bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of forest fire on the fluxes of CO2, CH4 and N2O in boreal forest soils, interior Alaska
Authors: Kim Yongwon , Tanaka Noriyuki
Published: May 2003

Flux measurements at sites of mixed hardwood and black spruce stands from an area (C4) of the Caribou‐Poker Creek Research Watershed (CPCRW), interior Alaska, in the summer seasons of 1998, 1999, and 2000 are used to estimate the fluxes of CO2, CH4, and N2O before and after forest fires. The FROSTFIRE burning experiment was executed in typical boreal forest from 8 to 15 July 1999. The forest fire significantly decreased soil CO2 and N2O emissions, by at most 50%. On the contrary, CH4 flux from the soil increased from 7 to 142%, suggesting that the forest fire plays a role in accelerative thawing of the frozen soil, and subsequently the release of CH4 from permafrost. Most of the CH4 was oxidized in the soil after the fire; however, some was released from the soil when the permafrost maximally thawed at the end of August 1999 and September 2000. Relationships between the fluxes of trace gases and soil temperature before and after the fire showed good exponential correlations, indicating that soil temperature was one of the factors determining the fluxes of trace gases on boreal forest soils. Also, the higher soil temperature after the fire may be led to the enhanced diffusion of CO2, CH4 and N2O by microbial activity between the atmosphere and the forest soils, and to the increased fluxes of trace gases in burned black spruce stand soils. In order to understand the roles of moss and lichen mats on the black spruce stands, the net respiration by mosses and lichens was estimated with light and dark chamber measurements. Net respiration corresponds to 42 to 58% of the total soil respiration before fire. Therefore, the net respiration by moss and lichen layers was responsible for one‐half of total soil CO2 emissions. The maximum regional net respiration rate by moss and lichen mats on black spruce forest floors of central Alaska was 0.018 ± 0.009 GtC/yr, an important source of atmospheric CO2 in boreal forests. After the prescribed burn, soil respiration was attributable only to respiration by roots and microbes. The microbial respiration estimated after the fire is almost three times as high as that the calculated respiration before the fire. This finding indicates the post‐fire condition may stimulate microbial respiration because of higher nutrients and substrates in remnant soils and enhanced soil temperature. The microbial respiration can be estimated 14.7 tC/ha in burned black spruce stands over a decade after the fire, suggesting burned black spruce forests in central Alaska are a crucial source of atmospheric CO2.
Reliability of biomass burning estimates from savanna fires: Biomass burning in northern Australia during the 1999 Biomass Burning and Lightning Experiment B field campaign
Authors: Russell‐Smith Jeremy , Edwards Andrew C. , Cook Garry D.
Published: May 2003

This paper estimates the two‐daily extent of savanna burning and consumption of fine (grass and litter) fuels from an extensive 230,000 km2region of northern Australia during August‐September 1999 encompassing the Australian continental component of the Biomass Burning and Lightning Experiment B (BIBLE B) campaign [Kondo et al., 2002]. The extent of burning for the study region was derived from fire scar mapping of imagery from the advanced very high resolution radiometer (AVHRR) on board the National Oceanic and Atmospheric Administration (NOAA) satellite. The mapping was calibrated and verified with reference to one Landsat scene and associated aerial transect validation data. Fine fuel loads were estimated using published fuel accumulation relationships for major regional fuel types. It is estimated that more than 43,000 km2was burnt during the 25 day study period, with about 19 Mt of fine (grass and litter) fuels. This paper examines assumptions and errors associated with these estimates. It is estimated from uncalibrated fire mapping derived from AVHRR imagery that 417,500 km2of the northern Australian savanna was burnt in 1999, of which 136,405 km2, or 30%, occurred in the Northern Territory study region. Using generalized fuel accumulation equations, such biomass burning consumed an estimated 212.3 Mt of fine fuels, but no data are available for consumption of coarse fuels. This figure exceeds a recent estimate, based on fine fuels only, for the combined Australian savanna and temperate grassland biomass burning over the period 1990–1999 but is lower than past estimates derived from classification approaches. We conclude that (1) fire maps derived from coarse‐resolution optical imagery can be applied relatively reliably to estimate the extent of savanna fires, generally with 70–80% confidence using the approach adopted here, over the major burning period in northern Australia and (2) substantial further field assessment and associated modeling of fuel accumulation, especially of coarse fuels, is required.
Speciation of volatile organic compound emissions for regional air quality modeling of particulate matter and ozone
Authors: Makar P. A. , Moran M. D. , Scholtz M. T. , Taylor A.
Published: May 2003

