Pawan K. Bhartia

Global distribution of UV‐absorbing aerosols from Nimbus 7/TOMS data

Global distributions of UV-absorbing aerosols are obtained using measured differences between the 340 and the 380 nm radiances from the Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) for the years 1979–1993. Time series are shown for major sources of biomass burning and desert dust giving the frequency of occurrence and areal coverage over land and oceans. Minor sources of UV-absorbing aerosols in the atmosphere are also discussed (volcanic ash and oil fires). Relative values of year-to-year variability of UV-absorbing aerosol amounts are shown for major aerosol source regions: (1) central South America (Brazil) near 10°S latitude; (2) Africa near 0°–20°S and 0° to 10°N latitude; (3) Saharan Desert and sub-Saharan region (Sahel), Arabian Peninsula, and the northern border region of India; (4) agricultural burning in Indonesia, Eastern China, and Indochina, and near the mouth of the Amazon River; and (5) coal burning and dust in northeastern China. The first three of these regions dominate the injection of UV-absorbing aerosols into the atmosphere each year and cover areas far outside of their source regions from advection of UV-absorbing particulates by atmospheric wind systems. During the peak months, smoke and dust from these sources are transported at altitudes above 1 km with an optical depth of at least 0.1 and can cover about 10% of the Earths surface. Boundary layer absorbing aerosols are not readily seen by TOMS because the small amount of underlying Rayleigh scattering leads to a small signal. Significant portions of the observed dust originate from agricultural regions frequently within arid areas, such as in the Sahel region of Africa, especially from the dry lake-bed near Lake Chad (13.5°N, 14°E), and intermittently dry drainage areas and streams. In addition to drought cycle effects, this suggests there may be an anthropogenic component to the amount of dust injected into the atmosphere each year. Detection of absorbing aerosols and calculation of optical depths are affected by the presence of large-scale and subpixel clouds in the TOMS field of view.

Derivation of aerosol properties from satellite measurements of backscattered ultraviolet radiation : Theoretical basis

We discuss the theoretical basis of a recently developed technique to characterize aerosols from space. We show that the interaction between aerosols and the strong molecular scattering in the near ultraviolet produces spectral variations of the backscattered radiances that can be used to separate aerosol absorption from scattering effects. This capability allows identification of several aerosol types, ranging from nonabsorbing sulfates to highly UV-absorbing mineral dust, over both land and water surfaces. Two ways of using the information contained in the near-UV radiances are discussed. In the first method, a residual quantity, which measures the departure of the observed spectral contrast from that of a molecular atmosphere, is computed. Since clouds yield nearly zero residues, this method is a useful way of separately mapping the spatial distribution of UV-absorbing and nonabsorbing particles. To convert the residue to optical depth, the aerosol type must be known. The second method is an inversion procedure that uses forward calculations of backscattered radiances for an ensemble of aerosol models. Using a look-up table approach, a set of measurements given by the ratio of backscattered radiance at 340-380 nm and the 380 nm radiance are associated, within the domain of the candidate aerosol models, to values of optical depth and single-scattering albedo. No previous knowledge of aerosol type is required. We present a sensitivity analysis of various error sources contributing to the estimation of aerosol properties by the two methods.

Science objectives of the ozone monitoring instrument

The Ozone Monitoring Instrument (OMI) flies on NASAs Earth Observing System AURA satellite, launched in July 2004. OMI is an ultraviolet/visible (UV/VIS) nadir solar backscatter spectrometer, which provides nearly global coverage in one day, with a spatial resolution of 13 km/spl times/24 km. Trace gases measured include O/sub 3/, NO/sub 2/, SO/sub 2/, HCHO, BrO, and OClO. In addition OMI measures aerosol characteristics, cloud top heights and cloud coverage, and UV irradiance at the surface. OMIs unique capabilities for measuring important trace gases with daily global coverage and a small footprint will make a major contribution to our understanding of stratospheric and tropospheric chemistry and climate change along with Auras other three instruments. OMIs high spatial resolution enables detection of air pollution at urban scales. Total Ozone Mapping Spectrometer and differential optical absorption spectroscopy heritage algorithms, as well as new ones developed by the international (Dutch, Finnish, and U.S.) OMI science team, are used to derive OMIs advanced backscatter data products. In addition to providing data for Auras prime objectives, OMI will provide near-real-time data for operational agencies in Europe and the U.S. Examples of OMIs unique capabilities are presented in this paper.

