bluuebunny/crossref_metadata_2025 huggingface dataset:
Individual aerosol particles from biomass burning in southern Africa: 1. Compositions and size distributions of carbonaceous particles
Authors: Pósfai Mihály , Simonics Renáta , Li Jia , Hobbs Peter V. , Buseck Peter R.
Published: May 2003

Individual aerosol particles in smoke plumes from biomass fires and in regional hazes in southern Africa were studied using analytical transmission electron microscopy (TEM), which allowed detailed characterization of carbonaceous particle types in smoke and determination of changes in particle properties and concentrations during smoke aging. Based on composition, morphology, and microstructure, three distinct types of carbonaceous particles were present in the smoke: organic particles with inorganic (K‐salt) inclusions, “tar ball” particles, and soot. The relative number concentrations of organic particles were largest in young smoke, whereas tar balls were dominant in a slightly aged (∼1 hour) smoke from a smoldering fire. Flaming fires emitted relatively more soot particles than smoldering fires, but soot was a minor constituent of all studied plumes. Further aging caused the accumulation of sulfate on organic and soot particles, as indicated by the large number of internally mixed organic/sulfate and soot/sulfate particles in the regional haze. Externally mixed ammonium sulfate particles dominated in the boundary layer hazes, whereas organic/sulfate particles were the most abundant type in the upper hazes. Apparently, elevated haze layers were more strongly affected by biomass smoke than those within the boundary layer. Based on size distributions and the observed patterns of internal mixing, we hypothesize that organic and soot particles are the cloud‐nucleating constituents of biomass smoke aerosols. Sea‐salt particles dominated in the samples taken in stratus clouds over the Atlantic Ocean, off the coast of Namibia, whereas a distinct haze layer above the clouds consisted of aged biomass smoke particles.
Semivolatile particulate organic material in southern Africa during SAFARI 2000
Authors: Eatough D. J. , Eatough N. L. , Pang Y. , Sizemore S. , Kirchstetter T. W. , Novakov T. , Hobbs P. V.
Published: May 2003

During August and September 2000, the University of Washington's (UW) Cloud and Aerosol Research Group (CARG) with its Convair‐580 research aircraft participated in the Southern African Fire‐Atmosphere Research Initiative (SAFARI) 2000 field study in southern Africa. Aboard this aircraft was a Particle Concentrator‐Brigham Young University Organic Sampling System (PC‐BOSS), which was used to determine semivolatile particulate material with a diffusion denuder sampler. Denuded quartz filters and sorbent beds in series were used to measure nonvolatile and semivolatile materials, respectively. Results obtained with the PC‐BOSS are compared to those obtained with conventional quartz–quartz and Teflon–quartz filter pack samplers. Various 10–120 min integrated samples were collected during flights through the free troposphere, in the atmospheric boundary layer, and in plumes from savanna fires. Significant fine particulate semivolatile organic compounds (SVOC) were found in all samples. The SVOC was not collected by conventional filter pack samplers and therefore would not have been determined in previous studies that used only filter pack samplers. The SVOC averaged 24% of the fine particulate mass in emissions from the fires and 36% of the fine particulate mass in boundary layer samples heavily impacted by aged emissions from savanna fires. Concentrations of fine particulate material in the atmospheric mixed layer heavily impacted by aged savanna fire emissions averaged 130 μg m−3. This aerosol was 85% carbonaceous material.
Coordinated airborne, spaceborne, and ground‐based measurements of massive thick aerosol layers during the dry season in southern Africa
Authors: Schmid B. , Redemann J. , Russell P. B. , Hobbs P. V. , Hlavka D. L. , McGill M. J. , Holben B. N. , Welton E. J. , Campbell J. R. , Torres O. , Kahn R. A. , Diner D. J. , Helmlinger M. C. , Chu D. A. , Robles‐Gonzalez C. , de Leeuw G.
Published: May 2003

During the dry season airborne campaign of the Southern African Regional Science Initiative (SAFARI 2000), coordinated observations were made of massive thick aerosol layers. These layers were often dominated by aerosols from biomass burning. We report on airborne Sun photometer measurements of aerosol optical depth (λ = 0.354–1.557 μm), columnar water vapor, and vertical profiles of aerosol extinction and water vapor density that were obtained aboard the University of Washington's Convair‐580 research aircraft. We compare these with ground‐based AERONET Sun/sky radiometer results, with ground based lidar data (MPL‐Net), and with measurements from a downward pointing lidar aboard the high‐flying NASA ER‐2 aircraft. Finally, we show comparisons between aerosol optical depths from the Sun photometer and those retrieved over land and over water using four spaceborne sensors (TOMS, MODIS, MISR, and ATSR‐2).
A multilayer biochemical dry deposition model 2. Model evaluation
Authors: Wu Yihua , Brashers Bart , Finkelstein Peter L. , Pleim Jonathan E.
Published: May 2003

