Particle extinction measured at ambient conditions with differential optical absorption spectroscopy 1 System setup and characterization

Abstract

We describe an instrument for measuring the particle extinction coefficient at ambient conditions in the spectral range from 270 to 1000 nm. It is based on a differential optical absorption spectroscopy (DOAS) system, which was originally used for measuring trace-gas concentrations of atmospheric absorbers in the ultraviolet-visible wavelength range. One obtains the particle extinction spectrum by measuring the total atmospheric extinction and subtracting trace-gas absorption and Rayleigh scattering. The instrument consists of two nested Newton-type telescopes, which are simultaneously used for emitting and detecting light, and two arrays of retroreflectors at the ends of the two light paths. The design of this new instrument solves crucial problems usually encountered in the design of such instruments. The telescope is actively repositioned during the measurement cycle. Particle extinction is simultaneously measured at several wavelengths by the use of two grating spectrometers. Optical turbulence causes lateral movement of the spot of light in the receiver telescope. Monitoring of the return signals with a diode permits correction for this effect. Phase-sensitive detection efficiently suppresses background signals from the atmosphere as well as from the instrument itself. The performance of the instrument was tested during a measurement period of 3 months from January to March 2000. The instrument ran without significant interruption during that period. A mean accuracy of 0.032 km(-1) was found for the extinction coefficient for an 11-day period in March.

Night-Time Ground Hyperspectral Imaging for Urban-Scale Remote Sensing of Ambient PM. I. Aerosol Optical Thickness Acquisition

“…Such systems have a dedicated reflector at the periphery of the (relatively short) open path (Paganini et al 2001;Müller et al 2005;Varma et al 2009). In contrast, here we portray the first steps toward the development of a novel passive RS approach for ground imaging spectroscopy procedure with high temporal resolution, applicable for night-time (i.e., dark scenes) urban monitoring of fine PM.…”

Night-Time Ground Hyperspectral Imaging for Urban-Scale Remote Sensing of Ambient PM—Modal Concentrations Retrieval

“…It is noteworthy that the lower spectral limit, 500 nm, stemmed from the choice of the halogen illumination source. In order to expand the spectral range to lower wavelengths, a different light source may be used, e.g., xenon lamps are commonly used in active RS technologies 29,30 and in optical particle counters. 34 The experimentally limited spectral range we could use both weakened the signatures' sensitivity to PM size and narrowed the retrievable range to 0.15 μm ≤ d p ≤ 1 μm (vertical dotted lines in Figure 5), as discussed previously by Etzion et al 32 Lienert et al 22 argued that a very accurate measurement of the spectral signature (∼0.5% RMS error) is necessary for an accurate PSD retrieval.…”

“…It is noteworthy that all these methods measure AOT through the whole atmospheric column. Hence, their correlations with surface PM concentrations are influenced by the vertical profile along the measured column and by local meteorology and seasonality. , However, ground spectral measurements have also been studied, in particular visibility measurements (extinction at 550 nm) and ground spectroscopy along horizontal open paths. − …”

Spectral Aerosol Extinction Monitoring System (SÆMS): setup, observational products, and comparisons

“…Foregoing efforts to develop and apply a DOAS technique at the Leibniz Institute for Tropospheric Research (TROPOS) for aerosol extinction measurements date back to the early 1990s (Flentje et al, 1997). A first test version of our long-path aerosol extinction spectrometer was constructed and successfully tested by Müller et al (2005).…”

“…At both towers, temperature and relative-humidity sensors are mounted close to the reflectors and measure these meteorological state parameters continuously. Following the denotation after Müller et al (2005), the measured spectral intensities I r (λ) and I m (λ) from the reference and measurement tower are given by…”