Raman lidar techniques for air pollution measurements

Abstract

Raman lidar has been demonstrated to provide vertical profiles of several of the key parameters needed for investigations of air quality. The time sequence of atmospheric profiles is most valuable for understanding the meteorological processes controlling the evolution of events and exposure associated with air pollution. The vibrational and rotational Raman lidar signals provide simultaneous profiles of meteorological data, ozone and measurements of airborne particulate matter. An operational prototype Raman lidar instrument makes use of 2nd and 4th harmonic generated laser beams of a Nd:YAG laser to provide both daytime and nighttime measurements. The Raman scatter signals from vibrational states of water vapor and nitrogen provide robust profiles of the specific humidity in the lower atmosphere. The temperature profiles are measured using the ratio of rotational Raman signals at 530 and 528 nm from the 532 nm beam of the Nd:YAG laser. In addition, the optical extinction profiles can be determined from the measured gradients in each of several molecular species scattering profiles compared to the molecular scale height. Wavelengths at 284 nm, 530 nm and 607 nm have been used routinely to determine profiles of optical extinction. The ozone profiles in the lower troposphere are measured using a DIAL analysis of the ratio of the vibrational Raman signals for nitrogen (284 nm) and oxygen (278 nm), which are on the steep side of the Hartley band of ozone. Examples from several data sets are used to demonstrate the utility of Raman lidar to describe the evolution of air pollution events. The examples presented have been selected to show the new level of understanding of air pollution events that is being gained from applications of lidar techniques.