The multilayer biochemical dry deposition model (MLBC) described in the accompanying paper was tested against half‐hourly eddy correlation data from six field sites under a wide range of climate conditions with various plant types. Modeled CO2, O3, SO2, and H2O (latent heat) fluxes were compared with measurements. Model outputs have good correlations with measurements at all locations. Correlation coefficients between model outputs and measurements at all sites range from 0.72 to 0.96 for CO2 flux, from 0.84 to 0.98 for H2O flux, from 0.77 to 0.95 for O3 flux, and from 0.36 to 0.86 for SO2 flux. Model sensitivity analyses were conducted to investigate the variation of model outputs due to measurement errors of input variables and to changes of environmental conditions such as changes in weather conditions. The MLBC outputs were also compared with outputs from the Multilayer Model (MLM) model [Meyers et al., 1998] and the Regional Acid Deposition Model (RADM) [Wesely, 1989] at two typical sites. Comparisons show that the MLBC performs better than the other two models. The model is suitable for use in nationwide dry deposition networks, for example, the Clean Air Status And Trends Network (CASTNet). It can be used to assist in describing total pollutant loadings to major ecosystems. With some modifications, the model may also be suitable for inclusion in region (meso‐) scale numerical models, for example, the Community Multiscale Air Quality (CMAQ) model.
Climatology and interannual variability of diurnal water vapor heating
Authors: Lieberman R. S. , Ortland D. A. , Yarosh E. S.
Published: May 2003

Tropospheric heating by water vapor absorption of near‐infrared (IR) radiation is a leading drive of the propagating diurnal tide. A climatology of monthly diurnal water vapor IR heating rates is derived using National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis and a global precipitable water (PW) data set. The new climatology updates an existing one compiled in 1982 that provides seasonally averaged IR heating between 35°S and 35°N. The new heating rates have greater temporal and spatial resolution and enable examination of intraseasonal and year‐to‐year variability over the 10‐year span of the PW data set. Our findings suggest that tropical tropospheric variability may be communicated to the diurnal tide (and possibly the middle and upper atmosphere) by means of water vapor heating.
Eddy flux and leaf‐level measurements of biogenic VOC emissions from mopane woodland of Botswana
Authors: Greenberg J. P. , Guenther A. , Harley P. , Otter L. , Veenendaal E. M. , Hewitt C. N. , James A. E. , Owen S. M.
Published: May 2003

Biogenic volatile organic compound (BVOC) emissions were measured in a mopane woodland near Maun, Botswana in January–February 2001 as part of SAFARI 2000. This landscape is comprised of more than 95% of one woody plant species, Colophospermum mopane (Caesalpinaceae). Mopane woodlands extend over a broad area of southern Africa. A leaf cuvette technique was used to determine the emission capacities of the major vegetation and the temperature and light dependence of the emissions. In addition, relaxed eddy accumulation (REA) measurements of BVOC fluxes were made on a flux tower, where net CO2 emissions were also measured simultaneously. Large light‐dependent emissions of terpenes (mostly α‐pinene and d‐limonene) were observed from the mopane woodland. The diurnal BVOC emissions were integrated and compared with the CO2 flux. Monoterpene flux exceeded 3000 μg C m−2 h−1 during the daytime period, comparable to isoprene fluxes and much higher than terpene fluxes measured in most areas. The terpene flux constituted approximately 25% of the diurnal net carbon exchange (CO2) during the experimental period. Other BVOC emissions may also contribute to the carbon exchange.
Variability of biomass burning aerosol optical characteristics in southern Africa during the SAFARI 2000 dry season campaign and a comparison of single scattering albedo estimates from radiometric measurements
Authors: Eck T. F. , Holben B. N. , Ward D. E. , Mukelabai M. M. , Dubovik O. , Smirnov A. , Schafer J. S. , Hsu N. C. , Piketh S. J. , Queface A. , Roux J. Le , Swap R. J. , Slutsker I.
Published: May 2003

Measurements of the column‐integrated aerosol optical properties in the southern African region were made by Aerosol Robotic Network (AERONET) Sun‐sky radiometers at several sites in August–September 2000 as a part of the Southern African Regional Science Initiative (SAFARI) 2000 dry season field campaign. Fine mode biomass burning aerosols dominated in the northern part of the study region (Zambia), which is an active burning region, and other aerosols including fossil fuel burning, industrial, and aeolian coarse mode types also contributed to the aerosol mixture in other regions (South Africa and Mozambique), which were not as strongly dominated by local burning. The large amount of smoke produced in the north lead to a north‐south gradient in aerosol optical depth (τa) in September, with biomass burning aerosol concentrations reduced by dispersion and deposition during transport. Large average diurnal variations of τa(typical diurnal range of 25%) were observed at all sites in Zambia as a result of large diurnal trends in fire counts in that region that peak in midafternoon. However, for all sites located downwind to the south, there was relatively little (∼5–10%) average diurnal trend observed as the aerosol transport is not strongly influenced by diurnal cycles. AERONET radiometer retrievals of aerosol single scattering albedo (ω0) in Zambia showed relatively constant values as a function of τafor τa440ranging from 0.4 to ∼2.5. The wavelength dependence of ω0varied significantly over the region, with greater decreases for increasing wavelength at smoke‐dominated sites than for sites influenced by a significant coarse mode aerosol component. Retrievals of midvisible ω0based on the fitting of Photosynthetically Active Radiation (PAR; 400–700 nm) flux measurements to modeled fluxes for smoke in Mongu, Zambia yielded an average value of 0.84. This is in close agreement with the estimated average of 0.85 derived from interpolation of the AERONET retrievals made at 440 and 675 nm for August–September 2000. The spectral dependence of ω0independently retrieved with the AERONET measurements and with diffuse fraction measurements in Mongu, Zambia was similar for both techniques, as a result of both methods retrieving the imaginary index of refraction (∼0.030–0.035 on one day) with very little wavelength dependence.
Trace gas measurements in nascent, aged, and cloud‐processed smoke from African savanna fires by airborne Fourier transform infrared spectroscopy (AFTIR)
Authors: Yokelson Robert J. , Bertschi Isaac T. , Christian Ted J. , Hobbs Peter V. , Ward Darold E. , Hao Wei Min
Published: May 2003

