bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectral reflectance of whitecaps: Their contribution to water‐leaving radiance
Authors: Moore Karl D. , Voss Kenneth J. , Gordon Howard R.
Published: September 2002

A radiometric system, deployed from a ship, is used to measure directly the influence of the presence of breaking waves (whitecaps) on the upwelling radiance above the sea surface. Estimates of their remote sensing augmented spectral reflectance, i.e., the temporally averaged or spatially averaged increase in the ocean's reflectance over and above the reflectance in the absence of breaking waves, are provided from measurements in the tropical Pacific. The accuracy of these estimates is dependent on their ability to determine radiometrically the background reflectance of the water. In the visible the remote sensing augmented spectral reflectance of whitecaps measured in the open ocean was found to be essentially independent of wavelength and in the range 0.001–0.002 for wind speeds of 9–12 m s−1. This is in reasonably good agreement (within a factor of 2) with earlier predictions based on the statistical relationship between fractional coverage and wind speed and the estimated average reflectance of individual whitecaps. In the near infrared (860 nm) the remote sensing augmented spectral reflectance falls to ∼80% of its value in the visible.
Modeling of sound propagation in the atmospheric boundary layer: Application of the MIUU mesoscale model
Authors: Hole L. R. , Mohr H. M.
Published: September 2002

A mesoscale atmospheric model, the Meteorological Institute of Uppsala University model, has been used to simulate local wind and temperature profiles in an area with hilly terrain in eastern Norway. Simulated wind and temperature profiles (up to 250 m height above ground) have been used as input to an acoustical model, called Oases. In this way, sound propagation out to 1.4 km distance along ground has been determined for a site situated in a valley bottom in the center of the model domain. Measured ground conditions are also included in the acoustical model. During the investigated period, meteorological measurements were carried out in conjunction with measurements of low‐frequency (≲100 Hz) impulse sound propagation out to a range of 1400 m along a flat, uniform, and snow‐covered ground. For a case with geostrophic winds of the order of 25 m s−1 the predicted profiles are compared with profiles measured by meteorological towers and a tethered balloon launched at the site. Acoustic predictions based on both measured and simulated atmospheric profiles agree well with measured single‐frequency sound levels in most cases. This suggests that forecasts of sound propagation using both a nested mesoscale atmospheric model and an acoustical model should be possible in future.
Lightning‐produced NOx (LINOX): Experimental design and case study results
Authors: Höller Hartmut , Finke Ullrich , Huntrieser Heidi , Hagen Martin , Feigl Christian
Published: September 2002

This paper investigates the role of lightning in the production of nitrogen oxides (NOx) and their subsequent distribution by thunderstorms. These questions were addressed by the field experiment LINOX (lightning produced NOx), which was performed in southern Germany in July 1996. The structure of thunderstorms was observed by radar and satellite, the lightning activity was recorded by a lightning detection network, and airborne chemical measurements were performed aboard a jet aircraft penetrating the storm anvils. NOx concentrations in the storm anvils were found to typically range from 1 to 4 parts per billion by volume. The NO contribution to the total NOx was found to be dominant in narrow peaks produced by flashes as well as near cloud boundaries, probably because of increased photolysis rates of NO2. Using CO2 as an air mass tracer, the lightning‐produced NOx amount was discriminated from the contribution due to transport of air from the boundary layer. It was found from a case study of a large storm anvil that lightning‐produced NOx was present in the same order of magnitude as the amount of NOx originating from lower levels; during later stages of cloud development, the content of the former even exceeded the latter one. A simple two‐dimensional model of advection and dispersion of the lightning‐produced NOx was able to reproduce the general structure of the anvil NOx plume. Some NOx peaks could directly be attributed to flash observations close to the aircraft track.
Global concentrations of tropospheric sulfate, nitrate, and ammonium aerosol simulated in a general circulation model
Authors: Adams Peter J. , Seinfeld John H. , Koch Dorothy M.
Published: September 2002

Global sulfate aerosol composition is simulated online in the Goddard Institute for Space Studies general circulation model II′ (GISS GCM II‐prime). Four sulfur species, hydrogen peroxide, gas phase ammonia, and particulate ammonium are the prognostic tracer species, the emissions, transport, and deposition of which are explicitly simulated. Nitric acid fields are prescribed based on a global chemical transport model. An online thermodynamic equilibrium calculation determines the partitioning of ammonia and nitrate between gas and aerosol phases, and the quantity of aerosol water based on the temperature, relative humidity, and sulfate concentration in each GCM grid cell. The total global burden of sulfate, nitrate, ammonium, and aerosol water is 7.5 Tg and is most sensitive to changes in sulfur emissions. Tropospheric lifetimes for ammonium and ammonia are 4.2 and 0.9 days, respectively; the tropospheric ammonium burden is 0.30 Tg N, compared with 0.14 Tg N for ammonia. Simulated ammonium concentrations are generally within a factor of 2 of observations. Subgrid variability in measured concentrations hinders comparison of observations to predictions. Ammonium nitrate aerosol plays an important role in determining total aerosol mass in polluted continental areas. In the upper troposphere and near the poles, cold temperatures allow unneutralized nitric acid to condense into the aerosol phase. Acidic aerosol species tend to be neutralized by ammonia to a greater degree over continents than over oceans. The aerosol is most basic and gas phase ammonia concentrations are highest over India. Water uptake per mole of sulfate aerosol varies by two orders of magnitude because of changes in relative humidity and aerosol composition. Spatial variations in aerosol composition and water uptake have implications for direct and indirect aerosol radiative forcing.
An analysis of rapid increases in condensation nuclei concentrations at a remote coastal site in western Ireland
Authors: Grenfell J. L. , Harrison R. M. , Allen A. G. , Shi J. P. , Penkett S. A. , O'Dowd C. D. , Smith M. H. , Hill M. K. , Robertson L. , Hewitt C. N. , Davison B. , Lewis A. C. , Creasey D. J. , Heard D. E. , Hebestreit K. , Alicke B. , James J.
Published: September 2002

