Vincent G. Brackett

Tropospheric ozone derived from TOMS/SBUV measurements during TRACE A

The use of data from Total Ozone Mapping Spectrometer (TOMS) and Solar Backscatter Ultraviolet (SBUV) measurements to develop daily maps of the amount of ozone in the troposphere during Transport and Atmospheric chemistry Near the Equator-Atlantic (TRACE A) is presented. By comparing these maps with available ozone measurements from sensors aboard the NASA DC-8, we show that the agreement between the satellite-derived measurements and the other data is generally very good. The TOMS/SBUV technique successfully captures large-scale gradients and the derived integrated tropospheric ozone amount is generally within 10–15% of the observed amounts. The largest discrepancy between the two data sets occurs when copious amounts of lower-tropospheric (1–6 km) aerosols (from intense widespread biomass burning) are present. The amount of ozone found over the TRACE A region for the September–October 1992 period agrees well with the climatological values published previously using TOMS and Stratospheric Aerosol and Gas Experiment (SAGE) measurements and with ozonesonde measurements from several tropical sites established as part of TRACE A. The satellite-derived ozone maximum observed during TRACE A is ∼1000 km north of the previously published climatological data and it is not obvious whether or not this northward shift is due to normal interannual variability, or if it is a result of systematic data processing differences between the SAGE and the SBUV data sets.

The climatological distribution of tropospheric ozone derived from satellite measurements using version 7 Total Ozone Mapping Spectrometer and Stratospheric Aerosol and Gas Experiment data sets

The climatological distribution of tropospheric ozone derived from satellite observations has been updated using the reprocessed version 7 Total Ozone Mapping Spectrometer (TOMS) gridded data. The use of version 7 of the archive has not changed any of the climatological features first identified using earlier archived versions of the TOMS data. In general, however, the absolute values of the amount of ozone in the troposphere are decreased by ∼5 Dobson units (DU). In areas of persistent marine stratus clouds, the climatological values are decreased by an additional 2–3 DU.

Observations of reduced ozone concentrations in the tropical stratosphere after the eruption of Mt. Pinatubo

The eruption of Mt. Pinatubo (15oN, 122oE) on June 15 and 16, 1991, placed a large amount of SO2 and crustal material in the stratosphere. Based on measurements of decreases of stratospheric ozone after previous volcanic eruptions, it was expected that the aerosols deposited into the stratosphere (both directly and as a result of SO2 conversion into particulate sulfate) by this eruption would give rise to significant ozone depletions. To check for such an effect, ozone profiles obtained from ECC sondes before and after the eruption at Brazzaville, Congo (4oS, 15oE), and Ascen- sion Island (8oS, 14oW), are examined. Aerosol profiles determined from a lidar system in the western Pacific (4 o- 6o1,,1, 125oE) show that most of the material injected into the stratosphere is located between 18 and 28 km with highest mounts at 24-25 km. For the period 3-6 months after the eruption, decreases in ozone are found at 16 to 29 km, with peak decreases as large as 20% found at 24 km. Integrated between 16 and 28 km, a decrease of 13-20 Dobson units is observed when the ozonesonde data after the Pinatubo eruption are compared with those prior to the eruption. The altitude at which the most pronounced ozone decrease is found strongly correlates with peak aerosol loading deter- mined by the lidar. In addition, a small increase in ozone density is found above about 28 kin. Mechanisms that might explain the results such as heterogeneous chemistry, radiative effects, and dynamics are discussed.

LASE Validation Experiment

An extensive validation experiment was conducted in September 1995 from Wallops Island, Virginia, to evaluate the performance of the Lidar Atmospheric Sensing Experiment (LASE) system for the measurement of water vapor profiles under a wide range of atmospheric and solar background conditions. These measurements were compared with many different in situ and remote measurements in the most extensive water vapor intercomparison ever conducted. The LASE water vapor measurements were found to have an accuracy of better than 6% or 0.01 g/kg, whichever is greater, across the entire troposphere.

