摘要:

The rapid climate changes at the terminations marking the end of the last glaciation are poorly understood. This study uses the Cambridge two‐dimensional atmospheric model to investigate the impact of a large marine sediment slump or permafrost rupture releasing 4000 Tg of CH4into the high latitudes of the northern hemisphere, with particular emphasis on the evolution of atmospheric CH4concentrations, which may have led to an associated direct radiative effect of the order of 1 W m−2, persisting for a few decades, because of enhanced levels of atmospheric CH4. The model‐generated CH4concentrations were converted into synthetic ice core signals for 66°N and 85°S, representing Arctic and Antarctic cores, respectively. A range of possible averaging times was considered. The results show that irrespective of the site or enclosure time chosen, with the current sampling interval of ∼300 years, it is not possible to discount completely the existence of a spike as a result of the instantaneous release of 4000 Tg of CH4. Detection or refutation of a CH4spike would, however, be probable if the sampling interval were reduced to around 50

ISSN:0148-0227    

DOI:10.1029/96JD02547

年代:1996

数据来源: WILEY

摘要:

Eddy correlation measurements of methane flux were made at a fen in central Saskatchewan, as part of the Boreal Ecosystem Atmosphere Study (BOREAS) in 1994. Data were collected from mid‐May to early October. The water table was above the average peat surface throughout the measurement period. Detailed (near continuous) measurements allowed examination of temporal variability at hourly and daily time‐scales. As compared with the average nighttime flux, the average daytime methane flux was 25–45% higher in July and in August and 5–15% higher earlier and later in the season. Distribution of midday (1130–1430 LT) methane emission showed varying trends in different parts of the season. From mid‐May to early July, methane flux gradually increased from near zero to 4.1 mg m−2h−1. The water table height (above an average hollow surface) varied from 0.09 to 0.18 m, but the trend in methane flux followed peat temperature (at 0.1‐m depth) more closely. The peat warmed from 3.4° to 16.3°C during this time period. Methane flux was negligible until peat temperature (at 0.1‐m depth) was above 12°C. From early July to early August there was a fivefold increase in methane flux from 4.1 to its seasonal peak of 19.5 mg m−2h−1on August 1. The water table ranged from 0.23 m to a brief seasonal plateau of 0.30 m on July 20–23. Sharp increases in the water table were followed by increasing trends in methane flux by approximately 12 days. Peat temperature reached its seasonal maximum (19.3°C) the same time when the methane flux peaked. After early August the methane flux declined steadily and reached a value of 2.4 mg m−2h−1in early October. The water table and peat temperature showed comparable trends and decreased steadily to 0.06 m and 5.7°C, respectively. The seasonally integrated methane emission (mid‐May to early October) was estimated at 16.3 g C m−2. Nonlinear regression analysis of methane flux against water table (lagged by 12 days) and peat temperature was performed for different periods of the season. Except for a brief period of very high water table (when many hummocks were inundated) the regression using water table and peat temperature explained between 68 and 94% of the variability in methane flux. The sensitivity of methane flux to water table (or the slope of the log CH4flux/water table relationship) obtained from our daily flux values

ISSN:0148-0227    

DOI:10.1029/96JD02751

年代:1996

数据来源: WILEY

摘要:

The compound 2‐butanol has been used to scavenge ≥95% of the OH radicals formed in the reactions of O3with the alkenes 2,3‐dimethyl‐2‐butene and 2‐methyl‐2‐butene and with the monoterpenes α‐pinene and sabinene at 296±2 K and atmospheric pressure of air, with the OH radical formation yields from these O3reactions being obtained from the amounts of 2‐butanone formed. The rate constant for the reaction of the OH radical with 2‐butanol was measured using a relative rate method; the rate constant was determined to be (9.2±2.4) × 10−12cm3molecule−1s−1at 296±2 K, where the indicated error includes the estimated overall uncertainties in the rate constant for the reference organic (cyclohexane). The formation yield of 2‐butanone from the reaction of the OH radical with 2‐butanol was determined to be 0.695±0.073. Using this 2‐butanone formation yield from the OH radical reaction with 2‐butanol, the OH radical formation yields from the reactions of O3 with 2,3‐dimethyl‐2‐butene, 2‐methyl‐2‐butene, α‐pinene, and sabinene were determined to be 0.80±0.12, 0.93±0.14, 0.76±0.11, and 0.33±0.06, respectively. These OH radical formation yields agree to within ±25% with the yields previously derived using cyclohexane to scavenge OH radicals and measuring the amounts of cyclohexanone plus cyclohexanol formed. Our present OH radical formation yield for 2,3‐dimethyl‐2‐butene is also in good agreement with the

ISSN:0148-0227    

DOI:10.1029/96JD02722

年代:1996

数据来源: WILEY

摘要:

Transport‐induced interannual variability of carbon monoxide (CO) is studied during 1989–1993 using the Goddard chemistry and transport model (GCTM) driven by assimilated data. Seasonal changes in the latitudinal distribution of CO near the surface and at 500 hPa are captured by the model. The annual cycle of CO is reasonably well simulated at sites of widely varying character. Day to day fluctuations in CO due to synoptic waves are reproduced accurately at remote North Atlantic locations. By fixing the location and magnitude of chemical sources and sinks, the importance of transport‐induced variability is investigated at CO‐monitoring sites. Transport‐induced variability can explain 1991–1993 decreases in CO at Mace Head, Ireland, and St. David's Head, Bermuda, as well as 1991–1993 increases in CO at Key Biscayne, Florida. Transport‐induced variability does not explain decreases in CO at southern hemisphere locations. The model calculation explains 80–90% of interannual variability in seasonal CO residuals at Mace Head, St. David's Head, and Key Biscayne and at least 50% of variability in detrended seasonal residuals at Ascension Island and Guam. Upper tropospheric interannual variability during October is less than 8% in the GCTM. Exceptions occur off the western coast of South America, where mixing ratios are sensitive to the strength of an upper tropospheric high, and just north of Madagascar, where concentrations are influenced by the strength of offshore

ISSN:0148-0227    

DOI:10.1029/96JD02984

年代:1996

数据来源: WILEY

摘要:

During a “locally occurring system” (LOS) in Amazonian forest characterized by an active mixing layer from surface to 1000 m capped by a fossil mixed layer between 1000 and 1500 m, the vertical mixing effects of a shallow cumulus are examined. The explicit redistribution of CO and O3has been studied with a two‐dimensional convective cloud model coupled with a chemical model including gas and aqueous phases, for a shallow convective situation taken from the GTE/ABLE 2B campaign. The chemistry describes the main oxidation chains of CH4and CO in presence of low NOxconcentration in a remote troposphere. Model results compare favorably with observations obtained after development of the shallow convection. The analysis of results explains how a growing and decaying cloud field allows exchanges between a mixing layer, a fossil mixed layer and the free troposphere. An inert tracer study has shown that the layer lying between surface to 500 m is transported up to 2000 m. Even small clouds are responsible for the transport and the transformation of chemical species. Sensitivity tests are performed to evaluate the relative importance of dynamical, microphysical and chemical processes. The cumulus venting is the main process which modifies the trace species redistribution. The O3and CO amounts which are transported by a fair‐weather cumulus through the boundary layer and free troposphere in the wet season, over a tropical rain forest, are respectively 1.2 × 1023molecules km−2h−1and 7.7 × 1023molecules km−2h−1. In tropical regions, over rain forest, even with low NOxconcentration, several cumulus exist every day and the vertical fluxes of some chemical species (like O3) ca

ISSN:0148-0227    

DOI:10.1029/96JD01867

年代:1996

数据来源: WILEY

摘要:

Boundary layer processes associated with three pollution events during the North Atlantic Regional Experiment (NARE) 1993 field campaign are examined using airborne measurements and a Lagrangian particle dispersion model in conjunction with a mesoscale model employing four‐dimensional data assimilation. This nonphotochemical modeling system was able to qualitatively reproduce many of the observed features during a 15‐day period of the NARE field campaign. Simulated particle releases from urban regions within the daytime convective boundary layer were transported to heights up to 2 km above ground level. Particles located in the upper part of the residual layer in the early evening hours were quickly advected by higher wind speeds aloft to the sampling domain; however, particles released within the nocturnal stable boundary layer or the marine boundary layer remained within 200 m of the surface and often exhibited complex circulation patterns. As a consequence of the diurnal boundary layer characteristics, emissions released within a relatively close time interval were often found to have significantly different trajectories. Mixing of particles from various source regions results in a plume that does not have a unique age but is better characterized by a distribution of ages which vary with altitude. It is shown that much of the layering over Yarmouth is well established by convective boundary layer processes and vertical wind shears prior to the air masses leaving land. As expected, sea surface temperatures were found to play an important role in defining the vertical gradient of potential temperature and hence the amount of vertical mixing over the Gulf of Maine. Peak particle concentrations within 1 km of the ocean were often associated with a low‐level jet over the Gulf of Maine. A common feature during the analysis period is synoptic‐scale lifting in advance of low‐pressure systems which appears to be partially responsible for lifting particles to the heights observed over

ISSN:0148-0227    

DOI:10.1029/96JD02958

年代:1996

数据来源: WILEY

摘要:

Ozone measurements taken with an aircraft over the western North Atlantic in the summer of 1992 revealed numerous profiles that consisted of two principal layers with different, nearly constant mixing ratios in each layer. The lower layer was characterized by relatively low mixing ratios (<25–30 parts per billion by volume), while the upper layer had values a factor of 2 or more higher. The depths of the lower layer frequently corresponded to the depths of the mixed layer over the ocean as estimated from potential temperature observations. The upper ozone layer extended at least to the maximum sampling height of the aircraft on these flights, approximately 2.5 km. A mesoscale model is used to analyze the meteorology over the area for a representative case study day. Trajectory analysis shows that the air in the aircraft sampling region originated near Hudson Bay, followed a path that avoided major anthropogenic sources of ozone precursors over land, and eventually moved out over the ocean. Because of inhomogeneities in the sea surface temperatures in the observation area, the spatial and temporal evolution of the boundary layer over the ocean differed significantly over distances of only a few hundred kilometers. These differences, coupled with vertical mixing in the boundary layer and large‐scale vertical motions arising from synoptic and terrain influences, are identified as mechanisms responsible for the development of the characteristic shapes of the ozone and potential temperature profiles. This conclusion is supported by additional results obtained from using the mesoscale model to drive a Lagrangian particle dispersion model to simulate the transport and diffusion of a passive tracer; the major features of the ozone profiles are reproduced in the particle profiles. These findings indicate the critical role played by meteorological processes, irrespective of the details of the photochemical reactions. The results also imply that a failure to incorporate detailed descriptions of meteorology in photochemical models may lead to erroneous interpretations of the d