Algorithm for the estimation of vertical ozone profiles from the backscattered ultraviolet technique

An implementation of the optimal estimation scheme to obtain vertical ozone profiles from satellite measurements of backscattered solar ultraviolet (buv) radiation is described. This algorithm (Version 6.0) has been used to produce a 15-year data set of global ozone profiles from Nimbus 7 SBUV, NOAA 11 SBUV/2, and Space Shuttle SSBUV instruments. A detailed discussion of the information content of the measurement is presented. Using high vertical resolution ozone profiles from the SAGE II experiment as “truth” profiles, it is shown that the buv technique can capture short-term variabilities of ozone in 5-km vertical layers, between 0.3 mbar and 100 mbar, with a precision of 5–15%. However, outside the 1–20 mbar range, buv-derived results are heavily influenced by a priori assumptions. To minimize this influence, it is recommended that the studies of long-term trends using buv data be restricted to 1–20 mbar range. Outside this range, only the column amounts of ozone between 20 mbar and surface, and above 1 mbar, can be considered relatively free of a priori assumptions.

Satellite estimation of spectral surface UV irradiance in the presence of tropospheric aerosols: 1. Cloud‐free case

The algorithm for determining spectral UVA (320-400 nm) and UVB (290-320 nm) flux in cloud-free conditions is discussed, including estimates of the various error sources (uncertainties in ground reflectivity, ozone amount, ozone profile shape, surface height, and aerosol attenuation). It is shown that the Brewer-measured spectral dependence of UV flux can be accurately reproduced using just total column ozone amount and the solar flux spectrum. The presence of aerosols tends to reduce the logarithm of the absolute UV flux linearly with aerosol optical depth. Using Brewer measurements of UV flux and aerosol optical depth on clear days at Toronto, the estimated slope falls in the range 0.2 to 0.3 (aerosol single-scattering albedo about 0.95). The Brewer measurements of UV flux can be reproduced using the aerosol model derived within uncertainties of the instrument calibration. We have applied the algorithm to the data collected by the total ozone mapping spectrometer (TOMS) instruments that have been flown by NASA since November 1978. It was demonstrated that in the absence of clouds and UV-absorbing aerosols, TOMS measurements of total column ozone and 380 nm (or 360 nm) radiances can be used in conjunction with a radiative transfer model to provide estimates of surface spectral flux to accuracies comparable to that of typical ground-based instruments. A newly developed technique using TOMS aerosol index data also allows estimation of UV flux transmission by strongly absorbing aerosols. The results indicate that over certain parts of the Earth, aerosols can reduce the UV flux at the surface by more than 50%. Therefore the most important need for reducing errors in TOMS-derived surface UVB spectra is to improve the understanding of UV aerosol attenuation.

Two new methods for deriving tropospheric column ozone from TOMS measurements: Assimilated UARS MLS/HALOE and convective-cloud differential techniques

This study introduces two new approaches for determining tropospheric column ozone from satellite data. In the first method, stratospheric column ozone is derived by combining Upper Atmosphere Research Satellite (UARS) halogen occultation experiment (HALOE) and microwave limb sounder (MLS) ozone measurements. Tropospheric column ozone is then obtained by subtracting these stratospheric amounts from the total column. Total column ozone in this study include retrievals from Nimbus 7 (November 1978 to May 1993) and Earth probe (July 1996 to present) total ozone mapping spectrometer (TOMS). Data from HALOE are used in this first method to extend the vertical span of MLS (highest pressure level 46 hPa) using simple regression. This assimilation enables high-resolution daily maps of tropospheric and stratospheric ozone which is not possible from solar occultation measurements alone, such as from HALOE or Stratospheric Aerosols and Gas Experiment (SAGE). We also examine another new and promising technique that yields tropospheric column ozone directly from TOMS high-density footprint measurements in regions of high convective clouds. We define this method as the convective cloud differential (CCD) technique. The CCD method is shown to provide long time series (essentially late 1978 to the present) of tropospheric ozone in regions dominated by persistent high tropopause-level clouds, such as the maritime tropical Pacific and within or near midlatitude continental landmasses. In this our first study of the CCD and MLS/HALOE methods we limit analyses to tropical latitudes. Separation of stratospheric from tropospheric column ozone in the eastern Pacific tropics for January 1979 to December 1997 shows that the dominant interannual variability of stratospheric ozone is the quasi-biennial oscillation (QBO), whereas for tropospheric ozone it is driven by El Nino events. For validation purposes, both the CCD and assimilated UARS MLS/HALOE results are compared with ozonesonde data from several southern tropical stations. Despite all three measurements being distinctly different in sampling and technique, all three show good qualitative agreement.

