bluuebunny/crossref_metadata_2025 huggingface dataset:
Seasonal snow cover on frozen ground: Energy balance calculations of a permafrost site near Ny‐Ålesund, Spitsbergen
Authors: Boike Julia , Roth Kurt , Ippisch Olaf
Published: May 2003

We apply an energy balance model to the snow cover for snowpack accumulation and ablation at a continuous permafrost site on Spitsbergen for the snow‐covered periods from fall 1998 to winter 2000. The model includes net radiative, turbulent, ground, snow, and rain heat flux. The balance yields two distinct types of snow ablation: winter and spring ablation. Energy transferred by sensible heat and rain input reduces the snow cover during the winter, creating internal ice lenses and basal ice. The snowpack ablates during spring in two stages in both years. During the first stage, surface melt and subsequent internal freezing compact and reduce the snow cover, but no runoff is produced. This phase lasts more than twice as long as the second stage. During the second stage, which takes 14 days in both years, melt rates from the snowpack are represented well using the energy balance model. Ground heat fluxes are comparable during spring in both years, but the long persistence of the snow cover in 2000 delays the thawing of the ground. Due to the duration of the snow cover during spring snow melt of both years, the total energy supplied to the ground is significant, between 30 and 50% of the total energy supplied by net radiation.
Vegetation, plant biomass, and net primary productivity patterns in the Canadian Arctic
Authors: Gould W. A. , Raynolds M. , Walker D. A.
Published: May 2003

We have developed maps of dominant vegetation types, plant functional types, percent vegetation cover, aboveground plant biomass, and above and belowground annual net primary productivity for Canada north of the northern limit of trees. The area mapped covers 2.5 million km2 including glaciers. Ice‐free land covers 2.3 million km2 and represents 42% of all ice‐free land in the Circumpolar Arctic. The maps combine information on climate, soils, geology, hydrology, remotely sensed vegetation classifications, previous vegetation studies, and regional expertise to define polygons drawn using photo‐interpretation of a 1:4,000,000 scale advanced very high resolution radiometer (AVHRR) color infrared image basemap. Polygons are linked to vegetation description, associated properties, and descriptive literature through a series of lookup tables in a graphic information systems (GIS) database developed as a component of the Circumpolar Arctic Vegetation Map (CAVM) project. Polygons are classified into 20 landcover types including 17 vegetation types. Half of the region is sparsely vegetated (<50% vegetation cover), primarily in the High Arctic (bioclimatic subzones A–C). Whereas most (86%) of the estimated aboveground plant biomass (1.5 × 1015 g) and 87% of the estimated above and belowground annual net primary productivity (2.28 × 1014 g yr−1) are concentrated in the Low Arctic (subzones D and E). The maps present more explicit spatial patterns of vegetation and ecosystem attributes than have been previously available, the GIS database is useful in summarizing ecosystem properties and can be easily updated and integrated into circumpolar mapping efforts, and the derived estimates fall within the range of current published estimates.
Southern Hemisphere Additional Ozonesondes (SHADOZ) 1998–2000 tropical ozone climatology 1. Comparison with Total Ozone Mapping Spectrometer (TOMS) and ground‐based measurements
Authors: Thompson Anne M. , Witte Jacquelyn C. , McPeters Richard D. , Oltmans Samuel J. , Schmidlin Francis J. , Logan Jennifer A. , Fujiwara Masatomo , Kirchhoff Volker W. J. H. , Posny Françoise , Coetzee Gert J. R. , Hoegger Bruno , Kawakami Shuji , Ogawa Toshihiro , Johnson Bryan J. , Vömel Holger , Labow Gordon
Published: May 2003

A network of 10 southern hemisphere tropical and subtropical stations, designated the Southern Hemisphere Additional Ozonesondes (SHADOZ) project and established from operational sites, provided over 1000 ozone profiles during the period 1998–2000. Balloon‐borne electrochemical concentration cell (ECC) ozonesondes, combined with standard radiosondes for pressure, temperature, and relative humidity measurements, collected profiles in the troposphere and lower to midstratosphere at: Ascension Island; Nairobi, Kenya; Irene, South Africa; Réunion Island; Watukosek, Java; Fiji; Tahiti; American Samoa; San Cristóbal, Galapagos; and Natal, Brazil. The archived data are available at: 〈http://croc.gsfc.nasa.gov/shadoz〉.1 In this paper, uncertainties and accuracies within the SHADOZ ozone data set are evaluated by analyzing: (1) imprecisions in profiles and in methods of extrapolating ozone above balloon burst; (2) comparisons of column‐integrated total ozone from sondes with total ozone from the Earth‐Probe/Total Ozone Mapping Spectrometer (TOMS) satellite and ground‐based instruments; and (3) possible biases from station to station due to variations in ozonesonde characteristics. The key results are the following: (1) Ozonesonde precision is 5%. (2) Integrated total ozone column amounts from the sondes are usually to within 5% of independent measurements from ground‐based instruments at five SHADOZ sites and overpass measurements from the TOMS satellite (version 7 data). (3) Systematic variations in TOMS‐sonde offsets and in ground‐based‐sonde offsets from station to station reflect biases in sonde technique as well as in satellite retrieval. Discrepancies are present in both stratospheric and tropospheric ozone. (4) There is evidence for a zonal wave‐one pattern in total and tropospheric ozone, but not in stratospheric ozone.
Polar stratospheric cloud microphysical properties measured by the microRADIBAL instrument on 25 January 2000 above Esrange and modeling interpretation
Authors: Brogniez C. , Huret N. , Eckermann S. , Rivière E. D. , Pirre M. , Herman M. , Balois J.‐Y. , Verwaerde C. , Larsen N. , Knudsen B.
Published: May 2003

