Mike Newchurch1, Shi Kuang1, John Burris2, Steve Johnson3, Stephanie Long1

1University of Alabama in Huntsville, 2NASA/Goddard Space Flight Center, 3NASA/Mashall Space Flight Center

IntroductionA tropospheric ozone Differential Absorption Lidar (DIAL) system developed jointly by NASA and the University of Alabama at Huntsville (UAH) uses two separate Nd:YAG-pumped dye laser systems to produce wavelengths of 285 and 291 nm at 20 Hz frequency. A 16’’ Newtonian telescope and Electron Tubes 9813 PMTs capture the mid- and upper-tropospheric channel signals above 3km. A 4’’ Cassegrain telescope and Hamamatsu R7400 PMTs capture the tropospheric low-altitude channels below 4km. This lidar measures tropospheric ozone profiles between ~500m and 10 km at Regional Atmospheric Profiling Laboratory for Discovery (RAPCD) on the UAHuntsville campus (ASL 206 m) under both daytime and nighttime conditions. The future plan is to add a 266nm laser to reduce the aerosol interference with dual DIAL technique.

Hardware Diagnosis

A small blue LED instrument was developed to investigate the nonlinear behavior of the system such as the dead-time and signal-induced bias (SIB). The digital waveforms are stored in a function generator to emit desired LED light intensities to simulate various signal returns .

Measurement Examples

Figure 5. Block diagram of the LED experimental setup..

Table 1. Characteristics of RAPCD-DIAL system

DIAL MEASUREMENTS OF TROPOSPHERIC OZONE PROFILES IN HUNTSVILLE, AL

Function Generator

LED

Telescope

PMT

Licel

Oscilloscope

Gate Circuit

ND

Computer

Pulse Generator

285 291

Nd:YAG pumped Dye laser

Nd:YAG

4

16” Telescope

Aft Optics

30mJ/Pulse

Licel

Computer

PMT

Nd:YAG pumped Dye laser

266

4” Telescope

5

1

2

3

4mJ/pulseFuture

Figure 1. RAPCD-DIAL configuration

System Specification

Lasers Nd:YAG, 20 Hz repetition rate, ~300 mJ pulse-1 at 1064 nm, 80 mJ pulse-1 at 532 nmDye Rhodamine 590 and 610Emitted energy 4-5 mJ pulse-1 at 285/291 nm

Telescope Newtonian, 40-cm diameter, f/4.5, 1.5-mrad FOV Cassegrain, 10-cm diameter, f/2.3, 4-mrad FOVFilter Barr band-pass filter (286.4/11nm) and neutral density filtersDetector Electron Tubes 9813QA, 20-30% quantum efficiency Hamamatsu R7400, ~20% quantum efficiencySignal Processing Licel TR40-80, 250 MHz, 12 bit

Figure 8. Channel-5 measurements. (left) Ten ozone DIAL retrieval profiles (solid lines) with 750m vertical resolution and 30-min integration time are compared with co-located ozonesonde measurements (dash line) with uncertainty (gray envelope). The lidar data were taken between 11:13 and 16:42 local time 23 Dec. 06. The ozonesonde measurement was made at 13:00 local time 23 Dec. 06. (b) Average DIAL retrieval of the ten profiles shown in (a) and its 1-sigma standard deviation.

285

291

Figure 4. Channel-1 measurements made on May 17 2008. The ozonesonde was launched at 13:00 local time. (left) Range-corrected signal, (right) ozone retrievals with a 750m vertical resolution and 10min integration time. Figure 3. Nd:YAG pump and dye laser

Figure 4. The 16’’ telescope and 4’’ mini receiver

24th International Laser Radar Conference, Boulder, CO, Jun. 23, 2008

www.nsstc.uah.edu/atmchem

0 5 10 15 Range (km)

3 (266nm)

5 (285/291nm)

2 (266nm)

1 (285/291nm)

4 (285/291nm)

Figure 2. Measurement ranges of different channels

Cou

nts/

sec

Diff

eren

ce (

%)

Time (μs) Time (μs)

Figure 6. Determination of the system dead-time with LED driven by a linearly increasing function. (Left) Comparison of the raw, nonparalyzable-dead-time corrected signals and model. (Right) Percentage error of the corrected signal.

SIB

(C

ount

s/se

c)

Time (μs)

Background

Figure 7. SIB of various pulse intensities.

The 16’’ telescope operates with two altitude channels using a 10/90% beam splitter. The 4’’ telescope operates with three PMTs to retrieve ozone in the lower troposphere. One PMT detects the signal of 285/291nm lasers. The other two will be used for the future 266nm laser to reduce the aerosol impact.

Conclusion

System Configuration

The RAPCD-DIAL system currently measures ozone profiles between ~500m and 10 km with errors ranging from ~5% at 4km to ~60% at 10 km.  The error sources include the statistical uncertainty, differential scattering and absorption from non-ozone species, uncertainty in ozone absorption cross section, and imperfection of the dead-time and SIB corrections. The error due to aerosol in the lower troposphere can be significant and depends primarily on the aerosol vertical structure. We are adding a 266nm transmitter to the current 285/291 system to reduce the aerosol impact with dual DIAL technique.

Gate