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1.
Reactive halogen species (RHS = X, XO, HOX, OXO; X = Cl, Br, I) are known to have an important influence on the chemistry in the polar boundary layer (BL), where they are responsible for ozone depletion events in spring. Recent field campaigns at Mace Head, Ireland, and the Dead Sea, Israel, identified for the first time iodine oxide (IO) at mixing ratios of up to 6.6 ppt and 90 ppt bromine oxide (BrO), respectively, by DOAS also at lower latitudes. These results intensified the discussion about the role of the RHS in the mid-latitude BL.Photochemical box model calculations show that the observed IO mixing ratios can destroy ~0.45 ppb ozone per hour. This is comparable to the rates of the known O3-loss processes in the boundary layer. The model studies also reveal that IO, at these levels, has a strong influence on the BL photochemistry, increasing the OH/HO2- and the NO2/NO - ratios. In combination these changes lead to a reduction of the photochemical ozone formation, which - in addition - reduces ozone mixing ratios by up to 0.15 ppb/h.The studies for the Dead Sea case give no information on the heterogeneous process responsible for the bromine release, but they show that a total of 2 – 4 ppb of total bromine have to be released to explain the observed complete depletion of 60 ppb ozone in 2 – 3 hours.  相似文献   

2.
Halogens in the atmosphere chemically destroy ozone. In the troposphere, bromine has higher ozone destruction efficiency than chlorine and is the halogen species with the widest geographical spread of natural sources. We investigate the relative strength of various sources of reactive tropospheric bromine and the influence of bromine on tropospheric chemistry using a 6-year simulation with the global chemistry transport model MOZART4. We consider the following sources: short-lived bromocarbons (CHBr3, CH2BrCl, CHBr2Cl, CHBrCl2, and CH2Br2) and CH3Br, bromine from airborne sea salt particles, and frost flowers and sea salt on or in the snowpack in polar regions. The total bromine emissions in our simulations add up to 31.7 Gmol(Br)/yr: 63 % from polar sources, 24.6 % from short-lived bromocarbons and 12.4 % from airborne sea salt particles. We conclude from our analysis that our global bromine emission is likely to be on the lower end of the range, because of too low emissions from airborne sea salt. Bromine chemistry has an effect on the oxidation capacity of the troposphere, not only due to its direct influence on ozone concentrations, but also by reactions with other key chemical species like HO x and NO x . Globally, the impact of bromine chemistry on tropospheric O3 is comparable to the impact of gas-phase sulfur chemistry, since the inclusion of bromine chemistry in MOZART4 leads to a decrease of the O3 burden in the troposphere by 6 Tg, while we get an increase by 5 Tg if gas-phase sulfur chemistry is switched off in the standard model. With decreased ozone burden, the simulated oxidizing capacity of the atmosphere decreases thus affecting species associated with the oxidation capacity of the atmosphere (CH3OOH, H2O2).  相似文献   

3.
The activation of Br- and Cl- to atomic Br and Cl in sea-spray aerosol was investigated in smog-chamber experiments. In the presence of O3, hydrocarbons and NaCl aerosol alone no activation was observed. By adding Br- to the aerosol, the chain reaction: Br + O3 BrO, BrO + HO2 HOBr, HOBr HOBr(aq), HOBr(aq) + H+ + Br- Br2 (6), HOBr(aq) + H+ + Cl- BrCl (7) was verified. The step from reaction (6) to (7) is accompanied by a decrease of the Br-/Cl- ratio from 1/600 to less than 1/2000. In the absence of sulphate, the chain is initiated by the reaction of OH(aq) with Br-. The pH value decreases to less than 2 during the first minutes of the experiment and later on to almost 1 (in the absence of NOx or SO2). This is caused by the formation of oxalic acid from alkanes and toluene. In stopped flow experiments, the reduction of Br2 by oxalic acid was observed to occur through a two-step mechanism: HC2O4 - + Br2 Br- + BrC2O4H (k22, k-22), BrC2O4H Br- + H+ + 2 CO2 (23) with the following rate constants and ratios of rate constants, k ± 2: k22k-23 / k-22 = (2.9 ± 0.3) · 10-4 s-1, k-22 / k-23 = 7000 ± 3000 13000 M-1, k22 = 2 ±-1 4 M-1 s-1, and k-23 > 0.1 s-1, k-22 > 600 M-1 s-1. Oxalic acid may be responsible for the inhibition of the chain reaction observed at the end of the experiments.  相似文献   