The balloonborne microRADIBAL instrument is a radiometer that measures the radiance and polarization of the sunlight scattered by the atmosphere, gas, and aerosols in a horizontal plane in the near‐infrared range. It was launched from Esrange, Sweden, on 25 January 2000 in the framework of the Third European Stratospheric Experiment on Ozone (THESEO) 2000 campaign, and performed measurements in the vicinity of a large polar stratospheric cloud (PSC). The measurements provide diagrams of the radiance versus scattering angle at several altitudes. The aerosol signature, derived from the radiance measurements, has been modeled via Mie theory and the T‐Matrix code. Three different size distributions of aerosols have been tested: monomodal and bimodal size distributions of spherical particles, and bimodal size distributions including a mode of spherical and a mode of nonspherical particles. The best agreement between the measured and modeled signatures is obtained considering a bimodal size distribution composed by a mode of medium spherical particles (median radius about 0.15 μm) and a second mode of larger nonspherical particles (median radius about 1.1 μm, aspect ratio about 0.6). Concentrations and surface densities of the PSC particles have been estimated. The existence of such particles has been tentatively explained using the Lagrangian Microphysical and Photochemical Lagrangian Stratospheric Model of Ozone (MiPLaSMO) model. On 25 January 2000 the polar stratospheric cloud detected by microRADIBAL is associated with a lee‐wave event. Temperature perturbations due to lee‐wave events were calculated using the National Research Laboratory Mountain Wave Forecast Model (MWFM) and have been included along trajectories. They are localized in a large region between the Norwegian mountains and Esrange. Their amplitude varies from 3 to 7 K. Detailed comparisons between measured and modeled surfaces and dimensional distributions of PSCs' particles are achieved. The two modes of particles detected by microRADIBAL can be interpreted from MiPLaSMO results considering different air masses located along the lines of sight. The air masses are characterized by two different temperature perturbations due to lee‐wave events. With a small temperature perturbation (∼3 K) that occurred just before the time of the measurement, supercooled ternary solution particles are predicted, and with a strong temperature perturbation (∼6 K) that occurred four hours before the measurement, nitric acid trihydate particles are formed.
Nonorographic generation of Arctic polar stratospheric clouds during December 1999
Authors: Hitchman Matthew H. , Buker Marcus L. , Tripoli Gregory J. , Browell Edward V. , Grant William B. , McGee Thomas J. , Burris John F.
Published: May 2003

During December 1999, polar stratospheric clouds (PSCs) were observed in the absence of conditions conducive to generation by topographic gravity waves. The possibility is explored that PSCs can be generated by inertia gravity waves (IGW) radiating from breaking synoptic‐scale Rossby waves on the polar front jet. The aerosol features on 7 and 12 December are selected for comparison with theory and with simulations using the University of Wisconsin Nonhydrostatic Modeling System (UWNMS). Consistent with Rossby adjustment theory, a common feature in the UWNMS simulations is radiation of IGW from the tropopause polar front jet, especially from sectors which are evolving rapidly in the Rossby wave breaking process. Packets of gravity wave energy radiate upward and poleward into the cold pool, while individual wave crests propagate poleward and downward, causing mesoscale variations in vertical motion and temperature. On 12 December the eastbound DC‐8 lidar observations exhibited a fairly uniform field of six waves in aerosol enhancement in the 14–20 km layer, consistent with vertical displacement by a field of IGW propagating antiparallel to the flow, with characteristic horizontal and vertical wavelengths of ∼300 and ∼10 km. UWNMS simulations show emanation of a field of IGW upward and southwestward from a northward incursion of the polar front jet. The orientation and evolution of the aerosol features on 7 December are consistent with a single PSC induced by an IGW packet propagating from a breaking Rossby wave over western Russia toward the northeast into the coldest part of the base of the polar vortex, with characteristic period ∼9 hours, vertical wavelength ∼12 km, and horizontal wavelength ∼1000 km. Linear theory shows that for both of these cases, IGW energy propagates upward at ∼1 km/hour and horizontally at ∼100 km/hour, with characteristic trace speed ∼30 m/s. The spatial orientation of the PSC along IGW phase lines is contrasted with the nearly horizontal filamentary structures in the PSC, which are indicative of flow streamlines. It is suggested that vertical displacement is a crucial factor in determining whether a PSC will form and that most PSCs are relatable to specific synoptic and mesoscale motions.
Urban/industrial pollution for the New York City–Washington, D. C., corridor, 1996–1998: 1. Providing independent verification of CO and PCE emissions inventories
Authors: Barnes Diana H. , Wofsy Steven C. , Fehlau Brian P. , Gottlieb Elaine W. , Elkins James W. , Dutton Geoffrey S. , Montzka Stephen A.
Published: May 2003

