Upper tropospheric ice sensitivity to sulfate geoengineering

Abstract. Aside from the direct surface cooling that sulfate geoengineering (SG) would produce, investigations of the possible side effects of this method are still ongoing, such as the exploration of the effect that SG may have on upper tropospheric cirrus cloudiness. The goal of the present study is to better understand the SG thermodynamical effects on the freezing mechanisms leading to ice particle formation. This is undertaken by comparing SG model simulations against a Representative Concentration Pathway 4.5 (RCP4.5) reference case. In the first case, the aerosol-driven surface cooling is included and coupled to the stratospheric warming resulting from the aerosol absorption of terrestrial and solar near-infrared radiation. In a second SG perturbed case, the surface temperatures are kept unchanged with respect to the reference RCP4.5 case. When combined, surface cooling and lower stratospheric warming tend to stabilize the atmosphere, which decreases the turbulence and updraft velocities (−10 % in our modeling study). The net effect is an induced cirrus thinning, which may then produce a significant indirect negative radiative forcing (RF). This RF would go in the same direction as the direct effect of solar radiation scattering by aerosols, and would consequently influence the amount of sulfur needed to counteract the positive RF due to greenhouse gases. In our study, given an 8 Tg-SO2 yr−1 equatorial injection into the lower stratosphere, an all-sky net tropopause RF of −1.46 W m−2 is calculated, of which −0.3 W m−2 (20 %) is from the indirect effect on cirrus thinning (6 % reduction in ice optical depth). When surface cooling is ignored, the ice optical depth reduction is lowered to 3 %, with an all-sky net tropopause RF of −1.4 W m−2, of which −0.14 W m−2 (10 %) is from cirrus thinning. Relative to the clear-sky net tropopause RF due to SG aerosols (−2.1 W m−2), the cumulative effect of the background clouds and cirrus thinning accounts for +0.6 W m−2, due to the partial compensation of large positive shortwave (+1.6 W m−2) and negative longwave adjustments (−1.0 W m−2). When surface cooling is ignored, the net cloud adjustment becomes +0.8 W m−2, with the shortwave contribution (+1.5 W m−2) almost twice as much as that of the longwave (−0.7 W m−2). This highlights the importance of including all of the dynamical feedbacks of SG aerosols

Impact of Stratospheric Volcanic Aerosols on Age-of-Air and Transport of Long-Lived Species

The radiative perturbation associated to stratospheric aerosols from major explosive volcanic eruptions may induce significant changes in stratospheric dynamics. The aerosol heating rates warm up the lower stratosphere and cause a westerly wind anomaly, with additional tropical upwelling. Large scale transport of stratospheric trace species may be perturbed as a consequence of this intensified Brewer–Dobson circulation. 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 at surface stations (approximately −8 and −0.4 ppbv/year for CH4 and N2O, respectively). This decline is partly driven by the increased mid- to high-latitude downward flux at the tropopause and also by an increased isolation of the tropical pipe in the vertical layer near the tropopause, with reduced horizontal eddy mixing. Results from a climate-chemistry coupled model are shown for both long-lived trace species and the stratospheric age-of-air. The latter results to be younger by approximately 0.5 year at 30 hPa for 3–4 years after the June 1991 Pinatubo eruption, as a result of the volcanic aerosols radiative perturbation and is consistent with independent estimates based on long time series of in situ profile measurements of SF6 and CO2. Younger age of air is also calculated after Agung, El Chichon and Ruiz eruptions, as well as negative anomalies of the N2O growth rate at the extratropical tropopause layer. This type of analysis is made comparing the results of two ensembles of model simulations (1960–2005), one including stratospheric volcanic aerosols and their radiative interactions and a reference case where the volcanic aerosols do not interact with solar and planetary radiation

Stratospheric Aerosols from Major Volcanic Eruptions: A Composition-Climate Model Study of the Aerosol Cloud Dispersal and e-folding Time

