Field Methods of Monitoring Atmospheric Systems Remote Sensing Copyright © 2006 by DBS

Remote Sensing Measuring light absorption through a section of atmosphere Extremely long pathlengths compensate for low concentrations Applications –Chimney plumes –General surveys –Vehicle exhausts

Instrumentation –Laser techniques and advanced signal processing Choose wavelength of light absorbed by the analyte and not by other components UV absorbers –Ammonia –Nitrogen dioxide –OH radical –Unsaturated organics –Mercury –Ozone Sulfur dioxide

Differential Optical Absorption Spectrometry (DOAS) Measures absorption of UV ovr fixed pathlength –Source and detector with mirror at end of pathlength or –Source and detector located at each end of the sampling path Sources: –Incandescent lamp –Laser Detector: –Spectrometer Pathlength affects detection efficiency e.g. O 3 limit of detection 4 ppb over 5 km

Light Detection and Ranging (LIDAR) (Laser altimeter) Similar to RADAR - uses pulsed laser Light is scattered by particles providing different absorption paths –Amount of light returned from each height is proportional to the atmospheric density –Knowing the speed of light, the time is converted into distance/height –Concentration profile is built over entire range DIAL (Differential Absorption LIDAR) uses two wavelengths –One max (absorbed) –One min (removes background)

Light Detection and Ranging (LIDAR) Used to measure distance/altitude NO 2, H 2 O, O 3, aerosols, and SO 2 in the troposphere and stratosphere DIAL showed clear destruction of O 3 in lower stratospheric polar vortex (Bromwell, 1989) Current projects: –NASA Lidar In-space Technology Experiment (LITE)

Tunable Diode Laser Absorption Spectroscopy (TDLAS) Infra-Red Useful for local monitoring Widespread use is hindered by absorption of H 2 O and CO 2 and overlap of absorption bands Under reduced pressure bands resolve

Question TDLAS has been used for measurements from balloons in the upper atmosphere. Why do you think this form of IR spectrometry can be used for direct atmospheric measurments in the upper atmosphere but not the lower atmosphere? Pressure decreases with altitude - Decreases bandwidth of absorption lines to remove band overlap

Remote Sensing J. Fishman, A. E. Wozniak, and J. K. Creilson, "Global Distribution of Tropospheric Ozone from Satellite Measurements Using the Empirically Corrected Tropospheric Ozone Residual Technique: Identification of the Regional Aspects of Air Pollution," Atmospheric Chemistry and Physics, 3, 893–907 (2003).

Further Reading Journal articles

Text Books Hesketh, H.D. (1994) Air and Waste Management: A Laboratory/Field Handbook. CRC Press, Florida. Lodge, J.P.A. ed. (1988) Methods of Air Sampling and Analysis, 3rd Edition. CRC Press, Florida. Wight, G.D. (1994) Fundamentals of Air Sampling. CRC Press, Florida. Winegar, E.D. and Keith, L.A. eds. (1993) Sampling and Analysis of Airborne Pollutants. CRC Press, Florida.