bluuebunny/crossref_metadata_2025 huggingface dataset:
Uncertainties in upper stratospheric ozone trends from 1979 to 1996
Authors: Cunnold D. M. , Newchurch M. J. , Flynn L. E. , Wang H. J. , Russell J. M. , McPeters R. , Zawodny J. M. , Froidevaux L.
Published: September 2002

The time series of differences in coincident measurements of ozone by Stratospheric Aerosol and Gas Experiment (SAGE) and by Solar Backscattered Ultraviolet (SBUV), SBUV/2, Umkehr and Microwave Limb Sounder (MLS) are analyzed, and the slopes in the differences are calculated. SAGE ozone measurements are also compared against those by HALOE. The purpose of these comparisons is to look for statistically significant nonzero slopes which could indicate long‐term calibration problems in one or more of the measurement systems. It is found that the slopes are remarkably similar between the Northern and Southern Hemisphere midlatitudes, and, apart from a few exceptions, the slopes are also similar in the tropics. Slopes of MLS‐SAGE differences and HALOE‐SAGE trends from approximately 1992 to 1996 have values of approximately −0.5±0.4% yr−1 (95% confidence limits) in Umkehr layers 7–9 (which are centered at ∼37, 42, and 47 km altitude). Umkehr‐SAGE slopes for 1979–1996, however, are almost all positive and in the range −0.1–0.4% yr−1 for Umkehr layers 4–8, while SBUV‐SAGE slopes for 1979–1989 are essentially zero in layers 4–7 and 0.3–0.4% yr−1 in layers 8 and 9. Averaging all these results with SBUV‐SAGE II slopes from 1985 to 1989, the other sensors minus SAGE slopes are most likely between 0.2 and −0.2% yr−1 from ∼20 to 40 km altitude. The results indicate slightly negative slopes in Umkehr layers 5–7 and positive slopes in the other three layers. There thus appears to be no overall drift in the SAGE ozone measurements from 1979 to 1996, but SAGE sunrise/sunset trend differences >40 km altitude, combined with the more accurate SBUV‐SAGE slopes for 1979–1989, suggest a most likely slope range of 0.4 to −0.4% yr−1 between 40 and 50 km altitude. SBUV/2 measurements from 1989 to 1994 have an upward trend with respect to SAGE measurements of ∼0.7% yr−1 with some altitudinal structure; this slope exceeds the estimated 95% uncertainties on the SBUV/2 trends.
Estimates of the seasonal cycle of mass and ozone transport at high northern latitudes
Authors: Rosenlof Karen H.
Published: September 2002

Zonally averaged transport of mass and ozone into high northern latitudes exhibits a pronounced annual cycle. During summer, downwelling is at a minimum, and horizontal bulk advection and eddy fluxes of ozone and mass are found to be significant. This study examines in detail the ozone budget between 58 and 67 hPa in a meridional annulus ranging from 60° to 70°N during summer. The area of the study was chosen to correspond with the Photochemistry of Ozone Loss in the Arctic Region in Summer high‐altitude aircraft experiment based in Fairbanks, Alaska, from April to September, 1997. It is found that the seasonal cycle in transport is an important contributor to the resultant seasonal march of ozone at high latitudes. The horizontal eddy contribution to the ozone tendency at high latitudes in the lower stratosphere is found to be large and negative (reducing ozone) during middle and late summer, where it more than offsets the increasing ozone tendency due to advection by the residual circulation. During early summer, the high‐latitude air mass appears to be relatively isolated, with in situ chemical processes driving the ozone tendency.
Simulation of stratospheric tracers using an improved empirically based two‐dimensional model transport formulation
Authors: Fleming Eric L. , Jackman Charles H. , Stolarski Richard S. , Considine David B.
Published: September 2002

We have developed a new empirically based transport formulation for use in our Goddard Space Flight Center (GSFC) two‐dimensional chemistry and transport model. In this formulation, we consider much of the information about atmospheric transport processes available from existing data sets. This includes zonal mean temperature, zonal wind, net heating rates, and Eliassen‐Palm flux diagnostics for planetary and synoptic‐scale waves. We also account for the effects of gravity waves and equatorial Kelvin waves by utilizing previously developed parameterizations in which the zonal mean flow is constrained to observations. This scheme utilizes significantly more information compared to our previous formulation and results in simulations that are in substantially better agreement with observations. The new model transport captures much of the qualitative structure and seasonal variability observed in stratospheric long lived tracers, such as isolation of the tropics and the southern hemisphere winter polar vortex, the well‐mixed surf‐zone region of the winter subtropics and midlatitudes, and the latitudinal and seasonal variations of total ozone. Model simulations of carbon 14 and strontium 90 are in good agreement with observations, capturing the peak in mixing ratio at 20–25 km and the decrease with altitude in mixing ratio above 25 km. We also find mostly good agreement between modeled and observed age of air determined from SF6 outside of the northern hemisphere polar vortex. However, inside the vortex, the model simulates significantly younger air compared to observations. This is consistent with the model deficiencies in simulating CH4 in this region and illustrates the limitations of the current climatological zonal mean model formulation. The model correctly propagates the phase of the lower stratospheric seasonal cycles in 2CH4+H2O and CO2. The model also qualitatively captures the observed decrease in the amplitude of the stratospheric CO2 seasonal cycle between the tropics and midlatitudes. However, the simulated seasonal amplitudes were attenuated too rapidly with altitude in the tropics. The generally good model‐measurement agreement of these tracer simulations demonstrate that a successful formulation of zonal mean transport processes can be constructed from currently available atmospheric data sets.
Daily ozone vertical profile model built on geophysical grounds, for column retrieval from atmospheric high‐resolution infrared spectra
Authors: De Maziere M. , Hennen O. , Van Roozendael M. , Demoulin P. , De Backer H.
Published: September 2002

