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1.
The concentration, radiocarbon (14C) and stable isotope (13C and 18O) content of CO have been determined in air samples collected across Russia (about 8,500 km) and along the Ob river during the summer of 1999 to study the CO sources and sinks. An instrumented carriage on the Trans-Siberian railway and a boat on the river Ob were used as atmospheric measurement platforms. In general, CO mixing ratios, CO stable isotope ratios, as well as the abundances of 14CO over West Siberia were similar to those found at remote northern hemispheric baseline monitoring stations. Identified sources of CO along the Ob appear to be connected to methane oxidation based on an inferred δ13Csource = −36.8 ± 0.6‰, while the value for δ18Osource = 9.0 ± 1.6‰ identifies it as burning. Thus flaring in the oil and gas production can be supposed to be a source. The extreme 13C depletion and concomitant 18O enrichment for two of the boat samples unambiguously indicates contamination by CO from combustion of natural gas (inferred values δ13Csource = −40.3‰ and δ18Osource = 17.5‰). For these two samples, that have strongly elevated 14CO concentrations, the industrial area near Tomsk is identified as a source area using meteorological calculations. Along the Trans-Siberian Railroad background CO was to various degrees contaminated with CO from methane combustion (δ13Csource = −35.7 ± 6.2‰ and δ18Osource = 10.3 ± 1.8‰). The impact of industrial burning was discernable in the vicinity of Perm-Kungur.  相似文献   

2.
Summary As an aspect of the LINEX field studies (1996–1997; Lindenberg near Beeskow, Germany), the characteristics of the internal boundary layer (IBL) that is associated with a step change of the surface roughnesses in neutral constant stress layers was investigated and is reported in this paper. Both smooth to rough (in 1996) and rough to smooth (in 1997) types of flow, have been studied based upon the profiles of mean wind and temperature realised from a 10-m mast and eddy correlation measurements taken at two levels (2 m and 5 m). Depending upon wind direction, the fetch at the site varied between 140 m and 315 m within the wind sector (200° to 340°) used for the field investigations. The height of the IBL, δ, had been determined from the intersect of the logarithmic wind-profiles below (< 2 m) and above (> 6 ) the interface. Values of δ obtained at the experimental site compared fairly well to the existing theoretical/empirical fetch-height relationships of the form: δ=aċx b , where a, b, are empirical constants. The ratio for the friction velocities below and above the IBL as measured directly by the eddy correlation techniques showed that for fetches less than 250 m there was an increase (decrease) of about 20% of the momentum flux arising from the smooth to rough (rough to smooth) transitions. Influences of distant obstructions (e.g., bushes, pockets of trees) on the surface flow were markedly important on the examined wind profiles and such can be indicative as multiple IBLs. Received September 1, 1997 Revised August 5, 1998  相似文献   

3.
We measured the surface energy budget of an Alpine grassland in highly complex terrain to explore possibilities and limitations for application of the eddy-covariance technique, also for CO2 flux measurements, at such non-ideal locations. This paper focuses on the influence of complex terrain on the turbulent energy measurements of a characteristic high Alpine grassland on Crap Alv (Alp Weissenstein) in the Swiss Alps during the growing season 2006. Measurements were carried out on a topographic terrace with a slope of 25◦ inclination. Flux data quality is assessed via the closure of the energy budget and the quality flag method used within the CarboEurope project. During 93% of the time the wind direction was along the main valley axis (43% upvalley and 50% downvalley directions). During the transition times of the typical twice daily wind direction changes in a mountain valley the fraction of high and good quality flux data reached a minimum of ≈50%, whereas during the early afternoon ≈70% of all records yielded good to highest quality (CarboEurope flags 0 and 1). The overall energy budget closure was 74 ± 2%. An angular correction for the shortwave energy input to the slope improved the energy budget closure slightly to 82 ± 2% for afternoon conditions. In the daily total, the measured turbulent energy fluxes are only underestimated by around 8% of net radiation. In summary, our results suggest that it is possible to yield realistic energy flux measurements under such conditions. We thus argue that the Crap Alv site and similar topographically complex locations with short-statured vegetation should be well suited also for CO2 flux measurements.  相似文献   