Atmospheric mixing ratios of carbon monoxide (CO) and perchloroethylene (PCE, C2Cl4) were measured above the canopy at Harvard forest, MA every half‐hour for 3 years starting in January 1996. Pollution enhancements are strongly correlated with winds from the southwest, the direction of the New York City–Washington, D. C., corridor, as compared to background levels observed during northwest winds traveling from Canada. We establish the ratio of CO to PCE pollution enhancements by wind direction, by season, and by year and use these results to test the quality of county‐level and national source emission inventories for these two gases. The EPA carbon monoxide emission county‐level inventories and the McCulloch and Midgley sales‐based national‐level PCE release estimates are found to be in accord with our independent observations of urban/industrial releases. For the New York City–Washington, D. C., corridor the inventory‐based COI/PCEI emissions ratio of 584 (kg/kg) for 1996 falls well within the range of observationally‐based ΔCO/ΔPCE pollution plume ratios of 388 to 706 (kg/kg) and is only 11% higher than the observed mean of 521 ± 90 (kg/kg). On the basis of this agreement, PCE emission estimates for 1997 and 1998 are derived from the CO inventory emissions values and the observed ΔCO/ΔPCE ratios in pollution plumes for those years; despite the call for voluntary cutbacks, urban/industrial emissions of PCE appear to be on the rise.
Impact of land use on Costa Rican tropical montane cloud forests: Sensitivity of cumulus cloud field characteristics to lowland deforestation
Authors: Nair U. S. , Lawton R. O. , Welch R. M. , Pielke R. A.
Published: May 2003

Recent studies have shown that there has been a reduction in dry season moisture input from direct interception of cloud water and wind‐blown mist at the lee edge of the Monteverde cloud forest, Costa Rica, since the mid 1970s. This reduction of moisture could be responsible for the population crashes of anurans observed in the region. It has been hypothesized that this behavior is a result of increases in cloud base height, linked to increased sea surface temperatures. In this study we present a complementary hypothesis, that deforestation upwind of the Monteverde cloud forest preserve is responsible for the observed changes in cloud base height. An automated cumulus cloud classification scheme extracts monthly spatial maps of the frequency of occurrence of cumulus cloudiness over Costa Rica from GOES 8 visible channel satellite imagery. We find that cumulus cloud formation in the morning hours over deforested regions is suppressed compared to forested areas. The degree of suppression appears to be related to the extent of deforestation. This difference in cloud formation between forested and deforested areas is a clear signal of land use change influencing the regional climate. Regional Atmospheric Modeling System numerical modeling simulations are used to explore the differences in cloud field characteristics over the lowland pasture and forest landscapes. Statistically significant differences in cloud base height and cloud thickness occur between the forest and pasture simulations. Clouds have higher base heights and are thinner over pasture landscapes than over forested ones. On the other hand, these simulations show no statistically significant differences in cloud top heights, cloud cover, mean cloud water mixing ratio, or cloud liquid water path between pasture and forest simulations. However, in the simulations there are enhanced sensible heat fluxes and reduced latent heat fluxes over pasture compared to forest. It is the drier and warmer air over pasture surfaces that results in the formation of elevated thinner clouds. This study suggests that deforestation results in warmer, drier air upwind of the Monteverde cloud forests and that this could influence the base height of orographic cloudbanks crucial to the region during the dry season.
In situ mountain‐wave polar stratospheric cloud measurements: Implications for nitric acid trihydrate formation
Authors: Voigt Christiane , Larsen Niels , Deshler Terry , Kröger Chris , Schreiner Jochen , Mauersberger Konrad , Luo Beiping , Adriani Alberto , Cairo Francesco , Di Donfrancesco Guido , Ovarlez Joelle , Ovarlez Henri , Dörnbrack Andreas , Knudsen Bjørn , Rosen Jim
Published: May 2003

Particle size distribution, composition, and optical properties of polar stratospheric clouds (PSCs) have been measured above northern Scandinavia during a nocturnal balloon flight within the polar vortex on 19 January 2000. The mountain‐wave PSC mainly consisted of nitric acid trihydrate (NAT) particles with number densities between 0.01 and 0.2 cm−3, median radii of 1 to 2 μm and volumes up to 1 μm3cm−3. A comparison between optical PSC data and optical simulations based on the measured particle size distribution indicates that the NAT particles were aspherical with an aspect ratio of 0.5. The NAT particle properties have been compared to another PSC observation on 25 January 2000, where NAT particle number densities were about an order of magnitude higher. In both cases, microphysical modeling indicates that the NAT particles have formed on ice particles in the mountain‐wave events. Differences in the NAT particle number density can be explained by the meteorological conditions. We suggest that the higher NAT number density on 25 January can be caused by stronger wave activity observed on that day, larger cooling rates and therefore higher NAT supersaturation.
Discrete measurements of reactive gases and fine particle mass and composition during the 1999 Atlanta Supersite Experiment
Authors: Baumann Karsten , Ift Frank , Zhao Jing Z. , Chameides William L.
Published: May 2003

