bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurement of 13CO and C18O in the free troposphere
Authors: Mak John E. , Brenninkmeijer Carl A. M.
Published: February 2004

We present δ13C, δ18O, and concentration measurements of atmospheric CO from several sampling trips over a broad range of latitude. More than 130 samples were collected from the middle troposphere and the boundary layer, from Antarctica to northern Sweden, during different seasons, between 1990 and 1993. Seasonally dependent spatial trends for all three species are observed in different regions, and reflect both the changing relative strengths of CO sources and the strength of the loss mechanism of CO+OH. For example, CO enriched in 18O resulting from fossil fuel combustion is clearly observed during winter in the northern hemisphere, in both remote and urban areas, whereas samples collected near Antarctica during the summer have very low concentrations of CO and depleted values of 18O. Of particular interest is the data set from the middle and high latitude southern hemisphere during the summer months, which is influenced both by fractionation effects during CO‐OH oxidation and by changing sources; the combination of isotopic values and concentration data suggests there may be a CO sink in addition to oxidation by reaction with OH. Relatively small changes were observed in the tropical Pacific between summer and winter months.
Possible laboratory evidence for multipole electric charge structures in thunderstorms
Authors: Jayaratne E. R.
Published: February 2004

The sign of electric charge separated during collisions between vapor‐grown ice crystals and graupel (soft hail) is determined principally by the temperature, cloud water content and supercooled droplet size spectrum. Experiments were conducted to show that, at a cloud water content of 1.0 g m−3 and a mean droplet diameter of about 10 μm, the charge acquired by a simulated graupel pellet was positive at temperatures above −18°C and negative below. This offers an explanation for the tripole charge distributions that have been reported from many thunderstorms. However, when the droplet spectrum was restricted to smaller sizes, the variation of the charging sign with temperature became more complex. When the mean droplet diameter was less than 4 μm, the charging current to the target showed four sign reversals within the temperature range 0°C to −36°C, starting from positive charging at the warmer extreme. These occurred at −9°C, −12°C, −17°C and −24°C. This suggests a possibility of finding up to six different horizontally layered charge centers in the specified temperature range within a thundercloud. It is proposed that variations in the droplet size spectrum in the vertical may offer a plausible explanation for the multipole electric charge distributions that have recently been observed in the stratiform regions and transition zones of mesoscale convective systems.
Photochemistry in biomass burning plumes and implications for tropospheric ozone over the tropical South Atlantic
Authors: Mauzerall Denise L. , Logan Jennifer A. , Jacob Daniel J. , Anderson Bruce E. , Blake Donald R. , Bradshaw John D. , Heikes Brian , Sachse Glenn W. , Singh Hanwant , Talbot Bob
Published: February 2004

Photochemistry occuring in biomass burning plumes over the tropical south Atlantic is analyzed using data collected during the Transport and Atmospheric Chemistry Near the Equator‐Atlantic aircraft expedition conducted during the tropical dry season in September 1992 and a photochemical point model. Enhancement ratios (ΔY/ΔX, where Δ indicates the enhancement of a compound in the plume above the local background mixing ratio,Yare individual hydrocarbons, CO, O3, N2O, HNO3, peroxyacetyl nitrate (PAN), CH2O, acetone, H2O2, CH3OOH, HCOOH, CH3COOH or aerosols andXis CO or CO2) are reported as a function of plume age inferred from the progression of Δnon‐methane hydrocarbons/ΔCO enhancement ratios. Emission, formation, and loss of species in plumes can be diagnosed from progression of enhancement ratios from fresh to old plumes. O3is produced in plumes over at least a 1 week period with mean ΔO3/ΔCO = 0.7 in old plumes. However, enhancement ratios in plumes can be influenced by changing background mixing ratios and by photochemical loss of CO. We estimate a downward correction of ∼20% in enhancement ratios in old plumes relative to ΔCO to correct for CO loss. In a case study of a large persistent biomass burning plume at 4‐km we found elevated concentrations of PAN in the fresh plume. The degradation of PAN helped maintain NOxmixing ratios in the plume where, over the course of a week, PAN was converted to HNO3. Ozone production in the plume was limited by the availability of NOx, and because of the short lifetime of O3at 4‐km, net ozone production in the plume was negligible. Within the region, the majority of O3production takes place in air above median CO concentration, indicating that most O3production occurs in plumes. Scaling up from the mean observed ΔO3/ΔCO in old plumes, we estimate a minimum regional O3production of 17×1010molecules O3cm−2s−1. This O3production rate is sufficient to fully explain the observed enhancement in tropospheric O3over the tropical South Atlantic during the dry season.
Laboratory studies of the influence of the rime accretion rate on charge transfer during crystal/graupel collisions
Authors: Saunders C. P. R. , Peck S. L.
Published: February 2004