The retrieval of column abundances of atmospheric gases from atmospheric infrared spectra necessarily depends on the choice of the a priori vertical profiles of the atmospheric species involved. Most commonly a standard profile is adopted, and some additional parameter is introduced in the retrieval algorithm for improving the final result. This parameter may be unrelated to geophysical arguments. For the retrieval of ozone columns, it is proposed here to use a model of a priori O3 vertical profiles which better reflect the real state of the atmosphere at the time and location of the measurement. The model profiles depend on the daily local thermal tropopause height and include a climatological seasonality. The model has been developed for the retrieval of ozone columns from high‐resolution Fourier transform infrared (FTIR) solar absorption measurements at the International Scientific Station of the Jungfraujoch in the Swiss Alps. It is demonstrated that the use of the model significantly improves the quality of the spectral fits while minimizing the need for adjusting additional nongeophysical parameters to compensate for atmospheric changes and that it enhances the accuracy of the retrieved O3 column data. Such is done by comparison of the retrieved FTIR O3 data with validated independent O3 data sets from co‐located Système d'Analyse par Observations Zénithales and nearby Brewer instruments. Contrary to some other approaches for improving the FTIR O3 retrieval, the model concept developed here can be applied to O3 observations at any midlatitude site, and it can be implemented in an operational retrieval procedure.
Analysis of radical propagation efficiency to assess ozone sensitivity to hydrocarbons and NOx: 2. Long‐lived species as indicators of ozone concentration sensitivity
Authors: Tonnesen Gail S. , Dennis Robin L.
Published: September 2002

We used an analysis of radical propagation efficiency and OH chain length in a simple trajectory model to propose combinations of long‐lived species that distinguish conditions in which O3 concentration ([O3]) is NOx‐limited and radical‐limited. We further examined these indicators in a three‐dimensional grid model. We proposed several new indicators including [H2O2]/([O3]+[NO2]), [O3]/[NOx], and a measure using the OH rate constant weighted concentrations of NO2 and hydrocarbons. Our analysis also supports the use of several indicators previously proposed by other researchers, including [O3]/[HNO3] and [H2O2]/[HNO3]. We found that [HCHO]/[NO2] was more useful than the previously proposed [HCHO]/[NOy]. We found that the indicators easily distinguished extremely NOx‐limited or extremely radical‐limited regimes but did not reliably distinguish conditions closer to the transition between these two regimes. We propose that a combined analysis using photochemical model simulations and a large set of indicators of both [O3] sensitivity and local odd oxygen production (P(Ox)) sensitivity to VOC and NOx provides the most complete and useful description of [O3] sensitivity. Time series of the indicators, at least from mid‐morning to late afternoon, provide useful information about the evolution of [O3] sensitivity during the day. Values of the indicators change depending on the [O3] level due in part to the effects of miscellaneous OH and HO2 termination reactions and to the effects of the composition of the HC mixture on P(Ox). Further evaluation of these indicators using modeling studies, measurements, and test cases with NOx or VOC emissions changes are needed to determine how reliably they distinguish NOx‐and radical‐limited conditions.
Stratospheric ozone destruction: The importance of bromine relative to chlorine
Authors: Daniel J. S. , Solomon S. , Portmann R. W. , Garcia R. R.
Published: September 2002

The relative effectiveness of bromine compared to chlorine for destroying stratospheric ozone is explored. Two definitions previously used to quantify this relative effectiveness, typically referred to as α, are compared and a definition is presented to calculate α values applicable for column and global ozone loss. Calculations of α are made with a two‐dimensional radiative/chemical/dynamical model and suggest that bromine is roughly 45 times more effective than chlorine for global ozone destruction. The physical processes underlying this result are probed, and sensitivity studies are presented that show that relatively large changes invoked in the modeled transport and heterogeneous chemistry lead to changes in this value of <15%.
A model mechanism to describe oxidation of monoterpenes leading to secondary organic aerosol: 1. α‐pinene and β‐pinene
Authors: Barthelmie R. J. , Pryor S. C.
Published: September 2002

A model mechanism to describe the oxidation of α‐pinene and β‐pinene has been developed based on the chemistry of lower alkenes and smog chamber studies of gas and aerosol phase products of monoterpene oxidation described in the literature. The mechanism is designed to replace single‐step aerosol yield or fractional aerosol coefficient approaches currently used in numerical atmospheric chemistry transport models. The mechanism predicts the concentrations of condensable products which are partitioned between gas and aerosol phases based on the absorption approach of Odum et al. [1996] and Hoffmann et al. [1997] but uses a six‐product rather than a two‐product model. Model‐predicted aerosol concentrations and yields are similar to those observed in the smog chamber, and the timing of aerosol formation is in reasonable accord with that observed. The monoterpene mechanism is also used to develop aerosol “isopleths” based on NOx and α‐pinene concentrations and temperatures representative of those found in the atmosphere.
Retrieval of downwelling UV actinic flux density spectra from spectral measurements of global and direct solar UV irradiance
Authors: Kazadzis S. , Bais A. F. , Balis D. , Zerefos C. S. , Blumthaler M.
Published: September 2002