4.
Summary This paper presents a study of the sensibility of the Penman-Monteith evapotranspiration model to climatic (available energy and vapour pressure deficit) and parametric (aerodynamic and canopy resistances, r a and r c respectively) factors in a semi-arid climate, for crops in contrasting water status (well irrigated and under water stress) and of different heights. Three experiments were carried out in southern Italy on reference grass (≈ 0.1 m), grain sorghum (≈ 1 m) and sweet sorghum (≈ 3 m). For this analysis the sensitivity coefficients, taken as hourly means, were evaluated during the growth season when the crops completely covered the soil. The relative errors on evapotranspiration were also evaluated for r a and r c . The results showed that, for reference grass, available energy and aerodynamic resistance play a major role. For crops under water stress the most important term to evaluate is canopy resistance. For a tall crop, as sweet sorghum, the role of the vapour pressure deficit is fundamental, both when the crop is in good water status and under water stress. Received July 14, 1997 Revised February 5, 1998  相似文献   

5.
We report on field observations in January 2009 (austral summer) of atmospheric dust devils in the northern part of the Atacama Desert in South America (≈20◦S). An extremely high level of dust-devil activity over the study site has been observed, dependent on local meteorological conditions. We found a high correlation between the dust-devil frequency of occurrence and the Obukhov length scale, L, calculated from meteorological gradient measurements, with a clear tendency for this frequency to increase with decreasing −L. The upper threshold values of −L ≈ 20–30 m, and the 2-m mean wind speed, V 2 ≈ 8m s−1, for dust-devil occurrence have been found, but the minimal V 2 threshold was not observed. Parallel routine meteorological measurements enabled us to calculate the main constituents of the surface energy balance, to obtain direct estimates of the surface albedo (α ≈ 0.21 at the solar noon) and to summarize the local conditions.  相似文献   

6.
Specific studies about the stable isotope composition (18O/16O and D/H) of atmospheric icy conglomerations are still scarce. The present work offers, for the first time, a very detailed analysis of oxygen and hydrogen isotopic signatures of unusually large ice conglomerations, or “megacryometeors”, that fell to the ground in Spain during January 2000. The hydrochemical analysis is based on the bulk isotopic composition and systematic selective sampling (deuterium isotopic mapping) of eleven selected specimens. δ18O and δD (V-SMOW) of all samples fall into the Meteoric Water Line matching well with typical tropospheric values. The distribution of the samples on Craig's line suggests either a variation in condensation temperature and/or different residual fractions of water vapour (Rayleigh processes). Three of the largest megacryometeors exhibited unequivocally distinctive negative values (δ18O = −17.2%0 and δD = −127 %0 V-SMOW), (δ18O = −15.6%0 and δD = −112%0 V-SMOW) and (δ18O = −14.4%0 and δD = −100%0 V-SMOW), suggesting an atmospheric origin typical of the upper troposphere. Theoretical calculations indicate that the vertical trajectory of growth was lower than 3.2 km. During the period in which the fall of megacryometeors occurred, anomalous atmospheric conditions were observed to exist: a substantial lowering of the tropopause with a deep layer of saturated air below, ozone depression and strong wind shear. Moreover, these large ice conglomerations occurred during non-thunderstorm conditions, suggesting an alternative process of ice growth was responsible for their formation.  相似文献   

