Factors controlling the distribution of ozone in the West African lower troposphere during the AMMA (African Monsoon Multidisciplinary Analysis) wet season campaign
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Abstract. Ozone and its precursors were measured on board the Facility for Airborne Atmospheric Measurements (FAAM) BAe 146 Atmospheric Research Aircraft during the monsoon season 2006 as part of the African Monsoon Multidisciplinary Analysis (AMMA) campaign. One of the main features observed in the west African boundary layer is the increase of the ozone mixing ratios from 25 ppbv over the forested area (south of 12° N) up to 40 ppbv over the Sahelian area. We employ a two-dimensional (latitudinal versus vertical) meteorological model coupled with an O3-NOx-VOC chemistry scheme to simulate the distribution of trace gases over West Africa during the monsoon season and to analyse the processes involved in the establishment of such a gradient. Including an additional source of NO over the Sahelian region to account for NO emitted by soils we simulate a mean NOx concentration of 0.7 ppbv at 16° N versus 0.3 ppbv over the vegetated region further south in reasonable agreement with the observations. As a consequence, ozone is photochemically produced with a rate of 0.25 ppbv h−1 over the vegetated region whilst it reaches up to 0.75 ppbv h−1 at 16° N. We find that the modelled gradient is due to a combination of enhanced deposition to vegetation, which decreases the ozone levels by up to 11 pbbv, and the aforementioned enhanced photochemical production north of 12° N. The peroxy radicals required for this enhanced production in the north come from the oxidation of background CO and CH4 as well as from VOCs. Sensitivity studies reveal that both the background CH4 and partially oxidised VOCs, produced from the oxidation of isoprene emitted from the vegetation in the south, contribute around 5–6 ppbv to the ozone gradient. These results suggest that the northward transport of trace gases by the monsoon flux, especially during nighttime, can have a significant, though secondary, role in determining the ozone gradient in the boundary layer. Convection, anthropogenic emissions and NO produced from lightning do not contribute to the establishment of the discussed ozone gradient.Keywords:
Mixing ratio
Deposition
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Atmospheric chemistry
Tropospheric ozone
Stratopause
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The trace gas observations from satellites can be the powerfull tools for the validation of tropospheric models, because of their global coverage in space and time. For verification of tropospheric models GOME spectrometer [1], which will be launched on ERS-2 satellite in 1994 should be able to measure the changes in tropospheric column densities of species occuring in the atmosphere. The trace species for which the tropospheric columns could be observed by GOME spectrometer are: ozone O 3 , nitrogen dioxide NO 2 , sulfur dioxide SO 2 and formaldehyde HCHO. In this study we tried to answer the question: what is the sensitivity of GOME spectrometer for the measurement of changes in the tropospheric column densities of these species. We applied only forward modeling to see how large should be the changes in tropospheric column densities of these species to cause the change of GOME signal which will exceed the noise level of the instrument. Though, it is the lowest boarder of detectibility, we tried to find out, and the error caused by the retrieval procedure must be added to our results.
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Tropospheric ozone
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Ozone Monitoring Instrument
Atmospheric chemistry
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Preface 1. Fundamental Quantities and Units 2. Data Regarding the Earth 3. Structure of the Atmosphere 4. Trace Gases in the Atmosphere 5. The Atmospheric Aerosol 6. Gas Phase Photochemistry 7. Rate Coefficients for Gas-Phase Reactions 8. Aqueous Phase Chemistry 9. The Upper Atmosphere 10. Measurement Techniques for Atmospheric Trace Species 11. Glossary of Atmospheric Chemistry Terms Index
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The Atmospheric Trace Molecule Spectroscopy (ATMOS) experiment, launched April 30, 1985, on Spacelab 3, measured vertical profiles at 30°N and 48°S of a host of reservoir gases, source gases, and other trace molecules important in the odd nitrogen, odd chlorine, and odd hydrogen chemical families of the middle atmosphere. The measurements included simultaneous observations of all the main elements of the NO y family (i.e., NO, NO 2 , HNO 3 , N 2 O 5 , HNO 4 , and ClONO 2 ), thereby giving a direct measurement of the total odd nitrogen mixing ratio for the first time. Some of these species (N 2 O 5 , HNO 4 , ClONO 2 ) represent first detections in the stratosphere, or confirmation of previously published tentative indentifications, and detailed discussion of each is not given here. The largest mixing ratios observed were ≈13±2.6 parts per billion by volume (ppbv) for [NO], 8.6±1.3 ppbv for [NO 2 ], 7.7±2.1 ppbv for [HNO 3 ], 1.6±0.8 ppbv for [N 2 O 5 ], 0.35±0.14 ppbv for HNO 4 , and 1.4±0.7 ppbv for [ClONO 2 ]. All of these peak values were found in northern hemisphere profiles, with the exception of HNO 3 and the implied value for N 2 O 5 . The maximum total odd nitrogen mixing ratio of ≈17±2.6 ppbv occurred at 43 km (≈2 mbar) and 30°N. These results have important implications because of the key role [NO y ] plays in buffering the odd chlorine depletion effect on ozone. The abundances of individual gases and total odd nitrogen levels measured by ATMOS have been compared with prior results obtained from balloon and satellite platforms. Since there are no prior [NO y ] measurements, comparisons in this case were made with a lower‐limit profile obtained from the sum of measured nighttime [NO 2 + HNO 3 ] and derived N 2 O 5 , obtained from model calculations initialized using earlier satellite results. The lower‐limit profile agrees with ATMOS data to within 16% up to 42 km altitude. Two‐dimensional model calculations of [NO y ] are as much as 40% larger than ATMOS values above 30 km and up to 40% less than the observations in the lower stratosphere.