Massive “bursts” in condensation nuclei (CN) concentration were recorded at a remote site on the west Irish coast during campaigns in summer 1996 and spring/summer 1997. Number concentrations of 3–7 nm diameter CN were observed to rise daily from 102–103 up to ∼105 /cm3 for 1–3 hours. Data were collected as part of the Atmospheric Chemistry Studies in the Oceanic Environment program. In a previous paper the burst phenomenon was linked to the movement of the tide, and it was suggested that enhanced biogenic emissions occurred near low tide with concomitant rapid homogeneous gas phase CN formation. In this paper possible chemical mechanisms for the burst phenomenon are investigated. Two approaches are adopted. First, by assuming a 20:80 sulfate:water molar composition and calculating the number distribution using data from condensation particle counters, the total mass of CN formed during a burst is evaluated. This is compared with that mass of sulfate produced by OH‐initiated dimethyl sulfide (DMS) oxidation. The procedure is termed “mass balance.” Second, a variety of chemical species are coplotted with tidal height. DMS oxidation is not believed to play a major role in CN formation at this site because (1) the mass balance calculations imply ambient DMS concentrations higher than those observed, and (2) gas phase HCl, HNO3, SO2, and NH3 did not exhibit any discernible correlation with tidal height. Further, none of the suite of observed nonmethane hydrocarbons or DMS showed a tidal relation. No mechanism has to date been convincingly identified for the burst phenomenon.
A study of sulfur dioxide oxidation pathways over a range of liquid water contents, pH values, and temperatures
Authors: Liang Jinyou , Jacobson Mark Z.
Published: September 2002

We examine factors controlling the photochemical oxidation of SO2 in tropospheric aerosols using a gas‐aqueous photochemical model. Over a range of liquid water contents (3×10−4 g H2O m−3 to 9 g H2O m−3) and pH values (0 to 8), we find that H2O2(aq) and O3(aq) provide the major sinks for SO2 in the aqueous phase when pH is held constant at below 5 and larger than 6, respectively. OH(aq) may be an important oxidant of SO2 in the aqueous phase when pH is held constant between 5 and 6 and H2O2 is depleted in an air parcel. When pH is allowed to vary during the integration, H2O2(aq) is the most important oxidant in the aqueous phase. O3(aq) is important primarily when the liquid water content is large (>1 g m−3) and the solution pH is above 4. O3(aq) is also important when the pH is initially high (>6) for quickly oxidizing SO2 and, thereby, reducing the pH into the pH region where H2O2(aq) is the most important oxidant. OH(aq) may be important when H2O2 is depleted and the liquid water content is large. When aerosols are present during noncloudy days in summer, the aqueous‐phase oxidation of SO2 is insignificant compared with the gas‐phase oxidation of SO2. We find, however, that the SO2 oxidation in wet aerosols may be enhanced in winter or when the temperature is low (273 K) and the relative humidity is high. Uncertainties in the reaction rate coefficients may significantly affect the concentrations of oxidants and other compounds of photochemical origin. Using a relatively stringent criterion, a compressed gas‐aqueous phase chemical mechanism for photochemical oxidation of SO2 is proposed for global tropospheric modeling.
Deliquescence, efflorescence, and supercooling of ammonium sulfate aerosols at low temperature: Implications for cirrus cloud formation and aerosol phase in the atmosphere
Authors: Cziczo D. J. , Abbatt J. P. D.
Published: September 2002

Motivated by the roles which tropospheric aerosols play in cloud formation processes and heterogeneous chemistry, low‐temperature phase transitions of submicron‐sized (NH4)2SO4 aerosols have been directly observed using Fourier transform infrared extinction spectroscopy. In particular, deliquescence relative humidities have been measured down to 254 K and efflorescence points to 238 K. In agreement with previous studies, it was observed that solid particles deliquesced at the thermodynamic value, whereas efflorescence of aqueous aerosols did not occur until a substantially lower relative humidity was reached. By observing the formation of spectral features due to ice, the same apparatus was used to monitor freezing of supercooled, micron‐sized aerosol particles over a composition range from 0 to 49 weight percent (wt%) (NH4)2SO4. The freezing of pure water aerosols occurred at 39 deg of supercooling, i.e., at a temperature of 234 K, in good agreement with the literature value. Ammonium sulfate aerosols exhibited significantly less than 39 deg of supercooling when referenced to the equilibrium freezing line. These results indicate that ammonium sulfate aerosols will promote cirrus cloud formation in the upper troposphere at higher temperatures and lower relative humidities than previously believed.
Southern Hemispheric halon trends (1978–1998) and global halon emissions
Authors: Fraser P. J. , Oram D. E. , Reeves C. E. , Penkett S. A. , McCulloch A.
Published: September 2002

