Hazardous anthropogenic aerosol particles constitute a major pollution component and cause a direct risk factor. However, many aerosols have natural origin and do usually not pose any direct hazards. Still, all particles in the atmosphere affect its physics and chemistry and thereby also the climate forcing effects, which are very much in focus of the discussions today.
The lidar technique has over many years developed to become an important method to measure the aerosol particle concentration with the help of the backscattered light.
The main advantage of the lidar approach is that, e.g., vertical concentration profiles can be obtained directly from ground-based measurement stations without any need for time consuming point sampling. The information about the particle concentration is, however, not directly obtained from lidar measurements, but extensive signal processing is necessary to evaluate the results.
What is generally shown when the aerosol particle concentration is measured with lidar is the extinction coefficient as a function of height. This coefficient tells how much light that is lost from a beam during a travel of a certain distance, e.g., 1 meter. The extinction coefficient is highly dependent on the amount of particles and is therefore a powerful measure of that. In the figure below, the part of the extinction coefficient that originates from aerosol particles is shown as a function of height and time for one measurement performed in a rural part of Sweden and one in an urban part of China. The very dark area (high extinction) close to ground indicates alot of particles at the urban site.
In atmospheric lidar applications we use the DIAL (DIfferential Absorption Lidar) technique to measure concentrations and fluxes of pollutants. Pulsed laser radiation on, and slightly off an absorption peak of the molecule under study is generated using an OPO (Optical Parametric Oscillator) and the backscattered radiation is detected with time resolution. The light on the absorption peak will be attenuated due to absorption and thus the concentration can be evaluated. The laser beam can be scanned through several different directions to create a 2D concentration map which can be combined with wind data to evaluate the pollutant flux.
Measurements were performed with the Lidar system placed on the aft deck of the research vessel Urania. The laser radiation was sent upwards while the boat traversed the plume and thus the sulphur dioxide flux from different volcanoes in the Mediterranean Sea could be determined.
Mercury is a severe pollutant and is often emitted as atoms (elemental gaseous mercury). The mercury absorption line at 254 nm has been used to measure elemental mercury fluxes – from factories (chlor-alkali industries), mines, geothermal fields, etc.
Lidar measurements of hydrocarbons are complicated by the abundance of several species with overlapping absorption features. By cleverly choosing a number (~20) of different wavelengths and using statistical methods to analyze the results the concentration of different species can be evaluated.