The process of thunderstorm electrification by charge transfers between ice crystals and riming graupel pellets (the noninductive process) has been the subject of extensive study in the laboratory in Manchester. Quantitative dependencies of the sign and magnitude of charge transfer have previously been determined as functions of ice crystal size, graupel/crystal relative velocity, temperature, and the effective liquid water content (EW) in the cloud experienced by the riming graupel pellets. We now present results of laboratory studies of thunderstorm charging in terms of the rime accretion rate (RAR = EW × V), which combines into one variable the velocity and EW dependence of the sign of graupel charging on temperature. The magnitude of the charge transfer can be determined from its dependence on the crystal size and graupel velocity, while the sign of the rimer charging can now be determined from a new figure showing the dependence of the charge sign on RAR and temperature. This figure may be used to compare charge transfer results from other laboratories obtained over a range of graupel/crystal velocities. These new experiments extend the temperature range of the previous studies and indicate that negative charging of graupel can occur at temperatures as high as −2°C in conditions of low RAR, while at temperatures below −30°C, more positive graupel charging is noted than in the earlier work.
Hemispheric asymmetries and seasonal variations of the lowermost stratospheric water vapor and ozone derived from SAGE II data
Authors: Pan Liwen , Solomon Susan , Randel William , Lamarque Jean‐Francois , Hess Peter , Gille John , Chiou Er‐Woon , McCormick M. Patrick
Published: February 2004

The distributions of water vapor and ozone in the lowermost stratosphere, the part of the stratosphere between the tropopause and the 380‐K potential temperature surface, are examined using the Stratospheric Aerosol and Gas Experiment II measurements for the period of March 1989 to February 1990. The monthly mean values of water vapor and ozone are computed for the stratospheric portions of the 320‐K and 350‐K potential temperature surfaces for both hemispheres. We find a strong seasonal cycle in the water vapor mixing ratio on the 320‐K potential temperature surface for both hemispheres, with maximum values in the summer season and minimum values in early spring. The seasonal cycle at the 350‐K isentrope is similar to that at the 320 K but with a reduced amplitude. Comparisons with their distributions on the 420‐K isentrope are made in order to explore information on stratosphere‐troposphere exchange via the extratropical tropopause. Both water vapor and ozone data suggest that troposphere to stratosphere exchange across the extratropical tropopause has a significant influence on the lowermost stratosphere, especially for the northern hemisphere in the summer season.
Environmental conditions required for contrail formation and persistence
Authors: Jensen Eric J. , Toon Owen B. , Kinne Stefan , Sachse Glen W. , Anderson Bruce E. , Chan K. Roland , Twohy Cynthia H. , Gandrud Bruce , Heymsfield Andrew , Miake‐Lye Richard C.
Published: February 2004

The ambient temperatures and humidities required for contrail formation and persistence are determined from in situ measurements during the Subsonic Aircraft: Contrail and Cloud Effects Special Study (SUCCESS) experiment. Ambient temperatures and water vapor concentrations were measured with the meteorological measurement system, a laser hygrometer, and a cryogenic hygrometer (all onboard the DC‐8). The threshold temperatures are compared with theoretical estimates based on simple models of plume evolution. Observed contrail onset temperatures for contrail formation are shown to be 0–2 K below the liquid‐saturation threshold temperature, implying that saturation with respect to liquid water must be reached at some point in the plume evolution. Visible contrails observed during SUCCESS persisted longer than a few minutes only when substantial ambient supersaturations with respect to ice existed over large regions. On some occasions, contrails formed at relatively high temperatures (≥−50°C) due to very high ambient supersaturations with respect to ice (of the order of 150%). These warm contrails usually formed in the presence of diffuse cirrus. Water vapor from sublimated ice crystals that entered the engine was probably necessary for contrail formation in some of these cases. At temperatures above about −50°C, contrails can only form if the ambient air is supersaturated with respect to ice, so these contrails should persist and grow.
Regional model simulations of New Zealand climate
Authors: Renwick James A. , Katzfey Jack J. , Nguyen Kim C. , McGregor John L.
Published: February 2004

Simulation of New Zealand climate is examined through the use of a regional climate model nested within the output of the Commonwealth Scientific and Industrial Research Organisation nine‐level general circulation model (GCM). R21 resolution GCM output is used to drive a regional model run at 125 km grid spacing over the Australasian region. The 125 km run is used in turn to drive a simulation at 50 km resolution over New Zealand. Simulations with a full seasonal cycle are performed for 10 model years. The focus is on the quality of the simulation of present‐day climate, but results of a doubled‐CO2 run are discussed briefly. Spatial patterns of mean simulated precipitation and surface temperatures improve markedly as horizontal resolution is increased, through the better resolution of the country's orography. However, increased horizontal resolution leads to a positive bias in precipitation. At 50 km resolution, simulated frequency distributions of daily maximum/minimum temperatures are statistically similar to those of observations at many stations, while frequency distributions of daily precipitation appear to be statistically different to those of observations at most stations. Modeled daily precipitation variability at 125 km resolution is considerably less than observed, but is comparable to, or exceeds, observed variability at 50 km resolution. The sensitivity of the simulated climate to changes in the specification of the land surface is discussed briefly. Spatial patterns of the frequency of extreme temperatures and precipitation are generally well modeled. Under a doubling of CO2, the frequency of precipitation extremes changes only slightly at most locations, while air frosts become virtually unknown except at high‐elevation sites.
A broadband lidar for the measurement of tropospheric constituent profiles from the ground
Authors: Povey I. M. , South A. M. , de Roodenbeke A. t'Kint , Hill C. , Freshwater R. A. , Jones R. L.
Published: February 2004

