Twin Otter Instrumentation
The primary instrument to be deployed on the aircraft will be a new down-looking ozone/aerosol DIAL system, which produces profiles of ozone and aerosol structure in the boundary layer and lower troposphere. The lidar system employs three tunable wavelengths in the ultraviolet spectral region between 280 and 300 nm. Each wavelength is characterized by a different ozone absorption crosssection, enabling measurements to be made over a wide range of ozone values. The multi-wavelength capability of the system also provides flexibility for the correction of potential errors in ozone calculations caused by aerosol backscatter gradients. Ozone measurements will be made at a horizontal resolution of approximately 600 m and vertical resolution of 90 m. Precision of the ozone measurement is expected to vary from about 3 to 10 ppbV, depending on range and amount of intervening ozone. The longest of the three wavelengths at approximately 300 nm, which is least absorbed by ozone, is used to measure aerosol backscatter profiles, after correction for ozone extinction. Resolution for the aerosol measurements will be 600 m horizontally and 15 m vertically. Onboard the aircraft, a global positioning system provides a precise location for each lidar measurement. Data are analyzed and displayed on board the aircraft in real time, enabling adjustment or changes in the science mission if unexpected features or events are observed.
A key measurement objective for the Houston study will be the characterization of the structure of the boundary layer, including mixing layer height. Mixing layer height is estimated from the gradient of the lidar aerosol signal, as indicated in Figure 16. Investigation of mixed layer properties over different surfaces and the relationship with ozone concentrations will be important for understanding layering, transport, and vertical mixing. To provide additional information on surface properties, we also plan to mount a downward looking infrared radiometer on the aircraft alongside the ozone lidar to measure surface skin temperature.
Flight plans for the Twin Otter will specifically address scientific objectives associated with the transport and evolution of pollution plumes, boundary layer structure, air quality forecasting, and intercomparison of observations.