Retrieval of Aerosol Optical Depth above Clouds from OMI Observations: Sensitivity Analysis and Case Studies

A large fraction of the atmospheric aerosol load reaching the free troposphere is frequently located above low clouds. Most commonly observed aerosols above clouds are carbonaceous particles generally associated with biomassburningandborealforestfires,andmineralaerosolsoriginatingin aridandsemiaridregionsand transported across large distances, often above clouds. Because these aerosols absorb solar radiation, their role in the radiative transfer balance of the earth‐atmosphere system is especially important. The generally negative (cooling) top-of-the-atmosphere direct effect of absorbing aerosols may turn into warming when the light-absorbing particles are located above clouds. The actual effect depends on the aerosol load and the single scattering albedo, and on the geometric cloud fraction. In spite of its potential significance, the role of aerosols above clouds is not adequately accounted for in the assessment of aerosol radiative forcing effects due to the lack of measurements. This paper discusses the basis of a simple technique that uses near-UV observations to simultaneously derive the optical depth of both the aerosol layer and the underlying cloud for overcast conditions. The two-parameter retrieval method described here makes use of the UV aerosol index andreflectancemeasurementsat 388 nm.Adetailedsensitivityanalysisindicatesthatthemeasuredradiances depend mainly on the aerosol absorption exponent and aerosol‐cloud separation. The technique was applied to above-cloud aerosol events over the southern Atlantic Ocean, yielding realistic results as indicated by indirect evaluation methods. An error analysis indicates that for typical overcast cloudy conditions and aerosol loads, the aerosol optical depth can be retrieved with an accuracy of approximately 54% whereas the cloud optical depth can be derived within 17% of the true value.

Satellite estimation of spectral surface UV irradiance. 2. Effects of homogeneous clouds and snow

This paper extends the theoretical analysis of the estimation of the surface UV irradiance from satellite ozone and reflectivity data from a clear-sky case to a cloudy atmosphere and snow-covered surface. Two methods are compared for the estimation of cloud-transmission factor CT, the ratio of cloudy to clear-sky surface irradiance: (1) the Lambert equivalent reflectivity (LER) method and (2) a method based on radiative transfer calculations for a homogeneous (plane parallel) cloud embedded into a molecular atmosphere with ozone absorption. The satellite-derived CT from the NASA Total Ozone Mapping Spectrometer (TOMS) is compared with ground-based CT estimations from the Canadian network of Brewer spectrometers for the period 1989 -1998. For snow-free conditions the TOMS derived CT at 324 nm approximately agrees with Brewer data with a correlation coefficient of ;0.9 and a standard deviation of ;0.1. The key source of uncertainty is the different size of the TOMS FOV (;100 km field of view) and the much smaller ground instrument FOV. As expected, the standard deviations of weekly and monthly C T averages were smaller than for daily values. The plane-parallel cloud method produces a systematic CT bias relative to the Brewer data (17% at low solar zenith angles to 210% at large solar zenith angles). The TOMS algorithm can properly account for conservatively scattering clouds and snow/ice if the regional snow albedo RS is known from outside data. Since RS varies on a daily basis, using a climatology will result in additional error in the satellite-estimated CT. The CT error has the same sign as the R S error and increases over highly reflecting surfaces. Finally, clouds polluted with absorbing aerosols transmit less radiation to the ground than conservative clouds for the same satellite reflectance and flatten spectral dependence of CT. Both effects reduce C T compared to that estimated assuming conservative cloud scattering. The error increases if polluted clouds are over snow.