We measured stable and reactive trace gases with an airborne Fourier transform infrared spectrometer (AFTIR) on the University of Washington Convair‐580 research aircraft in August/September 2000 during the SAFARI 2000 dry season campaign in Southern Africa. The measurements included vertical profiles of CO2, CO, H2O, and CH4up to 5.5 km on six occasions above instrumented ground sites and below the TERRA satellite and ER‐2 high‐flying research aircraft. We also measured the trace gas emissions from 10 African savanna fires. Five of these fires featured extensive ground‐based fuel characterization, and two were in the humid savanna ecosystem that accounts for most African biomass burning. The major constituents that we detected in nascent smoke were (in order of excess molar abundance) H2O, CO2, CO, CH4, NO2, NO, C2H4, CH3COOH, HCHO, CH3OH, HCN, NH3, HCOOH, and C2H2. These are the first quantitative measurements of the initial emissions of oxygenated volatile organic compounds (OVOC), NH3, and HCN from African savanna fires. On average, we measured 5.3 g/kg of OVOC and 3.6 g/kg of hydrocarbons (including CH4) in the initial emissions from the fires. Thus, the OVOC will have profound, largely unexplored effects on tropical tropospheric chemistry. The HCN emission factor was only weakly dependent on fire type; the average value (0.53 g/kg) is about 20 times that of a previous recommendation. HCN may be useful as a tracer for savanna fires. ΔO3/ΔCO and ΔCH3COOH/ΔCO increased to as much as 9% in <1 h of photochemical processing downwind of fires. Direct measurements showed that cloud processing of smoke greatly reduced CH3OH, NH3, CH3COOH, SO2, and NO2levels, but significantly increased HCHO and NO.
Evaluation of MODIS thermal IR band L1B radiances during SAFARI 2000
Authors: Moeller Christopher C. , Revercomb Henry E. , Ackerman Steven A. , Menzel W. Paul , Knuteson Robert O.
Published: May 2003

On 11 September 2000, a NASA ER‐2 aircraft underflew the Terra spacecraft during the Southern Africa Regional Science Initiative (SAFARI) 2000 field experiment. The Scanning High Resolution Interferometer Sounder (S‐HIS), Moderate Resolution Imaging Spectroradiometer (MODIS) Airborne Simulator (MAS), and Cloud Physics Lidar (CPL) instruments onboard the ER‐2 have provided a data set for a first look at the accuracy of the Collect 3 (L1B Version 3.X) Terra MODIS L1B radiances for the thermal infrared (TIR) bands. Based on comparisons between MODIS and S‐HIS nadir viewing data and an estimation of the existing uncertainties, the radiances of most MODIS TIR bands are found to be very near or within specification. There does not appear to be a significant influence by electronic cross talk on the midwave IR (MWIR) bands for the low reflectance, flat thermal scenes used in the evaluation. Longwave IR (LWIR) split window band residuals are within 0.1 K with an estimated uncertainty of ±0.13 K, raising confidence in their accuracy. Midtropospheric water vapor band residuals are also within specification, despite known detector striping in these bands. Residuals in the LWIR upper tropospheric CO2 bands 34–36 exceed specification. However, these are not considered strong indications of L1B performance issues due to possible undocumented uncertainty in the altitude correction for the unmeasured atmosphere above the ER‐2 aircraft level.
Water‐soluble organic components in aerosols associated with savanna fires in southern Africa: Identification, evolution, and distribution
Authors: Gao Song , Hegg Dean A. , Hobbs Peter V. , Kirchstetter Thomas W. , Magi Brian I. , Sadilek Martin
Published: May 2003

During the SAFARI 2000 field campaign, both smoke aerosols from savanna fires and haze aerosols in the boundary layer and in the free troposphere were collected from an aircraft in southern Africa. These aerosol samples were analyzed for their water‐soluble chemical components, particularly the organic species. A novel technique, electrospray ionization‐ion trap mass spectrometry, was used concurrently with an ion chromatography system to analyze for carbohydrate species. Seven carbohydrates, seven organic acids, five metallic elements, and three inorganic anions were identified and quantified. On the average, these 22 species comprised 36% and 27% of the total aerosol mass in haze and smoke aerosols, respectively. For the smoke aerosols, levoglucosan was the most abundant carbohydrate species, while gluconic acid was tentatively identified as the most abundant organic acid. The mass abundance and possible source of each class of identified species are discussed, along with their possible formation pathways. The combustion phase of a fire had an impact on the chemical composition of the emitted aerosols. Secondary formation of sulfate, nitrate, levoglucosan, and several organic acids occurred during the initial aging of smoke aerosols. It is likely that under certain conditions, some carbohydrate species in smoke aerosols, such as levoglucosan, were converted to organic acids during upward transport.
Aerosol size distribution during sea fog and its scavenge process of chemical substances over the northwestern North Pacific
Authors: Sasakawa Motoki , Ooki Atsushi , Uematsu Mitsuo
Published: May 2003