A new classification scheme for the speciation of organic compound emissions for use in air quality models is described. The scheme uses 81 organic compound classes to preserve both net gas‐phase reactivity and particulate matter (PM) formation potential. Chemical structure, vapor pressure, hydroxyl radical (OH) reactivity, freezing point/boiling point, and solubility data were used to create the 81 compound classes. Volatile, semivolatile, and nonvolatile organic compounds are included. The new classification scheme has been used in conjunction with the Canadian Emissions Processing System (CEPS) to process 1990 gas‐phase and particle‐phase organic compound emissions data for summer and winter conditions for a domain covering much of eastern North America. A simple postprocessing model was used to analyze the speciated organic emissions in terms of both gas‐phase reactivity and potential to form organic PM. Previously unresolved compound classes that may have a significant impact on ozone formation include biogenic high‐reactivity esters and internal C6–8 alkene‐alcohols and anthropogenic ethanol and propanol. Organic radical production associated with anthropogenic organic compound emissions may be 1 or more orders of magnitude more important than biogenic‐associated production in northern United States and Canadian cities, and a factor of 3 more important in southern U.S. cities. Previously unresolved organic compound classes such as low vapour pressure PAHs, anthropogenic diacids, dialkyl phthalates, and high carbon number alkanes may have a significant impact on organic particle formation. Primary organic particles (poorly characterized in national emissions databases) dominate total organic particle concentrations, followed by secondary formation and primary gas‐particle partitioning. The influence of the assumed initial aerosol water concentration on subsequent thermodynamic calculations suggests that hydrophobic and hydrophilic compounds may form external mixtures, and that separate treatment for these groups may be required in future air quality model simulations. The post‐processing model used here overestimates the organic particle formation relative to measurements, lacks the complexity of a regional air quality model, and is not intended as an alternative to the latter. Results from the post‐processing model do, however, provide guidance for the treatment of organic gases and particles in future air quality modeling work. Future air quality model simulations should attempt to speciate primary particulate organic compounds and include more detailed organic compound classes. Future emissions profile measurements should speciate gaseous high‐molecular‐mass organic compounds and primary organics emitted in particulate form (primary particle emissions are only available as a total particulate mass in currently available emissions data).
Emission estimates of selected volatile organic compounds from tropical savanna burning in northern Australia
Authors: Shirai T. , Blake D. R. , Meinardi S. , Rowland F. S. , Russell‐Smith J. , Edwards A. , Kondo Y. , Koike M. , Kita K. , Machida T. , Takegawa N. , Nishi N. , Kawakami S. , Ogawa T.
Published: May 2003

Here we present measurements of a range of carbon‐based compounds: carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), nonmethane hydrocarbons (NMHCs), methyl halides, and dimethyl sulfide (DMS) emitted by Australian savanna fires studied as part of the Biomass Burning and Lightning Experiment (BIBLE) phase B aircraft campaign, which took place during the local late dry season (28 August to 13 September 1999). Significant enhancements of short‐lived NMHCs were observed in the boundary layer (BL) over the region of intensive fires and indicate recent emissions for which the mean transport time was estimated to be about 9 hours. Emission ratios relative to CO were determined for 20 NMHCs, 3 methyl halides, DMS, and CH4based on the BL enhancements in the source region. Tight correlations with CO were obtained for most of those compounds, indicating the homogeneity of the local savanna source. The emission ratios were in good agreement with some previous measurements of savanna fires for stable compounds but indicated the decay of emission ratios during transport for several reactive compounds. Based on the observed emission ratios, emission factors were derived and compared to previous studies. While emission factors (g species/kg dry mole) of CO2varied little according to the vegetation types, those of CO and NMHCs varied significantly. Higher combustion efficiency and a lower emission factor for methane in this study, compared to forest fires, agreed well with results for savanna fires in other tropical regions. The amount of biomass burned was estimated by modeling methods using available satellite data, and showed that 1999 was an above average year for savanna burning. The gross emissions of the trace gases from Australian savanna fires were estimated.
Effect of stand age on whole ecosystem CO2exchange in the Canadian boreal forest
Authors: Litvak Marcy , Miller Scott , Wofsy Steve C. , Goulden Michael
Published: May 2003

One of the key steps to estimating the current and future contribution of boreal forests to the global carbon cycle is quantifying the role recovery from fire plays in stand level carbon dynamics. We used tower‐based eddy covariance to measure the CO2exchange above five black spruce stands in central Manitoba during the 1999 and 2000 growing seasons (June–September). Fluxes in the four youngest stands (11, 19, 36, and 70 year old burns) were measured using portable eddy flux systems stationed in each burn for 4 to 6 weeks. Fluxes in the oldest stand (∼1870 burn) were measured continuously throughout 1999 and 2000, providing a baseline for interpretation of the fluxes measured simultaneously in the younger stands. Light‐saturated CO2uptake was lowest in the 11 year old stand (−4.1 μmol m−2s−1), high in the 19 year old stand (−8.9 μmol m−2s−1), highest in the 36 year old stand (−10.1 μmol m−2s−1), and moderate in the 70 and 130 year old stands (−6.3 and −7.1 μmol m−2s−1, respectively). Whole‐ecosystem respiration was lowest in the youngest burn and consistently increased with stand age. Integrated daily carbon balance changed from a slight sink at the 11 year old stand (−0.20 g C m−2d−1) to a modest sink at the 19 year old stand (−1.9 g C m−2d−1) to a large sink at the 36 year old stand (−3.1 g C m−2d−1) to a modest sink at the 70 year old stand (−0.53 g C m−2d−1) to around zero at the 130 year old stand. The results from any single tower in a boreal region are therefore unlikely to be representative of the entire region. Reliable assessments of regional carbon balance will require an approach that incorporates information on the fractional coverage of stands in different age classes and measurements of ecosystem gas exchange by representative stands within each age class.
Influence of polar ozone loss on northern midlatitude regions estimated by a high‐resolution chemistry transport model during winter 1999/2000
Authors: Marchand Marion , Godin Sophie , Hauchecorne Alain , Lefèvre Franck , Bekki Slimane , Chipperfield Martyn
Published: May 2003

