Estimates of the global magnitude of atmospheric methane sources are currently mainly based on direct flux measurements in source regions. Their extrapolation to the entire globe often involves large ...uncertainties. In this paper, we present an inverse modeling approach which can be used to deduce information on methane sources and sinks from the temporal and spatial variations of atmospheric methane mixing ratios. Our approach is based on a three‐dimensional atmospheric transport model which, combined with a tropospheric background chemistry module, is also employed to calculate the global distribution of OH radicals which provide the main sink for atmospheric methane. The global mean concentration of OH radicals is validated with methyl chloroform (CH3CCl3) observations. The inverse modeling method optimizes the agreement between model‐calculated and observed methane mixing ratios by adjusting the magnitudes of the various methane sources and sinks. The adjustment is constrained by specified a priori estimates and uncertainties of the source and sink magnitudes. We also include data on the 13C/12C isotope ratio of atmospheric methane and its sources in the model. Focusing on the 1980s, two scenarios of global methane sources are constructed which reproduce the main features seen in the National Oceanic and Atmospheric Administration's Climate Monitoring and Diagnostics Laboratory (NOAA/CMDL) methane observations. Differences between these two scenarios may probably be attributed to underestimated a priori uncertainties of wetland emissions. Applying the inverse model, the average uncertainty of methane source magnitudes could be reduced by at least one third. We also examined the decrease in the atmospheric methane growth rate during the early 1990s but could not uniquely associate it with changes in particular sources.
A detailed set of reactions treating the gas and aqueous phase chemistry of the most important iodine species in the marine boundary layer (MBL) has been added to a box model which describes Br and ...Cl chemistry in the MBL. While Br and Cl originate from seasalt, the I compounds are largely derived photochemically from several biogenic alkyl iodides, in particular CH2I2, CH2ClI, C2H5I, C3H7I, or CH3I which are released from the sea. Their photodissociation produces some inorganic iodine gases which can rapidly react in the gas and aqueous phase with other halogen compounds. Scavenging of the iodine species HI, HOI, INO2, and IONO2 by aerosol particles is not a permanent sink as assumed in previous modeling studies. Aqueous-phase chemical reactions can produce the compounds IBr, ICl, and I2, which will be released back into the gas phase due to their low solubility. Our study, although highly theoretical, suggests that almost all particulate iodine is in the chemical form of IO-3. Other aqueous-phase species are only temporary reservoirs and can be re-activated to yield gas phase iodine. Assuming release rates of the organic iodine compounds which yield atmospheric concentrations similar to some measurements, we calculate significant concentrations of reactive halogen gases. The addition of iodine chemistry to our reaction scheme has the effect of accelerating photochemical Br and Cl release from the seasalt. This causes an enhancement in ozone destruction rates in the MBL over that arising from the well established reactions O(1D) + H2O arrow right 2OH, HO2 + O3 arrow right OH + 2O2, and OH + O3 arrow right HO2 + O2. The given reaction scheme accounts for the formation of particulate iodine which is preferably accumulated in the smaller sulfate aerosol particles.PUBLICATION ABSTRACT
ABSTRACT
Previous studies on ice core analyses and recent in situ measurements have shown that CH4 has increased from about 0.75–1.73 μmol/mol during the past 150 years. Here, we review sources and ...sink estimates and we present global 3D model calculations, showing that the main features of the global CH4 distribution are well represented. The model has been used to derive the total CH4 emission source, being about 600 Tg yr‐1. Based on published results of isotope measurements the total contribution of fossil fuel related CH4 emissions has been estimated to be about 110 Tg yr‐1. However, the individual coal, natural gas and oil associated CH4 emissions can not be accurately quantified. In particular natural gas and oil associated emissions remain speculative. Since the total anthropogenic CH4 source is about 410 Tg yr‐1 (∼70% of the total source) and the mean recent atmospheric CH4 increase is ∼20 Tg yr‐1 an anthropogenic source reduction of 5% could stabilize the atmospheric CH4 level. We have calculated the indirect chemical effects of increasing CH4 on climate forcing on the basis of global 3D chemistry‐transport and radiative transfer calculations. These indicate an enhancement of the direct radiative effect by about 30%, in agreement with previous work. The contribution of CH4 (direct and indirect effects) to climate forcing during the past 150 years is 0.57W m−2 (direct 0.44W m−2, indirect 0.13 W m−2). This is about 35% of the climate forcing by CO2 (1.6W m−2) and about 22% of the forcing by all long‐lived greenhouse gases (2.6 W m−2). Scenario calculations (IPCC‐IS92a) indicate that the CH4 lifetime in the atmosphere increased by about 25–30%during the past 150 years to a current value of 7.9 years. Future lifetime changes are expected to be much smaller, about 6%, mostly due to the expected increase of tropospheric O3 (→OH) in the tropics. The global mean concentration of CH4 may increase to about 2.55 μmol/mol, its lifetime is expected to increase to 8.4 years in the year 2050. Further, we have calculated a CH4 global warming potential (GWP) of 21 (kgCH4/kgCO2) over a time horizon of 100 years, in agreement with IPCC (1996). Scenario calculations indicate that the importance of the climate forcing by CH4 (including indirect effects) relative to that of CO2 will decrease in future; currently this is about 35%, while this is expected to decrease to about 15% in the year 2050.