ISSN:0148-0227    

DOI:10.1029/96JD02978

年代:1996

数据来源: WILEY

摘要:

A radiative transfer postprocessor calculates microwave brightness temperaturesTbfrom climate experiments investigating supersonic aircraft exhaust impacts with the Global Climate/Middle Atmosphere Model (GCMAM) at the NASA Goddard Institute for Space Studies. Microwave signals from the exhaust‐perturbed GCMAM atmospheres are contrasted with observed interannual variability (natural “noise”) for 1982–1991 as measured by microwave sounding unit (MSU) channels across the lower troposphere, midtroposphere, and lower stratosphere. Exaggerated ozone and water vapor perturbations at supersonic cruise altitudes produce microwave signals easily detected against natural noise. Removal of ozone greenhouse action between 200 and 50 hPa cools all MSU channels with greatest ΔTbof −8.3 K and signal‐to‐observed‐noise (S/N) ratios above 20 in the lower stratospheric channel. Doubling middle‐atmospheric water vapor above 100 hPa cools lower stratosphericTbvalues by 1.5 K while warming tropospheric channels, particularly the tropopause channel. Detectable S/N ratios of 2–4 occur over the tropics and subtropics in the lower‐to‐middle troposphere and lower stratosphere. Realistic ozone and water vapor perturbations are based on the High‐Speed Research Program/Atmospheric Effects of Stratospheric Aircraft reports. These realistic stratospheric ozone and water vapor changes produce ΔTbsignals under 0.6 K and negligible S/N ratios. The slight climatic forcings are overwhelmed by natural feedbacks of high and low cloud formation, sea ice formation, and snow coverage. Thus the modeled realistic ozone and water vapor perturbations produce small and conflicting microwave signals, undetectable against natural variability and other sources of ant

ISSN:0148-0227    

DOI:10.1029/96JD02721

年代:1996

数据来源: WILEY

摘要:

We present here the first ground‐based observations of stratospheric/mesospheric H2O emission at the 31,3→ 22,0resonance (183.3101 GHz of central frequency). The spectra were obtained in January 1991 and January 1994 using theInstitut de Radioastronomie Millimétrique(IRAM) 30‐m radiotelescope located on Pico Veleta, Spain, (altitude 2870 m, latitude 37°N), during very dry tropospheric conditions (total precipitable water within the range 0.2–0.3 mm above the observation site). Microwave studies of stratospheric and mesospheric H2O profile have been carried out so far mostly by means of observations of the weak 61,6→ 52,3resonance at 22.2351 GHz. Some measurements at 183 GHz were previously obtained from airborne observations. The water vapor abundance in the troposphere is, in most cases, so high that the atmosphere appears optically thick at 183 GHz. However, we show that emission from the stratosphere and mesosphere can be sometimes detected from the ground (allowing upper stratospheric and mesospheric water vapor profiles to be obtained). We use a generalized non linear radiative transfer code in order to perform a least squares fit of the observed spectra. The derived H2O abundances are consistent with most measurements at 22 GHz. We suggest the possibility of carrying out this kind of observation in some selected sites during a significant fraction of the whole winter season. The required integration times to get a given signal‐to‐noise ratio are much shorter at 183 GHz

ISSN:0148-0227    

DOI:10.1029/96JD02687

年代:1996

数据来源: WILEY

摘要:

A prototype robust three‐dimensional global chemical transport model (CTM) has been developed in order to facilitate a realistic simulation of stratospheric chemistry and dynamics. The current application is for a 100‐day run from August 10 to November 17 using the average (ensemble) of 10 dynamical runs (realizations) of the CCMl. The CTM results are compared with observations and two‐dimensional model results. A comparison of the midlatitude model results with satellite observations, stratospheric and mesospheric sounder (SAMS) and Halogen Occultation Experiment (HALOE), showed that the model can simulate relatively well, at least on the timescale of the simulation, the distribution and formation of long‐lived species (N2O, CH4, and H20). The distribution of short‐lived species was compared with some satellite measurements, limb infrared monitor of the stratosphere (LIMS) and Stratospheric Aerosol and Gas Experiment (SAGE II), and two‐dimensional model results, and was found to be

ISSN:0148-0227    

DOI:10.1029/96JD01550

年代:1996

数据来源: WILEY