In this paper we describe a novel lidar that combines differential UV‐visible absorption spectroscopy and the lidar technique. The critical and novel element of the system is the use of an imaging spectrometer in conjunction with a two‐dimensional CCD detector array to simultaneously spectrally and temporally resolve backscattered radiation. To exploit this approach, the lidar system utilizes a broadband laser output of 10–20 nm full width at half maximum tunable across the UV‐visible spectral region, thus allowing the simultaneous measurement of multiple molecular species by the differential optical absorption spectroscopy technique. To demonstrate the flexibility of the technique for tropospheric composition monitoring we present initial results for both elastic and inelastic (Raman) backscatter and for absorption studies in the spectral regions where NO3 and H2O absorb. In addition, the technique has applicability for a wide range of molecules including O3, NO2, and other spectrally structured absorbers and for atmospheric temperature sounding, which may be derived from either rotational Raman return or temperature dependent absorptions such as those of O2.
Trifluoroacetic acid from degradation of HCFCs and HFCs: A three‐dimensional modeling study
Authors: Kotamarthi V. R. , Rodriguez J. M. , Ko M. K. W. , Tromp T. K. , Sze N. D. , Prather Michael J.
Published: February 2004

Trifluoroacetic acid (TFA; CF3 COOH) is produced by the degradation of the halocarbon replacements HFC‐134a, HCFC‐124, and HCFC‐123. The formation of TFA occurs by HFC/HCFC reacting with OH to yield CF3COX (X = F or Cl), followed by in‐cloud hydrolysis of CF3COX to form TFA. The TFA formed in the clouds may be reevaporated but is finally deposited onto the surface by washout or dry deposition. Concern has been expressed about the possible long‐term accumulation of TFA in certain aquatic environments, pointing to the need to obtain information on the concentrations of TFA in rainwater over scales ranging from local to continental. Based on projected concentrations for HFC‐134a, HCFC‐124, and HCFC‐123 of 80, 10, and 1 pptv in the year 2010, mass conservation arguments imply an annually averaged global concentration of 0.16 μg/L if washout were the only removal mechanism for TFA. We present 3‐D simulations of the HFC/HCFC precursors of TFA that include the rates of formation and deposition of TFA based on assumed future emissions. An established (GISS/Harvard/ UCI) but coarse‐resolution (8° latitude by 10° longitude) chemical transport model was used. The annually averaged rainwater concentration of 0.12 μg/L (global) was calculated for the year 2010, when both washout and dry deposition are included as the loss mechanism for TFA from the atmosphere. For some large regions in midnorthern latitudes, values are larger, 0.15–0.20 μg/L. The highest monthly averaged rainwater concentrations of TFA for northern midlatitudes were calculated for the month of July, corresponding to 0.3–0.45 μg/L in parts of North America and Europe. Recent laboratory experiments have suggested that a substantial amount of vibrationally excited CF3CHFO is produced in the degradation of HFC‐134a, decreasing the yield of TFA from this compound by 60%. This decrease would reduce the calculated amounts of TFA in rainwater in the year 2010 by 26%, for the same projected concentrations of precursors.
Rayleigh lidar measurements of the temporal frequency and vertical wavenumber spectra in the mesosphere over the Rocky Mountain region
Authors: Gao X. , Meriwether J. W. , Wickwar V. B. , Wilkerson T. D. , Collins S.
Published: February 2004

Temporal and spatial spectral analysis techniques were applied to lidar data collected over a period of 18 months above the Rocky Mountain region at an altitude range from 45 to 70 km by a Rayleigh lidar system located in Logan, Utah (41.7°N, 111.8°W). Examination of the averaged temporal frequency F(ω) and vertical wavenumber F(m) spectra showed spectral slope values of −1.49±0.03 and −2.3±0.1, respectively. The observed slope for the overall averaged F(m) spectrum is considerably more positive than the value of −3 predicted by the linear instability theory but close to the value of ∼−2 that is predicted by the scale independent diffusive filtering theory using the measured F(ω) slope parameter. However, examination of the monthly averaged F(m) spectra for the transition from winter to summer showed the spectra became flatter suggesting that Doppler shift effects caused by the seasonal change in the magnitude and direction of the background wind field are significant. The characteristic vertical wavelength λ* was found to be ∼12 km for the altitude region of 45–70 km. Comparison of this value with the characteristic wavelengths from other lidar observations at lower and higher altitude ranges showed an overall increase of λ* with height. The observed enhancement of the F(m) spectral magnitude in winter is believed to be caused in part by the low–frequency wave activity observed in the temperature profiles.
Laboratory studies of the influence of cloud droplet size on charge transfer during crystal‐graupel collisions
Authors: Avila Eldo , Caranti Giorgio , Castellano Nesvit , Saunders Clive
Published: February 2004

Further laboratory measurements of charge transfer between ice crystals and riming graupel pellets, which are thought to be associated with the electrification processes within thunderstorms, have been carried out in the University of Manchester Institute of Science and Technology cloud chamber. In experiments with clouds in the temperature range −6°C to −26°C, the supercooled droplet spectrum has been extended to larger droplet sizes, above 60 μm maximum diameter, representative of the broadest spectrum observed in some thunderstorm cloud charging regions. The results indicate that at temperatures from −6°C to −18°C, broadening the droplet spectrum leads to negative graupel charging at higher values of cloud effective liquid water content than has been reported in previous laboratory studies. The significance of the result is that in order to ensure that laboratory experiments simulate as closely as possible the thunderstorm cloud microphysical environment, attention must be paid to the spectrum of droplets used. Two mechanisms of charge transfer that may account for this behavior are discussed, the relative growth rate theory and the surface splinter theory, and both are found to be compatible with the results on the assumption that the larger droplets lead to a reduction in the rate of vapor deposition to the riming surface. Analysis of the implications of these results to thunderstorm electrification requires more details of the evolution of droplet spectra in thunderclouds, their spatial and temporal development and location relative to observed regions of electrification.
Kinetic study of the formation of isotopically substituted ozone in argon
Authors: Sehested Jens , Nielsen Ole John , Egsgaard Helge , Larsen Niels Wessel , Andersen Thomas Sejner , Pedersen Thorvald
Published: February 2004

