Dan Peters

Abstract:

Lidars are ideally placed to investigate the effects of aerosol and cloud on the climate system due to their unprecedented vertical and temporal resolution. Dozens of techniques have been developed in recent decades to retrieve the extinction and backscatter of atmospheric particulates in a variety of conditions. These methods, though often very successful, are fairly ad hoc in their construction, utilising a wide variety of approximations and assumptions that makes comparing the resulting data products with independent measurements difficult and their implementation in climate modelling virtually impossible. As with its application to satellite retrievals, the methods of non-linear regression can improve this situation by providing a mathematical framework in which the various approximations, estimates of experimental error, and any additional knowledge of the atmosphere can be clearly defined and included in a mathematically ‘optimal’ retrieval method, providing rigorously derived error estimates. In addition to making it easier for scientists outside of the lidar field to understand and utilise lidar data, it also simplifies the process of moving beyond extinction and backscatter coefficients and retrieving microphysical properties of aerosols and cloud particles. Such methods have been applied to a prototype Raman lidar system. A technique to estimate the lidar’s overlap function using an analytic model of the optical system and a simple extinction profile has been developed. This is used to calibrate the system such that a retrieval of the profile extinction and backscatter coefficients can be performed using the elastic and nitrogen Raman backscatter signals.

Abstract:

A combination of techniques has been used to examine the composition of smoke generated by landing aircraft. A sample of dust from the undercarriage from several commercial airliners was examined with SEM/EDX (Scanning Electron Microscope/Energy Dispersive X-ray) to determine its elemental composition and also with an aerosizer/aerodisperser in order to measure the particle size spectrum. The observed size spectrum was bimodal with equal numbers of particles at peaks of aerodynamic diameter ∼10 μm and ∼50 μm. The EDX analysis suggested that the former peak is carbonaceous, while the latter consists of elements typical of an asphalt concrete runway. In the field, a scanning Lidar, in combination with optical and condensation particle counters, was deployed to obtain limits to the number concentration and size of such particles. Most of the (strong) Lidar signal probably arose from the coarser 50 μm aerosol, while respirable aerosol was too sparse to be detected by the optical particle counters. © 2011 American Chemical Society.

Abstract:

Traditionally the atmospheric physics department at the University of Oxford has applied optimal estimation techniques for the retrieval of atmospheric properties of gases; temperature, pressure and volume mixing ratio from satellite measurements. This paper describes the latest novel application of these techniques in deriving aerosol optical properties in laboratory experiments. Two examples are given; a spectral resolved technique allows an aerosol refractive index to be derived over a wide wavelength range and a method of deriving single particle refractive index and size from a novel aerosol instrument suitable for in situ aerosol monitoring.

Abstract:

The Robust And Compact Hybrid Environmental Lidar (RACHEL) is a 355nm, 4-channel Raman lidar system that has been developed for unattended, continuous measurement of the distributions of particulates, water vapour, and other pollutants in the boundary layer and troposphere, including the capacity for scanning the full hemisphere. The system has been designed to be portable and low-cost, providing the potential to investigate a wide range of environments with a single instrument. Deployment at the beginning of 2010 at the STFC Chilbolton Observatory has provided a unique opportunity to cross-compare the instrument against the numerous lidar and radar systems stationed at the observatory and to evaluate the implementation of various measurements into the data evaluation, such as radiosondes, radiometers, and aircraft observations. The system was deployed during the Eyjafjallajökull eruption of April 2010, observing the appearance and evolution of the ash plume over southern England.