Southern hemispheric data (1978‐1998) on four halons are reported. Halon‐derived bromine increased by a factor of 10 from 0.6–0.7 parts per trillion (ppt) in 1978 to more than 7 ppt in early 1998, currently growing at 3% yr−1. In 1997 the mixing ratios were 4.0 (CBrClF2, H‐1211), 2.1 (CBrF3, H‐1301), 0.4 (CBrF2CBrF2, H‐2402), and 0.04 ppt (CBr2F2, H‐1202), contributing ∼60, 30, 10 and 1% respectively to halon‐derived bromine (40% of background tropospheric bromine). The halons exhibit different growth patterns: CBrClF2 continues linearly (0.20 ppt yr−1), CBrF3 slows significantly (0.03 ppt yr−1, early 1998), CBrF2CBrF2 stops and CBr2F2 increases (17% yr−1 in early 1998). CBr2F2 shows a photochemically driven annual cycle. CBrClF2 and CBrF3 emissions (1963–2100) have been estimated from past and future production figures, in developed and developing countries, and from release estimates from the various halon banks. Mixing ratios have been calculated from a two‐dimensional model incorporating seasonal transport and halon photochemistry and a latitudinal source function. The model‐derived atmospheric lifetimes are 17 (CBrClF2), 62 (CBrF3), 20 (CBrF2CBrF2), and 2.9 (CBr2F2) years. Calculated and measured mixing ratios of CBrF3 agree reasonably but not for CBrClF2 in recent years. The model has been used to derive independently halon emissions that are consistent with the observed atmospheric trends. The continued growth of CBrClF2 in the background atmosphere could be due to enhanced production and emission in the Peoples' Republic of China, which may also explain the recent acceleration in growth of CBr2F2, a by‐product of CBrClF2 manufacture. The recent atmospheric growth of CBr2F2 may also be due to increased direct usage. The expected long‐term recovery of stratospheric ozone could be delayed if the current halon growth continues into the next decade.
Characterization of the chemical signatures of air masses observed during the PEM experiments over the western Pacific
Authors: Smyth S. , Sandholm S. , Shumaker B. , Mitch W. , Kanvinde A. , Bradshaw J. , Liu S. , McKeen S. , Gregory G. , Anderson B. , Talbot R. , Blake D. , Rowland S. , Browell E. , Fenn M. , Merrill J. , Bachmeier S. , Sachse G. , Collins J.
Published: September 2002

Extensive observations of tropospheric trace species during the second NASA Global Tropospheric Experiment Western Pacific Exploratory Mission (PEM‐West B) in February‐March 1994 showed significant seasonal variability in comparison with the first mission (PEM‐West A), conducted in September‐October 1991. In this study we adopt a previously established analytical method, i.e., the ratio C2H2/CO as a measure of the relative degree of atmospheric processing, to elucidate the key similarities and variations between the two missions. In addition, the C2H2/CO ratio scheme is combined with the back‐trajectory‐based and the LIDAR‐based air mass classification schemes, respectively, to make in‐depth analysis of the seasonal variation between PEM‐West A and PEM‐West B (hereinafter referred to as PEM‐WA and PEM‐WB). A large number of compounds, including long‐lived NMHCs, CH4, and CO2, are, as expected, well correlated with the ratio C2H2/CO. In comparison with PEM‐WA, a significantly larger range of observed C2H2/CO values at the high end for the PEM‐WB period indicates that the western Pacific was more impacted by “fresher” source emissions, i.e., faster or more efficient continental outflow. As in the case of PEM‐WA, the C2H2/CO scheme complements the back‐trajectory air mass classification scheme very well. By combining the two schemes, we found that the atmospheric processing in the region is dominated by atmospheric mixing for the trace species analyzed. This PEM‐WB wintertime result is similar to that found in PEM‐WA for the autumn. In both cases, photochemical reactions are found to play a significant role in determining the background mixing ratios of trace gases, and in this way the two processes are directly related and dependent upon each other. This analysis also indicates that many of the upper tropospheric air masses encountered over the western Pacific during PEM‐WB may have had little impact from eastern Asia's continental surface sources. NOx mixing ratios were significantly enhanced during PEM‐WB when compared with PEM‐WA, in the upper troposphere's more atmospherically processed air masses. These high levels of NOx resulted in a substantial amount of photochemical production of O3. A lack of corresponding enhancements in surface emission tracers strongly implies that in situ atmospheric sources such as lightning are responsible for the enhanced upper tropospheric NOx. The similarity in NOx values between the northern (higher air traffic) and southern continental air masses together with the indications of a large seasonal shift suggests that aircraft emissions are not the dominant source. However, photochemical recycling cannot be ruled out as this in situ source of NOx.
The Dole effect over the last two glacial‐interglacial cycles
Authors: Malaizé B. , Paillard D. , Jouzel J. , Raynaud D.
Published: September 2002

Detailed measurements of δ18O of atmospheric oxygen performed on air trapped in the Vostok ice cores (Antarctica) are used to extend the record of the Dole effect over two climatic cycles (back to 240 kyr B.P.). Except for glacial terminations I and II and for an unexpected minimum occurring around 175 kyr, the Dole effect shows small variations (ΔDole within ± 0.5‰). These small variations, however, show a well‐marked 23 kyr precessional periodicity, thus confirming the results obtained by Bender et al. [1994a] for the first climatic cycle. To explain the minimum value reached around 175 kyr, we invoke the possibility of a peak in the oceanic productivity linked to climatic events induced at low latitudes under glacial conditions.
Sampling errors in radar estimates of rainfall
Authors: Jordan Phillip , Seed Alan , Austin Geoff
Published: September 2002

