Vertical profiles of mineral dust in mixed East Asian pollution plumes based on Raman spectrometer lidar measurements

The height and geometric depth of even the thinnest aerosol layers can be detected by means of active remote sensing with lidar. Such measurements allow for a retrieval of aerosol optical properties under ambient atmospheric conditions. So far, the full potential for lidar observations is not yet exploited in Earth system monitoring and modelling as optical properties measured with lidar are not easily converted into microphysical or chemical information and vice versa. A shift of the lidar paradigm towards the profiling of actual chemical compounds rather than purely optical properties is necessary to truly integrate lidar into established atmospheric and air-quality monitoring networks.

Non-linear scattering by molecules of different chemical compounds causes characteristic frequency shifts of the known laser wavelength used in a lidar system. The detection of backscattered light at the shifted wavelength allows for a retrieval of the mass concentration of a target species from raw signals without the intermediate step of determining aerosol optical properties. The spectrometer lidar technique has been applied successfully to lidar measurements of non-linear (Raman) scattering by quartz molecules (silicone dioxide) to derive profiles of the atmospheric mass concentration of mineral dust (Müller et al., 2010; Tatarov et al., 2011). However, lidar techniques that allow for a comprehensive chemical aerosol characterization are still in their infancy and require refined methodological and instrumental specification.

We will present further results of spectrometer lidar measurements with the “Multiwavelength Raman/Spectrometer Lidar in East Asia” (MRS.LEA) which is based in Gwangju, Republic of Korea. The receiver system of MRS.LEA features a novel spectrometer/multichannel detector setup that can detect simultaneously more than two dozen Raman shifted wavelengths that can be attributed to species such as silicon dioxide which acts as tracer for mineral quartz, ozone, sulfur dioxide, nitrous oxides, and carbon. The high levels of pollution in East Asia with strong vertical variation and huge variance in relative humidity make Korea an excellent location for the test of such a revolutionary lidar technique. For instance the technique allows for identifying the mixing state of mineral dust with other chemical species in East Asian pollution. The methodology and measurement setup will have to be refined to be applicable to the lower levels of pollution outside of East Asia.

References

Müller et al. (2010), Mineral quartz concentration measurements of mixed mineral dust/urban haze pollution plumes over Korea with multiwavelength aerosol Raman-quartz lidar, Geophys. Res. Lett., 37, 2010GL044633.

Tatarov et al. (2011), Lidar measurements of Raman scattering at ultraviolet wavelength from mineral dust over East Asia, Opt. Expr. 19, 1569-1581.