Valutazione dell'impatto atmosferico della costruzione del MOSE

The construction activities of the mobile gates at the Venice Lagoon entrances might impact over several environmental matrixes; among them there are the atmospheric emissions due to off-road and maritime vehicles, heavy duty trucks, stones movement, ecc. Therefore the monitoring plan is made up of samplings of several pollutants. PM10 was analyzed on an hourly frequency at Punta Sabbioni by means of a light-scattering type sampler, while PM10 filters for elemental analysis were collected at the Punta Sabbioni, Malamocco and Chioggia inlets using low volume sequential aerosol samplers (Skypost PM-TCR Tecora). PAHs have been monitored by means of a High Volume sampler (Tisch Environment) using a monitoring strategy based on two sampling campaigns, usually carried out each year at Malamocco (September and January/February), and at Punta Sabbioni (November and February). Measurements of gaseous pollutant concentrations (CO, NO2 , NOx) were carried out on an hourly basis using tick film sensors (ETL2000, Unitech srl, Italy) at Punta Sabbioni and Malamocco, for 1 or 2 weeks each month. Furthermore, in order to put in evidence a possible contribution from the construction works of the MOSE, the data have been analysed separating the period of construction activity (between 8 am to 8 pm during working days) and in the period of inactivity (between 9 pm and 7 am during working days and all the day during holydays). Atmospheric deposition fluxes of organic and inorganic compounds have been measured by means of bulk deposimeters positioned at the three lagoon entrances and in a site far distant from the courtyard (elementary school S. Pertini, Punta Sabbioni). The last one is considered not influenced from the construction activities and therefore it gives the deposition reference flux. Two yearly sampling campaigns (June-October and December-March), each made of three samples (one month length) have been performed. The monitoring strategy is fulfilled by deploying numerical models (CALPUFF and CALMET) to compute the spatial distributions of the pollutants concentration levels according to different atmospheric dispersion conditions. The data elaboration is based on the comparison of each pollutant averaged concentration with its specific thresholds (in some cases legal ones, in other cases obtained through statistical elaboration of data obtained in absence of working activities) and on the correlation with the meteorological parameters (a meteorological mast has been positioned at Punta Sabbioni, while at Malamocco the MAV meteorological stations is used). The comparison of the real time monitored data with the meteorological conditions is very important in case of critical episodes, because it allows the discerning of them as if they are due to the MOSE work activities or as if they are a consequence of a regional bad air quality condition. Results of PM10 hourly concentrations show no statistically differences between working and no-working days. Therefore emission from working activities were not so important to give rise higher dust level concentrations (from 2005 to April 2014). An interesting feature of the PM10 monitoring is the correlation on a regional scale of the concentration levels. Results show a good correlation between the PM10 concentration recorded at Punta Sabbioni up to Rovigo, while for Reggio Emilia station the correlation coefficient is lower. That means a coupling of the local meteorology between the Venice Lagoon and Veneto Region stronger respect to Emilia Romagna Region. Most of the recorded PM10 concentration daily averaged limit exceedings at Punta Sabbioni, took place also at other Veneto cities allowing to establish that was not the conseguence of the MOSE work activity but, rather, a general pollution event on a regional scale. The comparison of the average concentrations of the different metals in the PM10 between cases in which the measurement sites are downwind and upwind of the MOSE construction yards have also been performed. However, the differences associated to the two wind direction sectors were not statistically significant so that a contribution of construction activities to metals concentrations was not evidenced. As far as the PAH difference in contaminants concentrations between the two sites is not evident, even if it is possible to notice that the concentrations found near the Lido inlet are basically lower than at the Malamocco inlet, especially in the last two years of sampling. The daily patterns of the gas concentrations at Punta Sabbioni are different with respect to those observed at Malamocco. At Malamocco, peaks in concentrations for all gas are visible in the morning between 8 am and 12 am. At both sites the trends observed in weekdays and holidays are similar. Concentrations during holidays are slightly lower with respect to weekdays at Malamocco site but are comparable for the two periods at Punta Sabbioni site. Average gaseous pollutant concentrations increase in nocturnal hours at both sites showing a correlation with the evolution of the atmospheric boundary-layer that could indicate a trapping of the pollutants in this mixed layer. An evidence of a direct effect of the MOSE construction activities in diurnal hours was not observed. With respect to the atmospheric deposition, for all pollutants here reported, both inorganic and organic, we cannot observe any long-term trend and neither an increase nor a decrease over the years has been put in evidence in the dataset. Results from experimental data obtained with field measurements are corroborated by concentration maps from dispersion model simulations, which show concentration levels comparable with the background outside the courtyards.