PNNL

Abstract

Modern robotic spectral solar instruments designed for retrievals of aerosol opt ical depth (AOD), such as the Multi-Filter Rotating Shadowband Radiometer (MFRSR ) (Harrison et al. 1994), usually operate in an unattended mode. Thus their raw data sets sample a wide range of atmospheric conditions, most of which are unde sirable for aerosol optical depth analysis. In addition, these instruments are o ften not calibrated for absolute irradiance, and must be calibrated for AOD anal ysis from their own operational data. For AOD retrievals, this involves extrapol ation to the value that the instrument would measure before the sun's beam enter s the earth's atmosphere, i.e., the extraterrestrial, or zero air mass signal (I 0). This value is inferred via the Langley method (Shaw 1983). Recently, a meth od that utilizes component solar measurements (direct and diffuse) to identify totally clear-sky and non-hazy periods (Long and Ackerman 2000) has been used suc cessfully to screen MFRSR data for spectral solar measurements suitable for cali bration Langley plots. This method was tested in a proof-of-concept mode on a t wo-month period during the Spring of 2001 with data from the Table Mountain SURF RAD station near Boulder, Colo. The resultant calibration is subsequently applie d to an Asian dust event that occurred within that period, and verified with ind ependent aerosol optical depth measurements from a nearby MFRSR and an automated sun photometer.