We investigated the chemical characteristics of sea fog and its generation mechanism over the northwestern North Pacific from 28 June to 10 July 2001. Continuous aerosol number measurements revealed that sea fog scavenged coarse particles (d > 0.50 μm) more efficiently than fine particles (d < 0.50 μm). Although there was no relationship between Na+ concentration and sea fog age, anthropogenic ions concentrations (NO3−, nss‐SO42−, and NH4+) decreased with an increase in sea fog age. Compared with nss‐SO42− and NH4+, NO3− was selectively removed by sea fog. Coarse particles (sea‐salt and NaNO3) act as condensation nuclei (CN) of sea fog droplets more preferentially than fine particles (H2SO4, (NH4)HSO4 and (NH4)2SO4) because of the effect of curvature. This is in harmony with the result of aerosol number measurement.
Tropospheric carbon monoxide measurements from the Scanning High‐Resolution Interferometer Sounder on 7 September 2000 in southern Africa during SAFARI 2000
Authors: McMillan W. W. , McCourt M. L. , Revercomb H. E. , Knuteson R. O. , Christian T. J. , Doddridge B. G. , Hobbs P. V. , Lukovich J. V. , Novelli P. C. , Piketh S. J. , Sparling L. , Stein D. , Swap R. J. , Yokelson R. J.
Published: May 2003

Retrieved tropospheric carbon monoxide (CO) column densities are presented for more than 9000 spectra obtained by the University of Wisconsin‐Madison (UWis) Scanning High‐Resolution Interferometer Sounder (SHIS) during a flight on the NASA ER‐2 on 7 September 2000 as part of the Southern African Regional Science Initiative (SAFARI 2000) dry season field campaign. Enhancements in tropospheric column CO were detected in the vicinity of a controlled biomass burn in the Timbavati Game Reserve in northeastern South Africa and over the edge of theriver of smokein south central Mozambique. Relatively clean air was observed over the far southern coast of Mozambique. Quantitative comparisons are presented with in situ measurements from five different instruments flying on two other aircraft: the University of Washington Convair‐580 (CV) and the South African Aerocommander JRB in the vicinity of the Timbavati fire. Measured tropospheric CO columns (extrapolated from 337 to 100 mb) of 2.1 × 1018cm−2in background air and up to 1.5 × 1019cm−2in the smoke plume agree well with SHIS retrieved tropospheric CO columns of (2.3 ± 0.25) × 1018cm−2over background air near the fire and (1.5 ± 0.35) × 1019cm−2over the smoke plume. Qualitative comparisons are presented with three other in situ CO profiles obtained by the South African JRA aircraft over Mozambique and northern South Africa showing the influence of theriver of smoke.
Regional fuel load for two climatically contrasting years in southern Africa
Authors: Hély C. , Dowty P. R. , Alleaume S. , Caylor K. K. , Korontzi S. , Swap R. J. , Shugart H. H. , Justice C. O.
Published: May 2003

Available fuel loads for burning in savanna ecosystems in the southern African region have been estimated using a new Fuel load‐Net Primary Production model based on ecophysiological processes such as respiration and Potential Evapotranspiration. The model outputs 15‐day standing available fuel load layers for an entire year (a total of 24 layers). Published data from the Southern African Fire‐Atmosphere Research Initiative (SAFARI‐92) project and from the Southern African Regional Science Initiative (SAFARI 2000) field campaigns were generally in agreement with the estimations. Consistently with previous studies, precipitation was recognized to be the major climatic driver for fuel production. As a consequence, even though there was a regional increase in precipitation in 1999–2000 as compared to the 1991–1992 periods, the temporal and spatial variability in precipitation at fine scales (site level) was important enough to restrict generalities over the entire region for fuel load production. Four areas of interest, Etosha National Park (Namibia), Mongu (Zambia), Kasama (Zambia), and Kruger National Park (South Africa), were selected to reconstruct an aridity gradient and analyze their fuel load variability over the two years. These areas presented contrasting fuel load distributions for the two very different studied periods with arid areas producing heavier fuel loads in 1999–2000, and the more humid areas producing heavier fuel loads in 1991–1992. The consequences of such fuel load variability and the use of such results are discussed.
Global frequency and distribution of lightning as observed from space by the Optical Transient Detector
Authors: Christian Hugh J. , Blakeslee Richard J. , Boccippio Dennis J. , Boeck William L. , Buechler Dennis E. , Driscoll Kevin T. , Goodman Steven J. , Hall John M. , Koshak William J. , Mach Douglas M. , Stewart Michael F.
Published: May 2003