During the Atlanta Supersite Experiment (ASSE) in August 1999, a three‐channel particle composition monitor (PCM) was used to measure PM2.5 mass and composition as well as gas‐phase NH3, HCl, HNO3, HONO, SO2, and acetic, formic, and oxalic acids over integrating periods of 10–24 hours. The period was characterized by stagnant periods associated with high temperatures, relative humidities, and UV radiation, underlying parameters causing photochemical activity with intense ozone, and PM2.5 formation. Analysis of diurnal measurements points to photochemical sources of HNO3, HONO, acetic, and formic acids. NO2 denuder artifact reactions are assessed, and it is found that the secondary heterogeneous O3 reaction potentially overcorrects the nitrite to nitrate oxidation step on the denuder walls, underestimating [HNO3] and correspondingly overestimating [HONO]. The high ambient humidity caused gravimetric mass to be systematically high by about 20 ± 8%, which was interpreted as hydrate artifact and corrected for. Despite this correction, and inclusion of other organic elements (OOE) (OOE = 0.6 * organic carbon (OC)), an average percent fraction unidentified mass of 13 ± 10% relative to the total reported mass concentration still remained. However, if semivolatile OC collected on an XAD‐coated quartz backup filter placed downstream of an XAD‐denuded quartz front filter are included, an almost complete closure in the mass balance is achieved. The possible use of different OOE factors for OC and semivolatile OC (here 0.6 and 0.4, respectively) is also investigated, taking into account the different physical properties of carbon species collected from quartz front and XAD‐coated quartz backup filters. It can be speculated that photochemically well‐aged and well‐mixed air masses contain organic compounds with more highly oxygenated and less volatile functional groups, whereas under more stagnant conditions, particle‐phase organics might be less oxygenated and therefore more volatile. Sulfate and all organic compounds (including particle bound light organic acids, organic carbon, and OOE) are the main identified contributors to overall PM2.5 mass with 32 ± 8 and 39 ± 12%, respectively. Acidity calculations based on the particle‐phase SO42−/NO3−/NH4+ system show slightly acidic conditions.
Intercomparison of near real time monitors of PM2.5 nitrate and sulfate at the U.S. Environmental Protection Agency Atlanta Supersite
Authors: Weber R. , Orsini D. , Duan Y. , Baumann K. , Kiang C. S. , Chameides W. , Lee Y. N. , Brechtel F. , Klotz P. , Jongejan P. , ten Brink H. , Slanina J. , Boring C. B. , Genfa Z. , Dasgupta P. , Hering S. , Stolzenburg M. , Dutcher D. D. , Edgerton E. , Hartsell B. , Solomon P. , Tanner R.
Published: May 2003

Five new instruments for semicontinuous measurements of fine particle (PM2.5) nitrate and sulfate were deployed in the Atlanta Supersite Experiment during an intensive study in August 1999. The instruments measured bulk aerosol chemical composition at rates ranging from every 5 min to once per hour. The techniques included a filter sampling system with automated water extraction and online ion chromatographic (IC) analysis, two systems that directly collected particles into water for IC analysis, and two techniques that converted aerosol nitrate or sulfate either catalytically or by flash vaporization to gaseous products that were measured with gas analyzers. During the one‐month study, 15‐min integrated nitrate concentrations were low, ranging from about 0.1 to 3.5 μg m−3 with a mean value of 0.5 μg m−3. Ten‐minute integrated sulfate concentrations varied between 0.3 and 40 μg m−3 with a mean of 14 μg m−3. By the end of the one‐month study most instruments were in close agreement, with r‐squared values between instrument pairs typically ranging from 0.7 to 0.94. Based on comparison between individual semicontinuous devices and 24‐hour integrated filter measurements, most instruments were within 20–30% for nitrate (∼0.1–0.2 μg m−3) and 10–15% for sulfate (1–2 μg m−3). Within 95% confidence intervals, linear regression fits suggest that no biases existed between the semicontinuous techniques and the 24‐hour integrated filter measurements of nitrate and sulfate;, however, for nitrate, the semicontinuous intercomparisons showed significantly less variability than intercomparisons amongst the 24‐hour integrated filters.
Galactic cosmic ray and El Niño–Southern Oscillation trends in International Satellite Cloud Climatology Project D2 low‐cloud properties
Authors: Marsh Nigel , Svensmark Henrik
Published: May 2003

The recently reported correlation between clouds and galactic cosmic rays (GCR) implies the existence of a previously unknown process linking solar variability and climate. An analysis of the interannual variability of International Satellite Cloud Climatology Project D2 (ISCCP‐D2) low‐cloud properties over the period July 1983 to August 1994 suggests that low clouds are statistically related to two processes, (1) GCR and (2) El Niño–Southern Oscillation (ENSO), with GCR explaining a greater percentage of the total variance. Areas where satellites have an unobstructed view of low cloud possess a strong correlation with GCR, which suggests that low‐cloud properties observed in these regions are less likely to be contaminated from overlying cloud. The GCR‐low cloud correlation cannot easily be explained by internal climate processes, changes in direct solar forcing, or UV‐ozone interactions. Instead, it is argued that a mechanism involving solar variability via GCR ionization of the atmosphere is consistent with these results. However, the results are marginal when including the recently extended ISCCP‐D2 data covering the period until September 2001. This, we believe, is related to problems experienced with the ISCCP intercalibration between September 1994 and January 1995.
Stratospheric influence on the northern North American free troposphere during TOPSE: 7Be as a stratospheric tracer
Authors: Dibb Jack E. , Talbot Robert W. , Scheuer Eric , Seid Garry , DeBell Linsey , Lefer Barry , Ridley Brian
Published: May 2003