7.
Summary ?By analyzing normalized variables, it was found that the latitudinal secular variations of the rainwater deuterium fractionation ratio δ2H, oxygen fractionation ratio δ18O, vapor pressure, and surface temperature were almost non-linear, occurred in parallel, and decreased with latitude. The rate of depletion around the equator is asymmetric and smaller to the south of 45° S than to the north of 45° N. In the east Mediterranean, the rate of change of δ18O with height was found to be −.2‰ per 100 m and that of δ2H is comparable with the dry lapse rate in the atmosphere. Analysis of the annual time series of δ2H at Alexandria has indicated that variations show sinusoidal waveform with a major cycle of two years that accounts for 68% of the total variance. Although the quasi-biannual cycle in the atmosphere has small amplitude in the lower layers of the atmosphere at East Mediterranean latitudes, the major cycle in annual series of δ2H or δ18O may be linked to the quasi-biannual oscillation in the atmosphere. It was also found that the first three Empirical Orthogonal Functions (EOF) account for 72% of the seasonal variation of δ2H and share 68% of the seasonal variation of δ18O. Share of variances of monthly EOF in the months of the year indicate that the main underlying factors that cause fractionation processes for δ2H and δ18O are similar across the east Mediterranean especially in late winter and early spring. Received May 13, 2002; revised July 8, 2002; accepted August 6, 2002  相似文献   

8.
Summary  Turbulent fluxes of CO2 were continuously measured by eddy correlation for three months in 1997 over a gramineous fen in a high-arctic environment at Zackenberg (74°28′12″N, 20°34′23″W) in NE-Greenland. The measurements started on 1 June, when there was still a 1–2 m cover of dry snow, and ended 26 August at a time that corresponds to late autumn at this high-arctic site. During the 20-day period with snow cover, fluxes of CO2 to the atmosphere were small, typically 0.005 mg CO2 m−2 s−1 (0.41 g CO2 m−2 d−1), wheres during the thawed period, the fluxes displayed a clear diurnal variation. During the snow-free period, before the onset of vegetation growth, fluxes of CO2 to the atmosphere were typically 0.1 mg CO2 m−2 s−1 in the afternoon, and daily sums reached values up to almost 9 g CO2 m−2 d−1. After 4 July, downward fluxes of CO2 increased, and on sunny days in the middle of the growing season, the net ecosystem exchange rates attained typical values of about −0.23 mg m−2 s−1 at midday and max values of daily sums of −12 g CO2 m−2 d−1. Throughout the measured period the fen ecosystem acted as a net-sink of 130 g CO2 m−2. Modelling the ecosystem respiration during the season corresponded well with eddy correlation and chamber measurements. On the basis of the eddy correlation data and the predicted respiration effluxes, an estimate of the annual CO2 balance the calender year 1997 was calculated to be a net-sink of 20 g CO2 m−2 yr−1. Received October 6, 1999 Revised May 2, 2000  相似文献   

9.
Mixing depth structure and its evolution have been diagnosed from radar wind profiler data in the Chamonix and the Maurienne valleys (France) during summer 2003. The behaviour of refractive index structure parameter C n 2 peaks coupled with the vertical velocity variance σ w 2 was used to estimate the height of the mixed layer. Tethersonde vertical profiles were carried out to investigate the lower layers of the atmosphere in the range of approximately 400–500 m above ground level. The tethersonde device was especially useful to study the reversal of the valley wind system during the morning transition period. Specific features such as wind reversal and the convective mixed layer up to approximately the altitude of the surrounding mountains were documented. The wind reversal was observed to be much more sudden in the Maurienne valley than in the Chamonix valley  相似文献   

10.
Summary  A K-type diffusion model coupled with a massconsistent wind model is applied for one of the rural biological waste disposal sites in Austrian Alps. The site is situated in the P?ls valley in the eastern Alps, 250 km south-west of Vienna in Austria Aim of the study is to demonstrate dispersion of H2S from the site to near by village. Model simulations are carried out each for an evening and a morning transition case characterized by flow reversals. The role of locally generated wind in changing the pollutant distribution over nearby residential area is investigated. Surface observations at two stations toward the open boundaries of the main valley are used to derive the turbulence parameters and then to obtain initial inputs of wind profiles. The turbulence parameters behave analogous to that over a plane terrain after the establishment of the valley wind. The model simulations are done for eight hours during the evening transition and eighteen hours for the morning transition by incorporating the wind field from a mass consistent wind model. The results are compared with SF6 tracer experiments conducted during those periods. The model outputs and the observations at various points inside the valley are in good correlation except for NW part of the valley after the reversal of valley wind. The results also reveal the potential of a simple approach with minimized inputs. Received August 15, 1997 Revised August 15, 1998  相似文献   

