Ozone and aerosol measurements with an airborne lidar system By Edward V. Browell

Ozone (O3) is an important component of atmospheric pollution. It indirectly controls the chemistry of the tropo­sphere (atmospheric region typically below about 10 km in altitude) and directly contributes to the greenhouse effect by being a radiatively active gas.

In the lower stratosphere (atmospheric region typically between 10-30 km in altitude), O3 is the principal absorber of solar ultraviolet radiation, and because of this, it pro­tects life at the surface from harmful ultraviolet radiation. In addition, it controls the temperature in the lower strato­sphere and is important in determining the chemistry of this region. Significant O3 enhancements in the tropo­sphere have been observed in both the northern and south­ern hemispheres,1 and significant O3 reductions in the stratosphere at mid-and high-northern latitudes have been measured, with very dramatic reductions in the polar re­gions.2 Therefore, it is urgent to determine the natural and anthropogenic (manmade) processes that are controlling the amount and distribution of O3 in both the troposphere and stratosphere. This can only be adequately studied by comprehensive aircraft field experiments with remote and in situ sensors. Twelve major field experiments (nine of

them international) have been conducted since 1980 with the NASA Langley Research Center's airborne differential absorption lidar (DIAL) system 3 , 4 to investigate the princi­pal processes controlling O3 in the troposphere and lower stratosphere from the tropics to the polar regions.4 - 1 6

Two of the most recent field experiments investigated the principal processes controlling O3 in the troposphere and lower stratosphere over the Arctic. The NASA Arctic Boundary Layer Experiment (ABLE-3A) was conducted near Barrow and Bethel, Alaska, from July 10 to Aug. 12, 1988, with a primary objective of investigating the sources and/ or sinks for tropospheric O3 in the Arctic regions of North America. 1 7 The airborne DIAL system was flown on the NASA Wallops Electra aircraft to provide simultaneous remote measurements of O3 and aerosols above and below the aircraft from near the surface to the tropopause re­gion.1 6 The NASA/NOAA Airborne Arctic Stratospheric Expedition (AASE) was conducted during the winter of 1988-89 to study the conditions leading to possible O3

destruction in the wintertime Arctic stratosphere.18 The mission was based in Stavanger, Norway, and aircraft flights into the polar vortex were conducted between Jan. 6 and Feb. 15, 1989. As part of this expedition, the airborne DIAL system was flown on the NASA Ames Research Center's DC-8 aircraft to make measurements of O3 and aerosol profiles from about 1 km above the aircraft to an altitude of about 24 km for O3 and to above 27 km for aerosols. 1 4 , 1 5 This paper describes the airborne DIAL system and discusses some of the results of O3 and aerosol measurements made during these investigations.

Airborne DIAL system A schematic of the NASA airborne DIAL system configured for tropospheric measurements is shown in Figure 1. Two frequency-doubled Nd:YAG lasers are used to pump two high-conversion-efficiency, frequency-doubled, tunable dye lasers. The four lasers are mounted on a structure that supports all of the laser power supplies, the laser beam transmitting optics, and the dual telescope and detector packages for simultaneous nadir and zenith O3; and aerosol measurements. One of the frequency-doubled dye lasers is operated at 286 nm for the DIAL "on-line" wavelength of O3;

FIGURE 1 SCHEMATIC OF NASA LANGLEY RESEARCH

CENTER'S AIRBORNE DIAL SYSTEM.

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the other one is operated at 300 nm for the "off-line" wavelength. The DIAL wavelengths are produced in se­quential laser pulses with a time separation of ~300 µs to ensure that the same atmospheric scattering volume is sampled at both wavelengths during the DIAL measure­ment. Half of each ultraviolet beam is transmitted in the zenith and nadir directions. The dye laser output at 600 and 582 nm that is left after the frequency-doubling process and the residual 1064-nm output from the frequency-doubled Nd:YAG laser are also transmitted for aerosol profile mea­surements. The output beams are transmitted out of the aircraft coaxially with the receiver telescopes through 40-cm-diameter quartz windows.