4.
Kinetics and products of the gas-phase reactions of dimethylsulphide (DMS), dimethylsulphoxide (DMSO) and dimethylsulphone (DMSO2) with Br atoms and BrO radicals in air have beeninvestigated using on-line Fourier Transform Infrared Spectroscopy (FT-IR) as analytical technique at 740 ± 5 Torr total pressure and at 296 ± 3 K in a480 L reaction chamber. Using a relative rate method for determining the rate constants; the following values (expressed in cm3molecule–1 s–1) were found: kDMS+Br = (4.9 ±1.0) ×10–14, kDMSO + Br < 6 × 10–14,kDMSO 2 + Br 1 × 10–15,kDMSO + BrO = (1.0 ± 0.3) × 10–14 andkDMSO 2 + BrO 3 × 10–15 (allvalues are given with one on the experimental data). DMSO, SO2, COS, CH3SBr andCH3SO2Br were identified as the main sulphur containing products of the oxidation of DMS by Br atoms. From the reaction between DMSO and Br atoms, DMSO2and CH3SO2Br were the only sulphur containing products thatwere identified. DMSO, DMSO2 and SO2 were identified as themain sulphur containing products of the reaction between DMS and BrO.DMSO2 was found to be the only product of the reaction between DMSO and BrO. For the reactions of DMSO2 with Br and BrO no products were identified because the reactions were too slow.The implications of these results for atmospheric chemistry are discussed.  相似文献   

5.
A one-dimensional coupled climate and chemistry model has been developed to estimate past and possible future changes in atmospheric temperatures and chemical composition due to human activities. The model takes into account heat flux into the oceans and uses a new tropospheric temperature lapse rate formulation. As found in other studies, we estimate that the combined greenhouse effect of CH4, O3, CF2Cl2, CFCl3 and N2O in the future will be about as large as that of CO2. Our model calculates an increase in average global surface temperatures by about 0.6°C since the start of the industrial era and predicts for A.D. 2050 a twice as large additional rise. Substantial depletions of ozone in the upper stratosphere by between 25% and 55% are calculated, depending on scenario. Accompanying temperature changes are between 15°C and 25°C. Bromine compounds are found to be important, if no rigid international regulations on CFC emissions are effective. Our model may, however, concivably underestimate possible effects of CFCl3, CF2Cl2, C2F3Cl3 and other CFC and organic bromine emissions on lower stratospheric ozone, because it can not simulate the rapid breakdown of ozone which is now being observed worldwide. An uncertainty study regarding the photochemistry of stratospheric ozone, especially in the region below about 25 km, is included. We propose a reaction, involving excited molecular oxygen formation from ozone photolysis, as a possible solution to the problem of ozone concentrations calculated to be too low above 45 km. We also estimate that tropospheric ozone concentrations have grown strongly in the northern hemisphere since pre-industrial times and that further large increases may take place, especially if global emissions of NOx from fossil fuel and biomass burning were to continue to increase. Growing NOx emissions from aircraft may play an important role in ozone concentrations in the upper troposphere and low stratosphere.  相似文献   

