Mapping Aerosols in the Atmosphere using Bistatic Polarimetric Aerosol Lidar (BiPAL)

David Wolinski mans the BiPAL receiver in Madison, WI, in summer 2023. The primary lidar is located 2.7 km from this location on top of the Space Science and Engineering Center. The receiver intercepted its laser beam at an altitude of 8.4 km.

Aerosols — small particles in the atmosphere – have a profound effect on many aspects of our environment. Some aerosols, such as sea spray, can impact short-term weather by causing clouds to form, while others, such as pollutants, can harm our lungs when inhaled. Aerosols might even have long-term impacts on Earth's energy balance: at high altitudes, they can reflect some of the Sun’s energy back into space, and at lower altitudes can seed water-droplet and ice-crystal formation. These effects can counteract the warming effects of carbon dioxide, leading some groups to study ways to engineer the Earth’s environment by intentionally releasing aerosols into the atmosphere at key locations. We have much more to learn about how aerosols interact with other components of the atmosphere, how they affect human health, and how they will impact the environment under various scenarios. It is widely agreed that novel approaches are needed understand all these effects. 

Researchers study atmospheric aerosols by using a combination of ground- and spaced-based assets. Space imagery, for example, can track wildfire smoke with full Earth coverage, high resolution, and frequent update. Multispectral imagery improves the ability to distinguish aerosol types. Not available from any imaging technology, however, is the ability to characterize the type and density of aerosols at various altitudes. This information can best be captured using aerosol lidars.   

Aerosol lidars work by probing the atmosphere with pulsed laser light, and then measuring the time-resolved backscatter from aerosol particles. The denser the aerosol, the brighter the return. Aerosol lidars have been deployed on the ground, in aircraft, and in space. 

BiPAL is an aerosol lidar that has both a backscatter receiver (as in a standard lidar) and a secondary polarization-sensitive receiver that views the laser beam at an oblique angle (15–35° from vertical). The primary receiver provides altitude-resolved aerosol density but only limited information about aerosol composition. The secondary receiver adds significant information due to optical polarization, which strongly depends on aerosol size, shape, and composition. Additional secondary detectors could be used to pull in more information in parallel. This approach could be compared to having multiple sets of eyes looking at the same thing at different angles: each has access to information that the other doesn’t, and the best picture comes from combining all the points of view. 

We recently fielded a simplified version of BiPAL at the University of Wisconsin (UW)–Madison. The receiver was set up at a distance to view the UW High Spectral Resolution Lidar laser beam emitting from a rooftop. This initial field trial demonstrated that even a relatively simple receiver has sufficient sensitivity to view the beam at up to 8.3 km altitude under light aerosol conditions at nighttime. This result has encouraged our team to perform a second field campaign in spring 2024 to collect additional data to further prove out the method.