The backscattered laser energy at the four wavelengths is collected by two back-to-back 36-cm telescopes. The lidar returns in the ultraviolet, visible, and infrared are separated with dichroic optics and directed onto different detectors. The analog signals from the detectors are digi­tized at 10 MHz to 12-bit accuracy, and the average digitized signals are stored every 3 seconds (average of 15 lidar returns) on 1600-bits/in 9-track magnetic tape. Ozone concentrations and aerosol distributions are calculated in real time, and the output can be displayed on a video screen or can be continuously plotted in color with two ink-jet plotters for in-flight and post-mission analysis. The vertical and horizontal resolution of the aerosol measurements can be as small as that defined by the digitization rate of the lidar return (15m) and the distance traveled by the aircraft in 3 seconds (~300 µs), respectively. The UV lidar data require vertical smoothing of at least 210 m and horizontal averaging over at least 1 minute (~6 km) to provide ad­equate signal statistics for an accurate O3 DIAL calculation. The actual resolution used in the DIAL calculation depends upon many factors during each mission, including the transmitted laser energy and solar background conditions that greatly affect the zenith measurements in particular. For the tropospheric measurements discussed here, the vertical resolution for the O3 DIAL calculation was 300 m for the nadir data and 500 m for the zenith data with a 1-minute horizontal averaging in both cases. Detailed characteris­tics of the current airborne DIAL system and the O3 DIAL technique are given in Ref. 4.

For stratospheric measurements, the airborne DIAL sys­tem is operated only in a zenith mode from the DC-8, with six laser beams being simultaneously transmitted at about 10 Hz. Lidar measurements of O3 from on- and off-line DIAL wavelengths of 301.5 and 311 nm, respectively; aerosol backscatter ratios at 603 and 1064 nm; and aerosol depolar­ization ratios at 603 and 1064 nm are obtained along the aircraft flight track. An O3 vertical resolution of 1 km up to 18 km and 2 km up to 26 km was used with a 5 minute averaging period (~70 km horizontal distance). In both tropospheric and stratospheric investigations, the O3 mix­ing ratio in parts per billion by volume (ppbv) was calcu­lated by dividing the DIAL derived O3 concentration by the atmospheric number density derived from a meteorologi­cal analysis for the same location.

Arctic tropospheric measurements A total of 23 flights were made in the vicinity of Barrow and Bethel, Alaska, between July 7 and Aug. 17, 1988, as part of the ABLE-3A field experiment. During this investigation, the tropospheric O3 budget over the Arctic was found to be strongly influenced by stratospheric intrusions.1 6 Regions of low aerosol scattering and enhanced O3 mixing ratios were correlated with descending air from the upper tropo­sphere or lower stratosphere. In the vicinity of Barrow, Alaska, O3 was generally in the range of 20-30 ppbv below 2 km; however, air with O3 above 40 ppbv was found as low as 1 km with continuity in air mass characteristics to the tropopause level.

Figure 2 presents an example of the O3 distribution ob­served in the vicinity of Barrow. Large areas with O3

exceeding 45 ppbv can be seen above 2 km near Point 1 and in a layer extending down to about 1 km on the right side of the figure. The areas with enhanced O3 also had low aerosol scattering associated with them. Using the zenith DIAL O3

data, these regions of elevated O3 were traced back to large-scale stratospheric intrusions that generally increased the O3 levels in the upper troposphere. Ozone mixing ratios >100 ppbv were found as low as 6 km in the presence of strong intrusion events. The variability in O3 in the mid-to upper troposphere affects the O3 distribution in the lower troposphere. Due to the frequency of the observed intru­sions, this process plays an important role in the tropo­spheric O3 budget at high latitudes.

Several cases of enhanced O3 were observed during ABLE-3A in conjunction with enhanced aerosol scattering in layers in the free troposphere. The airborne DIAL aerosol distribution given in the top of Figure 3 shows multiple aerosol layers from fires in Alaska, and the O3 distribution found over the same region (bottom of Fig. 3) shows en­hanced O3 associated with these layers. In some cases, the

FIGURE 2 AIRBORNE DIAL O3 DISTRIBUTION

OBTAINED NEAR BARROW, ALASKA, ON JULY 13, 1988.

October 1991 Optics & Photonics News 9

FIGURE 3 AIRBORNE DIAL O3 AND AEROSOL

MEASUREMENTS OF BIOMASS BURNING PLUMES OVER ALASKA ON

enhancement in O3 in plumes from biomass burning sources was found to be more than 50% larger than the average background O3 distribution over Alaska. The products of biomass burning can significantly alter the O3 concentra­tions in the troposphere in the Arctic, as was shown to happen over the Amazon Basin during the dry season.9

FIGURE 4 STRATOSPHERIC O3 DISTRIBUTION

FROM THE NORTH POLE SOUTHWARD ALONG THE 5°E MERIDIAN TO NEAR

STAVANGER, NORWAY, ON FEB. 9,

Arctic stratospheric measurements During the NASA/NOAA expedition, 15 long-range flights were made across the polar vortex between Stavanger, Norway, and the North Pole. The O3 distribution measured with the airborne DIAL system above the DC-8 was found to reflect the dynamics of the polar vortex prior to Feb. 9, 1989. During these earlier flights, the O3 mixing ratio decreased at each altitude upon going from outside to inside the polar vortex. This generally indicates the de­scent of the air inside the polar vortex as it cools in the darkness of the winter. At low altitudes within the vortex (<15 km), the structure in the O3 distribution was found to be correlated with the forcing of the lower stratosphere from tropospheric weather systems.

