM10 in Eastern Spain during regional episodes. This is due to the temperature increases that favour the turbulence dynamics in arid areas. Querol et al. (1998a) have shown that the load of crustal particulate matter in the Ebro basin increases during summer because of soil re-suspension. In addition, Rodr!ıguez et al. (2002) found 58% and 21% loads for secondary and crustal particulate matter in PM10 sampled in summertime in the study area.
4. Conclusions At rural sites in Eastern Spain in the flat Ebro basin (600 m a.s.l.) and in the Iberian and Catalan ranges (altitudes>1000 m a.s.l.), the concentrations of PM10 and TSP (PM) undergo a seasonal evolution that is characterised by a summer maximum. In the summer months, the highest PM events are reported during African dust outbreak events (daily concentrations 40– 60 mg/m3). The second highest PM events (daily concentrations 20–45 mg/m3) are reported during regional pollution episodes associated with ozone events. These regional events are induced by the specific Mediterranean context (such as orography, meteorology, and high degree of insolation). The lowest PM10 and TSP events (daily concentrations o20 mg/m3) are reported during abrupt entries of Atlantic air masses into the Western Mediterranean basin. The frequent occurrence of African dust outbreak and regional PM episodes has implications on air pollution regulation strategies, and accounts for the marked difference in the features of the airborne particulates between Southern and Northern Europe. The African dust events are more frequently observed in Mediterranean countries than in Central–Northern Europe. In Eastern Spain, around 10 African dust events are reported every year (Rodr!ıguez et al., 2001). The high load of mineral dust brought by the African air masses interferes with the monitoring of the anthropogenic fraction of PM10 in Mediterranean countries, and could result in exceedances of the forthcoming EU daily limit values for PM10 (50 mg/m3). Mineral dust concentrations >20 mg/m3 in PM10 and TSP during African dust outbreaks over Southern Europe have been reported by several authors (Molinaroli et al., 1993; Dulac et al., 1992; Kubilay et al., 2000; Rodr!ıguez et al., 2001, 2002). Regional PM episodes reach maximum intensity in summer. In this season, westerly air flow related to the mid-latitude general circulation (typically associated with intensive westerly winds, eastward moving depressions and Atlantic cold front passages and rain) shift northward to Central and Northern Europe. This
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westerly air flow favours the dilution of pollutants and the frequent renovation of air masses over the flat terrain, which characterises these European regions. In contrast, the Western Mediterranean remains under weak gradient conditions in summer, and the abrupt orography surrounding the Western basin induces a regional meteorology that favours the ageing and recirculation of polluted air masses. The variations in the meteorology and orography from the Western Mediterranean to Central–Northern Europe result in a different behaviour of the airborne particulate pollutants (such as seasonal evolution), and influence on the background levels of suspended particles. At rural sites in Central Europe, high PM events due to regional pollution sources are mostly recorded in winter during stagnant episodes caused by cold temperature inversions and low wind speed (Monn . et al., 2001). However, at rural sites et al., 1995; Ro. osli in the Western Mediterranean high PM events are reported throughout the summer owing to the higher frequency of the above-described regional pollution episodes. Moreover, when comparing two regions with the same degree of urban/industrial development (similar rate emissions of air pollutants) from the Western Mediterranean and from the Central–Northern Europe, higher background levels of airborne particulate pollutants are expected in the Mediterranean region given the smaller capacity of ‘‘self-cleaning’’ of the Mediterranean atmosphere.
Acknowledgements The authors wish to thank the Spanish Interministerial Commission for Science and Technology (REN2001-0659-C03-03), Ministry of the Environment, the Department of Environment of the Government of Valencia and ENDESA for supporting this study and for supplying the PM10, TSP and gaseous pollutant measurements. We are also indebted to Jay Herman (Goddard Space Flight Center, NASA) for kindly supplying TOMS maps over the Mediterranean.
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