Development of LED-DOAS system for observing aerosol optical properties in the lower troposphere

Aerosols play an important role in the Earth's radiation budget through the reflection of incoming solar radiation and formation of cloud droplets working as cloud condensation nuclei. The understanding on aerosol optical properties in troposphere, especially their behavior near the ground level, is still insufficient for precise evaluation of their impact. Although a sunphotometer can provide the aerosol optical thickness, its application is limited to daytime under near cloud free conditions. A visibility meter, on the other hand, can give the value of visibility, but the operation wavelength is limited to a single wavelength, e.g. 875 nm. To attain the multi-wavelength observation of aerosol extinction coefficient near the surface level, here we propose the use of a four-color light emitting diode (LED) source emitting at 455, 530, 590, and 625 nm as a light source for the long-path measurement of aerosol extinction. A near-horizontal light path with a round-trip distance of around 630 m has been established inside the campus of Chiba University. A collimated light beam is produced with a 130 mm diameter telescope, and the reflected beam from a retro-reflector is detected using a 200 mm diameter telescope connected to a photodiode. A sinusoidal wave modulation is applied to the LED source, and the resulting modulated signal amplitude is detected and recorded using a digital oscilloscope (Iwatsu, DS-5614A). The results of the recent observation are discussed in conjunction with the concurrent records of a visibility meter (Vaisala, PWD52) and a nephelometer (TSI3563) that can measure the aerosol scattering coefficient. (paper)