The Modèle Isentropique de transport Mésoéchelle de l'Ozone Stratosphérique par Advection avec CHIMie (MIMOSA‐CHIM) three‐dimensional high‐resolution chemical transport model has been developed to estimate the contribution of polar ozone destruction to the lower stratosphere ozone budget at midlatitudes. The ability of the model to reproduce the evolution of polar and midlatitude ozone during the 1999/2000 winter is first evaluated by comparisons against lidar and sonde measurements. The modeled potential vorticity (PV) fields are also compared with PV fields derived from ECMWF analyses and the chemical fields of the model are compared with the output of a large‐scale chemical transport model in order to highlight the interest of a high‐resolution model for resolving fine‐scale structures such as polar filaments. A PV‐based analysis is performed to estimate the area covered by polar air, vortex, and filaments in the 45°N–55°N latitude band at 475K and their contribution to ozone loss. The polar air contribution was found to represent usually between 20% and 40% of the total ozone loss in this latitude band but can reach 50% during large vortex intrusions. At 475K, the total chemical ozone loss in nonpolar air between 45°N and 55°N increases from 1% in mid‐December to 15% at the end of March. Several chemical ozone tracers are considered to investigate the origin of the ozone loss in nonpolar air. These tracers allow us to quantify the amount of chemical ozone destruction that occurred in the vortex, in the polar filamentary structures, and in the nonpolar air. Until February, the main contributor to the nonpolar ozone loss is in situ destruction at midlatitudes, but the contribution from the ozone destruction in the polar vortex increases steadily during the winter and represents about 50% of the total midlatitude ozone loss in April, after the vortex breakup. The contribution from the ozone destruction within filamentary structures is found to be quasi‐negligible as a result of the limited number of filaments.
Erythemally weighted UV variations at two high‐altitude locations
Authors: Zaratti F. , Forno R. N. , García Fuentes J. , Andrade M. F.
Published: May 2003

A field campaign, with the purpose of measuring the rate of integrated erythemally weighted ultraviolet radiation (UVR) with altitude, was carried out at two high‐altitude locations near La Paz, Bolivia. The horizontal distance between the two sites is 25 km and they differ in height by 2 km. Irradiance data were obtained only on clear days and were analyzed using three different methods: (1) by comparison of integrated erythemal solar UV irradiances (daily doses) at both locations; (2) by calculation of the slope of the plot of the irradiance at the higher location against the irradiance at the lower location at the same time; and (3) by the asymptotic ratio, based on Beer's law, of the irradiances. The results show that erythemally weighted UV irradiance increases with altitude at an approximate rate of 7% per kilometer. The experimental data were compared with the output of a well‐known radiative model for local conditions. A sensitivity study of the nonmeasured input parameters indicated that the modeled rates of increase are slightly lower than the measured ones. Even more, these rates are not very sensitive to reasonable variations of the unknown parameters.
Soil organic carbon and CO2 respiration at subzero temperature in soils of Arctic Alaska
Authors: Michaelson G. J. , Ping C. L.
Published: May 2003

Carbon dioxide respiration rates were measured in the laboratory at −2°C for 88 arctic Alaska soil horizon samples. Soil horizon samples represented most ATLAS C‐flux study sites to a 1‐m depth. Sites represented extend across the Alaska Arctic, from tundra at Prudhoe Bay (70°N, 149°W) in the northeast to the tundra–forest transition zone of the Seward Peninsula (64°N, 163°W) in the southwest, a range of nearly 1000 km. Results of an initial temperature sensitivity study indicated that at 4°C, organic horizons (>20% organic carbon [OC]) have average respiration rates that are twice as high as those found for the mineral horizons. However, as temperature dropped to −2°C, respiration in organic horizons was about the same as that for the low OC mineral horizons (with <5% OC). However, organic enriched mineral horizons (with OC 5–20%) maintained higher rates of respiration at the subzero temperature, and rates averaged 1.7 times higher than for the organics at −2°C. Results of −2°C incubations for all study samples indicated that, indeed, there is a good correlation between soil water‐soluble OC (wsOC) stocks and CO2 respiration rates in both organic surface horizons and among the high respiring organic enriched mineral horizons, R2 = 0.70 (p < 0.01) and R2 = 0.62 (p < 0.01), respectively. The presence of permafrost was associated with increased wsOC substrate levels, and the highest respiration rate found over all samples was in an organic enriched (17.8% OC) mineral horizon from the permafrost layer. High levels of substrates and high respiration rates were associated with surface organic horizons from sites at the forest–tundra transition (Council) for all sites with shrub vegetation and for Eriophorum tussocks at Toolik Lake. Stocks of wsOC were not correlated to soil total OC levels. These results point to the importance of a soil carbon–substrate shift with subzero temperature and its implications for cold‐season processes throughout the soil active layer.
Modeling biogeochemical responses of tundra ecosystems to temporal and spatial variations in climate in the Kuparuk River Basin (Alaska)
Authors: Le Dizès S. , Kwiatkowski B. L. , Rastetter E. B. , Hope A. , Hobbie J. E. , Stow D. , Daeschner S.
Published: May 2003

