Methane production from mixed tropical savanna and forest vegetation in Venezuela

International audienceMeasurements of methane concentrations in the boundary layer in the northern part of the Guayana shield, Venezuela, during the wet season (October 1988), showed the presence of substantial methane surface emissions. The measuring site is within the savanna climate region, but is affected by emissions from savanna and forest vegetation. From day versus night concentration measurements, with higher concentrations during night, a methane source strength near the site of 3?7×1011 molecules/cm2/s can be estimated, which includes emissions from small tracts of flooded soils, termites and especially tropical vegetation. Extrapolated to the entire savanna, this may imply a methane source of ~30?60 Tg yr?1 similar to the one calculated for tropical vegetation on the basis of recently published in vitro plant emission experiments by Keppler et al. (2006), which indicate emissions of ~30 Tg yr?1 for tropical savannas and grasslands and ~78 Tg yr?1 for tropical forests

Analysis of non-methane hydrocarbons in air samples collected aboard the CARIBIC passenger aircraft

The CARIBIC project (Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container) is a long-term monitoring program making regular atmospheric measurements from an instrument container installed monthly aboard a passenger aircraft. Typical cruising altitudes of the aircraft allow for the study of the free troposphere and the extra-tropical upper troposphere as well as the lowermost stratosphere. CARIBIC measurements include a number of real time analyses as well as the collection of aerosol and whole air samples. These whole air samples are analyzed post-flight for a suite of trace gases, which includes non-methane hydrocarbons (NMHC).<br> <br> The NMHC measurement system and its analytical performance are described here. Precision was found to vary slightly by compound, and is less than 2% for the C₂–C₆ alkanes and ethyne, and between 1% and 6% for C₇–C₈ alkanes and aromatic compounds. Preliminary results from participation in a Global Atmospheric Watch (WMO) VOC audit indicate accuracies within the precision of the system. Limits of detection are 1 pptv for most compounds, and up to 3 pptv for some aromatics. These are sufficiently low to measure mixing ratios typically observed in the upper troposphere and lowermost stratosphere for the longer-lived NMHC, however, in air samples from these regions many of the compounds with shorter lifetimes (&lt;5 days) were frequently below the detection limit. Observed NMHC concentrations span several orders of magnitude, dependent on atmospheric region and air mass history, with concentrations typically decreasing with shorter chemical lifetimes

Increasing concentrations of dichloromethane, CH2Cl2, inferred from CARIBIC air samples collected 1998–2012

Atmospheric concentrations of dichloromethane, CH2Cl2, a regulated toxic air pollutant and minor contributor to stratospheric ozone depletion, were reported to have peaked around 1990 and to be declining in the early part of the 21st century. Recent observations suggest this trend has reversed and that CH2Cl2 is once again increasing in the atmosphere. Despite the importance of ongoing monitoring and reporting of atmospheric CH2Cl2, no time series has been discussed in detail since 2006. The CARIBIC project (Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container) has analysed the halocarbon content of whole-air samples collected at altitudes of between ~10–12 km via a custom-built container installed on commercial passenger aircraft since 1998, providing a long-term record of CH2Cl2 observations. In this paper we present this unique CH2Cl2 time series, discussing key flight routes which have been used at various times over the past 15 years. Between 1998 and 2012 increases were seen in all northern hemispheric regions and at different altitudes, ranging from ~7–10 ppt in background air to ~13–15 ppt in regions with stronger emissions (equating to a 38–69% increase). Of particular interest is the rising importance of India as a source of atmospheric CH2Cl2: based on CARIBIC data we provide regional emission estimates for the Indian subcontinent and show that regional emissions have increased from 3–14 Gg yr^-1 (1998–2000) to 16–25 Gg yr^-1 (2008). Potential causes of the increasing atmospheric burden of CH2Cl2 are discussed. One possible source is the increased use of CH2Cl2 as a feedstock for the production of HFC-32, a chemical used predominantly as a replacement for ozone-depleting substances in a variety of applications including air conditioners and refrigeration

