Solar radiometry studies of Tucson´s Aspen fire smoke

The Aspen Fire started on June 17, 2003 in the mountains north of Tucson, Arizona, and smoke from this fire was carried into the Tucson valley allowing its measurement by a 10-channel solar radiometer operated by the Optical Sciences Center at the University of Arizona. This ten-band, "Reagan" solar radiometer has been used as part of regular monitoring of aerosols since the late 1990s. This work is an extension of work first started in the 1970s in Tucson and continued through portions of the 1980s and early 1990s. The historical work is crucial as a reference point to compare measurements made prior to the smoke being blown into the Tucson valley to those in July when a marked shift in wind direction carried the smoke into the line of sight of the solar radiometer. Optical depths during the smoke event exceeded average clear-sky values for the months of June and July by more than a factor of two. The retrieved Angstrom coefficient also exceeded the average values by more than one-standard deviation of the average of the previous year\´s data. This larger than average coefficient indicates small-sized particles typical of smoke near the point of origin. Results from known cirrus-cloud cases show that the retrieved Angstrom coefficient cannot be attributed to ice clouds, giving further confidence in the conclusion of smoke. The results from the Aspen fire are also used for comparison with data in the October 2003 time frame that coincided with the passage of a high optical depth case attributed to forest fires in Southern California. Measurements from multiple solar radiometers indicate that while there may have been small-sized smoke particles, the results show that large-sized particles dominated the spectral transmittance. These large particles are typical of dust and cloud conditions often seen during October in the desert southwest, but could also be indicative of larger-sized smoke particles seen at large distances from the smoke origin.