Figure 4 shows the O3 distribution observed on Feb. 9, 1989, on the last AASE mission to be flown deep into the polar vortex. The segment shown is for the return portion of the flight from the pole to Stavanger. The O3 mixing ratio was plotted against potential temperature because within the polar vortex, the O3 mixing ratio should be constant on an isentropic surface (constant potential temperature), unless there is diabatic heating/cooling of the air or a chemically-induced change in O3. The O3 mixing ratio generally increases on a potential temperature surface from outside to inside the vortex. The edge of the polar vortex near 69°N is clearly evident in the O3 data, and this again compares very well with the location of the edge from a dynamical analysis on the 440 K surface. The O3 distribu­tion at latitudes above 79°N is essentially constant on isentropic surfaces; however, in the range 71-79°N and between ~420-580 K, there is a decrease in the mixing ratio of O3 from the mixing ratio values at higher latitudes within

10 Optics & Photonics News October 1991

the vortex. The magnitude of the decrease is at a maximum at about 500 K (~20 km), and it represents a decrease of about 17% over the O3 levels farther inside the vortex.

Through the use of in situ measurements from a high-altitude aircraft, it was determined that this region of reduced O3 could not have been caused by dynamical or localized heating processes, and that the only possible explanation was that the O3 had been depleted in this region due to chemical processes.1 4 A region of lower O3 was also observed inside the polar vortex on the transit flight from Stavanger to California on Feb. 15, 1989. It was also located near the edge of the vortex, and it had a 21% decrease in O3

compared to levels farther inside the vortex. These were the only observations of large-scale regions inside the vortex that had lower O3 mixing ratios. The altitude region of the O3 decrease (~17-23 km) was found to correspond to the same altitude region where polar stratospheric clouds (PSCs) were observed prior to Feb. 3, 1989.15 This reflects the connection between the PSCs and the chemical pertur­bation of the polar vortex that led to the O3 depletion observed during this expedition.1 9 The DIAL O3 data ob­tained on Feb. 9 was the first confirmed observation of stratospheric O3 depletion in the Arctic and the first time where the full latitudinal extent of the O3 depletion was observed.

The polar stratospheric clouds that are associated with the chemical preconditioning of the stratosphere prior to O3 depletion were observed in the polar vortex on almost all flights prior to Feb. 3, 1989. The altitude range of the PSC observations was from 14-27 km, and the altitude of the most frequent PSC occurrence was near 20 km. Two types of large-scale PSCs (depth >2 km and horizontal extent >200 km) with distinctly different optical characteristics were observed for aerosols thought to be composed of nitric acid trihydrate (NAT). 1 5 Temperatures in the vicinity of the PSCs ranged from 190-198 K, which is consistent with the formation of NAT aerosols; however, the optical charac­teristics of the two types of the PSCs were markedly differ­ent. One type of NAT PSC (Type la) had low aerosol scattering ratios (<0.5 at 603 nm and <4 at 1064 nm) and high aerosol depolarizations (28-45%) at both laser wave­lengths. A second type of NAT PSC (Type 1b) had moderate aerosol scattering ratios (2-7 at 603 nm and 4-20 at 1064 nm) with low depolarizations (<4%). It has been suggested that these two types of NAT PSC's form different aerosol sizes, shapes, and number densities as a result of differences in cooling rates of the air.2 0

Since the temperatures in the Arctic stratosphere rarely decreased below the frost point for water (190 K), the water ice PSCs (Type 2) were rarely observed. Type 2 PSCs have aerosol scattering ratios > 10 and depolarizations generally >30%.

The data on PSCs obtained during this expedition have provided new insights into the optical and physical charac­teristics of PSCs and their formation properties. The PSCs provide an indication of the spatial extent of the chemical perturbation within the vortex, and this was reflected in the

spatial extent of the O3 depletion observed in the airborne lidar data near the end of the AASE field experiment.

Acknowledgements The author would like to thank R. J. Allen, C. F. Butler, A. F. Carter, T. H. Chyba, M. A. Fenn, W. B. Grant, N. S. Higdon, S. Ismail, M. Jones, S. A. Kooi, M. N. Mayo, S. D. Mayor, W. J. McCabe, B. L. Meadows, G. D. Nowicki, P. Ponsardin, J. H. Siviter, Jr., A. C. Weber, and J. A. Williams of the Lidar Applications Group in the Atmospheric Sciences Division at the NASA Langley Research Center for their assistance in developing the comprehensive measurement capabilities of the airborne DIAL system, integrating the system into the Electra and DC-8, operating the lidar in the field, and reduc­ing the lidar data after the mission. This research was supported by NASA's Global Tropospheric Research Pro­gram and NASA's Upper Atmospheric Research Program.

