Regional dispersion of pollutants with special reference to Etna emissions as measured at the Giordan Lighthouse GAW station

Abstract

Trace gas measurements of ozone, sulphur dioxide and nitrogen oxides together with meteorological parameters have been collected between 2011 and 2014 at Giordan Lighthouse Global Atmosphere Watch station in Gozo, Malta. Additionally, measurements of carbon monoxide, carbon dioxide, methane and water vapour were collected between 2012 and 2014. This research concentrates on the monitoring of these trace gases with a view to identify the origin of pollution sources and studying their dispersion. Ozone data gathered during this study was compared to that collected between 1997 and 2008 to analyse long term trends. A statistically significant decreasing trend was in fact found for ozone from 2003 to 2014 which was preceded by a statistically significant increase in the period between 1997-2003. Seasonal variations were evident in all the trace gas measurements and their variations with wind direction were also found to be statistically significant. The latter variations highlighted the effect of natural emissions from Etna, as well as local anthropogenic emissions from the main island of Malta as well as ship emissions in the Malta-Sicily channel. Etna' s volcanic eruptions were found to be one of the main natural sources of pollution affecting the Maltese Islands. Research was carried out to gather information about Etna's eruptions which involved the Maltese Islands, starting with historical eruptions dating back to the 14th century to more recent ones. The dispersion of Etna's ash plume was modelled using PUFF, which provided tephra deposit load and ash concentrations. Three different eruptive scenarios that characterize Etna's recent activity were considered; the first scenario representing the 2001 eruption (Sc1), the second scenario representing the July 1998 eruption (Sc2) whilst the third scenario represents the recent activity from 2011 onwards (Sc3). It was found that the time taken for the volcanic ash plume to reach the Maltese Islands when the wind direction is toward the Southwest ranges from 4 to 8 hours. The effect of wind speed and direction was also studied and it emerged that the probability that an Etna volcanic plume reaches Malta during an eruption is around 13% per annum. The now calibrated model produces daily forecasts of deposit load and cumulative area of volcanic ash dispersal. This allows for provision of adequate alerts to civil aviation authorities and Malta airport which is of direct use to local communities and aviation. Insights into the high levels of sulphur dioxide measured at Giordan Lighthouse were also explored by evaluating the potential effect of Etna's volcanic SO2 plume. Investigation was carried out by examining relationships between SO2 concentrations gathered at Giordan Lighthouse and those measured at Etna by the ultraviolet scanning spectrometer network FLAME (FLux Automatic Measurements) of the Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo (INGV-OE) between 2011 and 2013. Seventy case studies with anomalous S02 peaks observed in Gozo were inspected by evaluating the strength of association between Giordan Lighthouse and INGV-OE records using statistical analysis. Statistically significant correlation was found in 40 of these cases. Results show for the first time the impact of Etna's volcanic S02 plume on the island of Malta and its potential effects on the local environment. On the other hand, shipping in the Malta-Sicily channel was found to be a great source of anthropogenic emissions affecting the Maltese Islands. This channel is a very busy route, and thus an Automatic Identification System (AIS) receiver was installed to be able to gather information on the geographic distribution of maritime traffic and its activities. A database was developed to store this information which was used to come up with two methods of measuring ship's traffic near the Maltese Islands between 2012 and 2014. One procedure involves taking hourly counts of the number and type of vessels and averaging them on a daily basis. The other method entails the comparison of vessels present in a narrow strip between Malta and Sicily on a daily basis, to come up with a yearly average number of vessels crossing the channel. The first method resulted in an average of 256 vessels present at any point in time in the area surrounding the Maltese Islands, with the highest values observed during summer with an average peak in July of 356 vessels. The second method showed that an average of 40,000 vessels cross the Malta-Sicily channel annually. These figures, although high, only quantify a portion of the entire situation, since they are based on data gathered by one AIS antenna, and thus coverage was limited. The majority of vessels were found to be cargo vessels in both methods. Additionally, through a collaborative agreement, the AIS data for the year 2012 was inputted into the Ship Traffic Emission Model (STEAM), developed by the Finnish Meteorological Institute to give results of oxides of sulphur and nitrogen as well as carbon dioxide emissions. This was the first time that the STEAM model was used in the Mediterranean and that such a study related to shipping activities near the Maltese Islands was carried out.