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Photodiodes are widely used in LIDARs (Light Detection And Ranging) (Measures, 1988). In ordinary LIDARs, a pulsed laser source is used to probe the atmosphere, while a fast photomultiplier or Avalanche photodiode (APD) is used to receive the high-frequency return from the atmosphere. APDs are used mainly in the near infrared, where photomultipliers are blind. APDs were used in both analog (Porter et al., 2002) and photon counting mode ( Tatsumi & Tadashi, 1999) for the fast detection of IR LIDAR signals. In our simple telemetric LIDAR, a “vintage” technique used in the 1930s for pioneer studies on atmospheric aerosols (Duclaux,1936) and since then seldom reassessed (Meki et al.,1996), has been re-examined for the remote measurement of urban aerosols. Indeed, it represents a simplified and less expensive version of the elastic-backscatter LIDAR for short-range applications in which a continuous monitoring of particulate matter (PM) is required. It meets the requisite of being a simple instrument for the unattended, real time monitoring of PM to be used in urban pollution monitoring networks. For short-distance applications in which aerosols are to be measured within one hundred meters, a telemetric LIDAR can replace an ordinary LIDAR with a cost that is approximately 40 times lower than that of any ordinary LIDAR. The technique consists of illuminating the atmosphere with a laser beam modulated at low frequency, and then collecting the light scattered by aerosols by means of a photodiode array placed at the output of a telescope located at a certain distance from the laser. The observation angle defines the distance of the probed air volume through triangulation; the received intensity is related to PM10 in non-condensing conditions. The instrument is inexpensive, rugged, and suitable for outdoor operation, 24 h/day; it provides, moreover, all-weather measurement of PM with a time resolution of a few minutes. In the prototype, a green laser is modulated (on/off) at 620 Hz and emitted into the atmosphere. The choice of a visible wavelength simplifies both the alignment of the system and the calibration of the system in terms of volume backscatter (ch.2). The light backscattered by clean air and suspended matter is observed by means of a simple refractive telescope placed at a distance of 50 cm. The light received, which is filtered by means of an interference filter, is focused on a photodiode array placed on the telescope-focus surface. Each photodiode receives light scattered from different distances due to the telemetric geometry. A single photodiode may be selected for continuous measurements at a fixed

A Photodiode-Based, Low-Cost Telemetric- Lidar for the Continuous Monitoring of Urban Particulate Matter