In the present study, a method is proposed for retrieving UV actinic flux density spectra at ground level from global and direct UV spectral irradiance measurements. The most important parameters to establish the relation between downwelling actinic flux density and global UV irradiance are the ratio of direct to global irradiance and the description of the angular distribution of the diffuse radiation field. Under clear skies, retrieved spectral actinic flux densities agreed with model calculations to within ±5% for a variety of aerosol conditions. The assumption that the diffuse radiation is isotropic leads to a wavelength‐dependent overestimation of actinic flux densities from 10 to 15%, depending on solar zenith angle, especially in atmospheres with high aerosol content. Various sources of uncertainties, which are involved in the proposed methodology, are discussed and quantified.
Optimizing quantitative precipitation estimates using a noncoherent and a coherent radar operating on the same area
Authors: Gabella Marco , Joss Jürg , Perona Giovanni
Published: September 2002

A 40 hour period of rainfall during the 1994 Piemonte flood is analyzed; data from 69 rain gages were considered as well as 5 min maximum reflectivity maps acquired by an old, noncoherent radar and a “new‐generation” Doppler one. The two C‐band radars share a similar, hostile radar detection environment: the good view from the radar sites produces severe ground clutter contamination in an adverse orography. Furthermore, we are working at long ranges (distances between radar and gages up to 214 km). The data of both radars have been postprocessed with a texture‐based technique for ground clutter removal, because, especially for noncoherent radar data, postprocessing brings a significant improvement. The macroscopic “biases” related to the radar detection environment are reduced using a correction technique based on a weighted, multiple regression. The correction technique requires effective clutter suppression. Best results are obtained by weighing the regression with the total amount of rain estimated by radar. Within the 40 hour observation period the average precipitation measured by the gages is 160 mm. The average of uncorrected radar values at gage locations is about 40 mm only (for both radars). Thanks to the correction procedure, both the underestimation and the normalized standard error are reduced by approximately a factor of 3 (the normalized standard error is defined as the standard deviation between radar and gages divided by the mean radar values at gage locations).
Satellite monitoring of fire in the EXPRESSO study area during the 1996 dry season experiment: Active fires, burnt area, and atmospheric emissions
Authors: Pereira José M. C. , Pereira Bárbara S. , Barbosa Paulo , Stroppiana Daniela , Vasconcelos Maria J. P. , Grégoire Jean‐Marie
Published: September 2002

Fire activity in central Africa was monitored with NOAA advanced very high resolution radiometer (AVHRR) satellite imagery, acquired in situ during the 1996 dry season campaign of the Experiment for Regional Sources and Sinks of Oxidants (EXPRESSO). The extent of the area affected by fire was estimated with a contextual active fire detection algorithm, and with a burnt area mapping approach, based on multiple fuzzy thresholds. The latter was considered to produce more accurate results. In a study area of 2×106km2, and during a 5‐week period, the areas affected were estimated at 112,578 and 525,820 km2by the active fires and burnt area algorithms, respectively. Biomass densities, combustion factors, and emissions factors for Sudanian savanna, Guinean savanna, and dense tropical forest vegetation types were obtained from the literature, and used to estimate biomass burnt (228–371 Tg), and pyrogenic emissions of aerosols (1.8–2.6 Tg) and of the trace gases CO2(374–609 Tg), CO (29.2–39.0 Tg), CH4(2.05–2.73 Tg), and NOx(1.1–1.4 Tg). Because of its high biomass density, the tropical forest was a major source of atmospheric emissions, in spite of the relatively small extent of area burnt in this ecosystem. This highlights the need for particularly accurate estimates of area burnt, biomass density, combustion factor, and emissions factor for the dense tropical forest, as well as the potential for a significant increase in regional pyrogenic emissions as a consequence of deforestation.
Simulated tropospheric NOx: Its evaluation, global distribution and individual source contributions
Authors: Levy H. , Moxim W. J. , Klonecki A. A. , Kasibhatla P. S.
Published: September 2002

