1,720,980 research outputs found
Study of the impact of altered flight trajectories on soot-cirrus: a EC-REACT4C study
No full textThe emission, dispersion and transport of aviation black carbon particles in the atmosphere
may trigger additional cirrus clouds (‘soot-cirrus’) or change the background distribution
of upper tropospheric ice particles. These effects and the sensitivity of aerosol and ice accumulation
to changes in flight altitude have been studied with the University of L’Aquila climate-chemistry
model (ULAQ-CCM), using emission inventories from the collaborative European project ECREACT4C.
Formation of background upper tropospheric ice particles is included by means of homogeneous
and heterogeneous freezing of super-cooled aerosols: this scheme considers the basic
physical processes that eventually determine the number of ice crystals Ni forming during an adiabatic
ascent of air, including the link of Ni on temperature and updraft speed. Background ice particle
formation is dominated by the homogeneous freezing process, producing the largest population
in the tropical upper troposphere. Ice changes from aviation are dominated by the heterogeneous
freezing mechanism, with the ice particle number density increasing with increasing BC particles
from aircraft emissions. RF changes produced by flight vertical displacement are negative/positive
for upward/downward displacement. A 2000 ft upward shift of aircraft routes, in fact, brings more
emissions into the stratosphere, where lower amounts of condensable water vapour are present and
less amounts are transported from below due to a rapid decrease of sub-grid updraft velocities
above the tropopause
First results of the University of L’Aquila chemistry-transport model for the HTAP assessment project
No full textPerturbazioni chimico-climatiche prodotte dalle emissioni da trasporto aereo: effetti diretti e indiretti
No full textPerturbazioni chimico-climatiche prodotte dalle emissioni da trasporto aereo:effetti diretti ed indiretti
No full textA global model study of sulphate and black carbon aerosol perturbations due to aviation emissions and impact on ozone: a EC-REACT4C study,
No full textThe University of L’Aquila climate-chemistry model (ULAQ-CCM) is a global
coupled model including troposphere and stratosphere and a rather comprehensive aerosol module.
The validation of aerosol products from the ULAQ model has focused on surface aerosol mass
density on remote sites, total optical depth on a regional basis, aerosol extinction profiles using
SAGE-II and HALOE data and black carbon (BC) vertical profiles using recent aircraft campaign
data. Three numerical experiments were performed with the ULAQ model using emission
inventories from the collaborative European project EC-REACT4C: no aircraft emissions (EXP1),
NOx and H2O emissions only (EXP2), and including all gas and particle emissions (EXP3). These
experiments have the purpose to study the direct radiative forcing (RF) of sulphate and BC aerosols
and to study the indirect impact on the NOx-HNO3 balance (and hence on O3) via heterogeneneous
chemistry on the surface of sulphate particles. The ULAQ-CCM calculated changes of sulphuric
acid aerosol surface area density reach maximum values of 1.5 μm2/cm3 at about 10 km altitude in
the NH mid-latitudes. Aircraft emissions of BC particles may significantly affect the mass density
of carbonaceous aerosols (0.3 ng/m3 at the same location). One conclusion is that the impact of
aviation SO2 and freshly emitted ultrafine sulphuric acid aerosols is to reduce the net RF
associated to aviation emissions of NOx (i.e. O3 and CH4), H2O and sulphur from 4.5 mW/m2 to 3.1
mW/m2, via changes of heterogeneous chemistry and cooling due to additional or larger sulphate
particles
Stratospheric circulation changes due to major volcanic eruptions: impact on long-lived species transport and age of air
No full textLarge explosive volcanic eruptions are capable of injecting considerable amounts of particles and sulphur gases above the tropopause, causing increases in stratospheric aerosol optical depth even larger than one order of magnitude. Perturbations of stratospheric tracer species transport result from dynamical changes due to both local stratospheric heating and climate changes associated to the increasing scattering of incoming solar radiation by the volcanic aerosols. Summarizing, the dynamical perturbation of the volcanic aerosols is twofold: (a) the stratospheric mean meridional circulation is affected by local aerosol radiative heating and photo-chemically induced ozone changes; (b) the planetary wave propagation in the mid- to high-latitude lower stratosphere is altered as a consequence of perturbed atmospheric stability due to the climate perturbation. The additional heating rates produce a local temperature increase in the lower stratosphere and upset the stratospheric dynamics, since no radiative equilibrium is achieved. For tropical eruptions an additional residual upwelling motion is produced in the tropical stratosphere as a result of a stronger pole-to-equator gradient of net heating rates.
The radiatively forced changes of the stratospheric circulation during the first two years after the eruption of Mt. Pinatubo (June 1991) may help explain the observed trend decline of long-lived greenhouse gases (CH4 in particular), as a result of the increased mid- to high-latitude downward flux at the tropopause. Since the stratosphere contains lower methane mixing ratios, a decline in the observed trends could result from a higher degree of exchange between the stratosphere and the troposphere. Results from the ULAQ-CCM model, using an updated version with respect to the one that has participated to the CCMVal-1 and CCMVal-2 inter-comparison campaigns, are shown for long-lived species perturbations due to volcanic eruptions and also for the stratospheric age-of-air. This type of analysis is made by comparing the results of two model simulations of the ULAQ-CCM: a reference case (1960-2010) with no volcanic heating perturbations and a sensitivity experiment where Agung, El Chichon and Pinatubo eruptions have been included
Multi-model estimate of direct and indirect radiative impact of aviation aerosols
No full textAircraft emissions may perturb the global amount and the size distribution of atmospheric aerosols in two ways: (a) direct emission of ultrafine black carbon (BC) soot and sulphuric acid particles in aircraft plumes and (b) release of gas phase SO2, later dispersed on large atmospheric scales and oxidized in H2SO4 by the OH radical. Direct particle emissions are estimated to account for 4–15% of the overall aircraft emitted sulphur. These direct particle emissions do not significantly change aerosol mass and extinction but may substantially increase surface area density (SAD) in the Northern Hemisphere UTLS. Release of gas phase SO2, on the other hand, may increase the net production of H2SO4, thus enhancing the sulphate mass in the accumulation mode and consequently, the direct RF. It also may increase the gas phase contribution to SAD, in the range of 25% of the change produced by direct plume particle emission. The University of L’Aquila climate-chemistry coupled model (ULAQ-CCM) has calculated the accumulation of sulphate aerosols and BC and their globally averaged direct radiative forcing (RF) at the NCEP tropopause with temperature adjustment and in total sky conditions, i.e. -3.3 W/m2 and +1.1 W/m2, respectively (with forcing efficiencies of -140 W/g-SO4 and +2300 W/g-BC, respectively).
