The Gotthard Route closure in June 2006 as seen in NOAA AVHRR aerosol maps

EADS Astrium GmbH, Claude-Dornier-Strase, 88090, Immenstaad, GermanyKeywords: Air pollution, Monitoring, Optical Depth, Remote Sensing, Traffic.Remote sensing of aerosol properties with theAdvanced Very High Resolution Radiometer(AVHRR) on board the National Oceanic andAtmospheric Administration (NOAA) polar orbitingsatellites as well as on board the new MetOp seriesfrom the European Organisation for the Exploitationof Meteorological Satellites (EUMETSAT) has a longtradition (Knapp and Stowe, 2002). Aerosol detectionwas not the initial idea of the AVHRR instrument andnowadays there are several newer satellites whichsatisfy the needs of aerosol remote sensing muchbetter. Nonetheless, the large archive covering morethan 25 years of satellite data is an unique possibilityto identify both, long term trends and short scalesingular aerosol phenomena using NOAA AVHRRdata.Several studies focused on the detection ofaerosol optical depth (AOD) over oceans since it iseasier to retrieve the aerosol signal over areas withlow and mostly homogeneous surface reflectance.Hauser et al. (2005) developed an algorithm to extractAOD over land surfaces which is not limited to darksurface targets and does not need multi-spectralinformation about land surface properties. Only brightsurface types are excluded from the retrieval. End of May 2006 a fatal rockfall caused anearly one month closure of the Gotthard route in theReuss valley. A large part of the traffic used the detourover the San Bernardino route, passing the Rheinvalley to cross the Alps. Both are major routes for thecross-alpine traffic in the north-south direction. Theimpact of this traffic displacement on the regionalareas concerning air quality and traffic density isshown by Lotscher et al. (2007). This situation is anexcellent possibility to test the potential of the abovedescribed aerosol algorithm in an alpine environment.Herein we used two years of NOAA-18 dataand derived monthly mean AODs (cf. Figure 1) inorder to compare the two affected regions. Twolocations outside the influence of these valleys, theAerosol Robotic Network (AERONET, Holben et al.,1998) sites Laegeren and Ispra, serve as validationpoints and are used to estimate large scale aerosoltransport during the period of interest. Figure 1 showsthe reduction of AOD during June 2006 along theGotthard route with a local minimum. At the sametime we find a sharp increase in the Rhein valley. In2007 the behavior in the Reuss valley is totallydifferent with high values during June and July,exhibiting the regular traffic situation on one of themajor traffic axes in the Alps.The results from this test study demonstrate theability of the AOD-over-land-algorithm to detect evensuch small scale AOD variations. This underlines itspotential to detect long term aerosol trends in the Alpsusing the large satellite data archive over the past 25years.Figure 1. Mean monthly aerosol optical depth (AOD)from NOAA-18 along Gotthard (solid) and SanBernardino (dash-dotted) route. Arrows mark meanAOD for June 2006 and 2007.This work has been funded by the Swiss DefenceProcurement Agency.Hauser, A., D. Oesch, N. Foppa & Wunderle, S.(2005). NOAA AVHRR derived aerosol opticaldepth over land.