Description
Ceilometer networks are expanding rapidly worldwide and are increasingly used for aerosol studies because they provide continuous (24/7) autonomous measurements at substantially lower cost than conventional elastic lidars. However, retrieval of aerosol optical properties from ceilometers remains challenging because of their low signal-to-noise ratio (SNR), the need for instrument-specific calibration, and uncertainty in the lidar ratio required for elastic-lidar inversions. This work presents the calculation of the ceilometer system constant that is calibrated using raw ceilometer measurements, molecular backscatter coefficients derived from local radiosonde observations, and aerosol optical depth (AOD) from a sun photometer using both solar and lunar modes of the instrument. To enable retrievals in the low-altitude region, the ceilometer overlap profile is estimated using a modified two-stream method that combines upward-pointing ceilometer observations with coincident downward-looking measurements from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). We further develop a retrieval approach to estimate an effective lidar ratio from integrated attenuated backscatter measured by the ceilometer and coincident sun photometer solar and lunar AOD. The calibrated system constant and retrieved lidar ratio are then used in a modified Fernald inversion to derive aerosol backscatter and extinction coefficients from ceilometer observations. The feasibility of the approach is demonstrated using four case studies spanning a range of aerosol loading types over a diurnal period. Additionally, an analysis of 2.5 years of data is conducted to show seasonal trends over the Hampton University site. Results indicate that combined ceilometer and sun photometer observations can provide reasonable quantitative retrievals of aerosol optical properties over long periods for a variety of aerosol types.