Comparison of Aerosol Optical Properties and Water Vapor Among Ground and Airborne Lidars and Sun Photometers During TARFOX

We compare aerosol optical thickness (AOT) and precipitable water vapor (PWV) measurements derived from ground and airborne lidars and Sun photometers during the Tropo- spheric Aerosol Radiative Forcing Observational Experiment. Such comparisons are important to verify the consistency between various remote sensing measurements before employing them in any assessment of the impact of aerosols on the global radiation balance. Total scattering ratio and extinction profiles measured by the ground-based NASA Goddard Space Flight Center scan- ning Raman lidar system, which operated from Wallops Island, Virginia (37.86oN, 75.51 oW), are compared with those measured by the Lidar Atmospheric Sensing Experiment (LASE) airborne lidar system aboard the NASA ER-2 aircraft. Bias and root-mean-square differences indicate that these measurements generally agreed within about 10%. Aerosol extinction profiles and esti- mates of AOT are derived from both lidar measurements using a value for the aerosol extinction/ backscattering ratio Sa = 60 sr for the aerosol extinction/backscattering ratio, which was deter- mined from the Raman lidar measurements. The lidar measurements of AOT are found to be gen- erally within 25% of the AOT measured by the NASA Ames Airborne Tracking Sun Photometer (AATS-6). However, during certain periods the lidar and Sun photometer measurements of AOT differed significantly, possibly because of variations in the aerosol physical characteristics (e.g., size, composition) which affect Sa. Estimates of PWV, derived from water vapor mixing ratio profiles measured by LASE, are within 5-10% of PWV derived from the airborne Sun photometer. Aerosol extinction profiles measured by both lidars show that aerosols were generally concen- trated in the lowest 2-3 km.

Distribution of tropospheric ozone at Brazzaville, Congo, determined from ozonesonde measurements

An analysis of 33 ozonesonde launches in Brazzaville, Congo (4°S, 15°E), between June 1990 and May 1991 is presented. The data indicate highest tropospheric amounts between June and early October, coincident with the dry season and with the presence of enhanced widespread biomass burning. The seasonal cycle of ozone derived from the ozonesonde measurements is in good agreement with the climatological seasonal cycle inferred from the use of satellite data and both seasonal cycles peak in September. Averaged throughout the year, the integrated amount of ozone derived from the ozonesondes is 44 Dobson units (DU) and is 39 DU using the satellite data. Within the troposphere the highest partial pressures are generally found at pressure levels near 700 mbar (∼3 km). Using simultaneous ozonesonde data from Ascension Island (8°S, 15°W), examples are presented illustrating that differences in the troposphere are primarily responsible for the observed spatial gradients of total ozone observed by TOMS. Calculation of correlation coefficients suggests that total ozone measurements may be a better indicator of the amount of ozone in the troposphere than are surface measurements.

Large-scale air mass characteristics observed over the remote tropical Pacific Ocean during March-April 1999: Results from PEM-Tropics B field experiment

Eighteen long-range flights over the Pacific Ocean between 38oS to 20oN and 166oE to 90oW were made by the NASA DC-8 aircraft during the NASA Pacific Exploratory Mission (PEM) Tropics B conducted from March 6 to April 18, 1999. Two lidar systems were flown on the DC-8 to remotely measure vertical profiles of ozone (03), water vapor (H20), aerosols, and clouds from near the surface to the upper troposphere along their flight track. In situ measurements of a wide range of gases and aerosols were made on the DC-8 for comprehensive characterization of the air and for correlation with the lidar remote measurements. The transition from northeasterly flow of Northern Hemispheric (NH) air on the northern side of the Intertropical Convergence Zone (ITCZ) to generally easterly flow of Southern Hemispheric (SH) air south of the ITCZ was accompanied by a significant decrease in 03, carbon monoxide, hydrocarbons, and aerosols and an increase in H20. Trajectory analyses indicate that air north of the ITCZ came from Asia and/or the United States, while the air south of the ITCZ had a long residence time over the Pacific, perhaps originating over South America several weeks earlier. Air south of the South Pacific Convergence Zone (SPCZ) came rapidly from the west originating over Australia or Africa. This air had enhanced 0 3 and aerosols and an associated decrease in H20. Average latitudinal and longitudinal distributions of 0 3 and H20 were constructed from the remote and in situ 03 and H20 data, and these distributions are compared with results from PEM-Tropics A conducted in August- October 1996. During PEM-Tropics B, low 03 air was found in the SH across the entire Pacific Basin at low latitudes. This was in strong contrast to the photochemically enhanced 03 levels found across the central and eastern Pacific low latitudes during PEM-Tropics A. Nine air mass types were identified for PEM-Tropics B based on their 03, aerosols, clouds, and potential vorticity characteristics. The data from each flight were binned by altitude according to air mass type, and these results showed the relative observational frequency of the different air masses as a function of altitude in seven regions over the Pacific. The average chemical composition of the major air mass types was determined from in situ measurements in the NH and SH, and these results provided insight into the origin, lifetime, and chemistry of the air in these regions.