The Optical Transient Detector (OTD) is a space‐based instrument specifically designed to detect and locate lightning discharges as it orbits the Earth. This instrument is a scientific payload on the MicroLab‐1 satellite that was launched into a 70° inclination low Earth orbit in April 1995. Given the orbital trajectory of the satellite, most regions of the Earth are observed by the OTD instrument more than 400 times during a 1 year period, and the average duration of each observation is 2 min. The OTD instrument optically detects lightning flashes that occur within its 1300 × 1300 km2 field of view during both day and night conditions. A statistical examination of OTD lightning data reveals that nearly 1.4 billion flashes occur annually over the entire Earth. This annual flash count translates to an average of 44 ± 5 lightning flashes (intracloud and cloud‐to‐ground combined) occurring around the globe every second, which is well below the traditional estimate of 100 fl s−1 that was derived in 1925 from world thunder day records. The range of uncertainty for the OTD global totals represents primarily the uncertainty (and variability) in the flash detection efficiency of the instrument. The OTD measurements have been used to construct lightning climatology maps that demonstrate the geographical and seasonal distribution of lightning activity for the globe. An analysis of this annual lightning distribution confirms that lightning occurs mainly over land areas, with an average land/ocean ratio of ∼10:1. The Congo basin, which stands out year‐round, shows a peak mean annual flash density of 80 fl km−2 yr−1 in Rwanda, and includes an area of over 3 million km2 exhibiting flash densities greater than 30 fl km−2 yr−1 (the flash density of central Florida). Lightning is predominant in the northern Atlantic and western Pacific Ocean basins year‐round where instability is produced from cold air passing over warm ocean water. Lightning is less frequent in the eastern tropical Pacific and Indian Ocean basins where the air mass is warmer. A dominant Northern Hemisphere summer peak occurs in the annual cycle, and evidence is found for a tropically driven semiannual cycle.
Influence of planetary waves on noctilucent cloud occurrence over NW Europe
Authors: Kirkwood S. , Stebel K.
Published: May 2003

Observations of noctilucent clouds (NLC) from northwest Europe have been collected by a network of observers for almost 40 years. Previous analyses of the observations have found an apparent increasing frequency of occurrence, a 10–11 year modulation and evidence for 5‐day periodicity. Here we reexamine the observational data for NLC occurrence to test whether the observed variations can be explained by planetary wave activity in the middle atmosphere. Planetary wave amplitudes and phases in the lower mesosphere are derived from global meteorological assimilations from 1979–2000 and extrapolated to the mesopause. When the NLC observations are selected from a constant observing area, we find that there is no substantial trend in yearly NLC occurrence over the observation period, whereas the 10–11 year and 5‐day modulations remain significant. We find a strong correlation between the probability of observing NLC and the combined effects of stationary, 16‐day and 5‐day planetary waves at the NLC location. The most reasonable explanation for the correlation is that the probability of observing NLC depends on the strength of the wind from the north, which in turn depends on the amplitude and phase of the planetary waves. The influence of planetary waves on NLC occurrence may to a certain extent explain the 10–11 year periodicity in NLC. This possibility is a consequence of a strong correlation between the phase of the stationary planetary waves and the 10–11 year cycle of solar activity during the period 1979–2000.
Quantitative sampling using an Aerodyne aerosol mass spectrometer 2. Measurements of fine particulate chemical composition in two U.K. cities
Authors: Allan James D. , Alfarra M. Rami , Bower Keith N. , Williams Paul I. , Gallagher Martin W. , Jimenez Jose L. , McDonald Alan G. , Nemitz Eiko , Canagaratna Manjula R. , Jayne John T. , Coe Hugh , Worsnop Douglas R.
Published: May 2003

In part 1 of this series, techniques for generating quantitative information on fine airborne particulate‐size and chemically resolved mass concentration from an Aerodyne aerosol mass spectrometer were introduced. Presented here are the results generated using these techniques from sampling U.K. urban air with such an instrument in Edinburgh during October 2000 and in Manchester during July 2001 and January 2002. Data on the total mass concentrations and size‐resolved mass distributions of nitrate, sulfate, and organic compounds were obtained for all three campaigns and compared with data from other sources, including a micro‐orifice uniform deposit impactor, total particle numbers, CO and NOx concentrations, local wind speed and temperature, and back trajectory analysis. All three locations showed evidence for emissions from local transport, with a mass modal aerodynamic diameter of around 100–200nm. This mode was dominated by hydrocarbons showing little evidence of oxidization. The three sites also exhibited a larger mode consisting of inorganic chemicals and oxidized organics, which appeared to be governed by sources external to the cities and showed evidence of internal mixing. The mass modal aerodynamic diameter varied between approximately 200–500 nm during the winter and 500–800 nm during the summer. The summer also showed an increased mass loading without an increase in total particle number. Evidence of material building up and ageing in the atmospheric surface layer during periods of low wind speeds was also observed.
Aerosol single scattering albedo retrieved from measurements of surface UV irradiance and a radiative transfer model
Authors: Petters J. L. , Saxena V. K. , Slusser J. R. , Wenny B. N. , Madronich Sasha
Published: May 2003

Aerosol single scattering albedo (ω), the ratio of aerosol scattering coefficient to total aerosol extinction coefficient, at UV wavelengths is an important aerosol radiative parameter in determining surface UV irradiance. Surface measurements of total and diffuse UV irradiance in the summer and fall of 1999 at the seven narrowband wavelength channels of an UV multifilter rotating shadowband radiometer (UVMFR‐SR) at Black Mountain, N. C., were coupled with a tropospheric ultraviolet radiative transfer model to produce values of ω. Its value ranged from 0.65 to 0.91 at 300 nm, 0.71 to 0.96 at 305.5 nm, 0.73 to 0.97 at 311.4 nm, 0.74 to 0.91 at 317.6 nm, 0.76 to 0.96 at 325.4 nm, 0.77 to 0.97 at 332.4 nm, and 0.80 to 0.99 at 368 nm. Error in this procedure decreases with increasing aerosol optical depth (AOD), from ±0.63 at AOD = 0.05 to ±0.04 at AOD = 1.0 averaged over the seven wavelengths. The current values of ω have a slightly wider variation than values reported from a previous study at the same site. The lower values of ω could indicate that, over the site, preferential absorption of UV radiation by black carbon aerosols could be occurring. More values of ω in the UV spectrum will allow for better estimation of this parameter for UV radiative transfer modeling and will lessen error in estimation of surface UV irradiances.
MODIS observation of aerosols and estimation of aerosol radiative forcing over southern Africa during SAFARI 2000
Authors: Ichoku Charles , Remer Lorraine A. , Kaufman Yoram J. , Levy Robert , Chu D. Allen , Tanré Didier , Holben Brent N.
Published: May 2003