The kinetics of the formation of ozone was studied by using pulse radiolysis coupled with time‐resolved UV absorption at 275 nm and at T = 294.9±0.6 K. The rate constant for the formation of ozone 16O16O16O in argon was determined to be k3a = (3.38±0.04) × 10−34 cm6 molecule−2 s−1. The rate constants for the reactions 18O + 16O16O (k3b), 16O + 16O18O (k3c), 16O + 18O18O (k3d), 18O + 16O18O (k3e), and 18O + 18O18O (k3f) were studied, and the following parameters were determined: (k3b + k3d)/(2k3a) = (1.184±0.037), (k3c + k3e)/(2k3a) = (1.155±0.062), and k3f/k3a = (0.977±0.021). The values for (k3b + k3d)/(2k3a) and (k3c + k3e)/(2k3a) obtained here are equal to the values derived from the product studies and the recently reported relative rate study but higher than the reported values for (k3b + k3d)/(2k3a) and (k3c + k3e)/(2k3a) obtained by using CO2 as a third body. The parameter k3f/k3a = (0.977±0.021) is lower than the value of k3f/k3a obtained by using CO2 as a third body and the value derived from the product studies. These different values of k3f may be partly due to changes in third body efficiency or due to resonance interactions between the excited ozone molecules and the third body. The absolute measurements reported here together with literature data suggest that the nature of the third body is an important factor in controlling the enhancements of the rate constants for ozone formation and that asymmetry of neither ozone nor dioxygen ensure a fast ozone formation rate.
Microphysical properties of boundary layer clouds over the Southern Ocean during ACE 1
Authors: Boers Reinout , Krummel Paul B.
Published: February 2004

A survey is presented of microphysical properties of boundary layer clouds sampled over the Southern Ocean during the First Aerosol Characterization Experiment (ACE 1, November‐December 1995). Observations were confined between the latitudes of 40°S and 55°S and between the longitudes of 135°E and 160°E, which covers a region south of the island of Tasmania. Because of the remoteness of the region that was sampled, the microphysical cloud structure represents background conditions in which anthropogenic influences are minimal. Maximum cloud droplet concentrations for individual case studies ranged between 45 and 200 cm−3. These numbers are consistent with those reported from the summer phase of the Southern Ocean Cloud Experiment (SOCEX II, February 1995) and confirm that the droplet concentrations in an unpolluted atmosphere are largely constrained to values below 200 cm−3. The often observed weak decoupling of the cloud layer from the surface layer suggests that the entrainment of aerosol from the free atmosphere cannot be ruled out as a significant source of cloud condensation nuclei. The effective radius of the cloud droplets ranged from 4.5 to 16.6 μm. On many occasions the condition for cloud top entrainment instability was satisfied. This implies the occurrence of dry conditions aloft, turbulence generation due to entrainment, and more generally, conditions of broken cloud cover. It is premature to suggest that these conditions are prevalent over the Southern Ocean as the experiment may have inadvertently targeted atmospheric conditions with regions of broken clouds.
Eddy covariance measurement of isoprene fluxes
Authors: Guenther Alex B. , Hills Alan J.
Published: February 2004

A system has been developed to directly measure isoprene flux above a forest canopy by eddy covariance using the combination of a fast response, real‐time isoprene sensor and sonic anemometer. This system is suitable for making nearly unattended, long‐term, and continuous measurements of isoprene fluxes. Isoprene detection is based on chemiluminescence between isoprene and reactant ozone, which produces green light at 500 nm. The sensor has a noise level (1σ) of 450 pptv for a 1‐s integration which is dominated by random high‐frequency noise that does not significantly degrade eddy covariance flux measurements. Interference from the flux of other compounds is primarily due to the emission of monoterpenes, propene, ethene, and methyl butenol and the deposition of methacrolein and methyl vinyl ketone. The average total interference for North American landscapes in midday summer is estimated to be about 5% for emissions and −3% for deposition fluxes. In only a few North American landscapes, where isoprene emissions are very low and methyl butenol emissions are high, are interferences predicted to be significant. The system was field tested on a tower above a mixed deciduous forest canopy (Duke Forest, North Carolina, U.S.A.) dominated by oak trees, which are strong isoprene emitters. Isoprene fluxes were estimated for 307 half‐hour sampling periods over 10 days. Daytime fluxes ranging from 1 to 14 mg C m−2 h−1 were strongly correlated with light and temperature. The daytime mean flux of 6 mg C m−2 h−1 is similar to previous estimates determined by relaxed eddy accumulation by Geron et al [1997] at this site. Nighttime fluxes were near zero (0.01±0.03 mg C m−2 h−1).
Global sensitivity studies of the direct radiative forcing due to anthropogenic sulfate and black carbon aerosols
Authors: Haywood J. M. , Ramaswamy V.
Published: February 2004