Large explosive volcanic eruptions are capable of injecting considerable amounts of particles and sulfur gases above the tropopause, causing large increases in stratospheric aerosols. Five major volcanic eruptions after 1960 (i.e., Agung, St. Helens, El Chichon, Nevado del Ruiz and Pinatubo) have been considered in a numerical study conducted with a composition-climate coupled model including an aerosol microphysics code for aerosol formation and growth. Model results are compared between an ensemble of numerical simulations including volcanic aerosols and their radiative effects (VE) and a reference simulations ensemble (REF) with no radiative impact of the volcanic aerosols. Differences of VE-REF show enhanced diabatic heating rates; increased stratospheric temperatures and mean zonal westerly winds; increased planetary wave amplitude; and tropical upwelling. The impact on stratospheric upwelling is found to be larger when the volcanically perturbed stratospheric aerosol is confined to the tropics, as tends to be the case for eruptions which were followed by several months with easterly shear of the quasi-biennial oscillation (QBO), e.g., the Pinatubo case. Compared to an eruption followed by a period of westerly QBO, such easterly QBO eruptions are quite different, with meridional transport to mid- and high-latitudes occurring later, and at higher altitude, with a consequent decrease in cross-tropopause removal from the stratosphere, and therefore longer decay timescale. Comparing the model-calculated e-folding time of the volcanic aerosol mass during the first year after the eruptions, an increase is found from 8.1 and 10.3 months for El Chichon and Agung (QBO westerly shear), to 14.6 and 30.7 months for Pinatubo and Ruiz (QBO easterly shear). The corresponding e-folding time of the global-mean radiative flux changes goes from 9.1 and 8.0 months for El Chichon and Agung, to 28.7 and 24.5 months for Pinatubo and Ruiz

Sulfate Aerosols from Non-Explosive Volcanoes: Chemical-Radiative Effects in the Troposphere and Lower Stratosphere

SO2 and H2S are the two most important gas-phase sulfur species emitted by volcanoes, with a global amount from non-explosive emissions of the order 10 Tg-S/yr. These gases are readily oxidized forming SO42− aerosols, which effectively scatter the incoming solar radiation and cool the surface. They also perturb atmospheric chemistry by enhancing the NOx to HNO3 heterogeneous conversion via hydrolysis on the aerosol surface of N2O5 and Br-Cl nitrates. This reduces formation of tropospheric O3 and the OH to HO2 ratio, thus limiting the oxidation of CH4 and increasing its lifetime. In addition to this tropospheric chemistry perturbation, there is also an impact on the NOx heterogeneous chemistry in the lower stratosphere, due to vertical transport of volcanic SO2 up to the tropical tropopause layer. Furthermore, the stratospheric O3 formation and loss, as well as the NOx budget, may be slightly affected by the additional amount of upward diffused solar radiation and consequent increase of photolysis rates. Two multi-decadal time-slice runs of a climate-chemistry-aerosol model have been designed for studying these chemical-radiative effects. A tropopause mean global net radiative flux change (RF) of −0.23 W·m−2 is calculated (including direct and indirect aerosol effects) with a 14% increase of the global mean sulfate aerosol optical depth. A 5–15 ppt NOx decrease is found in the mid-troposphere subtropics and mid-latitudes and also from pole to pole in the lower stratosphere. The tropospheric NOx perturbation triggers a column O3 decrease of 0.5–1.5 DU and a 1.1% increase of the CH4 lifetime. The surface cooling induced by solar radiation scattering by the volcanic aerosols induces a tropospheric stabilization with reduced updraft velocities that produce ice supersaturation conditions in the upper troposphere. A global mean 0.9% decrease of the cirrus ice optical depth is calculated with an indirect RF of −0.08 W·m−2

Iloprost treatment in patients with Raynaud's phenomenon secondary to systemic sclerosis and the quality of life: a new therapeutic protocol

Objectives: to evaluate the clinical efficacy and the effects on the quality of life of Iloprost, a prostacyclin analogue, used, according to a new protocol, in patients with Raynaud’s phenomenon secondary to Systemic Sclerosis. Methods: in this randomized study we treated 30 patients with Iloprost given by intravenous infusion, at progressively increasing doses (starting from 0.5 ng/Kg/min up 2 ng/Kg/min) over a period of 6 hours a day for ten days in two consecutive weeks, with repeated cycles at regular intervals of three months for 18 months. The results were compared with those obtained in 30 other patients, who had received the same drug but with different posologic schemes. Results: the total average daily duration of the attacks, the average duration of a single attack and the average daily frequency of the attacks were reduced significantly in all groups of treatment, but the comparison between the groups demonstrated significant differences between patients treated with the new protocol and the others at later times (T12 and T18). The effects on the quality of life, evaluated by SF-36, demonstrate, in the group treated with the new protocol, a marked improvement both regarding the scale relative to the physical aspect of the illness, and especially regarding the scale relative to the mental aspect. Conclusions: in SSc patients, cyclic intravenous Iloprost infusion is efficacious in the treatment of Raynaud’s phenomenon. The protocol that we used, compared with others, is able to determine not only favourable clinical effects but also a marked improvement in the quality of life

