Handheld Sun Photometer Measurements from Light Aircraft*

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Abstract Handheld sun photometers are typically used to make aerosol optical depth measurements while on the ground. Various investigators, in unrelated efforts, have used handheld sun photometers to make aerosol optical depth measurements from light aircraft, but the strengths and weakness of this approach have not been characterized until now. While the ease and relatively low cost of an aircraft manual sun photometer are attractive, determining if the sun photometer was correctly pointed at the sun for each measurement is the biggest challenge. This problem can be partially addressed by collecting a large number of measurements at each altitude, then manually removing the largest optical depths (misalignment always results in erroneous larger values). Examples of past aircraft manual sun photometer measurements are demonstrating that it is possible to obtain quantitative measurements if sufficient sun photometer measurements are made at each elevation. In order to improve on manual sun photometer measurements, a small webcam was attached to the side of a Microtops sun photometer, and the Microtops sun photometer was triggered by computer control. By detecting the position of the sun in the webcam image, it is possible to determine whether the sun photometer was pointed at the sun correctly when the aerosol optical depth measurement was made. Unfortunately, it was found that the Microtops sun photometer takes ∼1.1 s to scan over the five wavelength channels. This 1.1-s delay proved to be too long, preventing the proposed approach from working as the aircraft was bouncing around.