MODIS provides almost complete global coverage daily. Aerosol optical thickness (AOT or τaλ) and other aerosol parameters are retrieved over land and ocean at a spatial scale of 10 km (level 2), then aggregated to a global grid of 1° spatial resolution on daily, weekly, and monthly time scales (level 3). The SAFARI 2000 ground‐based measurements of AOT from the Aerosol Robotic Network (AERONET) group of Sun photometers were used to compare MODIS over‐land AOT based on a spatiotemporal statistical technique. At low aerosol loading (when τa470 < 0.6), MODIS τaλ values agree with those of AERONET at the blue wavelength (λ = 440, 470 nm). But, at higher wavelengths and with increasing aerosol loading, MODIS AOT values underestimate those of AERONET. There are also regional variations in validation accuracy. This AOT underestimation by MODIS during SAFARI 2000 is attributed to the application of a constant single‐scattering‐albedo (SSA or ω0) value of 0.90 globally for smoke aerosol retrieval. Recent studies based on long‐term observations with AERONET Sun photometers suggest that lower ω0 values of 0.88 and 0.84 at 440 nm and 670 nm wavelengths respectively would be more applicable over southern Africa (particularly over Zambia, where most of the measurements were based). A column climate model, with MODIS aerosol information as input, is used to calculate aerosol radiative forcing over the (southern Atlantic) Ocean part of the SAFARI 2000 region. For September 2000, the results show a forcing of −10 W/m2 at the top of the atmosphere and approximately −26 W/m2 at the terrestrial surface.
The cloudy atmospheric boundary layer over the subtropical South Atlantic Ocean: Airborne‐spaceborne lidar observations and numerical simulations
Authors: Cosma‐Averseng Stéphanie , Flamant Cyrille , Pelon Jacques , Palm Stephen P. , Schwemmer Geary K.
Published: May 2003

This paper focuses on the structure of the marine boundary layer in an convergence region between easterly (trade winds) and westerly flows, over the Subtropical South Atlantic Ocean on 15 September 1994 using an unprecedented combination of spaceborne and airborne lidar observations, airborne Radar Ocean Wave Spectrometer and dropsondes system, and three‐dimensional modeling. Methodologies previously developed to determine the marine atmospheric boundary layer (MABL) depth and cloud top heights from airborne lidar measurements are successfully applied to spaceborne measurements. Nested numerical simulations have been performed in this case study. It is shown that the MABL structure in this region (observed with lidar and simulated) collapsed dramatically in connection with a marked decrease of wind speed and near‐zero turbulent kinetic energy due to specific synoptic conditions.
Seasonal and long‐term variations of PMSE from VHF radar observations at Andenes, Norway
Authors: Bremer J. , Hoffmann P. , Latteck R. , Singer W.
Published: May 2003

The observation of polar mesosphere summer echoes (PMSE) with VHF radars is an important possibility for the investigation of the polar mesospheric region during summer. This region is characterized by the lowest temperatures in the Earth's atmosphere and the existence of mesospheric ice clouds, visually or by lidars observed as noctilucent clouds (NLC). Using measurements at 53.5 MHz in Andenes, Norway, with the ALOMAR SOUSY radar between 1994–1997 and with the ALWIN radar between 1999–2001 mean seasonal, solar cycle and long‐term variations of PMSE have been derived. The seasonal variation of PMSE is characterized by a strong increase during end of May/beginning of June, a rather high level in June and July, and a more gradual decrease during August. The occurrence rate of PMSE is strongly positive correlated with the mesospheric ionization level mainly caused by solar cycle variations of the solar Lyman α radiation and also by the flux of precipitating high energetic particles. Long‐term trends of PMSE are only very small and not significant due to the limited data series.
Airborne lidar measurements of aerosol optical properties during SAFARI‐2000
Authors: McGill Matthew J. , Hlavka Dennis L. , Hart William D. , Welton Ellsworth J. , Campbell James R.
Published: May 2003

The Cloud Physics Lidar (CPL) operated onboard the NASA ER‐2 high‐altitude aircraft during the Southern African Regional Science Initiative (SAFARI)‐2000 field campaign. The CPL provided high spatial resolution estimates of aerosol optical properties at both 1064 and 532 nm. We present here results of planetary boundary layer (PBL) aerosol optical depth analysis and profiles of aerosol extinction. Variation of optical depth and extinction are examined as a function of regional location. The wide‐scale aerosol mapping obtained by the CPL is a unique data set that will aid in future studies of aerosol transport. Comparisons between the airborne CPL and ground‐based Micro‐Pulse Lidar Network (MPL‐Net) sites are shown to have good agreement.
Continuous record of microparticle concentration and size distribution in the central Greenland NGRIP ice core during the last glacial period
Authors: Ruth Urs , Wagenbach Dietmar , Steffensen Jørgen P. , Bigler Matthias
Published: May 2003

