Sources and processes affecting levels and composition of atmospheric particulate matter in the Western Mediterranean
Authors:   , Rodríguez González Sergio 
Published: September 2023

This study is focused on the identification of the sources and processes affecting levels and composition of PM10 and PM2.5 from air quality monitoring networks in Eastern Spain. This is a multidisciplinary study. Time series of TSP and PM10 recorded from 1996 to 2000 are interpreted to assess the role of local, regional and distant sources in PM levels in this region. To this is end, the influence of the meteorology on PM levels recorded at nineteen rural, urban and industrial monitoring stations is studied by means of synoptic charts, back-trajectories and local meteorological variables. The impact of African dust transport on PM levels is investigated by means of simulations in the SKIRON forecast system and by TOMS satellite observations. Moreover, a chemical characterisation of PM10, PM2.5 and TSP size segregated fractions and a daily PM10 and PM2.5 source apportionment by two receptor modelling techniques are performed. For this purpose, 24-hour samples of PM10 were collected for 16, 12 and 5 months at rural, urban and industrial sites, respectively. PM2.5 was sampled at the urban site simultaneously with PM10.<br/> The results showed that day-to-day variations in PM levels are highly influenced by the meteorology. From March to October, PM levels at rural, urban and industrial sites vary as a function of the concatenation of Atlantic air mass advections (Atlantic episodes with low PM levels) and regional circulations (Regional events with high PM and O3 levels) which induce the PM transport from urban/industrial to rural sites and the ageing of polluted air masses in the Western Mediterranean. From November to February low PM levels are recorded at rural sites, and variations in PM levels at urban/industrial sites are governed by the successive occurrence of Atlantic episodes and Local urban/industrial pollution events (with high PM, NOx and CO levels). The African dust events mainly occur from January to October and induce very high PM levels in all sites.<br/> As a result of the frequency and intensity of the Regional episodes, PM levels at rural sites undergo a seasonal evolution with a summer maximum. The difference between PM levels at urban and rural sites presents an autumn-winter maximum owing to the occurrence of intensive Local urban pollution events.<br/> The meteorological context in which the aforementioned episodes occur is discussed, with special emphasis on the synoptic scenarios giving rise to the African dust outbreaks in the different seasons.<br/> Levels of PM components at the different study sites were compared. The seasonal evolution and the grain size distribution of these components was studied and the form of occurrence was determined. The chemical characterisation shows that high levels of natural mineral dust components (e.g. Al, Fe, Mg, Ti, Sr, Ca) are simultaneously recorded in PM10 and PM2.5 at all sites during African episodes. Other interesting findings are: 1) a marked seasonal evolution of nitrate levels and grain size distribution due to the occurrence of ammonium-nitrate in autumn-winter, 2) an excess of Na with respect to the Na/Cl marine ratio in summer owing to reactions of acids with sea salt, and 3) high background levels of ammonium-sulphate in summer. Moreover, the levels of elemental, organic and mineral carbon were determined.<br/> At the rural site, the PM10 annual mean reaches 22µg/m3, the main contributions being secondary particles from industrial emissions (27% of PM10), vehicle exhausts (14-23%), natural mineral dust (23%) and sea spray (5-9%). At the urban kerbside station, the PM10 annual mean reaches 49µg/m3, the main contributions being vehicle exhausts (35-45% of PM10), secondary particles from industrial emissions (24-31%), natural + road dust (25%) and sea spray (4-6%). At the urban kerbside station, the PM2.5 annual mean reaches 34µg/m3, the main contributions being vehicle exhausts (41-53% of PM2.5), secondary particles from industrial emissions (29-35%) and mineral dust (9-11%).<br/> These results have important implications for the implementation of the PM10 EU standards. The natural load in ambient PM10 levels in Eastern Spain accounts for 30-40% of the 2010 EU annual PM10 limit value (20µg/m3). In this region it will not be easy to meet this limit value given that the annual PM10 levels are in the range 17-20µg/m3 at rural, 30-45µg/m3 at urban and 45-60µg/m3 at industrial sites. The high background levels of PM10 in this region are favoured by the specific orographic and meteorological context of the Mediterranean and by the high load of mineral dust caused by the soil re-suspension and the frequent occurrence of African dust events. At rural sites, 2-5 exceedances of the EU daily PM10 limit value (50µg/m3) are recorded every year during African dust outbreaks. On average, 15 African induced and 40-80 non-African induced exceedances of the EU daily PM10 limit value are recorded every year at the urban kerbside stations.<br/> The parameter selected for PM monitoring is a key factor. Most of the PM species resulting from combustion and vehicle exhaust emissions have a fine size distribution (<2.5µm), with exception of nitrate in summer, whereas sea spray and mineral dust present a coarse size distribution. The interference of African dust in the PM monitoring is significantly reduced when PM2.5 instead of PM10 is monitored. Natural mineral dust concentrations during African episodes are in the ranges 20-30µg/m3 in PM10 and 10-15µg/m3 in PM2.5. However, PM2.5 is not a suitable parameter for PM monitoring in all environments. The selection of PM10 or PM2.5 should be conditioned by the type of anthropogenic activity. At urban sites, PM2.5 contains mainly vehicle exhaust products, whereas the road traffic dust principally occurs in the 2.5-10µm fraction. PM2.5 is not a suitable parameter for monitoring some industrial activities (e.g. ceramics, cement production or mining) with primary PM emissions in the 2.5-10µm range.