Arnold L. Torres

ozone loss in the lower stratosphere over the United States in 1992–1993: Evidence for heterogeneous chemistry on the Pinatubo aerosol

Ozone profiles obtained at Boulder, Colorado and Wallops Island, Virginia indicate that ozone was about 25% below normal during the winter and spring of 1992–93 in the 12–22 km region. This large ozone reduction in the lower stratosphere, though sometimes partially compensated by higher than normal ozone above 24 km, was responsible for the low total column ozone values observed across the United States during this period. Normal temperatures throughout the low ozone region suggest that transport-related effects are probably not the most important cause of the ozone deficits. The region of low ozone at Boulder corresponds closely with the location of the enhanced H2SO4/H2O aerosol from the Pinatubo eruption of 1991 as measured near Boulder and at Laramie, Wyoming. Trajectory analyses suggest that except at low altitudes in spring, air parcels on the days of the ozone measurements generally arrived at Boulder from higher latitude, although seldom higher than 60°N, and hence may have been subjected to heterogeneous chemical processing on the surface of Pinatubo aerosol droplets resulting in chlorine-catalyzed ozone destruction, a process which is believed to be more effective under the lower winter temperatures and sunlight levels of higher latitudes.

Ozone observations and a model of marine boundary layer photochemistry during SAGA 3

A major purpose of the third joint Soviet-American Gases and Aerosols (SAGA 3) oceanographic cruise was to examine remote tropical marine O3 and photochemical cycles in detail. On leg 1, which took place between Hilo, Hawaii, and Pago-Pago, American Samoa, in February and March 1990, shipboard measurements were made of O3, CO, CH4, nonmethane hydrocarbons (NMHC), NO, dimethyl sulfide (DMS), H2S, H2O2, organic peroxides, and total column O3. Postcruise analysis was performed for alkyl nitrates and a second set of nonmethane hydrocarbons. A latitudinal gradient in O3 was observed on SAGA 3, with O3 north of the intertropical convergence zone (ITCZ) at 15–20 parts per billion by volume (ppbv) and less than 12 ppbv south of the ITCZ but never ≤3 ppbv as observed on some previous equatorial Pacific cruises (Piotrowicz et al., 1986; Johnson et al., 1990). Total column O3 (230–250 Dobson units (DU)) measured from the Akademik Korolev was within 8% of the corresponding total ozone mapping spectrometer (TOMS) satellite observations and confirmed the equatorial Pacific as a low O3 region. In terms of number of constituents measured, SAGA 3 may be the most photochemically complete at-sea experiment to date. A one-dimensional photochemical model gives a self-consistent picture of O3-NO-CO-hydrocarbon interactions taking place during SAGA 3. At typical equatorial conditions, mean O3 is 10 ppbv with a 10–15% diurnal variation and maximum near sunrise. Measurements of O3, CO, CH4, NMHC, and H2O constrain model-calculated OH to 9 × 105 cm−3 for 10 ppbv O3 at the equator. For DMS (300–400 parts per trillion by volume (pptv)) this OH abundance requires a sea-to-air flux of 6–8 × 109 cm−2 s−1, which is within the uncertainty range of the flux deduced from SAGA 3 measurements of DMS in seawater (Bates et al., this issue). The concentrations of alkyl nitrates on SAGA 3 (5–15 pptv total alkyl nitrates) were up to 6 times higher than expected from currently accepted kinetics, suggesting a largely continental source for these species. However, maxima in isopropyl nitrate and bromoform near the equator (Atlas et al., this issue) as well as for nitric oxide (Torres and Thompson, this issue) may signify photochemical and biological sources of these species.

Atmospheric SO/sub 2/ measurements on Project GAMETAG

On the 1978 Global Atmospheric Measurement Experiment of Tropospheric Aerosols and Gases (GAMETAG) flights, 201 measurements of the tropospheric concentration of SO/sub 2/ were made over a latitude range 57 degrees S to 70 degrees N. The area sampled included the central and the southern Pacific Ocean and the western section of the United States and Canada. Sulfur dioxide levels averaged 89 +- 69 pptv in the boundary layer and 122 +- 85 pptv in the free troposphere in the northern hemisphere. In the southern hemisphere, SO/sub 2/ concentrations averaged 57 +- 18 pptv in the boundary layer and 90 +- 21 pptv in the free troposphere. The mean concentration of the continental data was 112 +- 79 pptv in the boundary layer and 160 +- 100 pptv in the free troposphere. The SO/sub 2/ marine values were 54 +- 19 pptv in the boundary layer and 85 +- 28 pptv in the free troposphere. From a simple chemical model we conclude that a significant amount of background SO/sub 2/ may originate from the oxidation of OCS.