6.
As part of the Polar Sunrise Experiment (PSE) 1997, concentrations of halogen species thought to be involved in ground level Arctic ozone depletion were made at Alert, NWT, Canada (82.5°N, 62.3°W) during the months of March and April, 1997. Measurements were made of photolyzable chlorine (Cl2 and HOCl) and bromine (Br2 and HOBr) using the Photoactive Halogen Detector (PHD), and bromine radicals (BrOx) using a modified radical amplifier. During the sampling period between Julian Day 86 (March 27) and Day 102 (April 12), two ozone depletion episodes occurred, the most notable being on days 96-99, when ozone levels were below detectable limits (1 ppbv). Concentrations of BrOx above the 4 pptv detection limit were found for a significant part of the study, both during and outside of depletion events. The highest BrOx concentrations were observed at the end of the depletion event, when the concentration reached 15 pptv. We found substantial amounts of Br2 in the absence of O3, indicating that O3 is not a necessary requirement for production of Br2. There is also Br2 present when winds are from the south, implying local scale (e.g. from the snowpack) production. During the principal O3 depletion event, the HOBr concentration rose to 260 pptv, coincident with the BrOx maximum. This implies a steady state HO2 concentration of 6 pptv. During a partial O3 depletion event, we estimate that the flux of Br2 from the surface is about 10 times greater than that for Cl2.  相似文献   

7.
The halogen ions Br- and Cl- together with NO3 -, SO4 =, MSA- (methane sulfonate), Na+ and NH4 + were analysed by ion chromatography in extracts of more than 800 aerosol cellulose filter samples taken at Ny Ålesund, Svalbard (79°N, 12°E) in spring 1996 (March 27 - May 16) within the European Union project ARCTOC (Arctic Tropospheric Ozone Chemistry). Anticorrelated variations between f-Br (filterable bromine, i.e. water soluble bromine species that can be collected by aerosol filters) and ozone within the arctic troposphere were evaluated at a resolution of 1 or 2 hours for periods with depleted ozone and 4 hours at normal ozone. A mean f-Br concentration of 11 ng m-3 (0.14 nmol m-3) was observed for the whole campaign, while maximum concentrations of 80 ng m-3 (1 nmol m-3) were detected during two total O3-depletion events (O3 drop to mixing ratios below the detection limit of < 2 ppb). Anticorrelation between f-Br and O3 was also seen during minor O3-depletion episodes (sudden drop in O3 by at least 10 ppb, but O3 still exceeding the detection limit) and even for ozone variations near its background level (40-50 ppb). A time lag of about 10 hours between the change of ozone and of f-Br concentrations could only be found during a total ozone depletion event, when f-Br reached its maximum values several hours after ozone was totally destroyed. Bromine oxide (BrO) concentrations, measured by DOAS (Differential Optical Absorption Spectroscopy), and f-Br showed a coincident variability during almost the entire campaign (except in the case of total O3-loss). Frequently enhanced anthropogenic nitrate and sulphate concentrations were observed during O3-depletion periods. At O3 concentrations < 10 ppb sulphate and nitrate exceed their typical mean level by 54% and 77%, respectively. This may indicate a possible connection between acidity and halogen release.  相似文献   

8.
It is essential to quantify the background reactivity of smog-chambers, since this might be the major limitation of experiments carried out at low pollutant concentrations typical of the polluted atmosphere. Detailed investigation of three chamber experiments at zero-NO x in the European Photoreactor (EUPHORE) were carried out by means of rate-of-production analysis and two uncertainty analysis tools: local uncertainty analysis and Monte Carlo simulations with Latin hypercube sampling. The chemical mechanism employed was that for methane plus the inorganic subset of the Master Chemical Mechanism (MCMv3.1). Newly installed instruments in EUPHORE allowed the measurement of nitrous acid and formaldehyde at sub-ppb concentrations with high sensitivity. The presence of HONO and HCHO during the experiments could be explained only by processes taking place on the FEP Teflon walls. The HONO production rate can be described by the empirical equation W(HONO)EUPHORE dry = a × j NO 2× exp (− T 0/T) in the low relative humidity region (RH < 2%, a = 7.3×1021 cm−3, T 0 = 8945K), and by the equation W(HONO)EUPHORE humid = W(HONO)EUPHORE dry+ j NO 2× b × RH q in the higher relative humidity region (2% < RH < 15%, b = 5.8×108 cm−3 and q = 0.36, and RH is the relative humidity in percentages). For HCHO the expression W(HCHO)EUPHORE = c × j NO 2exp (− T′0/T) is applicable (c = 3.1×1017 cm−3 and T′0 = 5686 K). In the 0–15% relative humidity range OH production from HONO generated at the wall is about a factor of two higher than that from the photolysis of 100 ppb ozone. Effect of added NO2 was found to be consistent with the dark HONO formation rate coefficient of MCMv3.1.  相似文献   