The relatively slow rate of application of radar rainfall to operational hydrology is partially due to concerns about the measurement errors. The errors that result in a bias in the field mean have been studied extensively and can to some extent be treated, but errors that manifest themselves as more or less white noise become important when considering what scale is appropriate for spatially distributed hydrological modeling. This paper evaluates the errors that arise in radar estimates of rainfall as a result of temporal sampling, spatial averaging, measuring the field at some distance above the ground, and recording the reflectivity data with a limited radiometric resolution. By far the most significant source of error was found to be due to measuring the field at some height above the ground. The mean standard difference in rainfall rate between fields separated by 1 km in height at 1 km spatial resolution was found to be of the order of 100% of the mean rainfall rate. When the spatial resolution is reduced to 5 km the mean standard difference between the fields with the same 1 km vertical separation fell to about 50% of the mean rainfall rate. Temporal sampling was found to be quite sensitive to the intermittency of the rain field being sampled. The mean standard error caused by 2‐min sampling for 10‐min accumulations decreased from 14% for scattered rainfall to 8% for widespread rainfall.
Volatile organic compounds emitted after leaf wounding: On‐line analysis by proton‐transfer‐reaction mass spectrometry
Authors: Fall Ray , Karl Thomas , Hansel Armin , Jordan Alfons , Lindinger Werner
Published: September 2002

Volatile organic compounds (VOCs) released from vegetation, including wound‐induced VOCs, can have important effects on atmospheric chemistry. The analytical methods for measuring wound‐induced VOCs, especially the hexenal family of VOCs (hexenals, hexenols, and hexenyl esters), are complicated by their chemical instability and the transient nature of their formation after leaf and stem wounding. Here we demonstrate that formation and emission of hexenal family compounds can be monitored on‐line using proton‐transfer‐reaction mass spectrometry (PTR‐MS), avoiding the need for preconcentration or chromatography. These measurements allow direct analysis of the rapid emission of the parent compound, (Z)‐3‐hexenal, within 1–2 s of wounding of aspen leaves and then its disappearance and the appearance of its metabolites including (E)‐2‐hexenal, hexenols, and hexenyl acetates. Similar results were seen in wounded beech leaves and clover. The emission of hexenal family compounds was proportional to the extent of wounding, was not dependent on light, occurred in attached or detached leaves, and was greatly enhanced as detached leaves dried out. Emission of (Z)‐3‐hexenal from detached drying aspen leaves averaged 500 μg C g−1 (dry leaf weight). Leaf wound compounds were not emitted in a nitrogen atmosphere but were released within seconds of reintroduction of oxygen; this indicates that there are not large pools of hexenyl compounds in leaves. The PTR‐MS method also allows the simultaneous detection of less abundant hexanal family VOCs including hexanal, hexanol, and hexyl acetate and VOCs formed in the light (isoprene) or during anoxia (acetaldehyde). PTR‐MS may be a useful tool for the analysis of VOC emissions resulting from grazing, herbivory, and other physical damage to vegetation, from harvesting of crops, and from senescing leaves.
Consumption of atmospheric carbonyl sulfide by coniferous boreal forest soils
Authors: Simmons Jeffrey S. , Klemedtsson Leif , Hultberg Hans , Hines Mark E.
Published: September 2002

A static enclosure method was used to determine the role of saturated and unsaturated soils and soil‐plant systems in carbonyl sulfide exchange with the atmosphere. The influence of several environmental factors on these exchange processes was also investigated by measuring fluxes in catchments that were experimentally manipulated. Results indicated that both vegetated and nonvegetated soil surfaces were active sinks for OCS, with rates of consumption ranging between 3.6 and 77.7 nmol m−2 h−1. Both vegetation and microorganisms appeared to be involved in OCS uptake. Robust multiple regression analyses indicated that catchments receiving treatments of either nitrogen or lime consumed OCS 30% more rapidly than the control. Uptake of OCS in a recently clear‐cut site was 3.5 times greater than the control. Vegetated and water‐saturated soil surfaces consumed OCS 43% and 84% more rapidly than nonvegetated and unsaturated soils, respectively.
First measurement of the 18O/16O and 17O/16O ratios in stratospheric nitrous oxide: A mass‐independent anomaly
Authors: Cliff Steven S. , Brenninkmeijer Carl A. M. , Thiemens Mark H.
Published: September 2002

The first multi‐oxygen isotopic analysis of nitrous oxide samples from the lower stratosphere has a mass‐independent composition. This study extends the results from tropospheric nitrous oxide samples. Measurements are reported from two 8–12 km altitude airplane flights from New Zealand to the Antarctic in June and October 1993. The observed isotopic results strengthen the previous argument that an atmospheric process, source, sink, or exchange reaction, involving nitrous oxide must exist in the atmosphere, and they further define its signature.
A climatology of adverse winter‐type weather events
Authors: Déry Stephen J. , Yau M. K.
Published: September 2002

