CELiS (Compact Eyesafe Lidar System), a portable 1.5 μm elastic lidar system for rapid aerosol concentration measurement: Part 2, Retrieval of Particulate Matter Concentration

Abstract:

An elastic backscatter light detection and ranging (Lidar) system emits a laser pulse and measures the return signal from molecules and particles along the path. It has been shown that particulate matter mass concentrations (PM) can be retrieved from Lidar data using multiple wavelengths. In this paper we describe a technique that allows for semi-quantitative PM determination under a set of guiding assumptions using only one laser wavelength. The Space Dynamics Laboratory has designed an eye-safe (1.5 μm) single wavelength elastic Lidar system called CELiS (Compact Eye-safe Lidar System), which is described in a companion paper, to which this technique is applied.

Data utilized in the PM retrieval include the Lidar return signal, ambient temperature, ambient humidity, barometric pressure, particle size distribution, particle chemical composition, and PM measurements. Particle size distribution is measured with an optical particle counter. PM is measured with filter-based measurements. Chemical composition is determined through multiple analyses on exposed filter samples. Particle measurements are made both inside and outside of the plume of interest and collocated with the lidar beam for calibration.

The meteorological and particle measurements are used to estimate the total extinction (σ) and backscatter (β) for background and plume aerosols. These σ and β values are used in conjunction with the lidar return signal in an inversion technique based on that of Klett (1985, Appl. Opt., 1638-1643). Variable σ/β ratios over the lidar beam path are used to estimate the values of σ and β at each lidar bin. A relationship between β and PM mass concentrations at calibration points is developed, which then allows the β values derived over the lidar beam path to be converted to PM.

A PM-calibrated, scanning Lidar system like CELiS can be used to investigate PM concentrations and emissions over a large volume, a task that is very difficult to accomplish with typical PM sensors.