9.
Abstract

We describe a one‐dimensional (1‐D) numerical model developed to simulate the chemistry of minor constituents in the stratosphere. The model incorporates most of the chemical species presently found in the upper atmosphere and has been used to investigate the effect of increasing chlorofluorocarbon (CFC) emissions on ozone (O3).

Our calculations confirm previous results that O3 depletions in the 20–25 km region, the region of the O3 maximum, are very sensitive to the relative abundances of Clx and NOy in the lower stratosphere for high Clx amounts. The individual abundances of lower stratospheric Clx and NOy amounts are very sensitive to upper tropospheric mixing ratios, which, in turn, are determined largely by surface input fluxes and heterogeneous loss processes. Thus the behaviour of column O3 depletions at high Clx levels is greatly affected, albeit indirectly, by tropospheric processes. For high Clx levels the Ox flux from the stratosphere to the troposphere is dramatically reduced, leading to a large reduction in tropospheric O3. Some of the variation between different published 1‐D model results is most likely due to this critical dependence of O3 depletion on NOy‐Clx ratios.

Model simulations of time‐dependent CFC effects on ozone indicate that if CFCs were to remain at constant 1980 emission rates while N2O increased at 0.25% a?1 and CH4 increased at 1% a?1, we could expect a 2.2% decrease in total column O3 (relative to the 1980 atmosphere) by the year 2000. However, if CFC emission rates were to increase by 3% a?1 (current estimates are 5–6% a?1), we would predict a depletion of 2.7% by the year 2000. The calculations for times beyond the year 2000 suggest that the effects on total O3 will begin to accelerate. If methyl chloroform emissions are added at 7% a?1 (current estimates are 7–9% a?1) to the above CFC‐N2O‐CH4 scenario we calculate total O3 depletions by the year 2000 that are 41% larger than those calculated without. This suggests that if the emissions of methyl chloroform continue to increase at their present rate then methyl chloroform could have a significant effect upon total O3.  相似文献   

10.
基于美国宇航局NASA/AURA卫星臭氧监测仪OMI数据,分析了2005—2014年长三角地区及其典型城市对流层O_3、NO_2柱浓度和HCHO大气总柱浓度的时空分布特征。结果表明:10 a间,长三角地区对流层O_3柱浓度和HCHO总柱浓度呈现增长趋势,O_3增量为0.23ppbv/10 a,HCHO增量为0.07×10~(16) mol/(cm~2·10 a),对流层NO_2柱浓度呈现降低趋势,减量为0.06×10~(15)mol/(cm~2·10 a);长三角地区对流层O_3柱浓度最大值出现在3、4、5月,而对流层NO_2柱浓度最大值出现在1、12月,HCHO总柱浓度最大值出现在6、7月;对流层O_3柱浓度的高值区分布在长三角中部、北部区域,对流层NO_2柱浓度高值区分布于长三角中部,HCHO总柱浓度高值区相对分散,且四季的分布各不相同。O_3与NO_2和HCHO在时间和空间上呈现一定的相关性。  相似文献   

11.
Measurements of NO2, HCHO, and H2O2 were made by the highly specific method of mid infra-red absorption spectroscopy using tunable diode lasers (TDLAS) during the 1988 Polarstern expedition. The TDLAS data are compared to those obtained during the cruise using less direct methods. Southern Hemisphere NO2 levels suggest nett photochemical destruction of O3 in the boundary layer. Northern Hemisphere HCHO averaged 0.47±0.2 ppbv; the HCHO measurements are used in a simple calculation to estimate OH noontime maxima of 3–6×106 cm-3.  相似文献   