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Atmospheric composition is controlled by the emission, photochemistry and transport of many trace gases. Understanding the time scale as well as the chemical and spatial patterns of perturbations to trace gases is needed to evaluate possible environmental damage (e.g. stratospheric ozone depletion or climate change) caused by anthropogenic emissions. This paper reviews lessons learned from treating global atmospheric chemistry as a linearized system and analysing it in terms of eigenvalues. The results give insight into how emissions of one trace species cause perturbations to another and how transport and chemistry can alter the time scale of the overall perturbation. Further, the eigenvectors describe the fundamental chemical modes, or patterns, of the atmosphere's chemical response to perturbations.
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Changes in atmospheric concentrations of trace gases provided early evidence of widespread changes within the biosphere. Trace gas production by plants and in soils increased in response to human pressures. Long lived trace gases like nitrous oxide and methane are greenhouse gases and play an important role in stratospheric chemistry. Photochemically active compounds, isoprene, nitric oxide, and carbon monoxide, are determinants of tropospheric ozone concentrations and thus regulate the oxidizing capacity of the troposphere. Inclusion of isoprene produced by plants in 3-D chemical transport models increases atmospheric concentrations of ozone and carbon monoxide substantially. In return, terrestrial ecosystems are sensitive to atmospheric composition, responding to increased N deposition with increased C uptake, and soil acidification, and responding to increased ozone concentrations and UV-B with decreased plant production.
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Isoprene
Atmospheric chemistry
Nitrous oxide
Atmospheric carbon cycle
Tropospheric ozone
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The atmospheric importance of carbon dioxide, methane, nitrous oxide, and chlorofluorocarbons is discussed. Atmospheric concentrations of these species have increased because of human activities. Sources, lifetimes, atmospheric budgets, and global warming potentials are presented for the key trace gases. Nitrous oxide, a recently discovered industrial emission byproduct, is examined for its role in stratospheric ozone chemistry and global warming. The atmospheric chemistry of trace gases illustrates many fundamental chemical principles that help engender student interest. These topics are suited for introductory chemistry classes, as well as for advanced courses.
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The latitudinal variations of atmospheric trace gas column abundances have been measured during a ship cruise between 57°N and 45°S on the central Atlantic. The measurements were performed in October 1996 using high‐resolution solar absorption spectroscopy in the infrared. The analysis method employed permits the retrieval of the total column densities of 20 different trace gases and for a few compounds the vertical mixing ratio profiles. For CH 4 an interhemispheric difference of 3% was observed. The total columns of the shorter‐lived trace gases CO and C 2 H 6 , analyzed between 57°N and 45°S, reveal a slight maximum in the tropics and a substantial increase north of 45°N. The total columns of C 2 H 2 and HCN, detectable between 30°N and 30°S, reveal a maximum in the tropics of the Southern Hemisphere. For CH 2 O, studied between 57°N and 45°S, a well‐pronounced maximum is observed in the tropics. The profile retrieval gives high mixing ratios for CO, C 2 H 6 , and O 3 north of 40°N in the lower troposphere. In the tropics high concentrations are found for all three compounds in the entire troposphere, even above 12 km. The measurements have been used to estimate averaged mixing ratios of the trace gases for the free troposphere between 0 and 12 km. In the tropics the data give high values: for example, more than 200 pptv for HCN, 750 pptv for CH 2 O, 100 ppbv for CO and 100 pptv for C 2 H 2 . These values are comparable to or higher than what has been observed at midlatitudes, indicating the importance of biomass burning emissions on the tropospheric composition.
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We present a 15‐month record of mixing ratios of CO, CH 4 , N 2 O, and eight halogenated gases (CCl 3 F, CCl 2 F 2 , CCl 2 FCClF 2 , CH 3 CCl 3 , CCl 4 , CHCl 3 , C 2 Cl 4 , and SF 6 ) at a rural site in eastern North Carolina. The data result from hourly gas Chromatographic analyses of air sampled at three heights on a 610‐m‐tall telecommunications tower during November 1994 through January 1996. At night, most of these gases were more abundant near the ground (51 m) than aloft (496 m) because of the buildup of local and regional surface emissions in the shallow nocturnal stable layer. The abundance and variability of trace gases at this continental site were generally higher than those at similar latitude remote locations. Mixing ratios of most gases were well correlated in polluted air masses occasionally advected to the tower. Frequent, strong enhancements in CHCl 3 at the lower sampling level(s) indicate a local point source(s) of this gas that is not associated with combustion. Temporal trends of regional background mixing ratios at this continental site are, for the most part, in good agreement with recent trends of remote background mixing ratios in the northern hemisphere.
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