A novel laser microparticle detector used in conjunction with continuous sample melting has provided a more than 1500 m long record of particle concentration and size distribution of the NGRIP ice core, covering continuously the period approximately from 9.5–100 kyr before present; measurements were at 1.65 m depth resolution, corresponding to approximately 35–200 yr. Particle concentration increased by a factor of 100 in the Last Glacial Maximum (LGM) compared to the Preboreal, and sharp variations of concentration occurred synchronously with rapid changes in the δ18O temperature proxy. The lognormal mode μ of the volume distribution shows clear systematic variations with smaller modes during warmer climates and coarser modes during colder periods. We find μ ≈ 1.7 μm diameter during LGM and μ ≈ 1.3 μm during the Preboreal. On timescales below several 100 years μ and the particle concentration exhibit a certain degree of independence present especially during warm periods, when μ generally is more variable. Using highly simplifying considerations for atmospheric transport and deposition of particles we infer that (1) the observed changes of μ in the ice largely reflect changes in the size of airborne particles above the ice sheet and (2) changes of μ are indicative of changes in long range atmospheric transport time. From the observed size changes we estimate shorter transit times by roughly 25% during LGM compared to the Preboreal. The associated particle concentration increase from more efficient long range transport is estimated to less than one order of magnitude.
Interannual and seasonal variability of biomass burning emissions constrained by satellite observations
Authors: Duncan Bryan N. , Martin Randall V. , Staudt Amanda C. , Yevich Rosemarie , Logan Jennifer A.
Published: May 2003

We present a methodology for estimating the seasonal and interannual variation of biomass burning designed for use in global chemical transport models. The average seasonal variation is estimated from 4 years of fire‐count data from the Along Track Scanning Radiometer (ATSR) and 1–2 years of similar data from the Advanced Very High Resolution Radiometer (AVHRR) World Fire Atlases. We use the Total Ozone Mapping Spectrometer (TOMS) Aerosol Index (AI) data product as a surrogate to estimate interannual variability in biomass burning for six regions: Southeast Asia, Indonesia and Malaysia, Brazil, Central America and Mexico, Canada and Alaska, and Asiatic Russia. The AI data set is available from 1979 to the present with an interruption in satellite observations from mid‐1993 to mid‐1996; this data gap is filled where possible with estimates of area burned from the literature for different regions. Between August 1996 and July 2000, the ATSR fire‐counts are used to provide specific locations of emissions and a record of interannual variability throughout the world. We use our methodology to estimate mean seasonal and interannual variations for emissions of carbon monoxide from biomass burning, and we find that no trend is apparent in these emissions over the last two decades, but that there is significant interannual variability.
On the retrieval of columnar aerosol mass and CCN concentration by MODIS
Authors: Gassó Santiago , Hegg Dean A.
Published: May 2003

A parameterization is introduced for the derivation of columnar aerosol mass concentration (AMC) and cloud condensation nucleus concentration (CCNC) from the primary aerosol products of Moderate‐Resolution Imaging Spectroradiometer (MODIS). The method relies on the scaling between AMC and optical depth with a proportionality constant dependent on MODIS‐derived reff, η (contribution of the accumulation mode radiance to the total radiance), ambient RH, and an assumed constant aerosol composition. The CCNC is derived from a parameterization of concentration as a function of the retrieved aerosol volume. By comparing with in situ data, it is shown that retrievals in dry ambient conditions (dust) are improved when using a proportionality constant dependent on reff and η obtained in the same pixel. In high‐humidity environments, the improvement in the new method is inconclusive because of the difficulty in accounting for the uneven vertical distribution of relative humidity. Additionally, two detailed comparisons of AMC and CCNC retrieved by the MODIS Airborne Simulator (MAS) algorithm and the new method are shown. The new method and MAS retrievals of AMC are within the same order of magnitude with respect to the in situ measurements of aerosol mass. However, the proposed method is closer to the in situ measurements than the MODIS retrievals. The retrievals of CCNC are also within the same order of magnitude for both methods. Finally, the new method is applied to an actual MODIS retrieval. Although no in situ data is available for comparison, it is shown that the proposed method yields more credible values than the MODIS retrievals.
Observation of NOx species in a N2/O2/H2O microwave discharge plasma: A laboratory simulation of Earth's atmosphere
Authors: Fujii Toshihiro , Iwase Keiichiro , Selvin P. Christopher
Published: May 2003

During studies on the identification of the principal species produced in an N2/O2/H2O microwave discharge plasma, we succeeded in detecting the presence of NOx species, including NO2, in the gas phase. The Li+ ion adduct mass spectra of the discharge plasma, obtained by means of a direct sampling system, clearly showed various peaks, giving direct evidence that in addition to the familiar NO and N2O species, significant amounts of gas‐phase neutral NO2, NO3, HNO2, and HNO3 were produced and detected in the N2/O2/H2O plasma. The influence of certain discharge variables on the yield and concentration of NOxLi+ was investigated also. Comparison of the present results with field observations of Earth's atmosphere demonstrated that laboratory simulation experiments can yield species comparable to the reactive nitrogen oxide species found in the troposphere.
Spatial and seasonal variations in CCN distribution and the aerosol‐CCN relationship over southern Africa
Authors: Ross K. E. , Piketh S. J. , Bruintjes R. T. , Burger R. P. , Swap R. J. , Annegarn H. J.
Published: May 2003