The direct radiative forcing (DRF) of sulfate and black carbon (BC) aerosols is investigated using a new multispectral radiation code within the R30 Geophysical Fluid Dynamics Laboratory general circulation model (GCM). Two independent sulfate climatologies from chemical transport models are applied to the GCM; each climatology has a different atmospheric burden, vertical profile, and seasonal cycle. The DRF is calculated to be approximately −0.6 and −0.8 W m−2 for the different sulfate climatologies. Additional sensitivity studies show that the vertical profile of the sulfate aerosol is important in determining the DRF; sulfate residing near the surface gives the strongest DRF due to the effects of relative humidity. Calculations of the DRF due to BC reveal that the DRF remains uncertain to approximately a factor of 3 due to uncertainties in the total atmospheric burden, the vertical profile of the BC, and the assumed size distribution. Because of the uncertainties in the total global mass of BC, the normalized DRF (the DRF per unit column mass of aerosol in watts per milligram (W mg−1)) due to BC is estimated; the range is +1.1 to + 1.9 W mg−1 due to uncertainties in the vertical profile. These values correspond to a DRF of approximately +0.4 W m−2 with a factor of 3 uncertainty when the uncertainty in the total global mass of BC is included. In contrast to sulfate aerosol, the contribution to the global DRF from cloudy regions is very significant, being estimated as approximately 60%. The vertical profile of the BC is, once again, important in determining the DRF, but the sensitivity is reversed from that of sulfate; BC near the surface gives the weakest DRF due to the shielding effects of overlying clouds. Although the uncertainty in the estimates of the DRF due to BC remains high, these results indicate that the DRF due to absorption by BC aerosol may contribute a significant positive radiative forcing and may consequently be important in determining climatic changes in the Earth‐atmosphere system.
Stratospheric aerosol measurements by the Lidar in Space Technology Experiment
Authors: Osborn Mary T. , Kent Geoffrey S. , Trepte Charles R.
Published: February 2004

The Lidar in Space Technology Experiment (LITE) is a three‐wavelength backscatter lidar developed by NASA Langley Research Center to demonstrate and explore the capabilities of space lidar. LITE was flown on space shuttle Discovery in September 1994. Among the primary experimental objectives of LITE was the measurement of stratospheric aerosols. High‐quality stratospheric aerosol measurements at 532 nm and 355 nm were obtained during nighttime, high‐gain operation. These LITE data provide a detailed global view of the vertical structure and optical properties of the stratospheric aerosols. The data are also used to study the transport processes influencing the aerosol spatial distribution. LITE data compare well with measurements made by the Stratospheric Aerosol and Gas Experiment (SAGE) II. Individual profile comparisons and comparisons of more global features reinforce and extend the validation of the LITE stratospheric data. LITE demonstrates that a spaceborne lidar, with its high vertical resolution and global coverage, is a powerful tool for tracing atmospheric transport.
Long‐term measurements of alkyl nitrates in southern Germany: 1. General behavior and seasonal and diurnal variation
Authors: Flocke Frank , Volz‐Thomas Andreas , Buers Hermann‐Josef , Pätz Werner , Garthe Hans‐Jürgen , Kley Dieter
Published: February 2004

Continuous measurements of alkyl nitrates were made during the Tropospheric Ozone Research subproject of the European Experiment on Transport and Transformation of Environmentally Relevant Trace Constituents in the Troposphere Over Europe at the Schauinsland station in the Black Forest between June 1990 and May 1991, using an automated gas chromatograph with a NOy detector. More than 2000 samples were analyzed with a sampling frequency of six to nine samples per day. The mixing ratio of the sum of alkyl nitrates (C1‐C8) ranged between 30 and 630 parts per trillion (ppt) and averaged 120 ppt over the whole measurement period. The average contribution of alkyl nitrates to total odd nitrogen was about 3%, with little seasonal variation. The most abundant individual nitrates were 2‐propyl, 2‐butyl, and methyl nitrate. When lumped by carbon number, the sum of C5 nitrates represented the largest fraction after methyl nitrate. The mixing ratios of nitrates ≥C7 were very small. In polluted air masses, the mixing ratios of alkyl nitrates exhibited summer maxima and pronounced daily variations, with maxima in the early evening. The abundance increased from C3 to C5. In clean, photochemically aged air masses, alkyl nitrate mixing ratios showed a summer minimum and no pronounced diurnal variations. A decrease of abundance from C3 toward higher carbon chain lengths was observed, as is expected for air that is characterized by photochemical destruction. The measured concentration ratios between different alkyl nitrate isomers are utilized to resolve discrepancies in the existing data on branching ratios for the formation of alkyl nitrates from the reaction of peroxy radicals with NO and to classify air masses with respect to photochemical age. The observed behavior and mixing ratios of methyl and ethyl nitrate cannot be explained by formation solely from the reaction of peroxy radicals and NO.
Interannual variations in satellite‐sensed vegetation index data from 1981 to 1991
Authors: Myneni R. B. , Tucker C. J. , Asrar G. , Keeling C. D.
Published: February 2004