11.
We investigated the dendroclimatic potential of stable carbon (δ13C) and oxygen (δ18O) abundances in tree rings of Callitris columellaris F. Muell. Tree-ring chronologies were constructed from the central Pilbara, north-western Australia and span 1919–1999. Variation in δ18O was more strongly related to climate than δ13C; ecological and physiological factors may have dampened the climate signal in the δ13C chronology. Tree-ring δ18O was most strongly correlated with relative humidity (RH) and rainfall (r = −0.36 and −0.39) of the wettest months of the summer period, January and February. The correlation with RH reflects its effect on evaporative enrichment of leaf water. However, tree-ring δ18O may also partly reflect the variability in 18O signatures of rainfall, which are influenced by the amount of rainfall and atmospheric humidity. From the δ18O chronology, we inferred that from 1919 to 1955 summers were relatively dry and warm, but since 1955, summers in the Pilbara region have become increasingly cooler and more humid. Since 1980, conditions have been the wettest and coolest of the last 80 years. These inferred changes in climate correspond to a measured increase in rainfall since 1980 in north-western Australia associated with a greater intensity of tropical cyclones. We conclude that δ18O abundances in tree rings of C. columellaris have significant potential for reconstructing the climate of semi-arid Australia, a region for which observational climate records are sparse.  相似文献   

12.
Summary  The winter wind regime of G?teborg, located on the West coast of Sweden, is composed of three different wind systems besides the ambient wind; a nocturnal low level jet (NLLJ), a winter land breeze (WLB) and an urban heat island circulation (UHIC). An inversion divides the air column into two layers, one between 10 – 50 m and one between 50 – 100 m. The UHIC is located in the lower layer, the WLB in the top layer and the NLLJ above the top layer. The intensity of the interacting processes depends on the stability of each layer as calculated from the bulk Richardson number (BRilow and BRihigh) using continuous data collected during four years (1991 – 94) from two sites (one within and one outside the urban area) and sampled at three levels. In the evening the WLB develops from the ground level and increases in height until after midnight. At about the same time an UHIC develops in the urban area, below the WLB and causing an uplift of the latter. However, at both sites the WLB does not exceed the 100 m level. At this time BRi in both layers are below one resulting in continuous coupling between the WLB, the UHIC layers and the regional wind. Consequently, the exchange of momentum is still effective between all layers and this is highlighted by a change in the wind direction and a regulation of wind-speed to more constant levels. When BRihigh≥1, the layers become frictionally decoupled, as indicated by a return in the wind direction in the top level to the regional wind, and an acceleration of the top wind. The top level then becomes incorporated in to a nocturnal low-level jet (NLLJ) system. The normally acknowledged development of the NLLJ, with a start around sunset, is in this case delayed for several hours at the top level. The reason for this is that there are meso-scale/local wind systems present in layers beneath the jet causing an interaction between the layers. In the morning, when the layers are again coupled the top layer wind is once more influenced by the WLB and therefore changes direction and speed. The local and meso-scale wind systems thus delay the current nocturnal wind development. Received August 24, 1998 Revised March 17, 1999  相似文献   