We used a process‐based ecosystem model (Marine Biological Laboratory General Ecosystem Model (MBL‐GEM III)) to predict and analyze biogeochemical responses of Arctic tundra ecosystems to past (1921–2000) and future (2001–2100) changes in climate and atmospheric CO2 in the Kuparuk River Basin, Alaska. We first calibrated the model by deriving a single parameter set that closely simulated the response of moist tussock tundra to decade‐long experimental manipulations of nutrients, temperature, light, and atmospheric CO2 at Toolik Lake on the North Slope of Alaska. We then applied the parameterized model to the entire Kuparuk River Basin over 180 years. The model predicted that warming and drying resulted in a short‐term source of CO2 on annual timescales but resulted in a CO2 sink on decadal timescales. These predictions are consistent with recent measurements. A time series analysis has identified that while the immediate response to warming is to release C, the response a year later is to store C. This 1‐year lag is consistent with other work that has shown a similar lag in C storage and normalized difference vegetation index (NDVI) on a global scale. Our simulation results indicated that by 2100 high CO2 and warming will increase C sequestration, mostly as a result of (1) an increase in vegetation C:N ratio, which occurs across the Kuparuk Basin, and (2) a redistribution of N from soils (with low C:N ratios) to vegetation (with high C:N ratios), which occurs mainly in ecosystems in the basin that are initially productive, dry, and warm. These results are consistent with the observation of increased shrubiness in Alaskan tundra over the past few decades. Our application of the model has been hindered by the lack of climate data for the region, especially precipitation. A number of other general issues have been identified for making progress in modeling spatial and temporal C dynamics of Arctic tundra.
Impact of biomass burning on cloud properties in the Amazon Basin
Authors: Roberts G. C. , Nenes A. , Seinfeld J. H. , Andreae M. O.
Published: May 2003

We used a one‐dimensional (1‐D) cloud parcel model to assess the impact of biomass‐burning aerosol on cloud properties in the Amazon Basin and to identify the physical and chemical properties of the aerosol that influence droplet growth. Cloud condensation nuclei (CCN) measurements were performed between 0.15% and 1.5% supersaturation at ground‐based sites in the states of Amazonas and Rondônia, Brazil during several field campaigns in 1998 and 1999 as part of the Large‐Scale Biosphere–Atmosphere (LBA) Experiment in Amazonia. CCN concentrations measured during the wet season were low and resembled concentrations more typical of marine conditions than most continental sites. During the dry season, smoke aerosol from biomass burning dramatically increased CCN concentrations. The modification of cloud properties, such as cloud droplet effective radius and maximum supersaturation, is most sensitive at low CCN concentrations. Hence, we could expect larger interannual variation of cloud properties during the wet season that the dry season. We found that differences between CCN spectra from forested and deforested regions during the wet season are modest and result in modifications of cloud properties that are small compared to those between wet and dry seasons. Our study suggests that the differences in surface albedo, rather than cloud albedo, between forested and deforested regions may dominate the impact of deforestation on the hydrological cycle and convective activity during the wet season. During the dry season, on the other hand, cloud droplet concentrations may increase by up to 7 times, which leads to a model‐predicted decrease in cloud effective radius by a factor of 2. This could imply a maximum indirect radiative forcing due to aerosol as high as ca. −27 W m−2 for a nonabsorbing cloud. Light‐absorbing substances in smoke darken the Amazonian clouds and reduce the net radiative forcing, and a comparison of the Advanced Very High Resolution Radiometer (AVHRR) analysis and our modeling studies suggests that absorption of sunlight due to smoke aerosol may compensate for about half of the maximum aerosol effect. Sensitivity tests show that complete characterization of the aerosol is necessary when kinetic growth limitations become important. Subtle differences in the chemical and physical makeup are shown to be particularly influential in the activation and growth behavior of the aerosol. Knowledge of the CCN spectrum alone is not sufficient to fully capture the climatic influence of biomass burning.
Phytomass, LAI, and NDVI in northern Alaska: Relationships to summer warmth, soil pH, plant functional types, and extrapolation to the circumpolar Arctic
Authors: Walker D. A. , Epstein H. E. , Jia G. J. , Balser A. , Copass C. , Edwards E. J. , Gould W. A. , Hollingsworth J. , Knudson J. , Maier H. A. , Moody A. , Raynolds M. K.
Published: May 2003

We examined the effects of summer warmth on leaf area index (LAI), total aboveground phytomass (TAP), and normalized difference vegetation index (NDVI) across the Arctic bioclimate zone in Alaska and extrapolated our results to the circumpolar Arctic. Phytomass, LAI, and within homogeneous areas of vegetation on acidic and nonacidic soils were regressed against the total summer warmth index (SWI) at 12 climate stations in northern Alaska (SWI = sum of mean monthly temperatures greater than 0°C). SWI varies from 9°C at Barrow to 37°C at Happy Valley. A 5°C increase in the SWI is correlated with about a 120 g m−2 increase in the aboveground phytomass for zonal vegetation on acidic sites and about 60 g m−2 on nonacidic sites. Shrubs account for most of the increase on acidic substrates, whereas mosses account for most of the increase on nonacidic soils. LAI is positively correlated with SWI on acidic sites but not on nonacidic sites. The NDVI is positively correlated with SWI on both acidic and nonacidic soils, but the NDVI on nonacidic parent material is consistently lower than the NDVI on acidic substrates. Extrapolation to the whole Arctic using a five‐subzone zonation approach to stratify the circumpolar NDVI and phytomass data showed that 60% of the aboveground phytomass is concentrated in the low‐shrub tundra (subzone 5), whereas the high Arctic (subzones 1–3) has only 9% of the total. Estimated phytomass densities in subzones 1–5 are 47, 256, 102, 454, and 791 g m−2, respectively. Climate warming will likely result in increased phytomass, LAI, and NDVI on zonal sites. These changes will be most noticeable in acidic areas with abundant shrub phytomass.
Role of the biosphere in the mid‐Holocene climate of West Africa
Authors: Irizarry‐Ortiz Michelle M. , Wang Guiling , Eltahir Elfatih A. B.
Published: May 2003