Normalized difference vegetation index (NDVI) data processed from measurements of advanced very high resolution radiometers (AVHRR) onboard the afternoon‐viewing NOAA series satellites (NOAA 7, 9, and 11) were analyzed for spatial and temporal patterns comparable to those observed in atmospheric CO2, near‐surface air temperature, and sea surface temperature (SST) data during the 1981–1991 time period. Two global data sets of NDVI were analyzed for consistency: (1) the land segment of the joint NOAA/NASA Earth Observing System AVHRR Pathfinder data set and (2) the Global Inventory Monitoring and Modeling Studies AVHRR NDVI data set. The impact of SST events was found to be confined mostly to the tropical latitudes but was generally dominant enough to be manifest in the global NDVI anomaly. The vegetation index anomalies at latitudes north of 45°N were found to exhibit an increasing trend. This linear trend corresponds to a 10% increase in seasonal NDVI amplitude over a 9 year period (1981–1990). During the same time period, annual amplitude in the record of atmosphere CO2 measured at Point Barrow, Alaska, was reported to have increased by about 14%. The increase in vegetation index data between years was especially consistent through the spring and early summer time periods. When this increase was translated into an advance in the timing of spring green‐up, the measure (8±3 days) was similar to the recently published estimate of about 7 days in the advance of the midpoint of CO2 drawdown between spring and summer at Point Barrow, Alaska. The geographical distribution of the increase in vegetation activity was consistent with the reported patterns in springtime warming and decline of snow cover extent over the northern hemisphere land area.
Particle microphysics and chemistry in remotely observed mountain polar stratospheric clouds
Authors: Carslaw K. S. , Wirth M. , Tsias A. , Luo B. P. , Dörnbrack A. , Leutbecher M. , Volkert H. , Renger W. , Bacmeister J. T. , Peter T.
Published: February 2004

Polar stratospheric clouds (PSCs) at 22–26 km were observed over the Norwegian mountains by airborne lidar on January 15, 1995. Simulations using a mesoscale model reveal that they were caused by mountain‐induced gravity waves. The clouds had a highly detailed filamentary structure with bands as thin as 100 m in the vertical, and moved insignificantly over 4 hours, suggesting them to be quasi‐stationary. The aircraft flight path was parallel or close to parallel with the wind at cloud level. Such a quasi‐Lagrangian observation, together with the presence of distinct aerosol layers, allows an air parcel trajectory through the cloud to be constructed and enables the lidar images to be simulated using a microphysical box model and light scattering calculations. The results yield detailed information about particle evolution in PSCs and suggest that water ice nucleated directly from liquid HNO3/H2SO4/H2O droplets as much as 4 K below the ice frost point. The observation of solid nitric acid hydrate particles downwind of the mountains shows that such mesoscale events can generate solid PSC particles that can persist on the synoptic scale. We also draw attention to the possible role of mesoscale PSCs in chlorine activation and subsequent ozone destruction.
Latitude dependence of the quasi‐biennial oscillation and quasi‐triennial oscillation characteristics of total ozone measured by TOMS
Authors: Kane R. P. , Sahai Y. , Casiccia C.
Published: February 2004

The 12‐month moving averages of total ozone mapping spectrometer (TOMS) total ozone data for 14 years (1979–1992) were examined for quasi‐biennial oscillation (QBO) and quasi‐triennial oscillation (QTO) and compared with stratospheric low‐latitude zonal wind and equatorial eastern Pacific sea surface temperature (SST). The equatorial ozone had a strong QBO with a period of ∼30 months, and its maxima tallied with westerly wind maxima. At other latitudes the ozone maxima spacings were often different from 30 months, more so in the northern hemisphere. A spectral analysis showed that both hemispheres had one peak at ∼20 months and another peak at ∼30 months, only up to ∼50° latitude. At higher latitudes these peaks shifted to ∼23 and ∼36 months. For 0°–50° the ∼30‐month periodicity in the northern hemisphere showed abrupt phase changes, while the southern latitudes showed a roughly gradual phase shift. The northern hemisphere had an additional periodicity at ∼4 years, roughly matching the SST. Analysis of the same data without 12‐month averaging showed the above characteristics more clearly and revealed additional peaks at ∼0.7 years (8–9 months) and ∼1.30 years (16–17 months). All of these peaks showed shifts (mostly increases) at higher latitudes.
Isotope analysis based source identification for atmospheric CH4 and CO sampled across Russia using the Trans‐Siberian railroad
Authors: Bergamaschi P. , Brenninkmeijer C. A. M. , Hahn M. , Röckmann T. , Scharffe D. H. , Crutzen P. J. , Elansky N. F. , Belikov I. B. , Trivett N. B. A. , Worthy D. E. J.
Published: February 2004

The isotopic composition of carbon monoxide (13C, 14C, 18O) and methane (13C, D) was measured on air samples collected between Vladivostok and Moscow using the Trans‐Siberian railroad during August 1996. Apart from short term fluctuations in the direct vicinity of sources, continuous measurements of CO and CH4 showed sustained, elevated mixing ratios over several hundreds of kilometers indicating the large scale influencing of traversed air masses by significant sources. Persistent, enhanced CH4 levels were found over the west Siberian lowlands concurrent with significantly depleted δ13C and δD values. The derived isotopic signature of the CH4 source (δ13C = −62.5±4.7‰ V‐PDB; δD = −311±14‰ V‐SMOW) clearly indicates the dominance of biogenic CH4, with the west Siberian wetlands being the most likely candidate. A second major feature in the data set is the enormous enhancement of CO (up to 1500 nmol/mol) east of Chita, extending over a 2000 km section along the river Amur. The 14CO measurements and back trajectory analyses identify biomass burning as the origin of the highly elevated CO. This is further supported by the δ18O(CO) and the δ13C/δD signature of the accompanying moderate CH4 enhancement.
The physicochemical structure of the Greenland summer aerosol and its relation to atmospheric processes
Authors: Kerminen Veli‐Matti , Hillamo Risto E. , Mäkelä Timo , Jaffrezo Jean‐Luc , Maenhaut Willy
Published: February 2004