Using the European Centre for Medium‐Range Weather Forecasts Re‐Analysis gridded data, a global climatology of blowing snow, blizzard, and high‐windchill events is conducted for the period 1979–1993. The results show that these phenomena occur primarily over flat, open surfaces with long seasonal or perennial snow covers such as the Greenland and Antarctic ice fields as well as the Arctic tundra. On a regional scale, emphasis is given to the Mackenzie River Basin (MRB) of Canada, where fewer events take place within the boreal forest as opposed to the Arctic tundra. Interannual and monthly variabilities in the number of events are also evident and are due primarily to 10‐m wind speed anomalies at high latitudes for blowing snow and blizzard events, while high‐windchill events are more sensitive to air temperatures near the surface. We also find that high‐windchill episodes are the more frequent events, since they occur at 9.3% of all possible grid points and times on a yearly basis, while blowing snow at 6.5% and blizzards at 1.4% are less common events. Compositing of principal meteorological fields show that anticyclones and lee cyclones are prominent features associated with blowing snow events in some sections of the MRB.
Sensitivity of the simulated Asian summer monsoon to parameterized physical processes
Authors: Eitzen Zachary A. , Randall David A.
Published: September 2002

A study of the sensitivity of the simulated Asian summer monsoon to changes in general circulation model formulation is reported. The baseline version of the model fails to realistically simulate the precipitation, wind, and temperature fields. In one experiment the stratiform cloud parameterization was changed from a simple large‐scale saturation scheme to a scheme that prognostically determines cloud water, cloud ice, and rain. In a second experiment a parameter that relates the cumulus mass flux to the cumulus kinetic energy was altered so as to increase the convective adjustment time. These changes in the stratiform and cumuliform cloud parameterizations significantly improve the simulations of the precipitation and upper level wind fields, respectively.
Water adsorption by hydrophobic organic surfaces: Experimental evidence and implications to the atmospheric properties of organic aerosols
Authors: Thomas Elan , Rudich Yinon , Trakhtenberg Sofia , Ussyshkin Rachel
Published: September 2002

The adsorption of water on hydrophobic organized organic thin films, used as a proxy for atmospheric organic aerosols, was measured simultaneously with a molecularly controlled semiconductor resistor (MOCSER) and quartz crystal microbalance (QCM). Water adsorption was found to be reversible and dependent on relative humidity (RH). The MOCSER measurements show that the adsorption kinetics of the first water layer resembles Langmuir behavior. The QCM findings indicate that adsorption is proportional to the water vapor pressure and exceeds the amount equivalent to one monolayer. These results can be explained by the formation of small water clusters on imperfections or structural defects on the organic surface and that the water does not achieve complete surface coverage even at high relative humidity. This mechanism resembles previous observations of water adsorption to structural defects on inorganic crystals. It is also shown that water can penetrate through the organic surface and reach the inorganic surface, even with a closely packed organic coating. This paper presents applications of these findings to the properties of organic aerosols.
Portable detector‐based primary scale of spectral irradiance
Authors: Kärhä Petri , Kübarsepp Toomas , Manoocheri Farshid , Toivanen Pasi , Ikonen Erkki , Visuri Reijo , Ylianttila Lasse , Jokela Kari
Published: September 2002

A method developed at the Helsinki University of Technology (HUT) for calibrating standard lamps of spectral irradiance in UV‐A and UV‐B regions is presented. The method is based on a compact filter radiometer that is characterized absolutely to measure spectral irradiance. The filter radiometer can be brought to laboratories where accurate calibrations are needed. The method overcomes some of the instability problems encountered when using lamps as transfer standards, which makes it also useful in intercomparison campaigns. Test measurements are presented where two standard lamps issued by the National Institute of Standards and Technology (NIST), USA, are compared with the HUT scale held by the radiometer. The results suggest that the agreement between the two scales (HUT and NIST) is approximately 1%–2% in the wavelength region from 300 nm to 400 nm.
Stratocumulus processing of gases and cloud condensation nuclei: 2. Chemistry sensitivity analysis
Authors: Zhang Yiping , Kreidenweis Sonia M. , Feingold Graham
Published: September 2002

A trajectory ensemble model (TEM) is used to investigate aqueous processing of gases and cloud condensation nuclei (CCN) in the boundary layer. The coupled aqueous chemistry/cloud microphysics model driven by a set of boundary layer parcel trajectories derived from a large eddy simulation is used to study the effects of variations in the initial chemical fields and initial aerosol number concentration on chemical heterogeneity, and the broadening of CCN and drop spectra. The differences in the overall fractional conversion between the TEM and a single parcel experiencing mean conditions in a stratocumulus‐capped marine boundary layer are also investigated. Results show that the O3 oxidation rate is larger than the H2O2 oxidation rate in the base case, whereas the volume‐mean pH might suggest that H2O2 oxidation dominates. Aqueous chemistry contributes to broadening of the drop size distribution, but the magnitude of the broadening also depends on initial chemical conditions. Sensitivity tests show that the H2O2 oxidation adds sulfate mass evenly and continuously across the particle sizes, while the O3 oxidation adds sulfate mass near the mode of the CCN spectrum over a relatively short time. In cases where more mass is added onto large particles in the tail of the initial CCN spectrum, the broadening of the drop spectrum is most evident, and may even trigger the collision‐coalescence process and drizzle formation in stratocumulus clouds.
Comparison of modeled and observed values of NO2 and JNO2 during the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) mission
Authors: Del Negro L. A. , Fahey D. W. , Gao R. S. , Donnelly S. G. , Keim E. R. , Neuman J. A. , Cohen R. C. , Perkins K. K. , Koch L. C. , Salawitch R. J. , Lloyd S. A. , Proffitt M. H. , Margitan J. J. , Stimpfle R. M. , Bonne G. P. , Voss P. B. , Wennberg P. O. , McElroy C. T. , Swartz W. H. , Kusterer T. L. , Anderson D. E. , Lait L. R. , Bui T. P.
Published: September 2002