12.
The response of tropospheric ozone to a change in solar UV penetration due to perturbation on column ozone depends critically on the tropospheric NO x (NO+NO2) concentration. At high NO x or a polluted area where there is net ozone production, a decrease in column ozone will increase the solar UV penetration to the troposphere and thus increase the tropospheric ozone concentration. However, the opposite will occur, for example, at a remote oceanic area where NO x is so low that there is net ozone destruction. This finding may have important implication on the interpretation of the long term trend of tropospheric ozone. A change in column ozone will also induce change in tropospheric OH, HO2, and H2O2 concentrations which are major oxidants in the troposphere. Thus, the oxidation capacity and, in turn, the abundances of many reduced gases will be perturbed. Our model calculations show that the change in OH, HO2, and H2O2 concentrations are essentially independent of the NO x concentration.  相似文献   

13.
Photochemical indicators for determination of O3–NOx–ROG sensitivity and their sensitivity to model parameters are studied for a variety of polluted conditions using a comprehensive mixed-phase chemistry box model and the novel automatic differentiation ADIFOR tool. The main chemical reaction pathways in all phases, interfacial mass transfer processes, and ambient physical parameters that affect the indicators are identified and analyzed. Condensed mixed-phase chemical mechanisms are derived from the sensitivity analysis.Our results show that cloud chemistry has a significant impact on the indicators and their sensitivities, particularly on those involving H2O2, HNO3, HCHO, and NOz. Caution should be taken when applying the established threshold values of indicators in regions with large cloud coverage. Among the commonly used indicators, NOy and O3/NOy are relatively insensitive to most model parameters, whereas indicators involving H2O2, HNO3, HCHO, and NOz are highly sensitive to changes in initial species concentrations, reaction rate constants, equilibrium constants, temperature, relative humidity, cloud droplet size, and cloud water content.  相似文献   

14.
A detailed photochemical box model was used to investigate the key reaction pathways between OH, HO2 and RO2 radicals during the summer and winter PUMA field campaigns in the urban city-centre of Birmingham in the UK. The model employed the most recent version of the Master Chemical Mechanism and was constrained to 15-minute average measurements of long-lived species determined in situ at the site. The results showed that in the summer, OH initiation was dominated by the reactions of ozone with alkenes, nitrous acid (HONO) photolysis and the reaction of excited oxygen atoms atoms with water. In the winter, ozone+alkene reactions were the primary initiation route, with a minor contribution from HONO photolysis. Photolysis of aldehydes was the main initiation route for HO2, in both summer and winter. RO2 initiation was dominated by the photolysis of aldehydes in the summer with a smaller contribution from ozone+alkenes, a situation that was reversed in the winter. At night, ozone+alkene reactions were the main radical source. Termination, under all conditions, primarily involved reactions with NO (OH) and NO2 (OH and RCO3). These results demonstrate the importance of ozone+alkene reactions in urban atmospheres, particularly when photolysis reactions were less important during winter and at nighttime. The implications for urban atmospheric chemistry are discussed.  相似文献   

15.
Abstract

To evaluate future climate change in the middle atmosphere and the chemistry–climate interaction of stratospheric ozone, we performed a long-term simulation from 1960 to 2050 with boundary conditions from the Intergovernmental Panel on Climate Change A1B greenhouse gas scenario and the World Meteorological Organization Ab halogen scenario using the chemistry–climate model ECHAM5/MESSy Atmospheric Chemistry (EMAC). In addition to this standard simulation we performed five sensitivity simulations from 2000 to 2050 using the rerun files of the simulation mentioned above. For these sensitivity simulations we used the same model setup as in the standard simulation but changed the boundary conditions for carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and ozone-depleting substances (ODS). In the first sensitivity simulation we fixed the mixing ratios of CO2, CH4, and N2O in the boundary conditions to the amounts for 2000. In each of the four other sensitivity simulations we fixed the boundary conditions of only one of CO2, CH4, N2O, or ODS to the year 2000.

