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Atmospheric gravity waves, detected over Kiruna (67.8°N, 20.4°E) during geomagnetic storms, are presented and analysed. The data include direct measurements of the OI 630.0 nm emission line intensity, the <i>x</i>-component of the local geomagnetic field and thermospheric (meridional and zonal) wind velocities derived from the OI 630.0 nm Doppler shift observed with an imaging Fabry-Perot interferometer (IFPI). A low pass band filter technique was used to determine short-period variations in the thermospheric meridional wind velocities observed during geomagnetic storms. These short-period variations in the meridional wind velocities, which are identified as due to gravity waves, are compared to the corresponding variations observed in the OI 630.0 nm emission line intensity, <i>x</i>-component of the local geomagnetic field and the location of the auroral electrojet. A cross-correlation analysis was used to calculate the propagation velocities of the observed gravity waves.
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In this study, springtime coastal currents along the Latium coast and their relation to external forcings, mainly wind stress curl and atmospheric pressure, are analysed. As a main result, we find that hydrographical measurements reveal currents parallel to the bottom isobaths, but with isopycnal sloping upwards towards the coast, suggesting the importance of upwellings in determining the coastal currents. This is confirmed by thermal satellite data showing the presence of a ~10-km-wide patch of cold water east of Mount Argentario, <i>i.e.</i> a cyclonic vortex. The current meter data give rather small values of the time-averaged alongshore velocities (~2 cm s<sup>–1</sup> for most current meters and ~3 cm s<sup>–1</sup> for the current meter placed immediately off the Argentario) and also smaller values for the offshore velocities. The correlation between these two types of currents is rather poor; this is also due to the variability characteristic of a wind-induced upwelling. Finally, we obtain a value of 0.74 for the correlation between the alongshore current (first mode of Empirical Orthogonal Functions decomposition) and the wind stress if a 23-h time lag is assumed.
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Lacuna on ionograms indicate conditions where the usual ionospheric reflections of vertically propagating radio waves transmitted from and subsequently received by an ionosonde are missing over a range of frequencies. Reflections from the lowermost E-region should still be observable contrary to the more usual cases where absorption processes in the underlying D-region removes the low-frequency part of the ionospheric reflections appearing in ionograms. It is emphasized below that lacuna cases related to E-region processes should be distinguished from lacunae related to processes in the F1 and F2 regions. This distinction appears to have been neglected in a recent paper.
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The occurrence of PMSEs with time of day shows a semi-diurnal variation with minima at 8 and 20 h LT. PMSE layers observed for more than 30 min show an average rate of descent of 2 km h<sup>–1</sup>. These characteristics suggest the influence of tidal winds. When the observed steady wind and diurnal and semi-diurnal tides at EISCAT are added, the overall magnitude shows a time-variation which matches the occurrence of PMSEs, and the observed rate of descent, approximately 2 km h<sup>–1</sup>. Atmospheric gravity waves also contribute to the velocity of the neutral wind. When the wave reinforces the background wind, the PMSEs are stronger and descend more rapidly, but when the wave-related velocity opposes the background wind the PMSE is weaker and it descends more slowly.
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The stratosphere holds a variety of particulates like polar stratospheric clouds (PSCs) and sulphate aerosols which catalyse chemical reactions. These reactions cause changes in the composition of the stratosphere, including the redistribution of active chlorine which might lead to ozone destruction. As a result during recent years a lot of effort has been directed towards the quantification of the uptake of trace gases like ClONO<sub>2</sub>, HCl, etc. into these particulates. However, it has been observed that many of the two and three dimensional models used in such studies are constrained by the lack of adequate rate constant data. This paper describes a theoretical approach to estimate the reaction rate constants for 23 gases on both types of polar stratospheric clouds (type I and II). It is found that for gases like N<sub>2</sub>O<sub>5</sub>, ClONO<sub>2</sub> and HCl, diffusional uptake is important and contributes significantly to the heterogeneous reaction rate. A complete Lennard-Jones calculation is used to accurately compute the trace gas diffusion coefficients.