We use 7Be, with HNO3 and O3, to identify air masses sampled from the NCAR C‐130 during TOPSE that retained clear evidence of stratospheric influence. A total of 43 such air masses, spread fairly evenly across the February to May sampling period, and 40°N–86°N latitude range, were encountered. South of 55°N, nearly all clear stratospheric influence was restricted to altitudes above 6 km. At higher latitudes stratospherically influenced air masses were encountered as low as 2 km. Approximately 12% of all TOPSE sampling time at altitudes above 2 km was spent in stratospherically impacted air, above 6 km this increased to more than half of the time. Because it is not certain how much of this stratospherically influenced air irreversibly injected mass (and chemical compounds) into the troposphere, we estimate the stratospheric fraction of O3 in high latitude TOPSE samples based on a linear relationship to 7Be and compare it to in situ O3. This analysis indicates that the stratospheric source can account for a dominant fraction (>85%) of in situ O3 throughout TOPSE, but that the stratospheric contribution was nearly constant through the 4 month campaign. In February and March the 7Be based estimates of stratospheric O3 account for 10–15% more O3 than was measured, but by April and May there is up to about 10% more O3 than expected from the stratospheric source. This trend suggests that a seasonal transition from O3 depletion to photochemical production in the high latitude North American troposphere is the major cause of the springtime increase in O3.
Interannual changes of active fire detectability in North America from long‐term records of the advanced very high resolution radiometer
Authors: Csiszar Ivan , Abuelgasim Abdelgadir , Li Zhanqing , Jin Ji‐zhong , Fraser Robert , Hao Wei‐Min
Published: May 2003

This paper addresses practical issues related to the processing of 1‐km National Oceanic and Atmospheric Administration (NOAA) advanced very high resolution radiometer (AVHRR) data for producing a consistent, long‐term time series of active fire locations over the Continental United States and Canada. The effects of the interannual changes in measured background temperatures, caused by the orbital drift of the afternoon NOAA satellites and by environmental factors, are investigated. Background temperature changes are analyzed using a time series of monthly mean clear‐sky brightness temperatures from the NOAA National Environmental Satellite, Data, and Information Service (NESDIS) Pathfinder Atmosphere (PATMOS) data set at a 1° × 1° resolution. Examples of target areas over four predominant land cover types, as defined in the International Geosphere‐Biosphere Programme (IGBP) global 1 km data set, are presented. The results indicate that over forests (defined as >60% tree canopy cover) the contrast between nonburning background and fire pixels is nearly always sufficient for successful fire detection. Over nonforested areas, however, the low saturation temperature of the mid‐IR channel on the NOAA 7 to NOAA 14 satellites often sets a physical limit to the separation of valid fire pixels and false ones. Moreover, the severity of this effect changes over the years with the changing background temperatures. The results suggest that because of the potential spurious trends in the number of fires, nonforested areas be excluded from the multiyear analysis. However, a detailed assessment of the emissions from nonforest fires is needed to quantify the effect of this on continental‐scale emission estimates.
Ozone, aerosol, potential vorticity, and trace gas trends observed at high‐latitudes over North America from February to May 2000
Authors: Browell Edward V. , Hair Johnathan W. , Butler Carolyn F. , Grant William B. , DeYoung Russell J. , Fenn Marta A. , Brackett Vince G. , Clayton Marian B. , Brasseur Lorraine A. , Harper David B. , Ridley Brian A. , Klonecki Andrzej A. , Hess Peter G. , Emmons Louisa K. , Tie Xuexi , Atlas Elliot L. , Cantrell Christopher A. , Wimmers Anthony J. , Blake Donald R. , Coffey Michael T. , Hannigan James W. , Dibb Jack E. , Talbot Robert W. , Flocke Frank , Weinheimer Andrew J. , Fried Alan , Wert Bryan , Snow Julie A. , Lefer Barry L.
Published: May 2003

Ozone (O3) and aerosol scattering ratio profiles were obtained from airborne lidar measurements on thirty‐eight flights over seven deployments covering the latitudes of 40°–85°N between 4 February and 23 May 2000 as part of the Tropospheric Ozone Production about the Spring Equinox (TOPSE) field experiment. Each deployment started from Broomfield, Colorado, with bases in Churchill, Canada, and on most deployments, Thule Air Base, Greenland. Nadir and zenith lidar O3 measurements were combined with in situ O3 measurements to produce vertically continuous O3 profiles from near the surface to above the tropopause. Potential vorticity (PV) distributions along the flight track were obtained from several different meteorological analyses. Ozone, aerosol, and PV distributions were used together to identify the presence of pollution plumes and stratospheric intrusions. Ozone was found to increase in the middle free troposphere (4–6 km) at high latitudes (60°–85°N) by an average of 4.6 ppbv/mo (parts per billion by volume per month) from about 54 ppbv in early February to over 72 ppbv in mid‐May. The average aerosol scattering ratios at 1064 nm in the same region increased rapidly at an average rate of 0.36/mo from about 0.38 to over 1.7. Ozone and aerosol scattering were highly correlated over the entire field experiment, and PV and beryllium (7Be) showed no significant positive trend over the same period. The primary cause of the observed O3 increase in the mid troposphere at high latitudes was determined to be the photochemical production of O3 in pollution plumes with less than 20% of the increase from stratospherically‐derived O3.
Measurements of OH and HO2 radical concentrations and photolysis frequencies during BERLIOZ
Authors: Holland Frank , Hofzumahaus Andreas , Schäfer Jürgen , Kraus Alexander , Pätz Hans‐Werner
Published: May 2003