Using a novel experimental technique, based on proton transfer reaction mass spectrometry, from measurements of emissions from laboratory scale biomass burning experiments, we have estimated the ...source strengths of several potential HOx producing gases: formaldehyde, acetaldehyde, methanol and acetone. The derived global average emissions are 5–13; 3.8–10; 1.5‐4; 2.3‐6.1 Tg y−1, respectively. The resulting global average HOx production from photochemical decay of these gases is 3 × 109 molecules cm−2 s−1. Although relatively small in a global context, these emissions are significant for the photochemistry in fresh fire plumes. From our measurements are also estimated global source strengths from biomass burning for CH3CN and HCN of 0.4‐1.0; 0.2‐0.6 Tg y−1 respectively. The biomass burning emissions of CH3CN may well dominate the global source of this compound, which thus might well be a unique tracer for biomass burning. Some discrepancies between experimental studies must, however, be resolved.
Airborne measurements indicating the presence of high concentrations of acetone and methanol are reported. A 3D model is used to show that acetone photochemistry provides a quantitatively significant ...pathway for sequestering nitrogen oxides in the form of peroxyacetylnitrate.
Oxygenated volatile organic compounds (OVOC) can dominate atmospheric organic chemistry, but they are difficult to measure reliably at low levels in complex mixtures. Several techniques that have ...been used to speciate nonmethane organic compounds (NMOC) including OVOC were codeployed/intercompared in well‐mixed smoke generated by 47 fires in the U.S. Department of Agriculture Forest Service Fire Sciences Combustion Facility. The agreement between proton transfer reaction mass spectrometry (PTR‐MS) and open‐path Fourier transform infrared spectroscopy (OP‐FTIR) was excellent for methanol (PT/FT = 1.04 ± 0.118) and good on average for phenol (0.843 ± 0.845) and acetol (∼0.81). The sum of OP‐FTIR mixing ratios for acetic acid and glycolaldehyde agreed (within experimental uncertainty) with the PTR‐MS mixing ratios for protonated mass 61 (PT/FT = 1.17 ± 0.34), and the sum of OP‐FTIR mixing ratios for furan and isoprene agreed with the PTR‐MS mixing ratios for protonated mass 69 (PT/FT = 0.783 ± 0.465). The sum of OP‐FTIR mixing ratios for acetone and methylvinylether accounted for most of the PTR‐MS protonated mass 59 signal (PT/FT = 1.29 ± 0.81), suggesting that one of these compounds was underestimated by OP‐FTIR or that it failed to detect other compounds that could contribute at mass 59. Canister grab sampling followed by gas chromatography (GC) with mass spectrometry (MS), flame ionization detection (FID), and electron capture detection (ECD) analysis by two different groups agreed well with OP‐FTIR for ethylene, acetylene, and propylene. However, these propylene levels were below those observed by PTR‐MS (PT/FT = 2.33 ± 0.89). Good average agreement between PTR‐MS and GC was obtained for benzene and toluene. At mixing ratios above a few parts per billion the OP‐FTIR had advantages for measuring sticky compounds (e.g., ammonia and formic acid) or compounds with low proton affinity (e.g., hydrogen cyanide and formaldehyde). Even at these levels, only the PTR‐MS measured acetonitrile and acetaldehyde. Below a few ppbv only the PTR‐MS measured a variety of OVOC, but the possibility of fragmentation, interference, and sampling losses must be considered.
A model of atmospheric photochemistry and transport has been developed and applied toward investigating global tropospheric chemistry. The Model of Atmospheric Transport and Chemistry ‐ ...Max‐Planck‐Institute for Chemistry version (MATCH‐MPIC) is described and key characteristics of its global simulation are presented and compared to available observations. MATCH‐MPIC is an “offline” model which reads in gridded time‐dependent values for the most basic meteorological parameters (e.g., temperature, surface pressure, horizontal winds), then uses these to compute further meteorological parameters required for atmospheric chemistry simulations (convective transport, cloud microphysics, etc.). The meteorology component of MATCH‐MPIC simulates transport by advection, convection, and dry turbulent mixing, as well as the full tropospheric hydrological cycle (water vapor transport, condensation, evaporation, and precipitation). The photochemistry component of MATCH‐MPIC represents the major known sources (e.g., industry, biomass burning), transformations (chemical reactions and photolysis), and sinks (e.g., wet and dry deposition) which affect the O3hyphen;HOx‐NOy‐CH4‐CO photochemical framework of the “background” troposphere. The results of two versions of the model are considered, focusing on the more recent version. O3 is in relatively good agreement with observed soundings, although it is generally underestimated at low levels and overestimated at high levels, particularly for the more recent version of the model. We conclude that the simulated stratosphere‐troposphere flux of O3 is too large, despite the fact that the total flux is 1100 Tg(O3)/yr, whereas the upper limit estimated in recent literature is over 1400 Tg(O3)/yr. The OH distribution yields a tropospheric CH4 lifetime of 10.1 years, in contrast to the lifetime of 7.8 years in the earlier model version, which nearly spans the range of current estimates in the literature (7.5–10.2 years). Surface CO mixing ratios are in good agreement with observations. NO is generally underestimated, a problem similar to what has also been found in several other recent model studies. HNO3 is also considerably underestimated. H2O2 and CH3OOH, on the other hand, are in relatively good agreement with available observations, though both tend to be underestimated at high concentrations and overestimated at low concentrations. Possible reasons for these differences are considered.