Using the 11‐level Geophysical Fluid Dynamics Laboratory global chemical transport model, we simulate global tropospheric fields of NOx, peroxyacetyl nitrate (PAN), HNO3, and NOy, as well as the deposition of nitrate, extensively evaluate them against available observations from surface stations and aircraft missions, and quantify the contributions of individual natural and anthropogenic sources. The patterns and magnitudes of simulated and observed HNO3wet deposition are generally in good agreement around the globe. Scatterplots of model simulations versus aircraft observations for NOxand NOyfind ∼50% of the points within ±25%, find ∼75% within ±50%, and show no systematic global biases. Both simulated and observed vertical profiles have similar shapes with high levels (∼1 ppbv or greater) in the polluted boundary layer (BL), very low values in the remote BL, and values increasing from the middle to the upper troposphere. Simulated NOyHNO3, and NOx+ PAN are also in good agreement with extensive lower free tropospheric (FT) observations made at Mauna Loa Observatory. In general, the level of agreement between simulation and observation is as good as the agreement between separate, but simultaneous, observations of NO, NOx. or NOy. As previous studies have shown, fossil fuel combustion and biomass burning control NOxlevels in most of the lower half of the troposphere with a significant contribution from biogenic emissions. The exceptions are the remote low‐NOxregions where BL and FT sources make comparable contributions. Unlike most previous studies, we find that the much smaller in situ FT sources generally dominate in the upper half of the troposphere. Lightning dominates in the tropics and summertime midlatitudes, and stratospheric injection is the major source in the summer high latitudes. The exception is transported emissions from fossil fuel combustion, which dominate in winter high latitudes. Though seldom dominant, aircraft emissions do have a significant impact on the upper troposphere and lower stratosphere of the northern hemisphere extratropics.
Transport of ozone and pollutants from North America to the North Atlantic Ocean during the 1996 Atmosphere/Ocean Chemistry Experiment (AEROCE) intensive
Authors: Prados A. I. , Dickerson R. R. , Doddridge B. G. , Milne P. A. , Moody J. L. , Merrill J. T.
Published: September 2002

Research flights were conducted to investigate the episodic occurrence of enhanced ozone mixing ratios over the North Atlantic Ocean (NAO) during the spring as part of the Atmosphere/Ocean Chemistry Experiment (AEROCE). We measured meteorological variables, trace gases, and aerosol light scattering from the University of Wyoming King Air research aircraft during 16 research flights in April and early May 1996. The flights were conducted over eastern North America and over the NAO between the United States and Bermuda. On April 28, a cold front was located just west of Bermuda while a second system developed over the Midwest and central United States. In the midtroposphere, polluted layers with up to 250 ppbv CO, 150 pptv NO, and 1.8 ppbv NOy were observed within a deeper layer of enhanced O3 mixing ratios (up to 88 ppbv) over the NAO at around 32°N, 64–68°W. These aircraft observations, when coupled with ozone sondes, back trajectories, and satellite image loops, indicate two distinct meteorological mechanisms that combine to yield the observed chemical signature of the air. There is substantial meteorological evidence, supported by ozone sonde observations and earlier King Air flights, indicating that stratosphere/troposphere exchange associated with the upstream frontal system injects and advects dry, ozone‐rich air into midtropospheric regions over the continent. These subsiding air masses have the potential to result in deep layers of enhanced ozone in the offshore postfrontal area. Convection from the developing (upwind) system appears to lift air from the continental boundary layer into the area of subsidence in the mid to upper troposphere. The result is high concentrations of gaseous and particulate pollutants along with elevated quantities of ozone. Ozone mixing ratios exceed those attributable to boundary layer venting or in‐transit photochemical production. These meteorological processes lead to pollution and enhanced ozone from the stratosphere cooccurring in postfrontal air masses over the NAO.
Global gridded technical hexachlorocyclohexane usage inventories using a global cropland as a surrogate
Authors: Li Y. F.
Published: September 2002

A global cropland data set with 1° × 1° longitude and latitude resolution has been created by using an advanced very high resolution radiometer global land database with 1 km × 1 km resolution. By using this global gridded cropland data set as a surrogate, global technical HCH usage data for the years of 1950, 1960, 1970, 1980, and 1990 are distributed to a grid system with 1° × 1° longitude and latitude resolution. Usage data for these years versus latitude and longitude and for different continents are analyzed. A centroid analysis of the use trends of technical hexachlorocyclohexane (HCH) for these years versus latitude and longitude leads to the conclusion that the use of this insecticide was undergoing a “northward tilt” before 1960, a “southward tilt” after 1960, an “eastward tilt” between 1950 and 1980, and a “westward tilt” between 1980 and 1990. Air concentrations of α‐HCH, the major isomer of technical HCH, in the Arctic and Antarctic are also compared to the usage of technical HCH in the Northern and Southern Hemispheres, respectively. While the ratio between the total technical HCH usage in the Northern Hemisphere and that in the Southern Hemisphere is around 17 in 1980, 8 in 1987, and 9 in 1990, the ratio of air concentrations of α‐HCH in the Arctic to that in the Antarctic is around 14 between 1980 and 1981, 9 in 1987, and 11 between 1989 and 1990. This suggests that atmospheric concentrations of α‐HCH in the Arctic and the Antarctic have been responding to the technical HCH use in the Northern and the Southern Hemispheres, respectively. This also provides evidence that the quality of the global gridded technical HCH usage presented in this study is reasonably high.
Comparison of aerosol optical depth inferred from surface measurements with that determined by Sun photometry for cloud‐free conditions at a continental U.S. site
Authors: Bergin M. H. , Schwartz S. E. , Halthore R. N. , Ogren J. A. , Hlavka D. L.
Published: September 2002

