Dan Lubin

Indian Ocean Experiment: An integrated analysis of the climate forcing and effects of the great Indo-Asian haze

Every year, from December to April, anthropogenic haze spreads over most of the North Indian Ocean, and South and Southeast Asia. The Indian Ocean Experiment (INDOEX) documented this Indo-Asian haze at scales ranging from individual particles to its contribution to the regional climate forcing. This study integrates the multiplatform observations (satellites, aircraft, ships, surface stations, and balloons) with one- and four-dimensional models to derive the regional aerosol forcing resulting from the direct, the semidirect and the two indirect effects. The haze particles consisted of several inorganic and carbonaceous species, including absorbing black carbon clusters, fly ash, and mineral dust. The most striking result was the large loading of aerosols over most of the South Asian region and the North Indian Ocean. The January to March 1999 visible optical depths were about 0.5 over most of the continent and reached values as large as 0.2 over the equatorial Indian ocean due to long-range transport. The aerosol layer extended as high as 3 km. Black carbon contributed about 14% to the fine particle mass and 11% to the visible optical depth. The single-scattering albedo estimated by several independent methods was consistently around 0.9 both inland and over the open ocean. Anthropogenic sources contributed as much as 80% (±10%) to the aerosol loading and the optical depth. The in situ data, which clearly support the existence of the first indirect effect (increased aerosol concentration producing more cloud drops with smaller effective radii), are used to develop a composite indirect effect scheme. The Indo-Asian aerosols impact the radiative forcing through a complex set of heating (positive forcing) and cooling (negative forcing) processes. Clouds and black carbon emerge as the major players. The dominant factor, however, is the large negative forcing (-20±4 W m^(−2)) at the surface and the comparably large atmospheric heating. Regionally, the absorbing haze decreased the surface solar radiation by an amount comparable to 50% of the total ocean heat flux and nearly doubled the lower tropospheric solar heating. We demonstrate with a general circulation model how this additional heating significantly perturbs the tropical rainfall patterns and the hydrological cycle with implications to global climate.

The Deuterium to Hydrogen Abundance Ratio toward a Fourth QSO: HS 0105+1619

We report the measurement of the primordial D/H abundance ratio toward QSO HS 0105+1619. The column density of the neutral hydrogen in the z 2.536 Lyman limit system is high, log N = 19.422 ± 0.009 cm-2, allowing for the deuterium to be seen in five Lyman series transitions. The measured value of the D/H ratio toward QSO HS 0105+1619 is found to be D/H = 2.54 ± 0.23 × 10-5. The metallicity of the system showing D/H is found to be 0.01 solar, indicating that the measured D/H is the primordial D/H within the measurement errors. The gas that shows D/H is neutral, unlike previous D/H systems that were more highly ionized. Thus, the determination of the D/H ratio becomes more secure since we are measuring it in different astrophysical environments, but the error is larger because we now see more dispersion between measurements. Combined with prior measurements of D/H, the best D/H ratio is now D/H = 3.0 ± 0.4 × 10-5, which is 10% lower than the previous value. The new values for the baryon-to-photon ratio and baryonic matter density derived from D/H are η = 5.6 ± 0.5 × 10-10 and Ωbh2 = 0.0205 ± 0.0018, respectively.

Direct observations of aerosol radiative forcing over the tropical Indian Ocean during the January‐February 1996 pre‐INDOEX cruise