A sensitivity study of the treatment of isoprene and related parameters in 3D atmospheric models was conducted using the global model of tropospheric chemistry MATCH-MPIC. A total of twelve ...sensitivity scenarios which can be grouped into four thematic categories were performed. These four categories consist of simulations with different chemical mechanisms, different assumptions concerning the deposition characteristics of intermediate products, assumptions concerning the nitrates from the oxidation of isoprene and variations of the source strengths. The largest differences in ozone compared to the reference simulation occured when a different isoprene oxidation scheme was used (up to 30-60% or about 10 nmol/mol). The largest differences in the abundance of peroxyacetylnitrate (PAN) were found when the isoprene emission strength was reduced by 50% and in tests with increased or decreased efficiency of the deposition of intermediates. The deposition assumptions were also found to have a significant effect on the upper tropospheric HOx production. Different implicit assumptions about the loss of intermediate products were identified as a major reason for the deviations among the tested isoprene oxidation schemes. The total tropospheric burden of O3 calculated in the sensitivity runs is increased compared to the background methane chemistry by 26±9 Tg( O3) from 273 to an average from the sensitivity runs of 299 Tg(O3). % revised Thus, there is a spread of ± 35% of the overall effect of isoprene in the model among the tested scenarios. This range of uncertainty and the much larger local deviations found in the test runs suggest that the treatment of isoprene in global models can only be seen as a first order estimate at present, and points towards specific processes in need of focused future work.
The first global tropospheric forecasts of O3 and its precursors have been used in the daily flight planning of field measurement campaigns. The 3-D chemistry-transport model MATCH-MPIC is driven by ...meteorological data from a weather center (NCEP) to produce daily 3-day forecasts of the global distributions of O3 and related gases, as well as regional CO tracers. This paper describes the forecast system and its use in three field campaigns, MINOS, CONTRACE and INDOEX. An overview is given of the forecasts by MATCH-MPIC and by three other chemical weather forecast models (EURAD, ECHAM, and FLEXPART), focusing on O3 and CO. Total CO and regional CO tracers were found to be the most valuable gases for flight planning, due to their relatively well-defined anthropogenic source regions and lifetimes of one to a few months. CO was in good agreement with the observations on nearly all the flights (generally r > 0.7, and the relative RMS differences for the deviations from the means was less than 20%). In every case in which the chemical weather forecasts were primarily responsible for the flight plans, the targeted features were observed. Three forecasted phenomena are discussed in detail: outflow from Asia observed in the Mediterranean upper troposphere during MINOS, outflow from North America observed in the middle troposphere over northern Europe during CONTRACE, and the location of the "chemical ITCZ'' over the Indian Ocean during INDOEX. In particular it is shown that although intercontinental pollution plumes such as those observed during MINOS and CONTRACE occur repeatedly during the months around the campaigns, their frequency is sufficiently low (~10--30% of the time) that global chemical weather forecasts are important for enabling them to be observed during limited-duration field campaigns. The MATCH-MPIC chemical weather forecasts, including an interface for making customized figures from the output, are available for community use via http://www.mpch-mainz.mpg.de/~lawrence/forecasts.html.
Arctic ozone loss due to denitrification Waibel, A.E; Peter, T; Carslaw, K.S ...
Science (American Association for the Advancement of Science),
03/1999, Letnik:
283, Številka:
5410
Journal Article
Recenzirano
Measurements from the winter of 1994-95 indicating removal of total reactive nitrogen from the Arctic stratosphere by particle sedimentation were used to constrain a microphysical model. The model ...suggests that denitrification is caused predominantly by nitric acid trihydrate particles in small number densities. The denitrification is shown to increase Arctic ozone loss substantially. Sensitivity studies indicate that the Arctic stratosphere is currently at a threshold of denitrification. This implies that future stratospheric cooling, induced by an increase in the anthropogenic carbon dioxide burden, is likely to enhance denitrification and to delay until late in the next century the return of Arctic stratospheric ozone to preindustrial values.
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