Combining in Situ and Multi-Sensor Satellite Data to Assess the Impact of Atmospheric Deposition in Lake Garda
Giardino, Claudia1; Bresciani, Mariano1; Candiani, Gabriele1; Matta, Erica1; Di Nicolantonio, Walter2; Rampini, Anna1; Cacciari, Alessandra2; Ober, Giovanna3
1National Research Council, Institute for Electromagnetic Sensing of the Environment, ITALY; 2Compagnia Generale per lo Spazio, CGS-OHB, Bologna, ITALY; 3Compagnia Generale per lo Spazio, CGS-OHB, Milano, ITALY

There is a close link between the quality of air and the status of water: 1) chemical and nitrogen contaminants are two types of pollution that deteriorate both the air and the water; 2) air pollutants can fall to the ground in precipitation, in dust, or simply due to gravity; and 3) the aerosol production from surface waters results in the transfer of aquatic materials (including nutrients and bacteria) to air (Sabouri et al., 2011; EPA, 2003). In this study we focused on the second point, which is often called "atmospheric deposition" or "air deposition". In particular, we investigated the effect of aerosol of both continental (e.g., Saharan dust) and terrestrial origins (mainly anthropogenic sources) on the quality of a lake's water. The study is undertaken at Lake Garda, the largest lake in Italy (surface area: 370 km2, volume: 50 km3 and maximum depth: 346 m) that belongs to the Po River Basin. The Basin has a population of about 16 million although the territory is unequally populated and accounts for 40% of Italy's Gross Domestic Product. It is home to 37% of the country's industry, providing 46% of jobs, about 55% of livestock and 35% of the country’s agricultural production. In situ observations and satellite data are used to retrieve indicators of both water and air quality. For assessing water quality, MERIS full resolution data are corrected for radiometric effects (i.e. smile, adjacency and atmosphere) and then converted into chlorophyll-a concentration (chl-a) and water transparency by using bio-optical modeling and specific inherent optical properties of the lake. Then, chl-a is used as indicator of phytoplankton quantity whose growth might depend on iron (Fe) that is contained in Aeolian dust (Meskhidze et al., 2005); transparency is monitored hence its variation might depend on the deposition of aerosol on the lake's surface (Dolislager et al., 2012). In order to monitor the content and type of aerosols and, in particular, to detect the most important events such as desert storms, both active (Lidar) and passive (sun-photometer and satellite sensors) instruments are used. In particular, Lidar observations give the depolarization ratio, the dust aerosol optical depth (AOD) at 532 nm and the maximum/minimum heights of dusts. AERONET provides the AOD in seven bands and the Angstrom parameter. SEVIRI/MSG, MODIS and MERIS guarantee a suitable combination of channels at spatial/temporal scales useful to detect dust events and also to find out relations between AOD and atmospheric pollutants in terms of particulate matter (Janssen et al., 2008, Di Nicolantonio et al, 2009). The investigated time range is 2003-2011. The analysis of the trends of in situ data showed that 2005 and 2007 are the years with the highest frequency of atmospheric events characterized by elevated PM concentrations and Saharan dust episodes. Then, the analysis of optical satellite data showed that 2007 was the year with the highest number of cloud-free images and was hence selected to find out the relation between air and water quality. Although the impact of air deposition of these aerosols on water quality of Lake Garda is still under investigation, cross-correlation of data seems to indicate a response of the Lake Garda in terms of worsening of its water quality (i.e. increase of chl-a and decrease transparency) for increasing values of AOD. Reference list: Di Nicolantonio W., Cacciari A., and Tomasi C., 'Particulate Matter at Surface: Northern Italy Monitoring based on Satellite Remote Sensing, Meteorological Fields, and in-situ Samplings', Journal of Selected Topics in Applied Earth Observations and Remote Sensing (JSTARS), Vol. 2, NO.4, 2009, doi:10.1109/JSTARS.2009.2033948. Dolislager L. J, VanCuren R., Pederson J. R., Lashgari A., McCauley E. 2012. A summary of the Lake Tahoe Atmospheric Deposition Study (LTADS). Atmospheric Environment, 46, 618-630. EPA, Richardson et al., Estimating Estuarine Pollutant Loading From Atmospheric Deposition Using Casco Bay, Maine as a Case Study; May 2003, 25 p. Janssen S., Viaene P., Fierens F., Dumont G., Mensink C. 2008. MERIS AOD and PM10 in-situ measurements: data fusion in an operational air quality forecast model. Proc. of the '2nd MERIS/(A)ATSR User Workshop', Frascati, Italy, 22-26 September 2008 (ESA SP-666, November 2008). Meskhidze N., Chameides W. L., Nenes A., 2005. Dust and pollution: A recipe for enhanced ocean fertilization? Journal Geophysical Research, 110, D03301, doi:10.1029/2004JD005082. Sabouri R., Afkhami M., Zarasvandi A., Khodadadi M. 2011. Correlation Analysis of Dust Concentration and Water Quality Indicators. International Journal of Environmental Science and Development, 2, ISSN:2010-0264.