Simultaneous measurements of aerosol optical depth, size distribution, and incoming solar radiation flux were conducted with spectral and broadband radiometers over the coastal Indian region, Arabian Sea, and Indian Ocean in January-February 1996. Columnar aerosol optical depth, δa, at visible wavelengths was found to be 0.2–0.5 over the Arabian Sea and <0.1 over the equatorial Indian Ocean. Aerosol mass concentration decreased from about 80 μg/m3 near the coast to just a few μg/m3 over the interior ocean. The sub-micron-size particles showed more than an order of magnitude increase in number concentration near the coast versus the interior ocean. This large gradient in particle concentration was consistent with a corresponding large increase in the Sun-photometer-derived Angstrom exponent, which increased from 0.2 over the Indian Ocean to about 1.4 near the coast. The change in surface-reaching solar flux with δa was obtained for both the direct and the global solar flux in the visible spectral region. The solar-zenith-angle-normalized global and diffuse fluxes vary almost linearly with normalized δa. The direct visible (<780 nm) solar flux decreases by about 42±4 W m−2 and the diffuse sky radiation increases by about 30±3 W m−2 with every 0.1 increase in δa, for solar zenith angles smaller than 60°. For the same extinction optical depth the radiative forcing of the coastal aerosols is larger than the open ocean aerosol forcing by a factor of 2 or larger.

Ultraviolet Radiation and Its Effects on Organisms in Aquatic Environments

The problem of trying to determine the effect of solar ultraviolet radiation (UVR) on aquatic organisms is much more difficult than that of assessing the impact of UVR on terrestrial plants. The major reasons for this are that spectral irradiance changes dramatically with depth in the water column and that most aquatic organisms will be moving up and down in the upper water column, either through active motility processes or by physical mixing processes. It is thus not possible to determine the effect of UVR on planktonic organisms with any degree of certainty; the best one can do is to determine the effects under a wide variety of experimental techniques, and to estimate the potential damage to organisms when they are under completely natural conditions.

A climatologically significant aerosol longwave indirect effect in the Arctic

The warming of Arctic climate and decreases in sea ice thickness and extent observed over recent decades are believed to result from increased direct greenhouse gas forcing, changes in atmospheric dynamics having anthropogenic origin, and important positive reinforcements including ice–albedo and cloud–radiation feedbacks. The importance of cloud–radiation interactions is being investigated through advanced instrumentation deployed in the high Arctic since 1997 (refs 7, 8). These studies have established that clouds, via the dominance of longwave radiation, exert a net warming on the Arctic climate system throughout most of the year, except briefly during the summer. The Arctic region also experiences significant periodic influxes of anthropogenic aerosols, which originate from the industrial regions in lower latitudes. Here we use multisensor radiometric data to show that enhanced aerosol concentrations alter the microphysical properties of Arctic clouds, in a process known as the ‘first indirect’ effect. Under frequently occurring cloud types we find that this leads to an increase of an average 3.4 watts per square metre in the surface longwave fluxes. This is comparable to a warming effect from established greenhouse gases and implies that the observed longwave enhancement is climatologically significant.

ULTRAVIOLET RADIATION IN ANTARCTICA: INHIBITION OF PRIMARY PRODUCTION

With the seasonal formation of the ozone hole over Antarctica, there is much concern regarding the effects of increased solar UV‐B radiation (280–320 nm) on the marine ecosystem in the Southern Ocean. In situ incubations of natural phytoplankton assemblages in antarctic waters indicate that under normal ozone conditions UV‐B radiation is responsible for a loss of approximately 4.9% of primary production in the euphotic zone, whereas UV radiation with wavelengths between 320 and 360 nm causes a loss of approximately 6.2%. When combined with data on the action spectrum for photoinhibition by UV radiation, our data suggest that the enhanced fluence of UV‐B radiation under a well‐developed ozone hole (150 Dobson units) would decrease daily primary productivity by an additional amount of 3.8%. Calculations that take into consideration the extent and duration of low stratospheric ozone concentrations during September to November indicate that the decrease in total annual primary production in antarctic waters due to enhanced UV‐B radiation would be 0.20%.