In our model simulations the future evolution of greenhouse gases leads to significant cooling in the stratosphere and mesosphere. Increasing CO2 mixing ratios make the largest contributions to this radiative cooling, followed by increasing stratospheric CH4, which also forms additional H2O in the upper stratosphere and mesosphere. Increasing N2O mixing ratios makes the smallest contributions to the cooling. The simulated ozone recovery leads to warming of the middle atmosphere.

In the EMAC model the future development of ozone is influenced by several factors. 1) Cooler temperatures lead to an increase in ozone in the upper stratosphere. The strongest contribution to this ozone production is cooling due to increasing CO2 mixing ratios, followed by increasing CH4. 2) Decreasing ODS mixing ratios lead to ozone recovery, but the contribution to the total ozone increase in the upper stratosphere is only slightly higher than the contribution of the cooling by greenhouse gases. In the polar lower stratosphere a decrease in ODS is mainly responsible for ozone recovery. 3) Higher NOx and HOx mixing ratios due to increased N2O and CH4 lead to intensified ozone destruction, primarily in the middle and upper stratosphere, from additional NOx; in the mesosphere the intensified ozone destruction is caused by additional HOx. In comparison to the increase in ozone due to decreasing ODS, ozone destruction caused by increased NOx is of similar importance in some regions, especially in the middle stratosphere. 4) In the stratosphere the enhancement of the Brewer-Dobson circulation leads to a change in ozone transport. In the polar stratosphere increased downwelling leads to additional ozone in the future, especially at high northern latitudes. The dynamical impact on ozone development is higher at some altitudes in the polar stratosphere than the ozone increase due to cooler temperatures. In the tropical lower stratosphere increased residual vertical upward transport leads to a decrease in ozone.  相似文献   

16.
Abstract

The dependence of ozone formation on the mixing ratios of volatile organic compounds (VOCs) and nitrogen oxides (NOx) has been widely studied. In addition to the atmospheric levels of VOCs and NOx, the extent of photochemical processing of VOCs has a strong impact on ozone levels. Although methods for measuring atmospheric mixing ratios of VOCs and NOx are well established and results of those measurements are widely available, determination of the extent of photochemical processing of VOCs, known as photochemical age (PCA), is difficult. In this article a recently developed methodology for the determination of PCA for individual compounds based on the change in their stable carbon isotope composition is used to investigate the dependence between ozone and VOC or NOx mixing ratios at a rural site in Ontario, Canada, during fall and winter. The results show that under these conditions the variability in VOC mixing ratios is predominantly a result of the varying impact of local emissions and not a result of changes in the extent of atmospheric processing. This explains why the mixing ratio of ozone shows no systematic dependence on the mixing ratios of VOCs or NOx in this environment and at this time of the year.  相似文献   

17.
We have studied long-term changes in tropospheric NO2 over South India using ground-based observations, and GOME and OMI satellite data. We have found that unlike urban regions, the region between Eastern and Western Ghat mountain ranges experiences statistically significant decreasing trend. There are few ground-based observatories to verify satellite based trends for rural regions. However, using a past study and recent measurements we show a statistically significant decrease in NOX and O3 mixing ratio over a rural location (Gadanki; 13.48° N, 79.18° E) in South India. In the ground-based records of surface NOX, the concentration during 2010–11 is found to be lower by 0.9 ppbv which is nearly 60 % of the values observed during 1994–95. Small but statistically significant decrease in noon-time peak ozone concentration is also observed. Noon-time peak ozone concentration has decreased from 34?±?13 ppbv during 1993–96 to 30?±?15 ppbv during 2010–11. NOX mixing ratios are very low over Gadanki. In spite of low NOX values (0.5 to 2 ppbv during 2010–11), ozone mixing ratios are not significantly low compared to many cities with high NOX. The monthly mean ozone mixing ratio varies from 9 ppbv to 37 ppbv with high values during Spring and low values during late Summer. Using a box-model, we show that presence of VOCs is also very important in addition to NOX in determining ozone levels in rural environment and to explain its seasonal cycle.  相似文献   

