Sunphotometers measure transmission through the atmosphere of the direct solar beam in narrow bands of ultraviolet, visible, and infrared radiation. . Such measurements yield valuable information on the properties of aerosols (particulate matter suspended in the atmosphere) and such trace gases as water vapor and ozone.

A series of water vapor intensive operating periods (WVIOPs) were conducted at the Department of Energy Atmospheric Radiation Measurement (ARM) site in north-central Oklahoma between 1996 and 2000. The initial focus of these experiments was on water vapor in the lower troposphere where most of the water vapor resides. To this end, a large suite of instruments (installed permanently or deployed temporarily) was operated during the WVIOPs. This included radiosondes, microwave radiometers, infrared radiometers, Raman lidars, differential absorption lidars, Global Positioning System receivers, sunphotometers and in-situ probes.

Our group participated in the 1997 and 2000 IOPs conducting ground-based measurements with the Ames Airborne Tracking 6-channel Sunphotometer (AATS-6).

The 2000 Water Vapor IOP ran from September 18 to October 8, 2000. Our primary result was columnar water vapor (CWV) but we simultaneously measured aerosol optical depth (AOD) at five wavelengths. During the IOP, preliminary results of CWV and AOD were displayed in real-time at the operator station. The result files were made available to other investigators by noon of the next day. The preliminary results of AATS-6 CWV fell within the spread of values obtained from other techniques.

After conclusion of the 2000 WVIOP, AATS-6 was shipped directly to Mauna Loa, Hawaii for post-mission calibration. The updated calibration and a cloud screening technique for AOD were applied to the data set, which was then archived in the ARM External Data Center. The CWV in the archived version 0.1 differs by only 0.5% from the data produced during the IOP. However, we have to keep in mind that the absolute precision of sunphotometer-derived CWV is affected by ongoing significant changes in the H₂O spectroscopy. Currently three different but equally plausible spectroscopic databases are available.

Figure 1 shows a time series of CWV from AATS-6, and 2 GPS receivers and a transport model during the 2000 WVIOP.

Collaborators: Univ. of Wisconsin; Pacific Northwest, Argonne and Brookhaven National Labs; SUNY; University of Sao Paulo; AER Inc.; NASA Goddard; UCAR

Point of Contact: Beat Schmid, 650/604-5933, bschmid@mail.arc.nasa.gov