Stratospheric measurements of NO, NO2, O3, ClO, and HO2 were made during spring, early summer, and late summer in the Arctic region during 1997 as part of the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) field campaign. In the sunlit atmosphere, NO2 and NO are in steady state through NO2 photolysis and reactions involving O3, ClO, BrO, and HO2. By combining observations of O3, ClO, and HO2, observed and modeled values of the NO2 photolysis rate coefficient (JNO2), and model estimates of BrO, several comparisons are made between steady state and measured values of both NO2 and JNO2. An apparent seasonal dependence in discrepancies between calculated and measured values was found; however, a source for this dependence could not be identified. Overall, the mean linear fits in the various comparisons show agreement within 19%, well within the combined uncertainties (±50 to 70%). These results suggest that photochemistry controlling the NO2/NO abundance ratio is well represented throughout much of the sunlit lower stratosphere. A reduction in the uncertainty of laboratory determinations of the rate coefficient of NO + O3 → NO2 + O2 would aid future analyses of these or similar atmospheric observations.
Effect of clouds on UV irradiance: As estimated from cloud amount, cloud type, precipitation, global radiation and sunshine duration
Authors: Josefsson W. , Landelius T.
Published: September 2002

Ten years of measurements of UV irradiance, monitored by the Robertson‐Berger (RB) meter in Norrköping, 58.58°N, 16.15°E, Sweden, have been combined with concurrent synoptic cloud observations, measurements of sunshine duration, and global radiation to establish the relative influence of clouds on UV irradiance. It is shown that the cloud effect for UV wavelengths is less than for the whole solar spectrum (global radiation). Relations retrieved for global radiation may be used by correcting for the differences. High‐level clouds are more transparent than low‐ and medium‐level clouds. As expected, it was found that precipitating clouds in general are more opaque than nonprecipitating clouds. If there is any solar elevation dependency in the effect of clouds, it is small. Using only total cloud amount as parameter to model, the cloud effect on UV irradiance will give a substantial uncertainty, which can be decreased considerably using cloud type and/or information on precipitation conditions. It has also been shown that sunshine duration can be used in a similar way as cloud cover.
Results of the 1998 Ny‐Ålesund Ozone Monitoring Intercomparison
Authors: Steinbrecht W. , Neuber R. , von der Gathen P. , Wahl P. , McGee T. J. , Gross M. R. , Klein U. , Langer J.
Published: September 2002

The Ny‐Ålesund Ozone Monitoring Intercomparison (NAOMI) took place at Ny‐Ålesund, Spitsbergen (78.92°N, 11.95°E), from January 20 to February 10, 1998. This paper focuses on comparing stratospheric ozone profiles measured by the Alfred Wegener Institute differential absorption lidar (AWI DIAL), in routine Network for Detection of Stratospheric Change (NDSC) operation at Ny‐Ålesund, the mobile Goddard Space Flight Center DIAL (GSFC DIAL), the University of Bremen microwave radiometer (μWave), and electrochemical concentration cell (ECC) ozonesondes, flown routinely by AWI. Below 30 km the two DIALs and the ECC sondes give virtually the same results, with instrumental precision (repeatability) better than ±5% and no detectable bias. When their coarser altitude resolution is not accounted for, the μWave data show 15% low bias at 16 km and 15% high bias at 23 km. Considerably better agreement, better than ±5% around 20 km and above 30 km, is found when the altitude resolution of the other data is degraded to match that of the μWave. During NAOMI the μWave data show high bias of up to 10% in a mixing ratio plateau around 25 km. Such bias has not been seen in routine intercomparisons between μWave and ECC sonde data at Ny‐Ålesund. It is likely caused by an a priori profile 40% higher than the true profile during NAOMI. Above 30 km the μWave data show the best precision (repeatability), about ±3 to ±5%. Precision of the GSFC DIAL data decreases from better than ±5% at 30 km to about ±10% at 40 km, and the precision for the AWI DIAL data decreases from better than ±5% at 30 km to ±30% at 40 km. From 34 to 38 km the AWI profile is 12% lower than the GSFC profile. AWI DIAL measurements that are low at 35 km often end below 40 km or show high values at 40 or 45 km. This behavior seems related to the way in which the AWI processing algorithm changes altitude resolution for data with poor signal‐to‐noise ratio.
Satellite monitoring of vegetation fires for EXPRESSO: Outline of activity and relative importance of the study area in the global picture of biomass burning
Authors: Grégoire J.‐M. , Pinnock S. , Dwyer E. , Janodet E.
Published: September 2002