References 1. J . F ishman et al., "Dis t r ibut ion of t ropospher ic ozone determined f rom

satel l i te data," J . Geophys . Res. 95, 1990, 3599-3617. 2. Report of the International Ozone Trends Panel - 1988, W M O , Globa l

Ozone Research and Mon i to r ing Project - Report No. 18, 1988. 3. E. V . Browel l et al., " N A S A Mu l t ipu rpose A i rbo rne DIAL System and

measurements of ozone and aeroso l prof i les," A p p l . Opt . 22, 1983, 522-534. 4. E. V . Browel l , "Differential absorpt ion l idar sens ing of ozone," P roc . of the

IEEE 77, 1989, 419-432. 5. E. V . Browe l l , "Remote sens ing of t ropospher ic gases and aerosols w i th an

a i rborne DIAL sys tem," Optical Laser Remote Sensing, D. K. Ki l l inger and A . Moorad ian , eds., Spr inger-Verlag, New York , N.Y., 1983, 138-147.

6. R. C. Harr iss et al., "A tmospher i c t ranspor t of pol lutants f rom Nor th Amer i ca to the Nor th At lant ic Ocean , " Nature 308, 1984, 722-724.

7. E. V . Browe l l et al., "A i rborne L idar Measurements of Aeroso ls , M i xed Layer Heights, and Ozone dur ing the 1980 P E P E / N E R O S Summer Fie ld Exper iment , " NASA Ref. Pub l . , RP-1143, 1985.

8. E. V . Browe l l et al., "T ropopause fold st ructure determined from a i rborne l idar and in si tu measurements," J . Geophys . Res. 92, 1987, 2112-2120.

9. E. V . Browe l l et al., "T ropospher i c ozone and aeroso l d is t r ibut ions across the Amazon Bas in , " J . Geophys. Res. 93, 1988, 1431-1451.

10. J . K. S. Ch ing et al., "Ev idence for c loud vent ing of mixed layer ozone and aerosols , " A tmos . Env i ron . 22, 1988, 225-242.

11. E. V . Browe l l et al., "Large-scale var iat ions in ozone and po lar s t ratospher ic c louds measured wi th a i rborne l idar dur ing format ion of the 1987 Ozone Hole over Antarc t ica , " Po la r Ozone Workshop, N A S A Ref. Pub l . 10014, 1988, 61-64.

12. J . J . Margi tan et al., " In tercompar ison of ozone measurements over Antarc t ica , " J . Geophys . Res. 94, 1989, 16,557-16,569.

13. E. V . Browe l l et al., "Ozone and aeroso l d is t r ibut ions over the Amazon Bas in dur ing the wet season, " J . Geophys . Res. 95, 1990, 16,887-16,901.

14. E. V . Browel l et al., "A i rborne l idar observat ions in the winter t ime Arc t i c s t ratosphere: Ozone , " Geophys . Res. Lett. 17, 1990, 325-328.

15. E. V . Browe l l et al., "A i rborne l idar observat ions in the winter t ime Arc t i c s t ratosphere: Po lar s t ratospher ic c louds, " Geophys . Res. Lett. 17, 1990, 385-388.

16. E. V . Browe l l et al., "Large-scale var iab i l i ty of ozone and aerosols in the summer t ime Arc t i c t roposphere , " J . Geophys . Res., in press.

17. R. C. Harr iss et al., "Arc t i c Boundary Layer Exper iment (ABLE-3A): July-August 1988," submi t ted to J . Geophys . Res. , November 1990.

18. R. Tu r co et al., "The a i rborne A rc t i c s t ra tospher ic expedi t ion: Pro logue," Geophys . Res. Lett. 17, 1990, 313-316.

19. R. L. Jones et al., "On the inf luence of polar s t ratospher ic c loud format ion on chemica l compos i t i on dur ing the 1988/89 Arc t i c winter," Geophys . Res. Lett. 17, 1990, 545-548.

20. O.B. T o o n et al., " A n analys is of l idar observat ions of polar s t ra tospher ic c louds , Geophys . Res. Lett. 17, 1990, 393-396.

Edward V. Browell is Head, Lidar Applications Group, Atmo­spheric Sciences Division, NASA Langley Research Center, Hampton, Va.

October 1991 Opt ics & Photon ics News 11