B.3 Determination of the impact of natural sources (African dust, marine aerosol)

Sahara dust long range transport events

Forecasts of Sahara dust long-range transport events at the four studied cities (Porto, Barcelona, Florence and Athens) have been registered during the entire period of the 1-year field campaigns. Sahara dust long-range transport was forecasted by the use of the following prognostic tools: i)The Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) Model; ii)The Barcelona Supercomputing Centre (BSC) – DREAM8b v2.0 Atmospheric Dust Forecast System; iii) The Skiron forecast model of the National and Kapodistrian University of Athens;  iv) Flextra model.Based on the forecasts provided by the models, additional measurements of PM10 and PM2.5 were programmed and carried out during the 1-year campaigns in Florence and Athens, covering more than 50% of the year, instead of the regular 1 over 3 days monitoring.

According to the forecasts obtained prior to sampling (by HYSPLIT forecast trajectories, DREAM8b-v2.0 and SKIRON model) long-range dust transport was expected during 18%, 13%, 26% and 33% of the measurement days in Athens, Barcelona, Florence and Porto respectively. PM samples were collected on almost all these days of potential dust transport events. Some of these events covered periods of several days. The largest number of dust-transport episodes for all cities except Porto were observed during spring time and in some occasions lasted for periods of more than two weeks. After evaluating the results the actual effect from Saharan dust was identified for 14%, 10%, 25% and 23% of the measurement days in Porto, Barcelona, Florence and Athens respectively. The occurrence (number of days) of Saharan dust intrusion, as simulated by forecast models, was more pronounced during spring at all AIRUSE cities, but also in summer in Barcelona, Florence and Porto, and autumn in Athens and Porto

 

Analysis of the collected samples and advancement level of the chemical analysis

The samples collected during the 1-year campaigns in all four sites were analyzed for ion species, major and trace elements, and elemental and organic carbon. PM10 samples were also analyzed for carbonate anion (CO32-) which will be assessed as a potential tracer for Sahara dust long range transport. The chemical speciation databases were further used in the application of source apportionment by state of the art models, for the determination of the contribution of natural sources (African dust and marine aerosol) to the particle concentration levels.PM2.5 data from Milan were also used in the analysis, obtained by ARPA Lombardia who offered to collaborate in the project.

 

Contribution of natural sources to PM concentration levels

The application of receptor modeling allowed for the identification of the natural sources affecting the PM concentration levels: sea salt, soil dust and in some cases separately local dust and long-range transported Saharan dust. Contributions of these sources to the PM levels varied between AIRUSE cities, depending on the geography of each site.

Sea salt was found in a “fresh” or “aged” form, or both. Aged sea salt is depleted in chlorine and sodium is in the form of sodium nitrate. Fresh sea salt was identified at Porto, Florence and Athens, while aged sea salt was found at Florence, Barcelona and Milan. Sea salt was mostly related to the coarse mode, as expected. Mean contributions of fresh and aged sea salt in PM10 were found equal to 5.5 µg m-3 in Porto, 2.5 µg m-3 in Barcelona, 1.8 µg m-3 in Florence, and 1.0 µg m-3 in Athens. The respective contributions to PM2.5 were much lower: 1.1 µg m-3 in Porto, 0.4 µg m-3 in Barcelona and in Milan, 0.3 µg m-3 in Florence, and 0.1 µg m-3 in Athens. The results reflect the geographical distribution of AIRUSE sites: lower levels of sea salt at the inland Italian cities (Florence and Milan) and higher at the Mediterranean coastal sites, with the highest contribution observed at the Atlantic site (Porto). The contribution for Athens may be underestimated since another source, i.e. secondary nitrate, seems to be related to aged sea salt. The chemical profile of secondary nitrate obtained by receptor modeling includes major sea salt components such as Na and Mg, thus pointing to a mixing of urban gaseous emissions (from traffic, fossil fuel combustion or biomass burning) with marine aerosol. This source contributes by 3.2 µg m-3 in PM10 and 0.7 µg m-3in PM2.5.

 

Graf1

 

Saharan dust was found as a separate source by receptor modeling only in the case of Florence. In Barcelona and Athens one mineral dust profile was obtained and Saharan dust contribution was separated from the local dust contribution by considering Saharan dust long-range transport forecasts and applying the methodology for net dust calculation (Escudero et al., Atmospheric Environment, 41: 5516–5524, 2007). In Milan and Porto, the contribution of Saharan dust was estimated as negligible. Annual mean Saharan dust contribution during AIRUSE sampling days was estimated in 0.3 µg/m3 (1%) in Barcelona, 0.7 µg/m3 (4%) in Florence, and 3.0 µg/m3 (14%) in Athens. This large difference is due to the Southern location of Athens, and severity of some Saharan dust episodes in the eastern part of the Basin. Saharan dust contribution to PM2.5 was estimated only in Florence (0.2 µg/m3) and Athens (0.7 µg/m3), with PM2.5/PM10 ratio equal to 0.2 in both cases. Concerning PM exceedances, the relative burden of Saharan dust increases during exceedance days only in Athens. For the Athens suburban site, Saharan dust is on average the main source of PM10 (52%) and PM2.5 (45%) during exceedance days (PM10 concentration exceeding the daily limit value of 50 µg m-3). No contributions from Saharan dust were found in Florence and Barcelona during days with PM10 above 50 µg m-3and 40 µg m-3, respectively.

Graf2

 

Dissemination activities

The initial results obtained in the framework of Action B3 were presented in two conferences:

–        International Conference on Atmospheric Dust (DUST 2014), 1-6/6/2014, Castellaneta Marina, Italy

–        International Aerosol Conference, 28/2-2/9/2014, Busan, Korea.