In previous studies, a zonally symmetric, synchronously coupled biosphere‐atmosphere model (ZonalBAM), which includes explicit representation of ecosystem dynamics, has been developed and validated based on current conditions over the region of West Africa. Here, we use ZonalBAM to study the response of the coupled biosphere‐atmosphere system to changes in the Earth's orbital forcing during the Middle Holocene (6K yrs BP) and the relative contribution of vegetation feedbacks. Simulations in which vegetation conditions were fixed to the current distribution, show that an orbitally induced increased seasonality in insolation for the Middle Holocene, by itself, results in a 1.1° northward shift in the location of the southern margin of the Sahara as compared to current solar forcings. When vegetation is allowed to be dynamic, a 2.4° northward shift is simulated. However, when dynamic vegetation is initialized to palaeovegetation, a 5.1° northward shift is simulated, bringing results more consistent with palaeoevidence. Based on previous studies on the role of the gradient of moist static energy on the dynamics of large‐scale tropical circulations, a mechanism for the enhancement of the summer monsoon circulation has been developed. Our results suggest that multiple equilibria could have coexisted over the region of West Africa during the Middle Holocene. Furthermore, based on previous studies on the current climate over the region, we hypothesize that transitions between the different equilibria could have taken place during the Middle Holocene causing the southern desert margin to migrate between 18.1°N and 21.4°N and shaping climate variability.
Peroxy radicals during BERLIOZ at Pabstthum: Measurements, radical budgets and ozone production
Authors: Mihelcic D. , Holland F. , Hofzumahaus A. , Hoppe L. , Konrad S. , Müsgen P. , Pätz H.‐W. , Schäfer H.‐J. , Schmitz T. , Volz‐Thomas A. , Bächmann K. , Schlomski S. , Platt U. , Geyer A. , Alicke B. , Moortgat G.K.
Published: May 2003

The Photochemistry Experiment during BERLIOZ (PHOEBE) was conducted in July and August 1998 at a rural site located near the small village of Pabstthum, about 50 km northwest of downtown Berlin. In this paper, spectroscopic measurements of hydroxyl (OH) and peroxy radicals (HO2 and RO2) are discussed for two intensive days (20 and 21 July) of the campaign. On both days peak values of the radical concentrations were similar, reaching 6–8 × 106 cm−3 for OH and 20–30 ppt for RO2 and HO2. Fairly high OH concentrations were observed during the morning hours in the presence of high‐NOx mixing ratios (>20ppb). The “master chemical mechanism” (MCM) was used to calculate OH, HO2, and RO2 concentrations from the simultaneously measured data comprising a comprehensive set of speciated hydrocarbons and carbonyl compounds, O3, CO, NO, NO2, HONO, PAN, J(NO2), J(O1D), and meteorological parameters. The calculated OH concentrations are in excellent agreement with the measurements during the morning hours at high‐NOx (>10 ppb). However, at low NOx conditions the model overestimates OH by a factor 1.6. The modeled concentrations of HO2 and RO2 are in reasonable agreement with the measurements on 20 July. On the next day, when isoprene from nearby sources was the dominant VOC, the model overpredicted HO2 and RO2 in addition to OH. Radical budgets solely calculated from measured data show that a missing sink for OH must be responsible for the overestimation by MCM. Missing VOC reactivity is unlikely, unless these VOC would not lead to RO2 production upon reaction with OH. The measured RO2/HO2 ratio of about one is well reproduced by the MCM, whereas a simple model without recycling of RO2 from decomposition and isomerisation of alkoxy radicals underpredicts the measured ratio by about a factor of two. This finding highlights the importance of RO2 recycling in the chemical mechanism. The ozone production rate P(O3), calculated from the peroxy radical concentrations and NO, had a maximum of 8 ppb/hr at 0.5 ppb NO, which is in good agreement with results from previous campaigns at Tenerife and Schauinsland.
Central/eastern North Pacific photochemical precursor distributions for fall/spring seasons as defined by airborne field studies
Authors: DiNunno B. , Davis D. , Chen G. , Gregory G. , Sachse G. , Anderson B. , Vay S. , Avery M. , Ridley B. , Carroll M. , Walega J. , Montzka D. , Grahek F. , Bradshaw J. , Sandholm S. , Kondo Y. , Kok G. , Blake D. , Blake N. , Barrick J. , Fuelberg H. , Martin B. , Balok A.
Published: May 2003

