The study of particulate matter is of increasing interest to the scientific community (epidemiologists, researchers…) and to decision makers, in particular to determine the impact of this pollutant on human health [1-3]. It is thus important to conduct the monitoring of this pollutant especially in densely inhabited regions, in order to take adequate measures (e.g. prevent the surrounding population, in particular sensible persons such as children and old ones) if regulatory values are exceeded and also to study its long-term trend and effects. In this sense, since a few decades, decision makers have established regulatory values for the PM10 concentration levels, following the severity of the impact of this pollutant on human health and on the environment [4-6]. European directives, have set air quality standards (limit values, quality objectives) to be achieved for more than a dozen pollutants, among which the PM10 [7].

The European regulations for PM10 (e.g. Decree of 15th February 2002) is based on the following:

• Quality Objective (QO): 30 µg/m³ on annual average;

• Limit values (LV): 90.4% of the daily average must be below 50 µg/m³ (i.e., 35 days of exceedances allowed per calendar year) ;

• Limit values (LV): 40 µg/m³ on annual average.

Importantly, air quality organizations which provide air quality index daily to the population are confronted with the contribution of natural aerosols on particles measurements, especially in marine urban regions, which may in part hamper the air quality index.

Anthropogenic activities such as traffic circulation emit important amounts of particles and gases [8]. Several epidemiological studies indicate that lung cancer and other health effects are possibly associated with atmospheric particles [2,9-14]. In this sense, in June 2012, the International Agency for Research on Cancer (IARC) classified diesel engine exhaust as carcinogenic to humans (Group 1), based on sufficient evidence that this exposure is associated with an increased risk for lung cancer [3].

Natural sources are also a major cause for atmospheric particles [15-18]. In particular, marine aerosols contribute significantly to the global aerosols load and consequently influence the Earth’s radiative budget [19]. Some studies suggest that on coastal regions, sea salt aerosols may notably contribute to the annual mean particulate mass such as PM10 [20-22]. They emit a large amount of halogens, which are highly reactive and thus have a notable impact on the chemistry of hydrocarbons and/or ozone in the atmosphere [23]. Furthermore, sea salt plays an important role in a number of physical and chemical atmospheric processes [24,25]. For example, the reaction of sea salt particles with nitric acid forms sodium nitrate, which may be integrated in the particulate phase of the nitrogen budget [26]. The halogens released by the reaction of acidic gases with sea salt contribute, via heterogeneous reactions, to ozone destruction [27,28]. Sea salt may also indirectly impact vulnerable ecosystems via acid deposition [29,30]. Besides, sea salt aerosols contribute to corrosion of materials (e.g. metals) in coastal regions which cost billions for their rehabilitation [31]. Sea salt aerosols also contribute to cloud condensation nuclei (CCN), especially in marine regions [32,33]. Consequently, sea salt aerosols are essential components of atmospheric constituents at local, regional and global scales [34,35].

A number of studies have been dedicated to the modeling of atmospheric particles in general and sea salt particles in particular [36-38]. Many studies of marine aerosols and their role in the climate system were performed on the global scale [34,39-42]. For air quality assessment on local-to-regional scales, it is important to determine the gradients in sea salt levels and a number of regional models include a large description of sea salt [43-46]. The validation of these models is notably concerned by the limited number of available measurements. However, up to now, few atmospheric measurements have been undertaken with links to mixed sources (e.g. urban and marine aerosols) in particular the southern hemisphere [47-49].

The objective of this study is to quantify the sea salt contribution on the PM10 particles measured at two marine urban locations close to each other. Continuous PM10 measurements and chemical analysis of particles sampled daily on filters to determine the major ions (specific tracers of sea salt aerosols) was undertaken, to establish the distribution of sea salt aerosols on PM10 measured across Saint-Pierre city. These data were analyzed in conjunction with meteorological parameters in order to confirm the air masses origin. Finally, an assessment of the sea salt contribution based on the retrieval of the sea salt contribution on the PM10 measurements is proposed for the recalculation of the air quality index more realistic of anthropogenic activities.

Réunion Island is a French department located in the South-Western Indian Ocean (21 S; 55 E). This tiny (area: ~2500 km²) and mountainous island (highest point: ~3075 m asl (above sea level)) holds two agglomerations with ~100,000 inhabitants: Saint-Denis, located to the North and Saint-Pierre located to the South of it (see Figure 1). Figure 1(a) is a zoom on Saint-Pierre city illustrating the geographical context of the measurement locations, in particular with respect to the coasts.

Saint-Pierre city is exempt of light/heavy industries susceptible emitting any atmospheric pollution and a significant proportion (~49%) of private cars as well as quite all trucks and buses operate with diesel at Reunion Island [50].

Figure 1(b) shows the configuration of Bons Enfants (BON hereafter) station, highlighting in particular:

• Its position, located about 200 m from the south coast of Reunion Island;

• Its proximity (about 1.5 km) with another urban station Luther King (LUT hereafter), the latter not showing any limit value of PM10 exceeded since 2008 (see Section 4.1.4 here after);

• The two measurement stations are located in the same urban environment, with light traffic circulation and at more than 0.5 to 1 km away from heavy highways. Hence, they are deemed representative of the urban background pollution level linked to anthropogenic activities (mainly traffic circulation);

• The presence of shallow water between the coast and the coral reefs which contributes to an efficient daily sea spray via wind flows and a potential sea salt source over this area. This is not the case for LUT which is downstream the coast too but where there is deep sea water with quite no coral reef and thus much lower sea spray emissions than at BON.

The PM10 and NO₂ (nitrogen dioxide) concentrations

have been recorded continuously at LUT and BON since August 2007.

The PM10 measurements are subsequently compared with meteorological data. In September 2011, atmospheric samples were also collected on filter on a daily basis for subsequent laboratory analysis in order to characterize the chemical composition of the PM10 particles.