Evaluation of the forcing of climate by aerosol scattering of shortwave radiation in cloud‐free conditions (direct aerosol forcing) requires knowledge of aerosol optical properties on relevant spatial and temporal scales. It is convenient to measure these properties at the surface. However, before these measurements can be used to quantitatively estimate direct climate forcing, it is necessary to determine the extent to which these properties are representative of the entire atmospheric column. In this paper we compare aerosol optical depth (AOD) determined by Sun photometry at the Southern Great Plains (SGP) Atmospheric Radiation Measurement (ARM) site in north central Oklahoma for several cloud‐free days with estimates of AOD based on two methods. First, the aerosol extinction measured at the surface (taken as the sum of the aerosol scattering and absorption coefficients at instrumental relative humidity of∼20%) is multiplied by the mixing height determined from temperature profiles from radiosonde measurements. Even under conditions of vigorous midday mixing this approach underestimates AOD by as much as 70% using dry aerosol measurements and by roughly 40% when hygroscopic growth of aerosol under ambient relative humidity is taken into account. This discrepancy is attributed primarily to underestimation of aerosol column extinction, as confirmed by examination of normalized aerosol backscatter profiles obtained from micropulse lidar (MPL), which show substantial contributions of aerosol loading above the atmospheric boundary layer. The second approach uses MPL profiles of normalized aerosol backscatter to estimate the vertical profile of aerosol extinction using surface values. The resulting AOD's are on average 30% less than measured values. This discrepancy is attributed to hygroscopic growth of aerosols in the atmospheric column. The results show that at the SGP site even under conditions of vigorous mixing in the atmospheric boundary layer the aerosol optical depth cannot be estimated with surface measurements of aerosol extinction unless information on the vertical profile of aerosol extinction is taken into account.
Latitudinal distribution of peroxyacetyl nitrate (PAN) over the Atlantic Ocean
Authors: Jacobi H.‐W. , Weller R. , Bluszcz T. , Schrems O.
Published: September 2002

Atmospheric concentrations of peroxyacetyl nitrate (PAN) were measured during a cruise of RV Polarstern from Cape Town (South Africa) to Bremerhaven (Germany) in May/June 1998 and at Ny‐Ålesund, Svalbard, in March 1998. The continuous in‐situ measurements were using electron capture gas chromatography combined with a cryogenic preconcentration technique. The time resolution of the measurements was 10 min with a detection limit of 5 parts per trillion by volume (pptv). The mixing ratios of PAN exhibited a maximum of 1100 pptv in the English Channel and values less than 5 pptv in tropical latitudes between 10°N and 10°S. The latitudinal distribution of PAN showed significantly higher concentrations in the northern hemisphere compared to the southern hemisphere. North of 10°N, daily maximum PAN concentrations normally exceeded 100 pptv. Mean mixing ratios decreased from 244 pptv between 50°N and 54°N to 13 pptv between 20°N and 30°N. Enhanced concentrations were detected in continentally influenced air masses identified by trajectory analysis and simultaneous measurements of black carbon and ozone. Intrusions of air masses from the upper troposphere could not be inferred from trajectory analysis. In temperate northern latitudes, enhanced PAN mixing ratios can be used as an indicator of long‐range transport of photochemically active pollutants.
Subsidence, mixing, and denitrification of Arctic polar vortex air measured during POLARIS
Authors: Rex M. , Salawitch R. J. , Toon G. C. , Sen B. , Margitan J. J. , Osterman G. B. , Blavier J.‐F. , Gao R. S. , Donnelly S. , Keim E. , Neuman J. , Fahey D. W. , Webster C. R. , Scott D. C. , Herman R. L. , May R. D. , Moyer E. J. , Gunson M. R. , Irion F. W. , Chang A. Y. , Rinsland C. P. , Bui T. P.
Published: September 2002

We determine the degree of denitrification that occurred during the 1996–1997 Arctic winter using a technique that is based on balloon and aircraft borne measurements of NOy, N2O, and CH4. The NOy/N2O relation can undergo significant change due to isentropic mixing of subsided vortex air masses with extravortex air due to the high nonlinearity of the relation. These transport related reductions in NOy can be difficult to distinguish from the effects of denitrification caused by sedimentation of condensed HNO3. In this study, high‐altitude balloon measurements are used to define the properties of air masses that later descend in the polar vortex to altitudes sampled by the ER‐2 aircraft (i.e., ∼20 km) and mix isentropically with extravortex air. Observed correlations of CH4 and N2O are used to quantify the degree of subsidence and mixing for individual air masses. On the basis of these results the expected mixing ratio of NOy resulting from subsidence and mixing, defined here as NOy**, is calculated and compared with the measured mixing ratio of NOy. Values of NOy and NOy** agree well during most parts of the flights. A slight deficit of NOy versus NOy** is found only for a limited region during the ER‐2 flight on April 26, 1997. This deficit is interpreted as indication for weak denitrification (∼2–3 ppbv) in that air mass. The small degree of denitrification is consistent with the general synoptic‐scale temperature history of the sampled air masses, which did not encounter temperatures below the frostpoint and had relatively brief encounters with temperatures below the nitric acid trihydrate equilibrium temperature. Much larger degrees of denitrification would have been inferred if mixing effects had been ignored, which is the traditional approach to diagnose denitrification. Our analysis emphasizes the importance of using other correlations of conserved species to be able to accurately interpret changes in the NOy/N2O relation with respect to denitrification.
Tropical tropospheric ozone (TTO) maps from Nimbus 7 and Earth Probe TOMS by the modified‐residual method: Evaluation with sondes, ENSO signals, and trends from Atlantic regional time series
Authors: Thompson Anne M. , Hudson Robert D.
Published: September 2002