Data from 10 aircraft programs recorded over the central/eastern North Pacific (180°–120°W, 0°–45°N) were assembled for the purpose of assessing an atmospheric chemical climatology for this region. It represents an early effort at carrying out this task and, thus, it can be expected to undergo many updates in the future. Such a database is useful in both gaining further insights concerning the fundamental processes controlling the chemistry of this region as well as serving as an important baseline by which to evaluate future change. Critical photochemical precursors examined included O3, CO, NOx, and H2O. In addition, the distribution profiles for select nonmethane hydrocarbons were explored. The precursor data were analyzed according to latitude, pressure altitude, and season of the year. Contrasting the spring and fall measurements, major trends that surfaced in both ozone and CO included observing elevated levels in spring compared to fall, with the largest spring increase occurring at latitudes north of 15°N. Both NOxand H2O showed trends quite different from that of CO and O3, with an indication of higher mixing ratio levels during the fall season. Variations in precursor distribution patterns within a season will be explained based on climatological flow patterns for the region and the resulting connection to source regions. Where possible, comparisons were performed with other sampling strategies, including ground‐based observations, sonde, and satellite data, and data collected during shuttle missions. Overall, these comparisons revealed a reasonably high level of correspondence between the airborne ensemble data and those collected using other sampling strategies.
Urban–rural contrasts in mixing height and cloudiness over Nashville in 1999
Authors: Angevine Wayne M. , White Allen B. , Senff Christoph J. , Trainer Michael , Banta Robert M. , Ayoub Mohammed A.
Published: May 2003

Strong contrasts in daytime mixing height (boundary layer [BL] height or zi) between urban and rural areas were observed during the 1999 Nashville Summer Intensive field campaign of the Southern Oxidants Study. On occasion, the urban mixing height was as much as 45% (700 m) higher than that over the rural areas. The difference was quite persistent, showing strongly in statistical comparisons, with a mean difference over all hours available for comparison of 160 m. Clouds had higher bases and were more common over the urban area as well. In this paper, measurements from wind profiling radars, lidars, and aircraft are used to characterize mixing height and clouds. The urban–rural contrasts have important implications for regional air quality. The mixing height is a first‐order control on pollutant concentrations. The urban–rural contrast also results in the venting of urban pollutants, affecting the local concentrations and the regional background. Clouds affect air quality by changing the radiative input for photochemistry and through changes in mixing and venting.
Meridional distributions of aerosol particle number concentrations in the upper troposphere and lower stratosphere obtained by Civil Aircraft for Regular Investigation of the Atmosphere Based on an Instrument Container (CARIBIC) flights
Authors: Hermann M. , Heintzenberg J. , Wiedensohler A. , Zahn A. , Heinrich G. , Brenninkmeijer C. A. M.
Published: May 2003

Number concentrations of submicrometer aerosol particles at altitudes of 8.5–11.3 km were measured along a single flight route between Germany (∼50°N, ∼10°E) and the Indic (∼5°N, ∼80°E) over 3 years using a commercial aircraft platform (project Civil Aircraft for Regular Investigation of the Atmosphere Based on an Instrument Container (CARIBIC)). During 41 intercontinental flights (∼380 flight hours), more than one million individual particle concentration measurements were made, yielding a comprehensive and unique aerosol data set. Using these data, the first meridional and seasonal probability distributions for ultrafine particles and for Aitken mode plus accumulation mode particles in the upper troposphere (UT) and lower stratosphere (LS) were derived. High particle number concentrations were observed in summer at tropical latitudes over the Arabian Sea and at midlatitudes over Europe, in contrast to lower values in the subtropics over the Middle East. This distribution primarily reflects the vertical transport pattern of the atmosphere. In winter, number concentrations were generally lower. The Intertropical Convergence Zone (ITCZ) was identified as a region of pronounced particle formation, but particle nucleation also occurred in the upper troposphere at midlatitudes. At tropical latitudes, convective transport and photochemistry appear to be the main driving forces for particle formation in the free troposphere. Consistent with these observations, integral length scales calculated from the aerosol data ranged from 6 to 12 km. Compared to particle concentrations in the midlatitudinal upper troposphere, those in the lowermost stratosphere were lower in winter, but equal or higher in summer. This seasonality is likely caused by stronger in‐mixing of upper tropospheric air in summer.
Effects of wet deposition on optical properties of the atmosphere over Bermuda and Barbados
Authors: Todd Deborah L. , Keene William C. , Moody Jennie L. , Maring Hal , Galloway James N.
Published: May 2003

Substantial spatial and temporal variabilities in chemical and physical properties of aerosols complicate attempts to model associated influences on global climate. Although wet deposition is the major mechanism by which most aerosols are removed from the atmosphere, direct effects of precipitation on radiative properties of the atmosphere are not well understood. To address this issue, attenuation coefficients for total insoluble constituents (ACt) and for nonvolatile (at 500°C) insoluble constituents (ACnv; primarily crustal dust) of precipitation at Bermuda and Barbados were measured at six wavelengths between 414 and 859 nm. Coefficients for volatile (at 500°C) insoluble constituents (ACv; primarily carbonaceous species) were calculated by difference. Between April and September, ACt at Bermuda was dominated by mineral constituents transported from North Africa, whereas carbonaceous species from North America were relatively low and exhibited no systematic seasonal variability. ACt and ACnv at Barbados were dominated by mineral dust, especially between April and September. Relative to ACv, ACnv decreased more rapidly with increasing wavelength. The wavelength dependencies of ACs for volatile and nonvolatile constituents at Bermuda were statistically indistinguishable from those at Barbados. The optical and chemical characteristics of precipitation were compared with scattering and absorption by ambient aerosols in associated air parcels to evaluate the influence of scavenging on the radiative properties of air. Although discernible relationships were evident, the small number of cases limited their applicability as reliable empirical predictors. However, these data do provide useful constraints for validating models of aerosol scavenging and wet removal on the optical properties of the troposphere.
Hydrogen peroxide, organic peroxides, carbonyl compounds, and organic acids measured at Pabstthum during BERLIOZ
Authors: Grossmann D. , Moortgat G. K. , Kibler M. , Schlomski S. , Bächmann K. , Alicke B. , Geyer A. , Platt U. , Hammer M.‐U. , Vogel B. , Mihelcic D. , Hofzumahaus A. , Holland F. , Volz‐Thomas A.
Published: May 2003