13.
Summary This paper investigates the characteristics of channelled airflow in the vicinity of a junction of three idealized valleys (one valley carrying the incoming flow and two tributaries carrying the outflow), using a two-dimensional single-layer shallow water model. Particular attention is given to the flow splitting occurring at the junction. Nondimensionalized, the model depends on the valley geometry, the Reynolds number, which is related to the eddy viscosity, and on the difference of the hydrostatic pressure imposed at the exit of the tributaries. At the spatial scale considered in this study, the Rossby number relating the inertial and Coriolis forces is always larger than 1, implying that the effect of earth rotation can be neglected to a first approximation. The analysis of the flow structure within the three valleys as well as the calculation of the split ratio (fraction of the air flow diverted into one of the two downstream valleys with respect to the total mass flux in the upstream valley) show that (i) the flow pattern depends strongly on the Reynolds number while the split ratio is comparatively insensitive; (ii) the valley geometry and the difference between the upstream and downstream hydrostatic pressures affect the flow pattern, the location of the split point and the split ratio; (iii) the relative contribution of flow deflection by the sidewalls and the blocking/splitting mechanism differs between the settings of a “Y-shape” valley and a “T-shape” valley. Quantitative comparison of the present results with numerical simulations of realistic cases and with observations collected in the region of the Rhine and Seez valleys (Switzerland) (“Y-shape” valley) and in the region of the Inn and Wipp valleys (Austria) (“T-shape” valley) during the Mesoscale Alpine Programme (MAP) field experiment shows good agreement provided that the normalized valley depth NΔH/Uu significantly exceeds 1, i.e., when “flow around” is expected. A structural disagreement between the idealized simulations and the observed wind field is found only when NΔH/Uu ≃ 1, that is, in the “flow over” regime. This shows that the dimensionless valley depth is indeed a good indicator for flow splitting, implying that the stratification is a key player in reality.  相似文献   

14.
Summary The boundary-layer structure of the Elqui Valley is investigated, which is situated in the arid north of Chile and extends from the Pacific Ocean in the west to the Andes in the east. The climate is dominated by the south-eastern Pacific subtropical anticyclone and the cold Humboldt Current. This combination leads to considerable temperature and moisture gradients between the coast and the valley and results in the evolution of sea and valley wind systems. The contribution of these mesoscale wind systems to the heat and moisture budget of the valley atmosphere is estimated, based on radiosoundings performed near the coast and in the valley. Near the coast, a well-mixed cloud-topped boundary layer exists. Both, the temperature and the specific humidity do not change considerably during the day. In the stratus layer the potential temperature increases, while the specific humidity decreases slightly with height. The top of the thin stratus layer, about 300 m in depth, is marked by an inversion. Moderate sea breeze winds of 3–4 m s−1 prevail in the sub-cloud and cloud layer during daytime, but no land breeze develops during the night. The nocturnal valley atmosphere is characterized by a strong and 900 m deep stably stratified boundary layer. During the day, no pronounced well-mixed layer with a capping inversion develops in the valley. Above a super-adiabatic surface layer of about 150 m depth, a stably stratified layer prevails throughout the day. However, heating can be observed within a layer above the surface 800 m deep. Heat and moisture budget estimations show that sensible heat flux convergence exceeds cold air advection in the morning, while both processes compensate each other around noon, such that the temperature evolution stagnates. In the afternoon, cold air advection predominates and leads to net cooling of the boundary layer. Furthermore, the advection of moist air results in the accumulation of moisture during the noon and afternoon period, while latent heat flux convergence is of minor relevance to the moisture budget of the boundary layer. Correspondence: Norbert Kalthoff, Institut für Meteorologie und Klimaforschung, Universit?t Karlsruhe/Forschungszentrum Karlsruhe, Postfach 3640, 76021 Karlsruhe, Germany  相似文献   

15.
This study aims at evaluating the variability of the optical properties of chromophoric dissolved organic matter (CDOM) of rainwater during the cold season, specifically between Autumn and Winter periods. The spectroscopic characteristics of rainwater samples collected at a town (Aveiro) in western Portugal were assessed by UV-Vis absorbance and three-dimensional excitation-emission matrix (EEM) fluorescence spectroscopies. Rainwater samples showed similar characteristics to those of natural humic substances when analysed by UV-Vis absorbance spectroscopy, but a significant difference was observed in the volume weight average (VWA) of absorbances between Autumn and Winter. In general, the EEM fluorescence spectra of the Autumn and Winter samples disclosed the presence of six fluorophores with different VWA specific fluorescence intensities: three humic-like (λ excitation/λ emission ≈ 230/415 nm; 290/415 nm; and 340/415 nm) and three protein-like (λ excitation/λ emission ≈ 230/350 nm; 280/340 nm; and 225/300 nm), but one of the humic-like peaks (≈340/415 nm) does not always appear in the EEM fluorescence spectra of the Winter samples. During the cold season, chromophoric compounds are important constituents of rainwater dissolved organic matter and the presence of these highly absorbing and fluorescing compounds may exert a determining effect in atmospheric absorption of solar radiation.  相似文献   