Revisiting the Mystery of Recent Stratospheric Temperature Trends

Simulated stratospheric temperatures over the period 1979–2016 in models from the Chemistry-Climate Model Initiative are compared with recently updated and extended satellite data sets. The multimodel mean global temperature trends over 1979–2005 are -0.88 ± 0.23, -0.70 ± 0.16, and -0.50 ± 0.12 K/decade for the Stratospheric Sounding Unit (SSU) channels 3 (~40–50 km), 2 (~35–45 km), and 1 (~25–35 km), respectively (with 95% confidence intervals). These are within the uncertainty bounds of the observed temperature trends from two reprocessed SSU data sets. In the lower stratosphere, the multimodel mean trend in global temperature for the Microwave Sounding Unit channel 4 (~13–22 km) is -0.25 ± 0.12 K/decade over 1979–2005, consistent with observed estimates from three versions of this satellite record. The models and an extended satellite data set comprised of SSU with the Advanced Microwave Sounding Unit-A show weaker global stratospheric cooling over 1998–2016 compared to the period of intensive ozone depletion (1979–1997). This is due to the reduction in ozone-induced cooling from the slowdown of ozone trends and the onset of ozone recovery since the late 1990s. In summary, the results show much better consistency between simulated and satellite-observed stratospheric temperature trends than was reported by Thompson et al. (2012, https://doi.org/10.1038/nature11579) for the previous versions of the SSU record and chemistry-climate models. The improved agreement mainly comes from updates to the satellite records; the range of stratospheric temperature trends over 1979–2005 simulated in Chemistry-Climate Model Initiative models is comparable to the previous generation of chemistry-climate models

Aircraft emission mitigation by changing route altitude: A multi-model estimate of aircraft NOx emission impact on O3 photochemistry

The atmospheric impact of aircraft NOx emissions are studied using updated aircraft inventories for the year 2006, in order to estimate the photochemistry-related mitigation potential of shifting cruise altitudes higher or lower by 2000 ft. Applying three chemistry-transport models (CTM) and two climatechemistry models (CCM) in CTM mode, all including detailed tropospheric and stratospheric chemistry, we estimate the short-lived radiative forcing (RF) from O3 to range between 16.4 and 23.5 mW m 2, with a mean value of 19.5 mW m 2. Including the long-lived RF caused by changes in CH4, the total NOxrelated RF is estimated to about 5 mW m 2, ranging 1e8 mW m 2. Cruising at 2000 ft higher altitude increases the total RF due to aircraft NOx emissions by 2 ± 1 mW m 2, while cruising at 2000 ft lower altitude reduces RF by 2 ± 1 mWm 2. This change is mainly controlled by short-lived O3 and show that chemical NOx impact of contrail avoiding measures is likely small

A Modelling Study of the Impact of On-Road Diesel Emissions on Arctic Black Carbon and Solar Radiation Transfer

Market strategies have greatly incentivized the use of diesel engines for land transportation. These engines are responsible for a large fraction of black carbon (BC) emissions in the extra-tropical Northern Hemisphere, with significant effects on both air quality and global climate. In addition to direct radiative forcing, planetary-scale transport of BC to the Arctic region may significantly impact the surface albedo of this region through wet and dry deposition on ice and snow. A sensitivity study is made with the University of L’Aquila climate-chemistry-aerosol model by eliminating on-road diesel emissions of BC (which represent approximately 50% of BC emissions from land transportation). According to the model and using emission scenarios for the year 2000, this would imply an average change in tropopause direct radiative forcing (RF) of −0.054 W∙m−2 (globally) and −0.074 W∙m−2 over the Arctic region, with a peak of −0.22 W∙m−2 during Arctic springtime months. These RF values increase to −0.064, −0.16 and −0.50 W∙m−2, respectively, when also taking into account the BC snow-albedo forcing. The calculated BC optical thickness decrease (at λ = 0.55 µm) is 0.48 × 10−3 (globally) and 0.74 × 10−3 over the Arctic (i.e., 10.5% and 16.5%, respectively), with a peak of 1.3 × 10−3 during the Arctic springtime