Gas‐phase H2O2, organic peroxides, carbonyl compounds, and carboxylic acids were measured from mid‐July to early August 1998 during the Berlin ozone (BERLIOZ) campaign in Pabstthum, Germany. The rural site, located 50 km northwest from Berlin, was chosen to measure the pollutants downwind during a summer smog episode. The hydroperoxides showed pronounced diurnal variations with peak mixing ratios in the early afternoon. The maximum mixing ratios were 1.4 ppbv (H2O2), 0.64 ppbv (methylhydroperoxide), and 0.22 ppbv (hydroxymethyl‐hydroperoxide). H2O2 was formed through photochemical activity, but originated also from vertical transport from air residing above the local inversion layer in the morning hours. Sometimes a second maximum was observed in the late afternoon‐evening: This H2O2 might be formed from ozonolysis of biogenic alkenes. Ratios of H2O2/HNO3 were used as indicators for the determination of NOx‐sensitive versus volatile organic compound (VOC)‐sensitive regimes for photochemical production of ozone. Diurnal profiles were measured for alkanals (C2–C10), showing maximum mixing ratios decreased from C2 (0.6 ppbv) to C5 (0.1 ppbv) alkanals, which originate primarily from anthropogenic hydrocarbon degradation processes. However, higher C6, C9, and C10 alkanals show strong fluctuations (0.25, 0.17, and 0.13 ppbv, respectively), showing evidence of biogenic emissions. Both primary unsaturated carbonyl (methyl vinyl ketone, methacrolein) and secondary oxidation products of isoprene (hydroxyacetone and glycolaldehyde, up to 0.16 and 0.20 ppbv, respectively) showed excellent correlation. Diurnal profiles of glyoxal, methylglyoxal, biacetyl, benzaldehyde, and pinonaldehyde were also obtained. Formaldehyde was measured continuously by long‐path DOAS and by an instrument based on the “Hantzsch” reaction; however, mixing ratios measured by DOAS (maximum 7.7 ppbv) were systematically larger by a factor of 1.3 on average, but by a factor of 1.7 during high photochemical activity. Homologous series of monocarboxylic acids were determined: Formic and acetic acid varied between 0.6 and 3.0 ppbv. The mixing ratio of the other dropped from 0.1 to 0.2 ppbv for C3 to typical 0.01 to 0.03 ppbv for C6, and from 0.01 to 0.002 ppbv for C7 to C9 monocarboxylic acids.
Urban/industrial pollution for the New York City–Washington, D. C., corridor, 1996–1998: 2. A study of the efficacy of the Montreal Protocol and other regulatory measures
Authors: Barnes Diana H. , Wofsy Steven C. , Fehlau Brian P. , Gottlieb Elaine W. , Elkins James W. , Dutton Geoffrey S. , Montzka Stephen A.
Published: May 2003

Background concentrations, emission rates, and trends in emission rates for five trace gases are inferred for the northeastern United States from continuous atmospheric observations at Harvard Forest in central New England for 1996–1998. Mixing ratios of gases regulated by the Montreal Protocol (CFC‐11 (CCl3F), CFC‐12 (CCl2F2), CFC‐113 (CCl2F‐CClF2), CH3CCl3, and halon‐1211 (CBrClF2)) are referenced to CO and PCE (perchloroethylene, tetrachloroethene, C2Cl4) to determine their urban/industrial source strengths and to test existing estimates of U.S. emissions. Despite the full imposition by 1996 of the Montreal Protocol ban on production by developed countries, our data show that significant releases to the atmosphere continue and that only emissions of CFC‐12 and CH3CCl3 declined in the region during this period. A broader historical and geographical study of emissions reveals that from 1986 to 1996 the international treaty has reduced U.S. emissions of CFC‐11, CFC‐12, CFC‐113, and CH3CCl3 by 87%, 70%, 93%, and 87%, respectively, but current trends suggest persistent emissions for many years following.
Horizontal structure of the electric field in the stratiform region of an Oklahoma mesoscale convective system
Authors: Mo Qixu , Detwiler Andrew G. , Hallett John , Black Robert
Published: May 2003