The ultraviolet radiation environment of the Antarctic Peninsula : the roles of ozone and cloud cover

Abstract The National Science Foundation scanning spectroradiometer at Palmer Station, Antarctica (64°46′S, 64°04′W) provides hourly ground-based measurements of solar ultraviolet (UV) irradiance at the, earths surface. These measurements define the UV radiation environment of the region and, in conjunction with a daily record of sky conditions and radiative transfer modeling, permit a quantitative understanding of the role of cloud cover in regulating UV radiation levels at the Antarctic surface, including the period of the springtime ozone depletion. The transmission properties of cloud types over the Antarctic Peninsula are quantified by taking the ratio of UV-A irradiances measured under them to UV-A irradiances calculated for clear skies and the same solar zenith angle, and the results are then generalized to the UV-B. Under the averse overcast sky in the region, UV irradiance at all wavelengths is slightly greater than half of the value for clear skies. Under the thickest overcast layers, UV irradi...

Review of Big Bang Nucleosynthesis and Primordial Abundances

Big Bang Nucleosynthesis (BBN) is the synthesis of the light nuclei, Deuterium (D or 2H), 3He, 4He and 7Li during the first few minutes of the universe. This review concentrates on recent improvements in the measurement of the primordial (after BBN, and prior to modification) abundances of these nuclei. We mention improvement in the standard theory, and the non-standard extensions which are limited by the data. We have achieved an order of magnitude improvement in the precision of the measurement of primordial D/H, using the HIRES spectrograph on the W. M. Keck telescope to measure D in gas with very nearly primordial abundances towards quasars. From 1994 – 1996, it appeared that there could be a factor of ten range in primordial D/H, but today four examples of low D are secure. High D/H should be much easier to detect, and since there are no convincing examples, it must be extremely rare or non-existent. All data are consistent with a single low value for D/H, and the examples which are consistent with high D/H are readily interpreted as H contamination near the position of D. The new D/H measurements give the most accurate value for the baryon to photon ratio, η, and hence the cosmological baryon density. A similar density is required to explain the amount of Lyα absorption from neutral Hydrogen in the intergalactic medium (IGM) at redshift z 3, and to explain the fraction of baryons in local clusters of galaxies. The D/H measurements lead to predictions for the abundances of the other light nuclei, which generally agree with measurements. The remaining differences with some measurements can be explained by a combination of measurement and analysis errors or changes in the abundances after BBN. The measurements do not require physics beyond the standard BBN model. Instead, the agreement between the abundances is used to limit the non-standard physics. New measurements are giving improved understanding of the difficulties in estimating the abundances of all the light nuclei, but unfortunately in most cases we are not yet seeing much improvement in the accuracy of the primordial abundances. Since we are now interested in the highest accuracy and reliability for all nuclei, the few objects with the most extensive observations give by far the most convincing results. Earlier measurements of 4He may have obtained too low a value because the He emission line strengths were reduced by undetected stellar absorption lines. The systematic errors associated with the 4He abundance have frequently been underestimated in the past, and this problem persists. When two groups use the same data and different ways to estimate the electron density and 4He abundance, the results differ by more than the quoted systematic errors. While the methods used by Izotov and Thuan seem to be an advance on those used before, the other method is reasonable, and hence the systematic error should encompass the range in results. The abundance of 7Li is measured to high accuracy, but we do not know how much was produced prior to the formation of the stars, and how much was destroyed (depleted) in the stars. 6Li helps limit the amount of depletion of 7Li, but by an uncertain amount since it too has been depleted. BBN is successful because it uses known physics and measured cross-sections for the nuclear reactions. It gives accurate predictions for the abundances of five light nuclei as a function of the one free parameter η. The other initial conditions seem natural: the universe began homogeneous and hotter than T > 1011 K (30 Mev). The predicted abundances agree with most observations, and the required η is consistent with other, less accurate, measurements of the baryon density. Abundance measurements of the baryon density, from the CMB, clusters of galaxies and the Lyα forest, will give η. Although the accuracy might not exceed that obtained from D/H, this is an important advance because BBN then gives abundance predictions with no adjustable parameters. New measurement in the coming years will give improved accuracy. Measurement of D/H in many more quasar spectra would improve the accuracy of D/H by a factor of a few, to a few percent, but even with improved methods of selecting the target quasars, this would need much more time on the largest telescopes. More reliable 4He abundances might be obtained from spectra which have higher spectral and spatial resolution, to help correct for stellar absorption, higher signal to noise to show weaker emission lines, and more galaxies with low metal abundances, to minimize the extrapolation to primordial abundances. Measurements of 6Li, Be and Boron in the same stars and observations of a variety of stars should give improved models for the depletion of 7Li in halo stars, and hence tighter constraints on the primordial abundance. However, in general, it is hard to think of any new methods which could give any primordial abundances with an order of magnitude higher accuracy than those used today. This is a major unexploited opportunity, because it means that we can not yet test BBN to the accuracy of the predictions.