This paper presents the measurements of OH and HO2 radical concentrations as well as photolysis frequencies of different molecules during the Berliner Ozone (BERLIOZ) field experiment in July/August 1998 at the rural site Pabstthum about 50 km NW of Berlin. Radical concentrations were measured using laser‐induced fluorescence (LIF) spectroscopy, while filter radiometers and a scanning spectroradiometer were used to obtain photolysis frequencies. The radical data set covers the time period from 20 July to 6 August and consists of more than 6000 simultaneous measurements of OH and HO2 with a typical time resolution of about 90 s. The maximum OH and HO2 daytime concentrations were 8 × 106 and 8 × 108 cm−3, respectively. While nighttime values of OH were usually below the detection limit of our instrument (3.5 × 105 cm−3), HO2 did show significant concentrations throughout most of the nights (on average 3 × 107 cm−3). The OH concentration was mainly controlled by solar UV radiation and showed a high linear correlation with J(O1D). A deviation from this general behavior was observed around dawn and dusk, when OH concentrations well above the detection limit were observed, although J(O1D) was essentially zero. A comparison with data sets from previous campaigns revealed that even though the linear correlation is found in other environments as well the slope [OH]/J(O1D) differs significantly. The diurnal cycles of HO2 were less dependent on the solar actinic flux but were predominantly influenced by NO. During episodes of high NO, HO2 remained below the detection limit (1 × 107 cm−3) but started to rise rapidly as soon as NO started to decrease.
Retrieval of cloud droplet size from visible and microwave radiometric measurements during INDOEX: Implication to aerosols' indirect radiative effect
Authors: Liu Guosheng , Shao Hongfei , Coakley James A. , Curry Judith A. , Haggerty Julie A. , Tschudi Mark A.
Published: May 2003

The effective radius of water cloud droplets is retrieved using remotely sensed passive microwave and visible data collected by aircraft during the Indian Ocean Experiment. The purpose of this study is to assess the aerosols' effect on cloud microphysical and radiative properties. To study this effect, we investigate the relationships among effective radius, liquid water path and number concentration of cloud droplets. The effective radius retrieval uses imager observations of reflected sunlight at 0.64 μm and liquid water path derived from microwave measurements. Results of an error analysis show that retrieval error is the largest for thin clouds having small visible reflectances and small liquid water path. For this reason, only pixels with visible reflectances greater than 0.2 are used in our data analysis, so that the maximum RMS error in effective radius is limited to about 4 μm. The relation between liquid water path and effective radius is examined for four different latitudinal regions. Results show that for the same liquid water path, effective radii are significantly smaller in the north than in the south, in correspondence to the north‐south gradient of aerosol concentration in this region. In situ aircraft observations reveal larger cloud droplet number concentrations in the north than in the south. The north‐south gradient of these variables are consistent with the aerosols' effect on cloud microphysics, that is, higher aerosol concentration increases the number concentration of cloud droplets, which, in turn, reduces droplet sizes given the same liquid water path and cloud thickness. Results based on comparison between data collected from northern and southern hemispheres suggest that the increase in aerosol number concentration alters cloud droplet numbers and sizes while leaving liquid water contents approximately the same.
Observations of new particle formation in urban air
Authors: Alam Aftab , Shi Ji Ping , Harrison Roy M.
Published: May 2003

Measurements are reported of particle number concentration at two urban sites within Birmingham, UK. The measurements were made with a condensation particle counter (TSI Model 3022A) and an ultrafine particle counter (TSI Model 3025) operated in tandem. Other colocated measurements included particle surface area by epiphaniometer, sulfur dioxide, oxides of nitrogen and ozone, solar radiation or JO1D, and in some campaigns, particle number size distribution by Scanning Mobility Particle Sizer. Events were identified when substantial excursions in particle number count were accompanied by a divergence between the two particle counters indicating a large number of particles in the 3–7 nm diameter range. Criteria have been developed to distinguish between particles within this size range arising from homogeneous nucleation within the atmosphere and from emissions from point sources. Out of a total of 232 days sampling distributed throughout the year, events on only 8 days could be attributed fairly unequivocally to homogeneous nucleation processes, with a further 4 days possibly attributable. Nucleation events occur in winter as well as summer months, but depend, among other things, upon a low particle surface area. Modeling of aerosol dynamical processes for 20 June 1999, indicates that the observations can be explained by a nucleation rate of 6 × 106 cm−3 s−1 at a condensable vapor concentration (as H2SO4) of 6.5 × 107 cm−3. It is inferred that both the nucleation and particle growth processes involve condensable molecules other than, or in addition to, sulfuric acid.
Seasonal variations of C2–C4 nonmethane hydrocarbons and C1–C4 alkyl nitrates at the Summit research station in Greenland
Authors: Swanson Aaron L. , Blake Nicola J. , Atlas Elliot , Flocke Frank , Blake Donald R. , Rowland F. Sherwood
Published: May 2003