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It was noticed 60 years ago that at sunrise (i) the ionospheric critical frequency <i>fo</i>F2 increases faster in winter than in summer and (ii) the increase begins at a greater solar zenith angle in winter. It was later suggested that this "seasonal sunrise anomaly" is due to a seasonal change of atomic/molecular ratio in the neutral air in the F2-layer. This paper uses the Sheffield University plasmasphere-ionosphere computational model (SUPIM) with the MSIS thermosphere model to examine the relative importance at sunrise of production, loss and diffusion processes, and the effect of neutral air winds. The results show that both (i) and (ii) can be explained in terms of neutral composition changes.
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With a radiative transfer model, assessments are made of the radiative forcing in northern mid-latitudes due to aircraft emissions up to 1990. Considered are the direct climate effects from the major combustion products carbon dioxide, nitrogen dioxide, water vapor and sulphur dioxide, as well as the indirect effect of ozone production from NO<i><sub>x</sub></i> emissions. Our study indicates a local radiative forcing at the tropopause which should be negative in summer (–0.5 to 0.0 W/m<sup>2</sup>) and either negative or positive in winter (–0.3 to 0.2 W/m<sup>2</sup>). To these values the indirect effect of contrails has to be added, which for the North Atlantic Flight Corridor covers the range –0.2 to 0.3 W/m<sup>2</sup> in summer and 0.0 to 0.3 W/m<sup>2</sup> in winter. Apart from optically dense non-aged contrails during summer, negative forcings are due to solar screening by sulphate aerosols. The major positive contributions come from contrails, stratospheric water vapor in winter and ozone in summer. The direct effect of NO<sub>2</sub> is negligible and the contribution of CO<sub>2</sub> is relatively small.
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We use the HadGEM3-GA4, CESM1, and GISS ModelE2 climate models to investigate the global and regional aerosol burden, radiative flux, and surface temperature responses to removing anthropogenic sulfur dioxide (SO<sub>2</sub>) emissions from China. We find that the models differ by up to a factor of 6 in the simulated change in aerosol optical depth (AOD) and shortwave radiative flux over China that results from reduced sulfate aerosol, leading to a large range of magnitudes in the regional and global temperature responses. Two of the three models simulate a near-ubiquitous hemispheric warming due to the regional SO<sub>2</sub> removal, with similarities in the local and remote pattern of response, but overall with a substantially different magnitude. The third model simulates almost no significant temperature response. We attribute the discrepancies in the response to a combination of substantial differences in the chemical conversion of SO<sub>2</sub> to sulfate, translation of sulfate mass into AOD, cloud radiative interactions, and differences in the radiative forcing efficiency of sulfate aerosol in the models. The model with the strongest response (HadGEM3-GA4) compares best with observations of AOD regionally, however the other two models compare similarly (albeit poorly) and still disagree substantially in their simulated climate response, indicating that total AOD observations are far from sufficient to determine which model response is more plausible. Our results highlight that there remains a large uncertainty in the representation of both aerosol chemistry as well as direct and indirect aerosol radiative effects in current climate models, and reinforces that caution must be applied when interpreting the results of modelling studies of aerosol influences on climate. Model studies that implicate aerosols in climate responses should ideally explore a range of radiative forcing strengths representative of this uncertainty, in addition to thoroughly evaluating the models used against observations.