18.
Samples of interstitial air from within the snow pack on an ice floe on the Arctic Ocean were collected during the April 1994 Polar Sunrise Experiment. The concentrations of C2-C7 hydrocarbons are reported for the first time in the snow pack interstitial air. Alkane concentrations tended to be higher than concentrations in free air samples above the snow but very similar to winter measurements at various locations in the Arctic archipelago. However, ethyne concentrations in both interstitial and free air were highly correlated with ozone mixing ratios, consistent with previous demonstrations of the effects of Br atom chemistry. The analysis of total bromine within the snow pack indicate an enrichment in total Br at the interface layer between snow and free troposphere. The mixing ratios of some brominated compounds, such as CHBr3 and CHBr2Cl, are found to be higher in this top layer of snow relative to the boundary layer. Results were inconclusive due to the limited number of samples, but suggest the possible presence of active bromine in the snow pack and also that some differences exist between chemical reactions occurring in interstitial air compared to air in the boundary layer.  相似文献   

19.
The paper presents a coupled chemical-radiative one-dimensional model which is used to assess the steady-state and time-dependent composition and temperature changes in relation to the release in the atmosphere of chemicals such as CO2, N2O, CH4, NO x and chlorofluorocarbons.The model indicates that a doubling in CO2 leads to an increase in temperature of 12.7 K near the stratopause and to an increase in total ozone of 3.3% with a local enhancement of 17% at 40 km altitude. Additional release of N2O leads to an ozone reduction in the middle stratosphere. The reduction in the ozone column is predicted to be equal to 8.8% when the amount of N2O is doubled. The chemical effect of CH4 on ozone is particularly important in the troposphere. A doubling in the mixing ratio of this gas enhances the O3 concentration by 11% at 5 km. The predicted increase of the ozone column is equal to 1.4%. A constant emission of CFCl3 (230 kT/yr) and CF2Cl2 (300 kT/yr) leads to a steady-state reduction in the ozone column of 1.9% compared to the present-day situation. The effect of some uncertainties in the chemical scheme as well as the impact of a high chlorine perturbation are briefly discussed.Finally the results of a time dependent calculation assuming a realistic scenario for the emission of chemical species are presented and analyzed.  相似文献   

20.
Measurements of NOx,y were made at Alert, Nunavut, Canada (82.5° N, 62.3° W) during surface layer ozone depletion events. In spring 1998, depletion events were rare and occurred under variable actinic flux, ice fog, and snowfall conditions. NOy changed by less than 10% between normal, partially depleted, and nearly completely depleted ozone air masses. The observation of a diurnal variation in NOx under continuous sunlight supports a source from the snowpack but with rapid conversion to nitrogen reservoirs that are primarily deposited to the surface or airborne ice crystals. It was unclear whether NOx was reduced or enhanced in different stages of the ozone depletion chemistry because of variations in solar and ambient conditions. Because ozone was depleted from 15–20 ppbv to less than 1 ppbv in just over a day in one event it is apparent that the surface source of NOx did not grossly inhibit the removal of ozone. In another case ozone was shown to be destroyed to less than the 0.5 ppbv detection limit of the instrument. However, simple model calculations show that the rate of depletion of ozone and its final steady-state abundance depend sensitively on the strength of the surface source of NOx due to competition from ozone production involving NOx and peroxy radicals. The behavior of the NO/NO2 ratio was qualitatively consistent with enhanced BrO during the period of active ozone destruction. The model is also used to emphasize that the diurnal partitioning of BrOx during ozone depletion events is sensitive to even sub ppbv variations in O3.  相似文献   

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