Shikha Raizada, Craig Tepley and Jonathan Friedman

Cornell University, NAIC Arecibo Observatory, HC-03 Box 53995, Arecibo, PR 00612

The multi-metal resonance fluorescence lidar in operation at the Arecibo Observatory since 1997 employs a Nd:YAG pumped, dye laser as its transmitter. Its fundamental output at 589 nm has been used primarily for Na densities measurements. The neutral temperature of the atmosphere is also derived from both the Rayleigh portion of the backscattered signal at 589 nm, as well as from the frequency doubled output of the Nd:YAG laser at a wavelength of 532 nm.Recently, this lidar system was upgraded to enable Differential Absorption Lidar (DIAL) observations to study minor species of the lower atmosphere, such as water vapor, ozone, sulfur-dioxide, or nitrogen-dioxide. In a DIAL experiment, two closely spaced frequencies are transmitted, one tuned to an absorption line of a species of interest, and the other one tuned slightly off the first to lie near the wings of the absorption feature. The ratio of the two measured signals from the atmosphere allows us to determine the concentration of the gas responsible for the absorption.Large differences between the "on" and "off" wavelengths used in conventional DIAL studies can introduce uncertainties resulting from different volume backscattering and atmospheric attenuation originate from the time delay between transmission of the two wavelengths. These difficulties have been overcome in our DIAL system by adding a rotating optic installed within the laser cavity. The optic diverts the light to two separate tunable gratings and its speed of rotation is synchronized with the laser pulses. This allows us to transmit two different wavelengths on alternating pulses within the same dye fluorescence gain curve giving us an ability to perform DIAL studies at high temporal resolution. We will discuss our temperature measurements and our initial observations of various minor species in this paper.