The satellite monitoring of vegetation fires for the International Global Atmospheric Chemistry (IGAC) Experiment for Regional Sources and Sinks of Oxidants (EXPRESSO) was designed to assist the assessment of the fluxes of trace gases and aerosols emitted by savanna fires that occur during the dry season in Central Africa. It is of particular interest that the study area covers the transition zone between the savanna and the forest domains. Satellite remote sensing is the only technology that allows consistent data collection of the spatial and temporal distribution of fires at the scale required by the EXPRESSO experiment. A portable monitoring system was developed and operated in Central Africa for the in situ, real‐time acquisition and processing of National Oceanic and Atmospheric Administration‐advanced very high resolution radiometer (AVHRR) imagery during the EXPRESSO dry season field campaign, which lasted from October 1, 1996, to December 2, 1996. One AVHRR scene per day was processed to locate active fires and burnt areas. These results are compared with our daily global maps of vegetation fires derived from the International Geosphere‐Biosphere Programme‐Data and Information System Global AVHRR data set for the period April 1992 to December 1993. This comparison shows the EXPRESSO fires in the context of the African continent and of the total global fire activity. The EXPRESSO study area is part of one of the largest fire belts observed in Africa, and even over the globe, spreading from Senegal to Ethiopia. This study also shows a big increase in the number of fires in 1996 compared to 1993, from a total of 39,500 fire pixels detected in November 1993 to 124,500 in November 1996. We also show that the November period corresponds to a minimum of the global fire activity: only 3% of the fires detected during a complete year were detected during this month. Moreover, the number of fires detected in this particular month was 28% of those detected during the annual peak period observed in July‐August. Current activity is focusing on the implementation of the World‐Fire‐Web network: A system for globally mapping vegetation fires.
Fractionation of inorganic nitrates in winter Arctic troposphere: Coarse aerosol particles containing inorganic nitrates
Authors: Hara Keiichiro , Osada Kazuo , Hayashi Masahiko , Matsunaga Katsuji , Shibata Takashi , Iwasaka Yasunobu , Furuya Keiichi
Published: September 2002

Atmospheric aerosol particles and acidic gases were collected at Ny‐Ålesund, Norwegian high Arctic, in winter (December‐March) of 1994/1995, 1995/1996 and 1996/1997 to understand the sink processes of atmospheric inorganic nitrate and reactive nitrogen oxides. Atmospheric inorganic nitrates were mostly present as particulate NO3− in the coarse mode defined as diameter >2.3 μm) of which major constituents were sea‐salt compounds such as Na+ and Cl−. The number fraction of coarse aerosol particles containing NO3− increased in aged oceanic air masses. The internal mixing between sea‐salt particles and inorganic nitrates revealed by individual particle analysis implies that uptake of gaseous HNO3 and/or formation of particulate NO3− through heterogeneous processes occurs on sea‐salt particles. A simple uptake model was used to estimate the production rate of particulate NO3− due to these processes. Model calculations showed that the formation of particulate NO3− was mainly due to the uptake of gaseous HNO3 on sea‐salt particles in winter Arctic. However, the heterogeneous formation from N2O5 and NO3 also makes an important contribution to the formation of particulate NO3− under no and/or weak solar radiation in winter Arctic (polar night). Thus the heterogeneous formation of NO3− on sea‐salt particles may play an important role as a sink of reactive nitrogen oxide in winter Arctic.
A history of δ13C in atmospheric CH4 from the Cape Grim Air Archive and Antarctic firn air
Authors: Francey R. J. , Manning M. R. , Allison C. E. , Coram S. A. , Etheridge D. M. , Langenfelds R. L. , Lowe D. C. , Steele L. P.
Published: September 2002

Marine (baseline) air from Cape Grim (41°S), collected and archived in high‐pressure metal containers, provides a history of δ13C in atmospheric methane from 1978. A similar history is obtained from air pumped from different layers of the firn at Law Dome, Antarctica, after correction for diffusion and gravitational settling effects in the firn. The archive records are linked to measurements since 1992 using 5‐L glass flasks filled at Cape Grim, and compared to data since 1989 from a comparable site at Baring Head, New Zealand. Over 18 years the δ13C of atmospheric methane in the extratropical Southern Hemisphere has increased by ∼0.6‰ while the methane mixing ratio increased by ∼200 ppb. The δ13C growth rate decreases over the 18‐year period, but by relatively less than the simultaneous decrease in mixing ratio growth rate. The overall increase in δ13C is significantly smaller than, and the recent slowing is in conflict with, previous estimates [Stevens and Engelkemeir, 1989]. The long‐term trend in δ13C, and the different shape to the trend in mixing ratio, are shown to be consistent with constant global methane sources and sinks since 1982. The slower equilibration of observed δ13C, compared to that of the mixing ratio, is an example of an effect pointed out recently by Tans [1997]. The data presented here constrain changes in the relative mix of isotopically heavy and light sources to be small and suggest that there was little change in the ratio of anthropogenic to natural sources in the 1978 to 1995 period.
A sensitivity study of photolysis rate coefficients during POLARIS
Authors: Swartz William H. , Lloyd Steven A. , Kusterer Thomas L. , Anderson Donald E. , McElroy C. Thomas , Midwinter Clive
Published: September 2002