The modified‐residual (MR) method for retrieving time‐averaged stratospheric ozone and tropospheric ozone column amounts from the Total Ozone Mapping Spectrometer (TOMS) is applied to the 14 complete calendar years of Nimbus 7 observations (1979–1992). These are available as digital data at http://metosrv2.umd.edu/∼tropo/14y_data.d The MR method has also been used to produce real‐time maps of tropical tropospheric ozone (TTO) from TOMS on the Earth‐Probe (1996‐present) and ADEOS platforms (1996–1997). Evaluation of the TTO time series for 1979–1990 and 1997–1998 is presented here; it is limited to the few tropical ozonesonde stations operational during those years (Ascension Island; Natal, Brazil; Brazzaville). The standard deviation of the differences between TTO and the sondes is ±(6–7) Dobson units (DU), depending on location. Stratospheric column ozone, which is also derived by the modified‐residual method, compares favorably with sondes (to within 6–9 DU) and with stratospheric ozone inferred from other satellites (usually 8–15 DU lower than the latter). TTO time series and the magnitude of the tropospheric wave‐one pattern show El Niño‐Southern Oscillation (ENSO) signals during the period from 1979–1992. During 1997 the ENSO stands out at some stations, but not at others. Between 12°N and 12°S, zonally averaged TTO shows no significant trend from 1980–1990. Trends are also not significant during this period in localized regions, for example, from just west of South America across to southern Africa. This is consistent with the ozonesonde record at Natal, Brazil (the only tropical ozone data publicly available for the 1980s), which shows no significant trend. The lack of trend in tropospheric ozone agrees with a statistical analysis based on another method for deriving TTO from TOMS, the convective‐cloud‐differential approach of Ziemke et al. [1998].
Maintenance of high HCl/Cly and NOx/NOy, in the Antarctic vortex: A chemical signature of confinement during spring
Authors: Michelsen H. A. , Webster C. R. , Manney G. L. , Scott D. C. , Margitan J. J. , May R. D. , Irion F. W. , Gunson M. R. , Russell J. M. , Spivakovsky C. M.
Published: September 2002

Observations made in the 1994 Antarctic vortex show that Cly recovered completely into HCl following conversion of Cly reservoir species to active radicals, and NOx constituted a 4–5 times higher fraction of NOy inside the vortex than outside. Measurements made in October and November from the Airborne Southern Hemisphere Ozone Expedition/Measurements of the Atmospheric Effects of Stratospheric Aircraft (ASHOE/MAESA) ER‐2 aircraft mission, the third Atmospheric Laboratory for Applications and Science (ATLAS‐3) space shuttle mission, and the Upper Atmosphere Research Satellite (UARS) demonstrate that this unusual partitioning of Cly and NOy was maintained for at least 4 weeks in the springtime vortex. In response to severe ozone loss, abundances of HCl and NOx remained high despite temperatures low enough to reactivate Cly and convert NOx to HNO3 via heterogeneous processes. Thus, under severely ozone depleted conditions, high HCl and NOx abundances in the vortex are maintained until the vortex breaks up or an influx of ozone‐rich extravortex air is entrained into the vortex. These observations suggest that the flux of extravortex air entering the core of the lower stratospheric vortex was small or negligible above ∼400 K during late spring, despite weakening of the vortex during this time period. Results of a photochemical model constrained by the measurements suggest that extravortex air entrained into the vortex during October and early November made up less than 5% of the vortex core air at 409 K. The model results also show that heterogeneous chemistry has little effect on the Cly and NOy partitioning once high abundances of HCl have been attained under ozone depleted conditions, even when aerosol loading is high.
Biogenic volatile organic compound emissions in central Africa during the Experiment for the Regional Sources and Sinks of Oxidants (EXPRESSO) biomass burning season
Authors: Greenberg J. P. , Guenther A. B. , Madronich S. , Baugh W. , Ginoux P. , Druilhet A. , Delmas R. , Delon C.
Published: September 2002

The recent aircraft and ground‐based Experiment for the Regional Sources and Sinks of Oxidants (EXPRESSO) campaign in central Africa studied atmospheric trace gases and aerosols during the biomass burning season. Isoprene, emitted from vegetation, was the most abundant nonmethane hydrocarbon observed over the forest and savanna, even though intense biomass burning activity was occurring several hundred kilometers to the north. The isoprene flux, measured directly from midmorning to noon by a relaxed eddy accumulation technique, was approximately 890 μg isoprene m−2 h−1 from the tropical rain forest and semideciduous forest landscapes and 570 μg isoprene m−2 h−1 from transitional and degraded woodland landscapes. Model estimates derived from satellite landscape characterization coupled with leaf enclosure emission measurements conducted during EXPRESSO compared well with these measured fluxes. Isoprene concentrations and fluxes were used to determine the oxidant balance over the forest and savanna. Radiative transfer calculations indicate that the observed strong vertical gradient of the NO2 photolysis rate coefficient could be explained by the presence of substantial amounts of absorbing aerosols, probably from biomass burning. Chemical (box) model simulations of the planetary boundary layer (PBL), constrained by measured isoprene emission fluxes and concentrations, show that this suppression of photolytic radiation lowers OH concentrations by about a factor of 2 relative to aerosol‐free conditions. Consequently, the direct contribution of PBL photochemistry to ozone production, especially from biogenic isoprene, is small.
Effect of environmental conditions on volcanic plume rise
Authors: Graf Hans‐F. , Herzog Michael , Oberhuber Josef M. , Textor Christiane
Published: September 2002