Relationships between size‐resolved aerosol chemistry and various atmospheric processes were studied at Summit over the Greenland ice sheet. The Summit summer aerosol displayed an Aitken mode below 0.1 μm aerodynamic particle diameter, one or two modes in the accumulation size range (0.1–1 μm), and a supermicron mode. The Aitken mode contained little particulate mass (<5%) but accounted for most of the particle number concentration. The accumulation size range frequently had two overlapping modes with a minimum around 0.4 μm. A potential reason for this bimodality is fog processing which occurs frequently over the ice sheet during summer nights. Most of the particulate sulfate, ammonium, methane sulfonic acid (MSA), and dicarboxylic were found in the accumulation size range, where they displayed quite a similar modal structure, suggestive of an internal mixture of these compounds in submicron particles. The ratio of MSA to sulfate varied with particle size over the accumulation size range, so that different deposition pathways are likely to cause different MSA to sulfate ratios in the surface snow. Less than 20% of particulate sulfate and >95% of particulate nitrate were usually found in supermicrometer particles. It is likely that the supermicron sulfate and nitrate are produced when SO2 and HNO3 react with particles of mostly crustal origin. Supermicron sulfate centered at a somewhat smaller size (<2 μm) than nitrate (2–3 μm). The particulate phase contained only minor amounts of semi volatile acidic compounds, except when the ice sheet was impacted by apparent biomass burning plumes. These plumes are characterized by elevated levels of many low‐volatility compounds, and they may contain submicron ammonium nitrate and ammonium formate.
Local closure during the First Aerosol Characterization Experiment (ACE 1): Aerosol mass concentration and scattering and backscattering coefficients
Authors: Quinn P. K. , Coffman D. J.
Published: February 2004

The radiative effects of tropospheric aerosol particles are a complex function of the chemical, physical, and optical properties of the aerosol. Closure experiments provide a means for identifying and reducing uncertainties associated with these aerosol properties and hence with the estimation of aerosol radiative forcing. In a closure experiment, an aerosol property is measured by one or more methods and calculated from a model that is based on other independently measured properties. A comparison of the measured and calculated values can reveal inadequacies in either the measurements or the model. A goal of the Aerosol Characterization Experiments (ACE) is to reduce the uncertainty associated with estimating aerosol radiative forcing through both local and column closure experiments. The remote marine aerosol encountered during ACE 1 was well suited for such closure studies because of its relatively simple chemical composition. Local closure experiments were conducted on board the NOAA RV Discoverer focusing on the aerosol mass concentration and scattering and backscattering coefficients. Aerosol mass was determined by gravimetric analysis, ion chromatography, and by converting the number size distribution to a mass distribution. Scattering and backscattering coefficients were measured with an integrating nephelometer and calculated from a Mie scattering model. The different measures of mass agreed for both the submicron and supermicron aerosol indicating that within experimental uncertainty, the aerosol was composed entirely of ionic species and associated water mass. Measured and calculated values of scattering and backscattering coefficients for submicron aerosol agreed within experimental uncertainty. There was an offset, however, between the Mie‐calculated true values and the measured values that is not explained by nonidealities of the nephelometer response. Closure was not obtained for scattering or backscattering in the supermicron size range due to inadequacies of the techniques used to measure the size distribution and scattering in this size range. Both the mass and the scattering closure experiments indicate a need for an improved understanding of the response of aerosol growth, mixing state, and scattering to changes in relative humidity.
The Phanerozoic impact cratering rate: Evidence from the farside of the Moon
Authors: McEwen Alfred S. , Moore Jeffrey M. , Shoemaker Eugene M.
Published: February 2004

The relatively recent (<1 b.y.) flux of asteroids and comets forming large craters on the Earth and Moon may be accurately recorded by craters with bright rays on the Moon's farside. Many previously unknown farside rayed craters are clearly distinguished in the low‐phase‐angle images returned by the Clementine spacecraft. Some large rayed craters on the lunar nearside are probably significantly older than 1 Ga; rays remain visible over the maria due to compositional contrasts long after soils have reached optical maturity. Most of the farside crust has a more homogeneous composition and only immature rays are visible. The size‐frequency distribution of farside rayed craters is similar to that measured for Eratosthenian craters (up to 3.2 b.y.) at diameters larger than 15 km. The areal density of farside rayed craters matches that of a corrected tabulation of nearside Copernican craters. Hence the presence of bright rays due to immature soils around large craters provides a consistent time‐stratigraphic basis for defining the base of the Copernican System. The density of large craters less than ∼3.2 b.y. old is ∼3.2 times higher than that of large farside rayed craters alone. This observation can be interpreted in two ways: (1) the average cratering rate has been constant over the past 3.2 b.y. and the base of the Copernican is ∼1 Ga, or (2) the cratering rate has increased in recent geologic time and the base of the Copernican is less than 1 Ga. We favor the latter interpretation because the rays of Copernicus (800–850 m.y. old) appear to be very close to optical maturity, suggesting that the average Copernican cratering rate was ∼35% higher than the average Eratosthenian rate. Other lines of evidence for an increase in the Phanerozoic (545 Ga) cratering rate are (1) the densities of small craters superimposed on Copernicus and Apollo landing sites, (2) the rates estimated from well‐dated terrestrial craters (≤120 m.y.) and from present‐day astronomical observations, and (3) the Proterozoic rate suggested by the crater record of Australia. The hypothesis most consistent with several key observations is that the cratering rate has increased by ∼2x during the past ∼300 m.y‥
Volumes of lunar lava ponds in South Pole‐Aitken and Orientale Basins: Implications for eruption conditions, transport mechanisms, and magma source regions
Authors: Yingst R. Aileen , Head James W.
Published: February 2004