The increase of BC in the upper troposphere due to aviation emissions may trigger formation of ice cloud particles (i.e. aviation ‘soot-cirrus’). The formation of background upper tropospheric ice particles is produced by homogeneous and heterogeneous freezing of supercooled aerosols. The ULAQ-CCM considers the basic physical processes that eventually determine the number of ice crystals Ni forming during an adiabatic ascent, including the link of Ni on temperature and updraft speed. In normal conditions the homogeneous freezing mechanisms dominates, but under significant local emissions of BC from aircraft the competition of heterogeneous and homogeneous freezing mechanisms becomes important. In the parameterization used for the formation of aviation soot-cirrus particles in a model grid-box, the change of ice crystals number concentration ∆Ni-HET is calculated as a function of ∆NBC and PHET, where ∆NBC is the change of soot particles due to aviation emissions, assuming a 1% non-hydrophobic fraction of the particles that may act as ice nuclei. PHET, in turn, is the probability that heterogeneous freezing may occur at given grid-box in the model, calculated as the probability to have ice super-saturation for a given temperature (RHICE › 100%) and taking into account water vapour transport due to subgrid vertical updraft velocity. The ULAQ-CCM calculates a +4.9 W/m2 indirect RF of BC, through formation of soot-cirrus (with forcing efficiency of +5 W/g-ice). Uncertainties in this model calculation of soot cirrus RF are however rather large.
The feedback of aviation-produced sulphate aerosol SAD on heterogeneous NOx chemistry represents another significant indirect radiative impact of aviation aerosols, via O3 and CH4 changes produced by the aerosol induced NOx perturbation. Here the physical and chemical approaches and the final calculations are much more robust than for upper tropospheric ice. In this case the results of the University of Oslo models (i.e. UiO-CTM2 and UiO-CTM3) have been used together with those from the ULAQ-CCM used in CTM mode, with the following results: -0.8 ± 0.2 W/m2 and +0.5 ± 0.1 W/m2, for O3 and CH4, respectively, where the error bar is obtained from the three models dispersion. The net aviation-aerosol RF calculated in this study accounts to -2.2 W/m2 (direct) and +4.6 W/m2 (indirect, including soot-cirrus) or -0.3 W/m2 (indirect, not including soot-cirrus), that is (in total) +2.4 W/m2 (including soot-cirrus) or -2.5 W/m2 (not including soot-cirrus). Taking into account that the non-CO2 aviation RF of gas species as calculated in the ULAQ-CCM (i.e. O3 and CH4 from aviation NOx changes and stratospheric H2O) accounts to +9.0 W/m2, the net aerosol impact represents a relative correction of the radiative forcing equal to +27% (including soot-cirrus) and -28% (not including soot-cirrus)
Sensitivity of the Methane Lifetime to Sulfate Geoengineering: Results from the Geoengineering Model Intercomparison Project (GeoMIP)
No full textSulfate geoengineering, made by sustained injection of SO2 in the tropical lower stratosphere, may impact the abundance of tropospheric methane through several photochemical mechanisms affecting the tropospheric OH abundance and hence the methane lifetime. Geoengineered sulfate aerosols in the stratosphere are responsible of important radiative and chemical effects: (a) solar radiation scattering increases the planetary albedo and cools the surface, with a tropospheric water vapor decrease as a response to this cooling: less OH. (b) The tropospheric UV budget is upset by the additional aerosol scattering and stratospheric ozone changes: the net effect is meridionally not uniform, with a net decrease in the tropics, thus producing less tropospheric O(1D): less OH. (c) The extratropical downwelling motion from the lower stratosphere tends to increase the sulfate aerosol surface area density available for heterogeneous chemical reactions in the mid-upper troposphere, thus reducing the amount of NOx and tropospheric O3 production: less OH. (d) The tropical lower stratosphere is warmed by solar and planetary radiation absorption by the aerosols. The heating rates perturbation are strongly latitude dependent, producing a significant change of the pole-to-equator temperature gradient and mean zonal wind distribution, with a net increase of tropical upwelling. A stronger meridional component of the Brewer-Dobson circulation may affect the abundance of mid-upper tropospheric sulfate aerosols, NOy, and O3 (all possibly affecting the budget of tropospheric OH), as well as CH4 transport directly. Three climate-chemistry coupled models are used here to explore the above radiative, chemical and dynamical mechanisms affecting the methane lifetime (ULAQ-CCM, CCSM4, GEOSCCM). First results show that the CH4 lifetime may become significantly longer (Fig.1) with a sustained injection of 2.5 Tg-S/yr started in year 2020 (exp. G4), which implies an increase of tropospheric CH4 (Fig. 2) and a positive indirect RF of sulfate geoengineering due to CH4 changes, of the order of 10% the aerosols direct forcing, but with opposite sign
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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