Black carbon aerosol number and mass concentration measurements by picosecond short-range elastic backscatter lidar

Abstract

Black carbon aerosol emissions are recognized as contributors to global warming and air pollution. There remains, however, a lack of techniques to remotely measure black carbon aerosol particles with high range and time resolution. This article presents a direct and contact-free remote technique to estimate the black carbon aerosol number and mass concentration at a few meters from the emission source. This is done using the Colibri instrument based on a novel technique, referred to here as Picosecond Short-Range Elastic Backscatter Lidar (PSR-EBL). To address the complexity of retrieving lidar products at short measurement ranges, we apply a forward inversion method featuring radiometric lidar calibration. Our method is based on an extension of a well-established light-scattering model, the Rayleigh-Debye-Gans for Fractal-Aggregates (RDG-FA) theory, which computes an analytical expression of lidar parameters. These parameters are the backscattering cross-sections and the lidar ratio for black carbon fractal aggregates. Using a small-scale Jet A-1 kerosene pool fire, we demonstrate the ability of the technique to quantify the aerosol number and mass concentration with centimetre range-resolution and millisecond time-resolution.

Figure 1

Principle of operation of the Picosecond Short-Range Elastic Backscatter Lidar (PSR-EBL) technique, intended to measure BC aerosol number and mass concentration, $n_{\mathrm{o}}$ and $m_{\mathrm{o}}$, respectively. A picosecond laser pulse is emitted from the lidar transmitter to illuminate a column of BC aerosols in the direction ${\widehat{\mathbf{q}}}^{\mathrm{inc}}$. When a pulse arrives at a particle (shown inset) at a range r, it may be partly absorbed and will scatter in all directions $\widehat{\mathbf{q}}$. The lidar return signal is directly related to the light backscattered by the particle to the receiver’s area A, which defines the received solid angle $\mathrm{\Delta }\mathrm{\Omega }$. An example measurement of the return signal is shown at the bottom for a small-scale kerosene pool-fire at a range of 9 m from the instrument. Further description of the Colibri lidar is given in the Methods section.

Figure 2

Microphysical properties of BC particles from a Jet-A1 pool-fire. In (a) is a STEM/HAADF image of a typical BC aggregate, while (b) shows the size distribution, in radius, of the monomers (red bars) and its lognormal fit (blue). In (c), a HRTEM image of a monomer is shown illustrating an onion-like structure and (d) presents the C–K edge EEL spectra of a monomer in blue and for graphite in red as a reference.

Figure 3

Range and time-resolved number $n_{\mathrm{o}}(r,t)$ and mass $m_{\mathrm{o}}(r,t)$ concentration profiles from the PSR-EBL technique of BC aerosols emitted by a small-scale Jet A-1 pool-fire. To highlight the resolution obtained, the inset images show a magnified view of the plume occurring between 20-30 s.