Comparisons of LASE, aircraft, and satellite measurements of aerosol optical properties and water vapor during TARFOX

We examine aerosol extinction and optical thickness from the Lidar Atmospheric Sensing Experiment (LASE), the in situ nephelometer and absorption photometer on the University of Washington C-131A aircraft, and the NASA Ames Airborne Tracking Sun Photometer (AATS-6) on the C-131A measured during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) over the east coast of the United States in July 1996. On July 17 and 24 the LASE profiles of aerosol extinction and aerosol optical thickness (AOT) had a bias difference of 0.0055 km-1 (10%) and a root-mean-square difference of 0.026 km-1 (42%) when compared to corresponding profiles derived from the airborne in situ data when the nephelometer measurements are adjusted to ambient relative humidities. Larger differences for two other days were associated with much smaller aerosol optical thicknesses (July 20) and differences in the locations sampled by the two aircraft (July 26). LASE profiles of AOT are about 10% higher than those derived from the airborne Sun photometer, which in turn are about 10-15% higher than those derived from the airborne in situ measurements. These differences are generally within the error estimates of the various measurements. The LASE measurements of AOT generally agree with AOT derived from both the Along-Track and Scanning Radiometer 2 (ATSR 2) sensor flown on the European Remote Sensing Satellite 2 (ERS-2) and from the Moderate-Resolution Imaging Spectroradiometer (MODIS) airborne simulator (MAS) which flew with LASE on the NASA ER-2 aircraft. Effective particle sizes derived from the MAS data indicate that the LASE retrievals of AOT are valid for effective particle radii less than 0.4 μm. Variations in the relative humidity derived from the LASE water vapor measurements on July 26 are found to be highly correlated with variations in the effective particle size derived from the MAS. Copyright 2000 by the American Geophysical Union.

LASE measurements of aerosol and water vapor profiles during TARFOX

The Lidar Atmospheric Sensing Experiment (LASE) was operated autonomously from the NASA high-altitude ER-2 aircraft on nine flights during July 10–26, 1996, as part of the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX). LASE measured high-resolution profiles of water vapor and aerosols in regions of urban haze plumes over the U.S. eastern seaboard. Real-time LASE aerosol measurements were used to guide the in situ aircraft to sample haze layers. In this paper the vertical and horizontal distributions of aerosol backscatter measured by LASE are presented along with the temporal evolution of the haze layers. The aerosol backscatter data also identify the presence of gradients in the aerosol plumes, the presence of low-altitude clouds, and optically thin cirrus. This information is useful for many of the radiometeric observations made during TARFOX and can help explain observational differences among ground, airborne, and satellite observations. An iterative procedure is discussed which was used to invert lidar data to retrieve aerosol scattering ratios, extinction, and total optical depths from the LASE measurements. The sensitivity of these retrievals to assumed parameters is discussed and the results of retrievals are also compared to the well-known Bernoulli method. LASE water vapor measurements were made across the entire troposphere using a three “line pair” method to cover the range of water vapor mixing ratio from < 0.01 g/kg near the tropopause to ∼ 20 g/kg near the surface in a single aircraft pass over the experiment region. These measurements also show two-dimensional distributions of large spatial gradients in water vapor in the lower and upper troposphere. These observations are useful in the calculation of IR radiation fields and relative humidity fields, since relative humidity has a strong influence on the growth of aerosols and their scattering properties. Water vapor profiles, aerosol scattering ratios, aerosol extinction coefficients and aerosol optical depths were derived using the methodology presented in this paper from LASE measurements during TARFOX. These measurements are compared with other in situ and remote measurements during TARFOX in the companion papers [Ferrare et al., this issue (a, b)]

Remote sensing of total precipitable water vapor in the near-IR over ocean glint

A method for remote sensing of total precipitable water vapor using water vapor absorption band at 0.94 µm was previously developed for continental regions. Here we apply a similar technique for ocean areas over the glint region. The glint, or oceanic specular reflection, has a high value of surface reflectance and thus, can work as well as or better than applications over land regions. The method is applied for glint regions measured by the Moderate Resolution Imaging Spectro-radiometer (MODIS) simulator, an imager flown on the NASA ER-2 research aircraft and simulating the expected measurements from the MODIS instrument on board the Earth Observing System (EOS)-Terra satellite. The measurements are made for the Atlantic coast of the United States during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX). The remote sensing technique is compared with measurements of water vapor column by the Lidar Atmospheric Sensing Experiment (LASE) Differential Infrared Absorption Lidar (DIAL) lidar system also on board the ER-2. Water vapor was derived with an error of ±5 mm PW (precipitable water vapor). Most of the errors are associated with the limitations of an experiment that was not originally designed for this purpose. Much better performance is expected from the actual MODIS instrument.