We report measurements of light (C2–C4) nonmethane hydrocarbons (NMHCs) and C1–C4 alkyl nitrates made at Summit, Greenland, over a full annual cycle (June 1997–1998). The remoteness of the Summit camp from industrial source regions resulted in trends of these trace gases that showed a clear seasonal variation with low variability. Variability (calculated as the percentage 1‐sigma deviation of a running 10‐point mean) averaged 7–9% for ethane over the entire study. The shorter‐lived species (ethyne, propane, and the butanes) exhibited variability from 12–20% in winter to 20–100% in summer. The best fit curve to the annual cycle of ethane is a sinusoidal oscillation, but each of the shorter‐lived NMHCs exhibited flat periods of low concentrations during the summer months. The C2–C4 NMHCs peaked between 8 and 24 February, with the longer‐lived NMHCs maximizing latest. These data were broadly consistent with literature values, confirming that high‐latitude Northern Hemisphere (NH) emissions are similar year to year. Employing their linear fall accumulation rates, we calculated average NMHC ratios versus ethane of 0.59(±0.10):0.33(±0.05):0.26(±0.08):0.14(±0.04) for propane, ethyne, n‐butane, and i‐butane, respectively. We suggest that these ratios represent useful quantities with which to compare averaged mid‐ and high‐latitude NH emission ratios. We also report the first year‐round observations of the seasonal cycle of light C1–C4 alkyl nitrates. Similar to the NMHCs, the seasonal trend of these gases shows primary dependence on transport to Summit during winter, and photochemical removal during summer. Unlike their parent NMHCs, alkyl nitrate concentrations did not asymptote to low levels during summer and they exhibited winter maxima later with decreasing photochemical lifetime.
Dry deposition of airborne trace metals on the Los Angeles Basin and adjacent coastal waters
Authors: Lu R. , Turco R. P. , Stolzenbach K. , Friedlander S. K. , Xiong C. , Schiff K. , Tiefenthaler L. , Wang Guangyu
Published: May 2003

We present an assessment of the deposition rates of airborne trace metals onto the Los Angeles Basin and adjacent coastal waters. For this purpose, the UCLA Surface Meteorology and Ozone Generation (SMOG) air pollution modeling system has been used to simulate the geographical distributions of trace metals and their deposition fluxes. Calculations were performed for average summer and winter conditions, as well as for extreme meteorological events, in particular, for Santa Ana winds. Thus, a series of simulations were carried out that define the range of meteorological conditions contributing to dry deposition in the region. These predictions have been calibrated and validated using measurements collected in the LA area. Significant spatial and temporal variability are found in trace metal concentrations and deposition rates. Large spatial gradients occur near the coast as well as at the mountainous boundaries of the airshed. Considerable diurnal and seasonal variations in trace metal deposition are also noted. For example, the development of a daytime sea breeze, particularly in the warmer months, leads to greater deposition in the northern and eastern basin as well as in the high desert. A nighttime land breeze, especially in the colder months, enhances deposition onto coastal ocean surfaces. Large particles dominate local trace metal deposition in central urban (and adjacent) areas, while fine particles export metals over regional scales through long‐range advection. Since the majority of urban metal deposition occurs on particles larger than 10‐μm diameter, routine measurements of PM10 or PM2.5 concentrations for air quality characterization may not be reliable indicators of local sources. Some 35–45% of all trace metal emissions are deposited locally within the Los Angeles Basin on an annual basis. Santa Monica Bay and its watersheds receive about 6% of this amount, which can have a significant impact on trace metal concentrations in the surface waters of the bay, primarily through land runoff following storms.
Nitrate and oxidized organic ions in single particle mass spectra during the 1999 Atlanta Supersite Project
Authors: Lee Shan‐Hu , Murphy Daniel M. , Thomson David S. , Middlebrook Ann M.
Published: May 2003

We measured the chemical components of 0.35–2.5 μm diameter aerosols with a Particle Analysis by Laser Mass Spectrometry (PALMS) instrument during the Atlanta Supersite Project in August 1999. This paper presents the results for nitrate and oxidized organics. More than 20% of the negative ion spectra of single particles contained nitrate ion peaks. Nitrate peaks showed a clear maximum during the morning at the time of high ambient relative humidity. Nitrate peaks also had a small local maximum during the afternoon when the gas‐phase HNO3 concentrations were high. This afternoon maximum was more significant for the particles containing soot/hydrocarbons or aluminosilicate components than for the typical organic/sulfate particles. About 45% of negative spectra contained ions indicative of the oxidized organics. Oxidized organic peaks showed a diurnal variation similar to the nitrate: a morning time maximum during the relative humidity maximum and a small maximum in the afternoon. However, in contrast to the nitrate peaks, the afternoon local maximum of oxidized organic peaks was apparent on the organic/sulfate particles. Both nitrate and oxidized organic peaks had larger ion fractions in particles with higher scattered light intensities.
High temporal resolution of extreme rainfall rate variability and the acoustic classification of rainfall
Authors: Nystuen Jeffrey A. , Amitai Eyal
Published: May 2003

The underwater sound generated by raindrop splashes on a water surface is loud and unique allowing detection, classification and quantification of rainfall. One of the advantages of the acoustic measurement is that the listening area, an effective catchment area, is proportional to the depth of the hydrophone and can be orders of magnitude greater than other in situ rain gauges. This feature allows high temporal resolution of the rainfall measurement. A series of rain events with extremely high rainfall rates, over 100 mm/hr, is examined acoustically. Rapid onset and cessation of rainfall intensity are detected within the convective cells of these storms with maximum 5‐s resolution values exceeding 1000 mm/hr. The probability distribution functions (pdf) for rainfall rate occurrence and water volume using the longer temporal resolutions typical of other instruments do not include these extreme values. The variance of sound intensity within different acoustic frequency bands can be used as an aid to classify rainfall type. Objective acoustic classification algorithms are proposed. Within each rainfall classification the relationship between sound intensity and rainfall rate is nearly linear. The reflectivity factor, Z, also has a linear relationship with rainfall rate, R, for each rainfall classification.
A springtime cloud over the Beaufort Sea polynya: Three‐dimensional simulation with explicit spectral microphysics and comparison with observations
Authors: Khvorostyanov Vitaly I. , Curry Judith A. , Gultepe Ismail , Strawbridge Kevin
Published: May 2003