This analysis combines vertical electric field components Ez observed by two research aircraft flying horizontally at two levels, with vertical soundings of thermodynamic parameters and Ez made by five balloons, to produce a quasi‐three‐dimensional view of the space charge distribution in the trailing stratiform cloud region behind a mesoscale convective system (MCS) that developed in central Oklahoma late in the afternoon of 2 June 1991. The balloons were launched serially at one‐hour intervals from two sites separated by 80 km along a north‐south line as the MCS moved eastward, yielding two east‐west time‐height cross‐sections of the Ez structure within the quasi‐steady state trailing stratiform region behind the MCS. The balloon measurements are consistent with a vertical stack of five rearward‐ and downward‐sloping horizontal sheets of charge of alternating polarity, beginning at the bottom with a negative charge layer below the 0°C level and a positive layer near the 0°C level. This structure persisted for more than 2 hours. The two aircraft flew back and forth along a north‐south line through the balloon launch sites during the balloon launch period. Aircraft measurements demonstrated that the vertical electric field (Ez) at constant altitude varied in the north‐south direction. The peak magnitudes of Ez deduced from the airborne instrument systems agreed with the magnitudes deduced from the balloon measurements at the aircraft altitudes of 4.5 km and 5.8 km AGL. Rapid reversals in polarity of Ez with peak magnitude >50 kV m−1 observed by the aircraft at 4.5 km, just above the 0°C level, confirms the thin concentrated positive charge layer observed there by balloons and suggests that this charge layer is undulating above and below 4.5 km altitude, at least in the north‐south direction. Microphysically, this layer contained large aggregates and pockets of low cloud liquid water concentration. At the 5.8 km level, the polarity of Ez was always positive but the magnitude varied from zero to 25 kV m−1. Aircraft‐observed Ez at both altitudes varied on horizontal scales of ∼10 km or greater at both levels, suggesting that the charge density derived using the one‐dimensional infinite‐layer Gauss's law approximation applied to the balloon soundings of Ez is valid in this study. These observations show that layers of charge can persist for hours as they advect rearward in a storm‐relative sense, possibly due to continuing in situ charge separation, and/or due to weak dispersion, slow recombination and slow settling of charge attached to low mobility low terminal velocity ice hydrometeors.
Effects of underrepresented hydrometeor variability and partial beam filling on microwave brightness temperatures for rainfall retrieval
Authors: Harris Daniel , Foufoula‐Georgiou Efi , Kummerow Chris
Published: May 2003

Cloud models run at 3 km resolution (typical resolution used for microwave radiative transfer applications) apart from ignoring subgrid variability (< 3 km); they also underrepresent variability of cloud particles between the scales of 3 and 15 km. In a previous study by the authors, evidence was presented that neglecting subgrid variability in modeled clouds results in considerable biases in microwave radiance computations. In this paper we present evidence that biases of the same order (−2 to −3 K for 10.7 GHz and + 4 to 5 K for 85.6 GHz) can result from underrepresented variability at scales of 3–15 km. In addition, this study reveals significant differences between modeled and observed precipitating fields at the “edges” of the storm (regions that border on zero precipitation) and documents the effects that these differences have on radiative transfer computations. It is found that biases due to underrepresented variability within the storm body are of the opposite direction to biases due to “edge” effects where partial beamfilling occurs over a field of view, and can counteract to give a misleading overall insignificant bias. However, having these two types of biases present in the (Tb‐R) databases (formed by radiative transfer through 3 km modeled clouds) can have a significant effect on rainfall retrievals and can be the source of drastically different and apparently unexplainable biases from region to region and storm to storm.
Lower stratospheric temperature differences between meteorological analyses in two cold Arctic winters and their impact on polar processing studies
Authors: Manney Gloria L. , Sabutis Joseph L. , Pawson Steven , Santee Michelle L. , Naujokat Barbara , Swinbank Richard , Gelman Melvyn E. , Ebisuzaki Wesley
Published: May 2003

A quantitative comparison of six meteorological analyses is presented for the cold 1999/2000 and 1995/1996 Arctic winters. Using different analyzed data sets to obtain temperatures and temperature histories can have significant consequences. The area with temperatures below a polar stratospheric cloud (PSC) formation threshold commonly varies by ∼25% between the analyses, with some differences over 50%. Biases between analyses vary from year to year; in January 2000, Met Office analyses were coldest and National Centers for Environmental Prediction (NCEP) analyses were warmest, while NCEP analyses were usually coldest in 1995/1996 and NCEP/National Center for Atmospheric Research Reanalysis (REAN) were usually warmest. Freie Universität Berlin analyses are often colder than others at T ≲ 205 K. European Centre for Medium‐Range Weather Forecasts (ECMWF) temperatures agreed better with other analyses in 1999/2000, after improvements in the assimilation system, than in 1995/1996. Temperature history case studies show substantial differences using Met Office, NCEP, REAN, ECMWF, and NASA Data Assimilation Office (DAO) analyses. In January 2000 (when a large cold region was centered in the polar vortex), all analyses gave qualitatively similar results. However, in February 2000 (a much warmer period) and in January and February 1996 (comparably cold to January 2000 but with the cold region near the polar vortex edge), distributions of “potential PSC lifetimes” and total time spent below a PSC formation threshold varied significantly between the analyses. Largest peaks in “PSC lifetime” distributions in January 2000 were at 4–6 and 11–14 days, while in 1996 they were at 1–3 days. Different meteorological conditions in comparably cold winters have a large impact on expectations for PSC formation and on the effects of discrepancies between different meteorological analyses. Met Office, NCEP, REAN, ECMWF, and DAO analyses are commonly used in modeling polar processes; the choice of analysis can strongly influence the results of such studies.