In an effort to characterize individual eruptive phases and events, 86 isolated mare deposits (ponds) in the lunar South Pole‐Aitken and Orientale regions were analyzed to obtain information on areas, volumes, and other characteristics. Deposits likely to represent single eruptive episodes have area mean values of ∼2000 km2 in the South Pole‐Aitken Basin and ∼1100 km2 in the Orientale Basin. Pond volumes range from 35 to 8745 km3, with a mean value of 860 km3 for South Pole‐Aitken, and 10 to 1280 km3, with a mean value of 240 km3 for the Orientale region. No evidence was found for shallow crustal magma reservoirs. The relatively common occurrence of sinuous rilles in Orientale is consistent with very high effusion rates, and the large volumes of individual eruptive episodes (tens to many hundreds of km3) are comparable to flood basalt eruption volumes on Earth. Pond morphologies are consistent with extrusion from deep, probably subcrustal reservoirs. Distribution of deposits suggests that many ponds may be derived from single reservoirs. Comparison of ponds in both basins shows a higher areal density and average volume of lava ponds in the South Pole‐Aitken basin relative to the Orientale area. This is plausibly attributed to the extreme depths of the South Pole‐Aitken basin and the correspondingly thinner crust there relative to the Orientale region. These observations are consistent with magma ascent and eruption mechanisms that are strongly dependent on the overpressurization of deep‐seated source regions, the subsequent propagation of dikes, and the thickness of the intervening lunar crust through which these dikes must rise.
Model near‐infrared optical constants of olivine and pyroxene as a function of iron content
Authors: Lucey Paul G.
Published: February 2004

The imaginary coefficients of the complex index of refraction of olivine, low‐Ca orthopyroxene, and high‐Ca clinopyroxene are derived as a function of iron content and wavelength over the spectral region 0.4 to 2.5 μm from reflectance spectra of powders and Hapke radiative transfer theory. Reflectance spectra as a function of composition and grain size are computed and compared. I find that while spectral effects of variations in these parameters are similar, they are distinguishable. Variations in iron content do not explain all the observed spectral variations in the spectra of the samples investigated.
Sputtering of the atmosphere of Mars: 1. Collisional dissociation of CO2
Authors: Johnson R. E. , Liu M.
Published: February 2004

Ejection of atoms and molecules from the atmosphere of Mars is caused by energetic O+ pickup‐ion bombardment [Luhmann et al., 1992; Jakosky et al., 1994], a process referred to as atmospheric sputtering. In an atmosphere which is primarily a mix of O and CO2, collisional dissociation of CO2 may have a large effect on the sputtering efficiency [Kass and Yung, 1995, 1996]. In this paper, we calculate dissociation cross sections for O + CO2, O + CO, CO + CO2, and CO2 + CO2 collisions in the energy range 20 eV to 1 keV. Semiempirical interaction potentials are used from which cross sections are calculated via classical molecular dynamics. The O + CO2 dissociation cross sections and the energy transfer to the fragments are compared to that calculated using the binary encounter approximation (BEA) [e.g., Sieveka and Johnson, 1984] and to the dissociation cross section model used by Kass and Yung [1995] in their calculation of atmospheric loss from Mars. The former are useful well above the dissociation threshold energy, whereas the latter significantly overestimate dissociation at all energies. Modifications of the BEA model for atom‐molecule collisions and that of Kass and Yung for molecular‐molecular collisions are suggested. The cross sections created here will be used in a Monte Carlo calculation of the atmospheric sputtering rate.
Sputter contribution to the atmospheric corona on Mars
Authors: Johnson R. E. , Luhmann J. G.
Published: February 2004

Pickup‐ion‐induced sputtering, a potentially important nonthermal loss mechanism for the atmosphere of Mars, will also increase the content of the oxygen corona. This atmospheric sputtering process is shown to provide a small fraction of the corona in the present epoch but is equivalent to or dominates the dissociative recombination contribution for the earlier epochs modeled by Zhang et al. [1993]. The addition of sputtered atoms to the corona initiates feedback processes which can enhance the atmospheric sputtering rate in the present epoch but limits it in the earliest epoch considered. It is also shown that the sputter contribution to the corona has the same dependence on altitude as the dissociative recombination component and it may be a significant contribution during contemporary solar maximum conditions. The possible detection of the sputter‐produced component of atomic O in the corona during solar maximum conditions may be the most promising way of confirming the significance of this nonthermal loss mechanism at Mars.