Ozone climatology at Natal, Brazil, from in situ ozonesonde data

Results are presented from analysis of a large ozone profile data set obtained from balloon ozonesonde soundings made at Natal, Brazil (6°S, 35°W) during the last 10 years (1978–1988). The measurements have been made through an Instituto Nacional de Pesquisas Espaciais (INPE)/NASA long-term collaboration program. The balloons were released by the Brazilian Air Force at the Natal rocket range. The data set is sufficiently large to provide useful climatology on the average ozone concentration behavior and its seasonal variation. The day-to-day ozone concentration variability in the troposphere is rather large, giving standard deviations of about 30–40% for seasonal averages. Maximum ozone concentrations occur during local spring, September–October, and minimum concentrations during late autumn, April-May. The seasonal variation in the troposphere is much larger than in the stratosphere. If there were no seasonal variation at all in the stratosphere, the seasonal variation observed in the troposphere alone would be strong enough to drive a total ozone column variation of about 5%, which is about one half the size of the variation observed in the Natal Dobson spectrophotometer data. The ozone concentration at Natal increases with height between the surface and about 500 mbar, almost linearly, from about 15 parts per billion by volume (ppbv) to about 38 ppbv, in autumn. For the spring average the ozone concentration increases from about 25 ppbv at the surface to about 66 ppbv at 500 mbar. The sonde data suggest that limitations in aneroid pressure sensors used until 1986 caused the Natal sondes to indicate too much ozone above 6 mbar. Because of the relative sparsity and uneven distribution in time of the ozone soundings, the data are not adequate to study ozone trends. The Dobson data time series shows no definitive ozone trend but displays a pronounced quasi-biennial oscillation in ozone.

Stratospheric Ozone Intercomparison Campaign (STOIC) 1989: Overview

The NASA Upper Atmosphere Research Program organized a Stratospheric Ozone Intercomparison Campaign (STOIC) held in July–August 1989 at the Table Mountain Facility (TMF) of the Jet Propulsion Laboratory (JPL). The primary instruments participating in this campaign were several that had been developed by NASA for the Network for the Detection of Stratospheric Change: the JPL ozone lidar at TMF, the Goddard Space Flight Center trailer-mounted ozone lidar which was moved to TMF for this comparison, and the Millitech/LaRC microwave radiometer. To assess the performance of these new instruments, a validation/intercomparison campaign was undertaken using established techniques: balloon ozonesondes launched by personnel from the Wallops Flight Facility and from NOAA Geophysical Monitoring for Climate Change (GMCC) (now Climate Monitoring and Diagnostics Laboratory), a NOAA GMCC Dobson spectrophotometer, and a Brewer spectrometer from the Atmospheric Environment Service of Canada, both being used for column as well as Umkehr profile retrievals. All of these instruments were located at TMF and measurements were made as close together in time as possible to minimize atmospheric variability as a factor in the comparisons. Daytime rocket measurements of ozone were made by Wallops Flight Facility personnel using ROCOZ-A instruments launched from San Nicholas Island. The entire campaign was conducted as a blind intercomparison, with the investigators not seeing each others data until all data had been submitted to a referee and archived at the end of the 2-week period (July 20 to August 2, 1989). Satellite data were also obtained from the Stratospheric Aerosol and Gas Experiment (SAGE II) aboard the Earth Radiation Budget Satellite and the total ozone mapping spectrometer (TOMS) aboard Nimbus 7. An examination of the data has found excellent agreement among the techniques, especially in the 20- to 40-km range. As expected, there was little atmospheric variability during the intercomparison, allowing for detailed statistical comparisons at a high level of precision. This overview paper will summarize the campaign and provide a “road map” to subsequent papers in this issue by the individual instrument teams which will present more detailed analysis of the data and conclusions.

Analysis of the ozone soundings made during the first quarter of 1989 in the Arctic

A total of 197 ozone profiles were obtained from the nine Arctic ozone sounding stations of Alert, Resolute, Ny Alesund, Heiss Island, Bear Island, Ammassalik, Scoresbysund, Sodankyla, and Lerwick during January–March 1989. The sounding stations cover the latitude range from 60°N to 83°N and the longitude range from 95°W to 58°E. It was, for the first time, possible to get a detailed picture with a 150-m resolution of the vertical distribution of ozone during winter and spring over extended areas of the north polar region. The fine structure of the Arctic ozone distribution shows considerable layering in profiles obtained outside and at the edge of the polar vortex, whereas little layering is found in profiles obtained well inside the vortex. The lowest temperatures were found at the edge of the vortex, e.g., at or below −90°C during January 30–31 at Lerwick and close to −90°C on January 24 at Bear Island at the altitudes of 20–25 km. Temperatures below −77°C, the condition for the formation of the type I polar stratospheric clouds (PSCs), were common during January and early February 1989 in the Arctic. Possibly as a consequence of the heterogeneous processes in the presence of PSCs, the ozone mixing ratios decreased, especially during late winter when the suns irradiation intensifies in high latitudes. A trend of −0.1%/d around 80°N latitude was found in the altitude range of 420–600 K potential temperature during the period January 10 to February 13, 1989, increasing to −0.4%/d when the later period January 24 to February 13, 1989, is considered. If one were to take into account the downward diabatic motion (2.6 km/month) the negative trends would increase further by about −0.4%/d in magnitude.