Cloud Slicing: A New Technique to Derive Upper Tropospheric Ozone from Satellite Measurements

A new technique called cloud slicing has been developed for measuring upper tropospheric O3. Cloud slicing takes advantage of the opaque property of water vapor clouds to ultraviolet wavelength radiation. Measurements of above-cloud column O3 from the Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) instrument are combined together with Nimbus 7 temperature-humidity and infrared radiometer (THIR) cloud-top pressure data to derive O3 column amounts in the upper troposphere. In this study, tropical TOMS and THIR data for the period 1979–1984 are analyzed. By combining total tropospheric column ozone (referred to as TCO) measurements from the convective cloud differential (CCD) method with 100- to 400-hPa upper tropospheric column O3 amounts from cloud slicing, it is possible to estimate 400- to 1000-hPa lower tropospheric column O3 and evaluate its spatial and temporal variability. Results for both the upper and lower tropical troposphere show a year-round zonal wave number 1 pattern in column O3 with the largest amounts in the Atlantic region (up to ∼15 DU in the 100- to 400-hPa pressure band and ∼25–30 DU in the 400- to 1000-hPa pressure band). Upper tropospheric O3 derived from cloud slicing shows maximum column amounts in the Atlantic region in the June-August and September-November seasons which are similar to the seasonal variability of CCD-derived TCO in the region. For the lower troposphere, the largest column amounts occur in the September-November season over Brazil in South America and also southern Africa. Localized increases in the tropics in the lower tropospheric O3 are found over the northern region of South America around August and off the west coast of equatorial Africa in the March-May season. Time series analysis for several regions in South America and Africa show an anomalous increase in O3 in the lower troposphere around the month of March which is not observed in the upper troposphere. The eastern Pacific indicates weak seasonal variability of upper, lower, and total tropospheric O3 compared with the western Pacific, which shows the largest TCO amounts in both hemispheres around spring months. O3 variability in the western Pacific is expected to have greater variability caused by strong convection, pollution and biomass burning, land-sea contrast, and monsoon developments.

Calibration of the NOAA 11 solar backscatter ultraviolet (SBUV/2) ozone data set from 1989 to 1993 using in‐flight calibration data and SSBUV

Total ozone and ozone profiles are currently being measured by solar backscatter ultraviolet (SBUV/2) instruments onboard NOAA polar orbiting spacecraft using the backscattered ultraviolet technique. The NOAA 11 SBUV/2 operational data set was reprocessed from January 1989 to May 1993 and is now called version 6. The version 6 data include an updated algorithm and revised prelaunch and postlaunch calibrations of the geometrical albedo observations used to derive ozone values. Only the calibration revisions are described in this paper. The postlaunch revisions remove time dependent errors in the ozone amounts due to instrument drift, while the revised prelaunch calibration corrects the absolute value of retrieved ozone. The prelaunch corrections are a result of calibration checks from in-orbit comparisons of ultraviolet geometric albedos measured by shuttle SBUV (SSBUV) and the NOAA 11 SBUV/2. Geometric albedo comparison data are further corrected using a radiative transfer code to account for the small difference in observing conditions between the two spacecraft. The postlaunch corrections rely on in-flight calibration and solar irradiance data to account for time dependent changes in instrument gain, thermal response, and instrument diffuser degradation over time. Comparison of data from three SSBUV flights, which occurred about one year apart, with concurrent SBUV/2 data provided an independent check of the time dependent change derived from the in-flight calibration data. Time independent corrections result in an increase of about 1% for total ozone, 5% for ozone at 1 mbar, and near 0% at 15 mbar. The time dependent corrections amount to an increase of 2% for total ozone, 10% for ozone near 1 mbar, and 3% at 15 mbar at the end of the current record in May 1993. Recent laboratory studies indicate that the absolute radiance calibrations may still be in error by a few percent which results in ozone profile values that are too low. The SBUV/2 total and ozone profile data are compared to the Nimbus SBUV data during the period when the data overlapped. Total ozone values agree to about 1%, while ozone profile differences range from −4% to +6%, depending on latitude and altitude, relative to SBUV. These differences are not statistically significant given the uncertainties of the two data sets.