A discussion on the determination of atmospheric OH and its trends

The oxidation efficiency of the troposphere is largely determined by the hydroxyl radical and its global distribution. Its presence limits the lifetime of most trace gases. Because of the great importance of several of these gases for climate, ozone budget and OH itself, it is of fundamental importance to acquire knowledge about atmospheric OH and possible trends in its concentrations. In the past, average concentrations of OH and trends were largely derived using industrially produced CH₃CCl₃ as a chemical tracer. The analyses have given valuable, but also rather uncertain results. In this paper we describe an idealized computer aided tracer experiment which has as one of its goals to derive tracer concentration weighted, global average <<i>k</i>(OH)>, where the temporal and spatial OH distribution is prescribed and <i>k</i> is the reaction rate coefficient of OH with a hitherto never produced (Gedanken) tracer, which is injected at a number of surface sites in the atmosphere in well known amounts over a given time period. Using a three-dimensional (3-D) time-dependent chemistry transport model, <<i>k</i>(OH)> can be accurately determined from the calculated 3-D tracer distribution. It is next explored how well <<i>k</i>(OH)> can be retrieved solely from tracer measurements at a limited number of surface sites. The results from this analysis are encouraging enough to actually think about the feasibility to carry out a global dedicated tracer experiment to derive <<i>k</i>(OH)> and its temporal trends. However, before that, we propose to test the methods that are used to derive <<i>k</i>(OH)>, so far largely using CH₃CCl₃, with an idealized tracer experiment, in which a global chemistry transport model is used to calculate the ``Gedanken'' tracer distribution, representing the real 3-D world, from which <<i>k</i>(OH)> is derived, using only the tracer information from a limited set of surface sites. We propose here that research groups which are, or will be, involved in global average OH studies to participate in such an inter-comparison of methods, organized and over-seen by a committee appointed by the International Global Atmospheric Chemistry (IGAC) program

Airborne multi-axis DOAS measurements of atmospheric trace gases on CARIBIC long-distance flights

A DOAS (&lt;i&gt;Differential Optical Absorption Spectroscopy&lt;/i&gt;) instrument was implemented and operated onboard a long-distance passenger aircraft within the framework of the CARIBIC project (&lt;i&gt;Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container&lt;/i&gt;). The instrument was designed to keep weight, size and power consumption low and to comply with civil aviation regulations. It records spectra of scattered light from three viewing directions (nadir, 10&amp;deg; above and below horizon) using a miniaturized telescope system. The telescopes are integrated in the main pylon of the inlet system which is mounted at the belly of the aircraft. Fibre bundles transmit light from the telescopes to spectrograph-detector units inside the DOAS container instrument. The latter is part of the removable CARIBIC instrument container, which is installed monthly on the aircraft for a series of measurement flights. &lt;br&gt;&lt;br&gt; During 30 flight operations within three years, measurements of HCHO, HONO, NO&lt;sub&gt;2&lt;/sub&gt;, BrO, O&lt;sub&gt;3&lt;/sub&gt; and the oxygen dimer O&lt;sub&gt;4&lt;/sub&gt; were conducted. All of these trace gases except BrO could be analysed with a 30 s time resolution. HONO was detected for the first time in a deep convective cloud over central Asia, while BrO, NO&lt;sub&gt;2&lt;/sub&gt; and O&lt;sub&gt;3&lt;/sub&gt; could be observed in tropopause fold regions. Biomass burning signatures over South America could be seen and measurements during ascent and descent provided information on boundary layer trace gas profiles (e.g. NO&lt;sub&gt;2&lt;/sub&gt; or HCHO)

A reconstruction of the past trend of atmospheric CO based on firn air samples from Berkner Island, Antarctica

International audienceAlthough for several atmospheric trace gases trends over the past 100 year have been reconstructed using firn air analyses, little is known about one of the chemically most significant trace gases, namely CO. Among the 3 Antarctic drilling expeditions reported, the one from Berkner Island appears to have given results of sufficient analytical quality to warrant a modelling with the aim to reconstruct past changes in atmospheric CO. Based on our reconstructions, CO in high latitudes of the Southern Hemisphere has been increasing since beginning of the 20th century from ~38 ppbv to a recent value of about 52.5 ppbv. The increase in CO is mainly explained by the known increase in CH4, with biomass burning output being most likely responsible for an additional increase. Which, if any, role changes in OH have played cannot be derived

Consistent simulation of bromine chemistry from the marine boundary layer to the stratosphere – Part 2: Bromocarbons