16.
Summary  Net ecosystem CO2 exchange was measured over a mountain birch forest in northern Finland throughout the growing season. The maximal net CO2 uptake rate of about − 0.5 mg(CO2) m−2 s−1 was observed at the end of July. The highest nocturnal respiration rates in early August were 0.2 mg(CO2) m−2 s−1. The daily CO2 balances during the time of maximal photosynthesis were about −15 g(CO2) m−2 d−1. The mountain birch forest acted as a net sink of CO2 from 30 June to 28 August. During that period the net CO2 balance was −448 g(CO2)m−2. The interannual representativeness of the observed balances was studied using a simplified daily balance model, with daily mean global radiation and air temperature as the input parameters. The year-to-year variation in the phenological development was parameterised as a function of the cumulative effective temperature sum. The daily balance model was used for estimating the variability in the seasonal CO2 balances due to the timing of spring and meteorological factors. The sink term of CO2 in 1996 was lower than the 15-year mean, mainly due to the relatively late emergence of the leaves. Received October 11, 1999 Revised April 25, 2000  相似文献   

17.
Aerosol and rain samples were collected between 48°N and 55°S during the KH-08-2 and MR08-06 cruises conducted over the North and South Pacific Ocean in 2008 and 2009, to estimate dry and wet deposition fluxes of atmospheric inorganic nitrogen (N). Inorganic N in aerosols was composed of ~68% NH4+ and ~32% NO3– (median values for all data), with ~81% and ~45% of each species being present on fine mode aerosol, respectively. Concentrations of NH4+ and NO3− in rainwater ranged from 1.7–55 μmol L−1 and 0.16–18 μmol L−1, respectively, accounting for ~87% by NH4+ and ~13% by NO3− of total inorganic N (median values for all data). A significant correlation (r = 0.74, p < 0.05, n = 10) between NH4+ and methanesulfonic acid (MSA) was found in rainwater samples collected over the South Pacific, whereas no significant correlations were found between NH4+ and MSA in rainwater collected over the subarctic (r = 0.42, p > 0.1, n = 6) and subtropical (r = 0.33, p > 0.5, n = 6) western North Pacific, suggesting that emissions of ammonia (NH3) by marine biological activity from the ocean could become a significant source of NH4+ over the South Pacific. While NO3− was the dominant inorganic N species in dry deposition, inorganic N supplied to surface waters by wet deposition was predominantly by NH4+ (42–99% of the wet deposition fluxes for total inorganic N). We estimated mean total (dry + wet) deposition fluxes of atmospheric total inorganic N in the Pacific Ocean to be 32–64 μmol m−2 d−1, with 66–99% of this by wet deposition, indicating that wet deposition plays a more important role in the supply of atmospheric inorganic N than dry deposition.  相似文献   

18.
Summary  Measurements of landscape-scale methane emission were made over an aapa mire near Kaamanen in Finnish Lapland (69° 8′ N, 27° 16′ E, 155 m ASL). Emissions were measured during the spring thaw, in summer and in autumn. No effect of water table position on CH4 emission was found as the water table remained at or above the surface of the peat. Methane emission fluxes increased with surface temperature from which an activation energy of −99 kJ mol−1 was obtained. Annual emission from the site, modelled from temperature regression and short-term flux measurements made in three separate years, was calculated to be 5.5 ± 0.4 g CH4 m−2 y−1 of which 0.6 ± 0.1 g CH4 m−2 y−1 (11%) was released during the spring thaw which lasted 20 to 30 days. The effect of global warming on the CH4 budget of the site was estimated using the central scenario of the SILMU (Finnish Research Programme on Climate Change) model which predicts annual mean temperature increases of 1.2, 2.4 and 4.4 °C in 2020, 2050 and 2100, respectively. Maximum enhancements in CH4 emission due to warming were calculated to be 18, 40 and 84% for 2020, 2050 and 2100, respectively. Actual increases may be smaller because prediction of changes in water table are highly uncertain. Received September 17, 1999 Revised October 16, 2000  相似文献   

