Airborne Lidar Investigation of Ozone and Aerosols During the
NASA GTE/PEM-Tropics B Field Experiment

Edward V. Browell, Principal Investigator*

William B. Grant, Co-Principal Investigator

Atmospheric Sciences Division

NASA Langley Research Center

Hampton, VA 23681-0001

*Phone: 757-864-1273; Fax: 757-8647790

E-Mail: e.v.browell@larc.nasa.gov

Large-scale distributions of ozone (O₃) and aerosols will be investigated over the tropical Pacific Ocean with the NASA Langley airborne differential absorption lidar (DIAL) system as part of the NASA GTE/PEM-Tropics B field experiment to be conducted in February-April 1999. During this investigation, we specifically will be studying the vertical and horizontal distributions of O₃ and aerosol concentrations over the tropical Pacific Ocean during the austral late-summer and fall seasons. The relationship of these distributions to sources and sinks for tropospheric O₃ and aerosols are of interest, and they will be compared with similar measurements obtained over the tropical Pacific in the austral late-winter and early-spring seasons during PEM-Tropics A. Airborne measurements of O₃ and aerosols will be recorded from the DC-8 during all regional and survey flights made as part of PEM-Tropics B. These data will provide high-resolution cross sections of O₃ (300 m in the vertical and 70 km in the horizontal) and aerosols (60 m in the vertical and 470 m in the horizontal) from the ocean surface to above the tropopause along the DC-8 flight track. Flights will be performed over the tropical Pacific Ocean to examine natural processes that contribute to determination of the distribution of O₃ and aerosols in a region that is the most remote from continental influences. Air mass types will be examined to determine the large-scale impact on the tropospheric O₃ budget over the tropical Pacific Ocean as a result of photochemical production/destruction and transport of O₃. Natural processes related to aerosol production and transport will also be investigated. Finally, the role of the inter-tropical convergence zone (ITCZ) as a barrier to transport from mid-latitudes to the tropics will be investigated.

Measurements of O₃ and aerosol distributions will be correlated to several factors. Those of primary concern are the atmospheric structure; the vertical transport of gases and aerosols between the marine mixed layer and the free troposphere; exchange between the troposphere and stratosphere; and the subsequent long-range advection of gases and aerosols in the free troposphere. Multiple-wavelength lidar measurements will be used to study characteristics of natural and anthropogenic aerosols. The O₃ and aerosol data will also be used to provide information on the physical and chemical structure of the troposphere for each comprehensive set of in situ aircraft measurements.

Characteristics of the Airborne UV DIAL System

An advanced UV-DIAL system is currently being developed and is expected to be used in PEM-Tropics B. The laser transmitter consists of two frequency-doubled dye lasers that are pumped by a new frequency-doubled, diode-pumped Nd:YAG laser that is double pulsed. The new laser is under development at Decade Optical Systems, Inc., and it is scheduled for delivery at the end of August 1998. The support structure for the combined laser transmitter system is being developed in advance of the laser delivery so it can be integrated together with a minimum of time delay.

In this system, the new double-pulsed Nd:YAG laser sequentially pumps the two high-conversion-efficiency tunable dye lasers. The dye lasers are frequency-doubled into the UV to produce the DIAL on-line (288 nm) and off-line (300 nm) wavelengths. The residual 1064-nm beams from the frequency-doubling process of the Nd:YAG laser are transmitted along with the residual visible beams from the frequency-doubling process of the dye lasers. In total there are four wavelengths (288, 300, 600, and 1064 nm) transmitted simultaneously into the atmosphere below and above the aircraft for lidar backscatter measurements. The DIAL on and off wavelengths are produced in sequential laser pulses with a time separation of 500 s. This close spacing ensures that the same atmospheric scattering volume is sampled at both wavelengths during the DIAL measurement. The four laser beams are transmitted collinearly with the receiver telescope through 40-cm diameter fused silica windows in the top and bottom of the aircraft. The receiver system consists of two 35-cm diameter Cassegrain telescopes (one pointed in the zenith and the other in the nadir) with optics to direct the received signals through narrowband optical filters and onto detectors. The detectors include gateable photomultiplier tubes for the UV and 600-nm returns, and avalanche photodiodes (APD) for the 1064-nm returns. Four different detectors can be accommodated simultaneously in each direction. The transmitter and receiver characteristics for the UV DIAL system are summarized in Table 1, and Figure 1 shows a block diagram of the system. The measurement capabilities of the airborne UV DIAL system are given in Table 2.

Table 1. Airborne UV DIAL System Characteristics

TRANSMITTER
Pump laser(1)	Decade Optical Systems, Inc.
Pulse energy at 532	650 mJ
Pulse separation of double pulses	500 s
Pulse length	8 ns
Repetition rate of double pulses	30 Hz
Residual transmitted laser energy at 1064 nm (each direction)	350 mJ
Dye laser(2)	Continumm ND-6000
Dye laser output energy, fundamental wavelength	220 mJ
Frequency-doubled energy per pulse	80 mJ
Residual transmitted energy (each direction),fundamental	40 mJ
Laser linewidth, fundamental,	<8 pm

RECEIVER
	WAVELENGTH REGIONS
	289-300 nm	572-600 nm	1064 nm
Efficiency to detector (3), %	31	40	31
Detector quantum efficiency, %	21 (PMT)	8 (PMT)	40 (PMT)
Total field of view (selectable), mrad	6.5	3.2	12.4
Receiver field of view (selectable), mrad	<1.5	<1.5	<1.5

PHYSICAL PARAMTERS
Total weight	2175 (lbs)
Dimensions of lasers and laser support structure (L x W xH) (telescope included in length, but not height)	110 x 40 x 43 (in.)
Power requirements	11(kW)

⁽¹⁾ Near 600 nm; ⁽²⁾ Near 300 nm; ⁽³⁾ Includes filter transmission for daytime operation.

Figure 1. Side-view configuration of the airborne UV DIAL system on the DC-8 aircraft.

Table 2. Ozone and aerosol measurement parameters for airborne UV DIAL system.