This paper addresses the formation of a cloud system associated with an arctic polynya in Beaufort Sea during springtime. Data were obtained as a part of the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE) Arctic Clouds Experiment from the Canadian Convair 580 aircraft during 25 April 1998. These data include in situ observations of cloud microphysics and meteorological variables and remote measurements from satellite and airborne lidar. A three‐dimensional cloud‐resolving model with explicit bin‐resolving cloud microphysics is used to simulate the atmospheric boundary layer and cloud evolution associated with the polynya. After initialization with the aircraft sounding profiles, a quasi‐steady polynya‐induced atmospheric boundary layer (ABL) and a cloud system form. Strong turbulence in the ABL occurs above the polynya, the internal thermal boundary layer (ITBL) develops and grows upward in the downwind direction, and the ABL downwind of the polynya is separated into two decoupled layers. The cloud forms in the middle of the polynya, and its upper boundary lifts downwind while the lower boundary lowers and reaches the surface beyond the polynya southern boundary as a light fog. Farther to the south, the fog evaporates and transforms into an elevated cloud plume that extends for several tens of kilometers downwind. This cloud morphology is in good agreement with the lidar observations, which showed a cloud layer extending for more than 100 km downwind, and with the numerous previous observations. The simulated microphysical properties of a mixed cloud are similar to those observed. A new ice nucleation scheme resulted in cloud plume gradual crystallization along the wind and eventual transformation into diamond dust. Detailed evaluation of the water budget, supersaturation, and crystal size spectra showed that the ice crystal supersaturation relaxation times are 10–60 min; thus deposition on the crystals is rather slow, and only 1–5% of the available vapor is deposited on the crystals. The large ice crystal supersaturation relaxation time explains the relatively slow growth and gravitational fallout of the ice crystals, as well as the extensive propagation downwind of the plume.
Transport and mixing between airmasses in cold frontal regions during Dynamics and Chemistry of Frontal Zones (DCFZ)
Authors: Esler J. G. , Haynes P. H. , Law K. S. , Barjat H. , Dewey K. , Kent J. , Schmitgen S. , Brough N.
Published: May 2003

The passage of two cold front systems over the United Kingdom are compared and contrasted, using the results of a detailed aircraft and ground‐based study. The measurements are interpreted by means of three‐dimensional, reverse‐domain‐filling trajectories using both global models and limited‐area mesoscale models. This method provides a three‐dimensional picture of the interleaving air‐masses in the frontal zone as defined by their Lagrangian histories. The two systems studied differ in that the first is associated with an intense surface low in January and the second is associated with a relatively weak surface low in April. In the intense surface low case the trajectory study suggests that a dry intrusion with stratospheric characteristics penetrated deep into the troposphere along the upper level front. Measurements indeed revealed an unsaturated layer with anomalously high ozone. This layer was intersected at four levels in the troposphere (at 8.5, 7.1, 5.2 and 3.7 km), and the lower the intersection, the lower the measured anomalous ozone and the higher the water vapor content. It is argued that this is best explained by the dry‐intrusion layer becoming mixed with background air by three‐dimensional turbulence, also encountered by the aircraft, along the upper level front. Evidence for this mixing is apparent on tracer‐tracer scatterplots. In the weak surface low case, by contrast, the dry intrusion has a more complex structure, with up to three separate layers of enhanced ozone and low humidity. Strong evidence for mixing was apparent only in the lowest layer. The weaker system may therefore be much more efficient at transporting upper tropospheric/stratospheric ozone to the lower troposphere. The transport of boundary layer air to the upper troposphere in the warm conveyor belt (WCB), however, was found to be around 8 times stronger in the intense system. Sonde measurements suggested that the WCB was ventilated by convection from the surface front in some regions to about 5–6 km, while it was stably stratified in other regions, suggesting layerwise long‐range transport.
Simulation of the combined effects of solar cycle, quasi‐biennial oscillation, and volcanic forcing on stratospheric ozone changes in recent decades
Authors: Lee H. , Smith A. K.
Published: May 2003

Stratospheric ozone responses to the 11‐year solar flux variation are calculated from two different decadal scale satellite ozone data sets by multiple regression analysis. The results show consistent dipole structures with solar regression coefficients that are positive in midlatitudes and negative in the equatorial lower stratospheric region. Because of the limited duration of the data record, the regression analysis may not completely separate variability from other processes. Other phenomena that could contribute to the observed pattern include the ozone variations associated with the quasi‐biennial oscillation (QBO) and with two major volcanic eruptions: El Chichón in 1982 and Mount Pinatubo in 1991. A fully interactive NCAR two‐dimensional chemical‐dynamical‐radiative model (Simulation of Chemistry, Radiation, and Transport of Environmentally Important Species (SOCRATES)) is used to investigate the effects of the equatorial QBO and the major volcanic eruptions on the 11‐year solar cycle analysis. When both effects are considered in the model simulation, the resulting ozone solar signal shows a dipole pattern similar to that observed. When the 11‐year solar flux variation is considered as the only external forcing, the resulting ozone solar cycle shows a monopole structure whose maximum is located in the equatorial upper stratosphere and whose response is uniformly positive.