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Ground-based observation of the polarization properties of aerosol particles using a polarization optical particle counter (POPC) was made from 27 October 2013, to 31 December 2015, at a suburban site in the Kyushu area of Japan. We found that the depolarization ratio (DR, the fraction of <i>s</i>-polarized signal in the total backward light scattering signal) of aerosol particles showed prominent seasonal variability, with peaks in spring (0.21–0.23) and winter (0.19–0.23), and a minimum value (0.09–0.14) in summer. The aerosol compositions in both fine mode (aerodynamic diameter of particle, <i>D</i><sub>p</sub> < 2.5 µm) and coarse mode (2.5 µm < <i>D</i><sub>p</sub> < 10 µm), and the size-dependent polarization characteristics were analyzed for long-range transport dust particles, sea salt, and anthropogenic pollution-dominant aerosols. The DR value increased with increasing particle size, and DR = 0.1 was a reliable threshold value to identify the sphericity of supermicron (<i>D</i><sub>p</sub> > 1 µm) particles. Occurrence of substandard air quality days in Kyushu was closely related with mixed type (coexistence of anthropogenic pollutants and dust particles in the atmosphere), especially in winter and spring, indicating that dust events in the Asian continent played a key role in the cross-boundary transport of continental pollution. Backward trajectory analysis demonstrated that air masses originating from the western Pacific contained large amounts of spherical particles due to the influence of sea salt, especially in summer; however, for air masses from the Asian continent, the dependence of number fraction of spherical particles on air relative humidity was insignificant, indicating the predominance of less-hygroscopic substances (e.g., mineral dust), although the mass concentrations of anthropogenic pollutants were elevated.
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Ambient air pollution from ground-level ozone and fine particulate matter (PM<sub>2.5</sub>) is associated with premature mortality. Future concentrations of these air pollutants will be driven by natural and anthropogenic emissions and by climate change. Using anthropogenic and biomass burning emissions projected in the four Representative Concentration Pathway scenarios (RCPs), the ACCMIP ensemble of chemistry–climate models simulated future concentrations of ozone and PM<sub>2.5</sub> at selected decades between 2000 and 2100. We use output from the ACCMIP ensemble, together with projections of future population and baseline mortality rates, to quantify the human premature mortality impacts of future ambient air pollution. Future air-pollution-related premature mortality in 2030, 2050 and 2100 is estimated for each scenario and for each model using a health impact function based on changes in concentrations of ozone and PM<sub>2.5</sub> relative to 2000 and projected future population and baseline mortality rates. Additionally, the global mortality burden of ozone and PM<sub>2.5</sub> in 2000 and each future period is estimated relative to 1850 concentrations, using present-day and future population and baseline mortality rates. The change in future ozone concentrations relative to 2000 is associated with excess global premature mortality in some scenarios/periods, particularly in RCP8.5 in 2100 (316 thousand deaths year<sup>−1</sup>), likely driven by the large increase in methane emissions and by the net effect of climate change projected in this scenario, but it leads to considerable avoided premature mortality for the three other RCPs. However, the global mortality burden of ozone markedly increases from 382 000 (121 000 to 728 000) deaths year<sup>−1</sup> in 2000 to between 1.09 and 2.36 million deaths year<sup>−1</sup> in 2100, across RCPs, mostly due to the effect of increases in population and baseline mortality rates. PM<sub>2.5</sub> concentrations decrease relative to 2000 in all scenarios, due to projected reductions in emissions, and are associated with avoided premature mortality, particularly in 2100: between −2.39 and −1.31 million deaths year<sup>−1</sup> for the four RCPs. The global mortality burden of PM<sub>2.5</sub> is estimated to decrease from 1.70 (1.30 to 2.10) million deaths year<sup>−1</sup> in 2000 to between 0.95 and 1.55 million deaths year<sup>−1</sup> in 2100 for the four RCPs due to the combined effect of decreases in PM<sub>2.5</sub> concentrations and changes in population and baseline mortality rates. Trends in future air-pollution-related mortality vary regionally across scenarios, reflecting assumptions for economic growth and air pollution control specific to each RCP and region. Mortality estimates differ among chemistry–climate models due to differences in simulated pollutant concentrations, which is the greatest contributor to overall mortality uncertainty for most cases assessed here, supporting the use of model ensembles to characterize uncertainty. Increases in exposed population and baseline mortality rates of respiratory diseases magnify the impact on premature mortality of changes in future air pollutant concentrations and explain why the future global mortality burden of air pollution can exceed the current burden, even where air pollutant concentrations decrease.