In this second part of a series of articles dedicated to a detailed analysis of bromine chemistry in the atmosphere we address one (out of two) dominant natural sources of reactive bromine. The two main source categories are the release of bromine from sea salt and the decomposition of bromocarbons by photolysis and reaction with OH. Here, we focus on C&lt;sub&gt;1&lt;/sub&gt;-bromocarbons. We show that the atmospheric chemistry general circulation model ECHAM5/MESSy realistically simulates their emission, transport and decomposition from the boundary layer up to the mesosphere. We included oceanic emission fluxes of the short-lived bromocarbons CH&lt;sub&gt;2&lt;/sub&gt;Br&lt;sub&gt;2&lt;/sub&gt;, CH&lt;sub&gt;2&lt;/sub&gt;ClBr, CHClBr&lt;sub&gt;2&lt;/sub&gt;, CHCl&lt;sub&gt;2&lt;/sub&gt;Br, CHBr&lt;sub&gt;3&lt;/sub&gt; and of CH&lt;sub&gt;3&lt;/sub&gt;Br. The vertical profiles and the surface mixing ratios of the bromocarbons are in general agreement with the (few available) observations, especially in view of the limited information available and the consequent coarseness of the emission fields. For CHBr&lt;sub&gt;3&lt;/sub&gt;, CHCl&lt;sub&gt;2&lt;/sub&gt;Br and CHClBr&lt;sub&gt;2&lt;/sub&gt; photolysis is the most important degradation process in the troposphere. In contrast to this, tropospheric CH&lt;sub&gt;2&lt;/sub&gt;Br&lt;sub&gt;2&lt;/sub&gt;, CH&lt;sub&gt;3&lt;/sub&gt;Br and CH&lt;sub&gt;2&lt;/sub&gt;ClBr are more efficiently decomposed by reaction with OH. In the free troposphere approximately 40% of the C&lt;sub&gt;1&lt;/sub&gt;-bromocarbons decompose by reaction with OH. Our results indicate that bromoform contributes substantial amounts of reactive bromine to the lower stratosphere and thus should not be neglected in stratospheric simulations

A climatology of surface ozone in the extra tropics: cluster analysis of observations and model results

Important aspects of the seasonal variations of surface ozone are discussed. The underlying analysis is based on the long-term (1990&amp;ndash;2004) ozone records of the Co-operative Programme for Monitoring and Evaluation of the Long-range Transmission of Air Pollutants in Europe (EMEP) and the World Data Centre of Greenhouse Gases, which provide data mostly for the Northern Hemisphere. Seasonal variations are pronounced at most of the 114 locations at all times of the day. A seasonal-diurnal variations classification using hierarchical agglomeration clustering reveals 6 distinct clusters: clean background, rural, semi-polluted non-elevated, semi-polluted semi-elevated, elevated and polar/remote marine. For the &quot;clean background&quot; cluster the seasonal maximum is observed in March-April, both for night and day. For those sites with a double maximum or a wide spring-summer maximum, the spring maximum appears both for day and night, while the summer maximum is more pronounced for daytime and hence can be attributed to photochemical processes. The spring maximum is more likely caused by dynamical/transport processes than by photochemistry as it is observed in spring for all times of the day. We compare the identified clusters with corresponding data from the 3-D atmospheric chemistry general circulation model ECHAM5/MESSy1 covering the period of 1998&amp;ndash;2005. For the model output as for the measurements 6 clusters are considered. The simulation shows at most of the sites a spring seasonal maximum or a broad spring-summer maximum (with higher summer mixing ratios). For southern hemispheric and polar remote locations the seasonal maximum in the simulation is shifted to spring, while the absolute mixing ratios are in good agreement with the measurements. The seasonality in the model cluster covering background locations is characterized by a pronounced spring (April&amp;ndash;May) maximum. For the model clusters which cover rural and semi-polluted sites the role of the photochemical production/destruction seems to be overestimated. Taking into consideration the differences in the data sampling procedure, the comparison demonstrates the ability of the model to reproduce the main regimes of surface ozone variations quite well

Accelerating growth of HFC-227ea (1,1,1,2,3,3,3-heptafluoropropane) in the atmosphere

We report the first measurements of 1,1,1,2,3,3,3-heptafluoropropane (HFC-227ea), a substitute for ozone depleting compounds, in air samples originating from remote regions of the atmosphere and present evidence for its accelerating growth. Observed mixing ratios ranged from below 0.01 ppt in deep firn air to 0.59 ppt in the current northern mid-latitudinal upper troposphere. Firn air samples collected in Greenland were used to reconstruct a history of atmospheric abundance. Year-on-year increases were deduced, with acceleration in the growth rate from 0.029 ppt per year in 2000 to 0.056 ppt per year in 2007. Upper tropospheric air samples provide evidence for a continuing growth until late 2009. Furthermore we calculated a stratospheric lifetime of 370 years from measurements of air samples collected on board high altitude aircraft and balloons. Emission estimates were determined from the reconstructed atmospheric trend and suggest that current "bottom-up" estimates of global emissions for 2005 are too high by a factor of three