Richards, LW; Weiss, RE; Waggoner, AP
This paper describes the Radiance Research Model 903 Integrating Nephelometer (RR903), which measures light scattering by particles (bsp). It is a light-weight (3.4 kg), low-power (4 watts, 12 VDC) instrument designed for portable operation as well as general environmental monitoring. A flash lamp is used for the light source and a photomultiplier tube for the detector. The effective wavelength is 530 nm and inadvertent heating of the sample airflow is negligible. The nephelometer can measure bsp from less than 1 Mm-1 (less than 10 percent of Rayleigh scattering) to more than 1 km-1. It has been operated at temperatures ranging from -10 degrees C - 50 degrees C (14 - 122 degrees F). The nephelometer electronics are computer based, allowing flexibility in averaging times, time constants, flash rate, background and dark current correction, output ranges, etc. An LCD display shows the current date, time, and bsp reading, and the display can be cycled through a series of screens to view instrument operation to and modify many operating parameters. The internal data logger has a built-in, real-time clock and battery-supported RAM capable of storing about two weeks of 5-min average data for bsp, temperature, pressure, and relative humidity (RH) before the newest data begin to overwrite the oldest data. An RS-232 serial port provides communication, and can be used to download data and set operating parameters. An internal temperature sensor is standard; the RH and pressure sensors are optional. The nephelometer calculates Rayleigh scattering from the temperature and the manually set (or measured) pressure, and subtracts it from the measured total scattering to calculate bsp. Both the zero and span values are automatically corrected for drift, and the calibrations remain stable until dust accumulation changes the wall scattering. In typical urban environments, the zero remains stable to within 2 Mm-1, and the span to within 10 percent, for months. The output of the RH sensor can be used to control a smart heater to heat the sample airflow only at high RH. The controller can be set so heating begins at about 60 percent RH, and the sample airflow RH does not exceed about 70 percent in dense fogs. The heating at high RH and in fogs eliminates measurement interferences and instrument problems caused by water absorption. The lack of sample airflow heating at lower RH minimizes the volatilization of nitrates, organics, and similar volatile species.
In: Regional haze and global radiation balance - aerosol measurements and models: closure, reconciliation and evaluation [proceedings of an international symposium]; October; Bend. OR. Pittsburgh, PA: Air & Waste Management Association; VIP-105-CD.