Abstract

Continuous observation and quantitative retrieval of aerosol backscatter coefficients are important in the study of air quality and climate in metropolitan areas such as New York City. Ceilometers are ideal for this application, but aerosol backscatter coefficient retrievals from ceilometers are challenging and require proper calibration. In this study, we calibrate the ceilometer (Lufft CHM15k, 1064 nm) system constant with the molecular backscatter coefficient and evaluate the calibrated profiles with other independent methods, including the water-phase cloud method and comparison with the NASA Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) attenuated backscatter coefficient profile. Multiple-day calibration results show a stable system constant with a relative uncertainty of about 7%. We also evaluate the overlap correction for the CHM15k ceilometer (provided by Lufft) with a Vaisala CL-31 ceilometer, and the results show good consistency between two ceilometers’ range-corrected signal (RCS) profiles above 200 m. Next, we implement a forward iterative method to retrieve aerosol backscatter coefficients from continuous ceilometer measurements. In the retrieval, the lidar ratio is constrained by the co-located NASA AERONET radiometer aerosol optical depth (AOD) retrieval and agrees with the AERONET lidar-ratio products, derived from aerosol microphysical parameters. The aerosol backscatter coefficient retrievals are validated with co-located elastic-Raman lidar retrievals and indicate a good correlation (R2≥0.95) in the planetary boundary layer (PBL). Furthermore, a case study shows that the ceilometer retrieved aerosol extinction coefficient profiles can be used to estimate the AOD of the PBL and the aloft plumes. Finally, simulations of the uncertainty of aerosol backscatter coefficient retrieval show that a calibration error of 10% results in 10–20% of relative error in the aerosol backscatter coefficient retrievals, while relative error caused by a lidar-ratio error of 10% is less than 4% in the PBL.

Highlights

  • Ceilometers have been widely used to observe cloud base height and mixing layer height (MLH) and are deployed as networks due to unattended operation and easy maintenance [1,2]

  • We present a forward iterative method for aerosol backscatter coefficient retrieval applied to a Lufft CHM15k ceilometer which can retrieve aerosol backscatter coefficients during the daytime and below the clouds

  • The system constant calibrated by the Rayleigh method is consistent with the result of using a liquid cloud method and the calibrated attenuated backscatter coefficient profile of CHM15k is comparable to the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) co-located attenuated backscatter coefficient profile at 1064 nm

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Summary

Introduction

Ceilometers have been widely used to observe cloud base height and mixing layer height (MLH) and are deployed as networks due to unattended operation and easy maintenance [1,2]. The MLH and cloud base product can be potentially assimilated into the weather research and air quality model [10]. Continuous ceilometer monitoring can provide range-resolved aerosol distribution in the planetary boundary layer (PBL) and lower troposphere. Vertical distribution and mixing of aerosols and other pollutants in the PBL are critical to characterize air pollution formation and dispersion processes. Aerosol vertical distribution or range-resolved information is important to better connect satellite-measured aerosol optical depth (AOD) to the near-surface PM2.5 (particulate matter with diameter equal or less than 2.5 μm) [11,12].

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