Sensitivity studies were performed with a complex nonhydrostatic volcano plume model that explicitly treats turbulence and microphysics. The impact of environmental conditions such as wind, temperature and humidity profiles was studied for standard observational data. To investigate the wind effects, a two‐dimensional Cartesian formulation of the model was used, while for the temperature and humidity effects a cylindrical coordinate system had to be applied, since this treats the entrainment process more realistically. It was found that horizontal wind generally reduces the height of the ash plume. The gaseous part of the plume sometimes may rise higher than without wind owing to the more effective separation between gas and solid material. Besides reduced static stability, the absolute temperature and humidity also increase the plume height. All environmental impacts strongly depend on the strength of entrainment and thus on the quality of the prognostic turbulence.
Actinic fluxes and photodissociation coefficients in cloud fields embedded in realistic atmospheres
Authors: Trautmann Thomas , Podgorny Igor , Landgraf Jochen , Crutzen Paul J.
Published: September 2002

The spatial distribution of the actinic flux is investigated for a realistic scattering and absorbing atmosphere with embedded two‐dimensional (2‐D) clouds. Three different models are intercompared for computing the actinic flux: MCC4, a versatile Monte Carlo code, SHDOM, a freely available code for solving the multidimensional integral form of the radiative transfer equation, and DISORT, a widely used discrete ordinate code for plane‐parallel media. Three different cloud scenarios are chosen; plane‐parallel clouds, internally homogeneous rectangular cloud bands, and a realistically variable stratocumulus cloud field. For scattered cloud fields the results show that both Rayleigh scattering as well as ground reflection significantly smooth out the actinic flux field. For cloud fields with realistic spatially inhomogeneous liquid water distributions, local maxima of the actinic flux are strongly correlated with the corresponding maxima in cloud water. The spatial patterns of the actinic flux field as computed with MCC4 and SHDOM agree very well. The horizontally averaged flux results of MCC4 and SHDOM are within 1–2% of each other. For the 2‐D cloud cases considered, SHDOM turned out to be 1 to 2 orders of magnitude faster than MCC4. For the stratocumulus cloud photodissociation coefficients for NO2 and O(1D) were computed with SHDOM and compared with the plane‐parallel (PP), independent pixel (IP), and locally smoothed IP (SIP) approximations. The PP is characterized by a general underestimation of the photodissociation coefficients in and below the cloud, whereas the IP reproduces horizontal averages satisfactorily well. Local biases of the IP near cloud top can be significantly reduced by employing the SIP.
Observational study of transport and photochemical formation of ozone over northern Europe
Authors: Laurila Tuomas
Published: September 2002

Seven years of ozone and total nitrate (the sum of gaseous HNO3 and particulate NO3−) data are analyzed to quantify the seasonal cycles of ozone production and transport in the boundary layer at 60°N over the Baltic Sea. This analysis is based on the statistical correlations between ozone and total nitrate and the covariances between trace species and wind components. Ozone production efficiencies are highest near the summer solstice, decreasing to low values in March, April, and October. In winter, precursor emissions effectively deplete ozone. Horizontal average and transient fluxes of ozone and total nitrate in the boundary layer were estimated as the covariances between trace gas concentrations and horizontal wind components. A year‐round northward flux of total nitrate, 2.0 μg(N) m−2 s−1 as annual average, was observed. This is the sum of a northeastward flux by the average wind field and a northwestward flux by synoptic‐scale transient eddies. Embedded in an east‐northeastward flux of background ozone, the sums of the components of the transient and estimated average flux divergence over Europe show large northward fluxes in summer and southward fluxes in winter. In summer, these fluxes are due to photochemically produced elevated ozone concentrations. In winter, high winds advect background ozone into the European continent which then acts as a sink area. The average effect of European precursor emissions on the horizontal flux was estimated assuming that it is the sum of the transient eddy component and the estimated average upwind divergence component over Europe. The northward component of these fluxes is higher in summer, with a highest monthly average in July of 49 μg m−2s−1, compared to the southward fluxes in winter, with a highest value of 44 μg m−2 s−1 in December, resulting in a northward yearly average flux of 6 μg m−2 s−1. The sum of the transient eddy flux component and the estimated average divergence flux component over Europe is estimated to affect monthly horizontal total ozone fluxes by 32–68% in the period April‐September, but only by 3–23% during the winter months.
Ternary nucleation of H2SO4, NH3, and H2O in the atmosphere
Authors: Korhonen P. , Kulmala M. , Laaksonen A. , Viisanen Y. , McGraw R. , Seinfeld J. H.
Published: September 2002