The sources and transport of cloud condensation nuclei (CCN) over southern Africa have been investigated using in situ measurements collected during the Aerosol Recirculation and Rainfall Experiment (ARREX) wet season projects and during the Southern African Regional Science Initiative (SAFARI‐2000) intensive dry season campaign. CCN concentrations over the subcontinent are generally higher in the dry season than in the wet season and exceed 2000 cm−3in highly polluted air masses. In the late dry season, CCN concentrations are highest in the northern regions of the subcontinent due to the burning of savanna biomass. Emissions from industries and power plants on the South African Highveld are a prolific year‐round source of CCN and are sufficient to account for CCN levels south of 20°S throughout the year. Most CCN are contained within the mixing layer, which extends to an altitude of 3000–4000 m over the plateau and is capped by a temperature inversion. Multiple inversions and absolutely stable layers control the stratification of aerosols and CCN. Biomass burning particles are efficient CCN, and the median diameter of the accumulation mode is large (up to 0.19 μm). Recently emitted industrial aerosols are less soluble and have a smaller median diameter (0.11 μm). Twice as many aerosols act as CCN in the dry season (68%) than in the wet season (34%). The fraction is highest in the dry season over the tropical regions (>80%), where smoke aerosol predominates. Elevated aerosol and CCN concentrations are expected to have implications for direct radiative forcing, especially in the dry season, and for indirect forcing in the wet season.
The response time of PMSE to ionospheric heating
Authors: Belova E. , Chilson P. B. , Kirkwood S. , Rietveld M. T.
Published: May 2003

During July of 1999, experiments were conducted in northern Norway to investigate the effects of ionospheric heating on polar mesosphere summer echoes (PMSE). The experiments were conducted using the European incoherent scatter (EISCAT) VHF radar and heating facility. It was shown that heating can dramatically reduce the backscattered echo power of PMSE. Here, we reexamine the high temporal resolution data of the PMSE backscattered power from three of the experiments as a function of ionospheric heating. Particular attention is paid to the transitions from the heater off‐to‐on and on‐to‐off states. The transition times of the PMSE echo power from high to low and low to high, respectively, is estimated in both cases to be less than 30 ms. It is suggested that enhancement of the electron diffusivity during heating is unlikely to account for such a fast decrease of radar backscattered power when the heater is switched on. We consider that an increase of the electron Debye length up to a significant fraction of a radar wavelength due to electron heating will change scattering character that might explain the observed heating effect on PMSE.
Mesospheric turbulence measurements from persistent Leonid meteor train observations
Authors: Kelley M. C. , Kruschwitz C. A. , Gardner C. S. , Drummond J. D. , Kane T. J.
Published: May 2003

Long‐duration meteor trains have fascinated observers for many years. The great Leonid meteor storms of 1866–1888 were the first to spark organized scientific study on the subject, but despite years of study, more than a century later, persistent trains remain for the most part a mystery. Over the last few years, however, the heightened Leonid activity has fueled considerable research efforts, much of it dealing with persistent trains. Some of the results of a comprehensive study of persistent trains conducted at the Starfire Optical Range (SOR) on Kirtland Air Force Base, New Mexico, during the 1998 and 1999 Leonid showers are reported here. For the first time the time evolution of persistent trains is used to determine the eddy diffusion coefficient at mesospheric heights. In three of the four trains studied, portions of the train exhibited molecular diffusion while the remainder of the train, as well as the entire fourth train, exhibited eddy diffusion. The eddy diffusion coefficients were several hundred m2 s−1, two orders of magnitude higher than the molecular rates. The sodium density in the train was sufficient to use it as a passive scalar tracer of turbulent fluctuations. The spectra are well modeled by the Heisenberg turbulence model, and the values found for the energy dissipation rate are in agreement with the eddy diffusion coefficient estimates. The gradient Richardson number and Brunt‐Väisällä frequency were determined from lidar measurements and indicated regions of convective and dynamic instability.
10Be/7Be tracer of atmospheric transport and stratosphere‐troposphere exchange
Authors: Jordan C. E. , Dibb J. E. , Finkel R. C.
Published: May 2003

The 10Be/7Be ratio is a sensitive tracer of atmospheric transport and stratosphere‐troposphere exchange (STE). Data from five NASA aircraft field missions (PEM: West A and B, Tropics A; SONEX; and SUCCESS) have been assembled to produce the largest data set of 10Be, 7Be, and their ratio collected to date (>300 samples). Ratios near 0.60 are indicative of tropospheric air with little stratospheric influence, while higher ratios are found in stratospheric air. Samples from the lower stratosphere were all collected within 2.5 km of the tropopause and had ratios >1.27. Of these lower stratosphere samples only 16% had ratios in excess of 3.0, suggesting that higher ratio air resides away from the tropopause. Seasonality observed in the 10Be/7Be ratios results from the downwelling of air with elevated ratios from higher in the stratosphere in the spring and summer (midlatitudes) and from the decay of 7Be during descent in the winter polar vortex (high latitudes). Our results illustrate the complexity of STE and some of the mechanisms through which it occurs, including tropopause folding, mixing associated with subtropical jets, and the effect of synoptic systems such as hurricanes and northeasters. The 10Be/7Be ratio provides important information beyond that which can be derived from studies that rely on chemical mixing ratios alone.