Initial Results Obtained from a Differential Absorption Lidar (DIAL) to Measure Tropospheric Ozone

A ground based Differential Absorption Lidar (DIAL) system has been built at the NASA Goddard Space Flight Center (Beltsville, MD 38.99 N, 76.84 W) and has had preliminary results from 200 meters to 18 km. The importance of this project is that current atmospheric satellites cannot peer through the optically thick stratospheric ozone layer to remotely sense tropospheric ozone, which is currently one of the most important greenhouse gases for climate change studies. Additionally, ozone can be formed in the troposphere due to complex interactions with sunlight, NOx and Volatile Organic Compounds (VOCs). Ozone has also been shown to mix down from the lower stratosphere and cause high levels of ozone at the surface. For these reasons, the origination, formation, and amount of ozone in the troposphere is therefore an important measurement in atmospheric sciences. In order to monitor this lower ozone more effectively, the ground based Tropospheric Ozone Lidar Network (NASA TOLNET) was created.

The usefulness of the DIAL method is that it exploits the difference in signal returned between one “on” channel and one “off” channel to obtain the ozone number density. Ozone, which easily absorbs light near 289 nm (the “on” channel), is more blind to photons around 299 nm (the “off” channel). To obtain these specific wavelengths, Raman cells were manufactured to enhance Stimulated Raman Scattering (SRS) in a Raman-active media. With the knowledge of the ozone absorption coefficient at these two wavelengths, the vertical number density can then be derived.