Global surface ultraviolet radiation climatology from TOMS and ERBE data

A global climatology of biologically active solar ultraviolet radiation (UVR) at the Earths surface is derived using NASA total ozone mapping spectrometer (TOMS) measurements of column ozone abundance and NASA Earth Radiation Budget Experiment (ERBE) measurements of solar reflectance from the Earth-atmosphere system. These two sources of satellite data are used as input to a delta-Eddington radiative transfer model to estimate climatological cloud opacity and thereby demonstrate how surface UVR varies with geography and season. The surface UVR fluxes are spectrally resolved to enable weighted integration with any biological action spectrum. Solar elevation is shown to be more important than total column ozone abundance in governing the variability of surface UVR over large geographic areas, although some regions with pronounced local minima in ozone (30 Dobson units or more) will cause noticeable enhancements of integrated UV-B (280–315 nm) flux relative to UV-A (315–400 nm). The greatest variability in surface UVR within a given climate zone is induced by cloud cover. During summer, regions that show lower surface UVR fluxes relative to their surrounding regions include the eastern United States (versus the western United States), India, China (in the vicinity of the Yangtze River), and Japan (relative to the surrounding oceans). Cloud cover over tropical rainforest areas reduces the surface UVR flux relative to ocean areas at the same latitudes. The UVR cloud transmission derived from the TOMS and ERBE data correlates with an independent climatology of global cloud coverage. The UVR mapping method, based on the TOMS and ERBE data, allows a direct investigation of diurnal variability and a rigorous calculation of the biologically relevant integrated daily dose of UVR. However, it is shown that a UVR mapping method based on TOMS data alone, which is limited to only local noon satellite measurements, can make defensible estimates of the integrated daily UVR dose and the instantaneous local noon UVR surface flux.

Spectral Signatures of Coral Reefs: Features from Space

The spectral signatures of coral reefs and related scenes, as they would be measured above the Earths atmosphere, are calculated using a coupled atmosphere-ocean discrete ordinates radiative transfer model. Actual measured reflectance spectra from field work are used as input data. Four coral species are considered, to survey the natural range of coral reflectance: Montastrea cavernosa, Acropora palmata, Dichocoenia stokesii, and Siderastrea siderea. Four noncoral objects associated with reefs are also considered: sand, coralline algae, green macroalgae, and algal turf. The reflectance spectra as would be measured at the top of the atmosphere are substantially different from the in situ spectra, due to differential attenuation by the water column and, most importantly, by atmospheric Rayleigh scattering. The result is that many of the spectral features that can be used to distinguish coral species from their surroundings or from one another, which have been used successfully with surface or aircraft data, would be obscured in spectral measurements from a spacecraft. However, above the atmosphere, the radiance contrasts between most coral species and most brighter noncoral objects remain noticeable for water column depths up to 20 m. Over many spectral intervals, the reflectance from dark coral under shallow water is smaller than that of deep water. The maximum top-of-atmosphere radiances, and maximum contrasts between scene types, occur between 400 nm and 600 nm. This study supports the conclusions of recent satellite reef mapping exercises, suggesting that coral reef identification should be feasible using satellite remote sensing, but that detailed reef mapping (e.g., species identification) may be more difficult.