19.
The carbon kinetic isotope effects (KIEs) in the reactions of several unsaturated hydrocarbons with chlorine atoms were measured at room temperature and ambient pressure using gas chromatography combustion isotope ratio mass spectrometry (GCC-IRMS). All measured KIEs, defined as the ratio of the rate constants for the unlabeled and labeled hydrocarbon reaction k 12/k 13, are greater than unity or normal KIEs. The KIEs, reported in per mil according to Cl ɛ = (k 12/k 13−1) × 1000‰ with the number of experimental determinations in parenthesis, are as follows: ethene, 5.65 ± 0.34 (1); propene, 5.56 ± 0.18 (2); 1-butene, 5.93 ± 1.16 (1); 1-pentene, 4.86 ± 0.63 (1); cyclopentene, 3.75 ± 0.14 (1); toluene, 2.89 ± 0.31 (2); ethylbenzene, 2.17 ± 0.17 (2); o-xylene, 1.85 ± 0.54 (2). To our knowledge, these are the first reported KIE measurements for reactions of unsaturated NMHC with Cl atoms. Relative rate constants were determined concurrently to the KIE measurements. For the reactions of cyclopentene and ethylbenzene with Cl atoms, no rate constant has been reported in refereed literature. Our measured rate constants are: cyclopentene (7.32 ± 0.88) relative to propene (2.68 ± 0.32); ethylbenzene (1.15 ± 0.04) relative to o-xylene (1.35 ± 0.21), all × 10−10 cm3 molecule−1 s−1. The KIEs in reactions of aromatic hydrocarbons with Cl atoms are similar to previously reported KIEs in Cl-reactions of alkanes with the same numbers of carbon atoms. Unlike the KIEs for previously studied gas-phase hydrocarbon reactions, the KIEs for alkene–Cl reactions do not exhibit a simple inverse dependence on carbon number. This can be explained by competing contributions of normal and inverse isotope effects of individual steps in the reaction mechanism. Implications for the symmetries of the transition state structures in these reactions and the potential relevance of Cl-atom reactions on stable carbon isotope ratios of atmospheric NMHC are discussed.  相似文献   

20.
In the present study, an attempt has been made to examine the governing photochemical processes of surface ozone (O3) formation in rural site. For this purpose, measurements of surface ozone and selected meteorological parameters have been made at Anantapur (14.62°N, 77.65°E, 331 m asl), a semi-arid zone in India from January 2002 to December 2003. The annual average diurnal variation of O3 shows maximum concentration 46 ppbv at noon and minimum 25 ppbv in the morning with 1σ standard deviation. The average seasonal variation of ozone mixing ratios are observed to be maximum (about 60 ppbv) during summer and minimum (about 22 ppbv) in the monsoon period. The monthly daytime and nighttime average surface ozone concentration shows a maximum (55 ± 7 ppbv; 37 ± 7.3 ppbv) in March and minimum (28 ± 3.4 ppbv; 22 ± 2.3 ppbv) in August during the study period. The monthly average high (low) O3 48.9 ± 7.7 ppbv (26.2 ± 3.5 ppbv) observed at noon in March (August) is due to the possible increase in precursor gas concentration by anthropogenic activity and the influence of meteorological parameters. The rate of increase of surface ozone is high (1.52 ppbv/h) in March and lower (0.40 ppbv/h) in July. The average rate of increase of O3 from midnight to midday is 1 ppbv/h. Surface temperature is highest (43–44°C) during March and April months leading to higher photochemical production. On the other hand, relative humidity, which is higher during the rainy season, shows negative correlation with temperature and ozone mixing ratio. It can be seen that among the two parameters are measured, correlation of surface ozone with wind speed is better (R 2=0.84) in compare with relative humidity (R 2=0.66).  相似文献   

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