Classical theory of binary homogeneous nucleation is extended to the ternary system H2SO4‐NH3‐H2O. For NH3 mixing ratios exceeding about 1 ppt, the presence of NH3 enhances the binary H2SO4‐H2O nucleation rate by several orders of magnitude. The Gibbs free energies of formation of the critical H2SO4‐NH3‐H2O cluster, as calculated by two independent approaches, are in substantial agreement. The finding that the H2SO4‐NH3‐H2O ternary nucleation rate is independent of relative humidity over a large range of H2SO4 concentrations has wide atmospheric consequences. The limiting component for ternary H2SO4‐NH3‐H2O nucleation is, as in the binary H2SO4‐H2O case, H2SO4; however, the H2SO4 concentration needed to achieve significant nucleation rates is several orders of magnitude below that required in the binary case.
Studying ozone climatology with a regional climate model: 2. Climatology
Authors: Bouchet V. S. , Laprise R. , Torlaschi E. , McConnell J. C. , Plummer D. A.
Published: September 2002

The Canadian Regional Climate Model with oxidant chemistry (CRCM) has been used to study the climatology and transport of ground‐level ozone in eastern North America. The climatology is derived from simulations of five consecutive Julys from 1990 to 1994. Biogenic emissions are calculated each time step, while anthropogenic emissions are taken to be identical for each simulated July. Comparisons of daily ozone maximum for each individual month suggest that the model is simulating satisfactorily the month‐long periods and usually correctly reproduces the high and low ozone regimes associated with the different parts of the domain and/or the various synoptic conditions, the Ohio region being the exception. The climatological number of exceedences above 80 ppbv, calculated as the monthly average for the 5 years, yields better agreement with the number of exceedences calculated from the observed climatological O3 field than those for the individual months. This behavior suggests that a large fraction of the errors present in each simulation are randomly distributed. Thus it should be possible to obtain fields that are more representative of reality, for applications such as emission reduction scenario studies, by averaging over several long simulations.
Nonlocal thermodynamic equilibrium vibrational, rotational, and spin state distribution of NO(ν = 0, 1, 2) under quiescent atmospheric conditions
Authors: Funke B. , López‐Puertas M.
Published: September 2002

A nonlocal thermodynamic equilibrium (non‐LTE) model for the NO(ν ≤ 2, J, S) state distributions for altitudes from ground to 200 km for nonauroral conditions is presented. The model includes (1) vibrational excitation due to collisions of NO with O and O2, (2) excitation due to photolysis of NO2 and due to NO2 + O→ NO + O2 in the stratosphere, and (3) excitation due to N(4S,2 D) +O2 → NO + O in the thermosphere. Spontaneous emission, induced emission, and absorption of tropospheric and solar radiation are included. Intravibrational spin and rotational relaxations are included by means of an exponential power gap law approach. This model improves upon previous work mainly through its completeness. It provides non‐LTE distributions of the three spin, rotational, and vibrational degrees of freedom covering the whole atmosphere up to the upper thermosphere. All previous knowledge of non‐LTE excitation processes of NO has been incorporated. The most important results of the model are (1) the stratospheric daytime vibrational distribution departs from local thermodynamic equilibrium (LTE) due to NO(ν≥1) production by NO2 photolysis; (2) the rotational and spin distributions of the NO(ν = 1, 2) are in non‐LTE above ∼110 km; (3) a 90% propensity to conserve the spin orbit state in intravibrational NO+O collisional relaxation was determined by comparison of the model results to cryogenic infrared radiance for shuttle (CIRRIS‐1A) data. The implications of the non‐LTE model results to remote sensing of stratospheric NO abundances from spectrally resolved 5.3 μm limb radiances are discussed. The neglect of thermospheric rotational/spin non‐LTE as well as the neglect of NO(ν>1) production by NO2 photolysis map into errors in the modeled spectra of 20–25%.
A method to derive long‐term time series and trends of UV‐B radiation (1968–1997) from observations at Hohenpeissenberg (Bavaria)
Authors: Gantner Leonhard , Winkler Peter , Köhler Ulf
Published: September 2002

The aim of this study is to estimate the solar ultraviolet (UV) radiation reaching the ground during the last 30 years. Solar UV‐B irradiance (290–320 nm) measured by the Brewer spectrophotometer 10 at Hohenpeissenberg, Germany (48°N, 11°E), was compared with other ground‐based measurements at Hohenpeissenberg. Eight years (1990–1997) of operationally measured UV were used to establish statistical relations between spectral UV and total ozone amount. Data were included in the analysis only if the sunshine recorder reported 10/10. This criterion allows clouds to be present, but only optically thin clouds may cover the Sun. The resulting data set contains approximately one fourth of all available measurements. The atmospheric conditions were classified by four well‐defined groups. Turbidity classes were derived from the ratio of global and diffuse radiation, which is well correlated with aerosol optical thickness during sunshine. Measurements of total ozone, global, and diffuse radiation are available for the last 30 years. Thus it is possible to calculate the mean maximum UV based on monthly mean total ozone values during this period from the statistical model. The mean maximum UV is characterized by a value of solar zenith angle (SZA) which roughly represents the monthly mean of the daily SZA at local noon. The decrease in total ozone has led to an increase in UV. The spectral global irradiance at 300 nm at a solar zenith angle of 40° increased in April by about 100±10% and in August by about 25±3% in the 30 year period. At 305 nm we found an increase of about 40±4% in April and of only about 10±2% in August. In general, the increase in UV radiation is stronger early in the year and at short wavelength (300 nm) than in summer and autumn and at longer wavelength.