Recent improvements in the agreement between observation‐derived and modeled photolysis rate coefficients (j‐values) have allowed for the close examination of the sensitivity of j‐values to variations in physical parameters influencing their magnitude and temporal(spatial variability. Altitude and solar zenith angle profiles of j‐values for two photolytic processes, NO2 → NO + O(3P) and O3 → O2 + O(1D), are modeled, varying surface albedo, atmospheric baseheight, total column ozone, and ozone and temperature altitude profiles over the ranges observed during the NASA Photochemistry of Ozone Loss in the Arctic Region In Summer (POLARIS) high‐altitude ER‐2 aircraft campaign. The effect of atmospheric refraction at high solar zenith angles is also addressed. Modeled j‐values using measured ozone/albedo input from the Composition and Photodissociative Flux Measurement (CPFM) spectroradiometer on board the ER‐2 exceed those derived from CPFM flux measurements by 6% for jNO2 and 14% for jO3, within experimental uncertainties. The individual effects of albedo, baseheight, and ozone on j‐values along specific ER‐2 flight tracks are modeled and related to the temporal and spatial variability observed. For jNO2, surface albedo has the greatest effect; for jO3, the ozone above the aircraft and surface albedo are the most important.
Validation and intercomparison of wet and dry deposition schemes using 210Pb in a global three‐dimensional off‐line chemical transport model
Authors: Giannakopoulos C. , Chipperfield M. P. , Law K. S. , Pyle J. A.
Published: September 2002

We have used 210Pb, a tracer originating from the radioactive decay of 222Rn emitted from soils, to validate different wet and dry deposition schemes in our global three‐dimensional off‐line chemical transport model, TOMCAT. We have tested two different parameterizations for the dry deposition, one with the turbulent exchange in the boundary layer and one without. When dry deposition is linked to the boundary layer scheme, the model shows the ability to resolve shallow mixing depths in winter with enhanced lifetimes and concentrations of surface aerosol. Three conceptually different wet deposition schemes have been implemented and tested in TOMCAT. In the first, where wet removal is assumed to be proportional to the local gradient of the specific humidity, the model has a global mean bias for the surface concentrations of −40%. Using this scheme, the observed surface concentrations are seriously underpredicted, and most of the aerosol burden is scavenged within the boundary layer. In the second scheme, where scavenging is parameterized proportional to the model‐derived total precipitation rate (large scale and convective), the model exhibits a +26% bias. The concentrations are overpredicted, especially in continental areas as this scheme fails to capture the coupling between vertical transport and rainout. In the third scheme, wet removal is coupled with the vertical transport inside convective clouds. The model then shows the best performance with only a −4% bias for the concentrations. Furthermore, other regional discrepancies between model and observations point to a variability in 222Rn emissions, which was not taken into account with our simple 222Rn emission scenario, and also to anomalies in our model‐derived precipitation rate. The aerosol residence times are realistic only when the wet removal schemes use the precipitation rate rather than the specific humidity. The dry subtropical and polar regions can be captured with lifetimes at 500 hPa of more than 50 days whereas in regions of high precipitation the lifetimes are less than 5 days.
Methane uptake by a selection of soils in Ghana with different land use
Authors: Priemé Anders , Christensen Søren
Published: September 2002

We measured the oxidation of atmospheric methane in tropical soils in Ghana covering a moisture gradient from the moist forest zone to the savanna zone at the onset of the rainy season. Land use at the sites covered undisturbed (forest and savanna) and cultivated soil, including burning. Generally, the methane oxidation rates in the tropical forest and savanna soils were low (range from 9 to 26 μg CH4m−2h−1) compared to, for example temperate forest soils. In the savanna soil, annual fire had decreased soil methane oxidation rates to 5 μg CH4m−2h−1compared to 9 μg CH4m−2h−1at a site not subjected to fire for 6 years. In paired sites of moist forest and arable soils, methane oxidation rates were lower by >60% in the arable soils. Methane oxidation rates in three arable soils in the savanna zone soils ranged from 7 to 11 μg CH4m−2h−1before the first rain but increased to 23–28 μg CH4m−2h−1after the rain. These rates are comparable to other reports from arable soils in tropical and temperate regions. Thus arable agriculture and, to a lesser extent, biomass burning decreased methane oxidation rates by the investigated soils.
Langley method of calibrating UV filter radiometers
Authors: Slusser James , Gibson James , Bigelow David , Kolinski Donald , Disterhoft Patrick , Lantz Kathleen , Beaubien Arthur
Published: September 2002

The Langley method of calibrating UV multifilter shadow band radiometers (UV‐MFRSR) is explored in this paper. This method has several advantages over the traditional standard lamp calibrations: the Sun is a free, universally available, and very constant source, and nearly continual automated field calibrations can be made. Although 20 or so Langley events are required for an accurate calibration, the radiometer remains in the field during calibration. Difficulties arise as a result of changing ozone optical depth during the Langley event and the breakdown of the Beer‐Lambert law over the finite filter band pass since optical depth changes rapidly with wavelength. The Langley calibration of the radiometers depends critically upon the spectral characterization of each channel and on the wavelength and absolute calibration of the extraterrestrial spectrum used. Results of Langley calibrations for two UV‐MFRSRs at Mauna Loa, Hawaii were compared to calibrations using two National Institute of Standards and Technology (NIST) traceable lamps. The objectives of this study were to compare Langley calibration factors with those from standard lamps and to compare field‐of‐view effects. The two radiometers were run simultaneously: one on a Sun tracker and the other in the conventional shadow‐band configuration. Both radiometers were calibrated with two secondary 1000 W lamp, and later, the spectral response functions of the channels were measured. The ratio of Langley to lamp calibration factors for the seven channels from 300 nm to 368 nm using the shadow‐band configuration ranged from 0.988 to 1.070. The estimated uncertainty in accuracy of the Langley calibrations ranged from ±3.8% at 300 nm to ±2.1% at 368 nm. For all channels calibrated with Central Ultraviolet Calibration Facility (CUCF) lamps the estimated uncertainty was ±2.5% for all channels.