3 research outputs found
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Stratospheric dynamics and midlatitude jets under geoengineering with space mirrors and sulfate and titania aerosols
The impact on the dynamics of the stratosphere of three approaches to geoengineering by solar radiation management is investigated using idealized simulations of a global climate model. The approaches are geoengineering with sulfate aerosols, titania aerosols, and reduction in total solar irradiance (representing mirrors placed in space). If it were possible to use stratospheric aerosols to counterbalance the surface warming produced by a quadrupling of atmospheric carbon dioxide concentrations, tropical lower stratospheric radiative heating would drive a thermal wind response which would intensify the stratospheric polar vortices. In the Northern Hemisphere this intensification results in strong dynamical cooling of the polar stratosphere. Northern Hemisphere stratospheric sudden warming events become rare (one and two in 65 years for sulfate and titania, respectively). The intensification of the polar vortices results in a poleward shift of the tropospheric midlatitude jets in winter. The aerosol radiative heating enhances the tropical upwelling in the lower stratosphere, influencing the strength of the Brewer-Dobson circulation. In contrast, solar dimming does not produce heating of the tropical lower stratosphere, and so there is little intensification of the polar vortex and no enhanced tropical upwelling. The dynamical response to titania aerosol is qualitatively similar to the response to sulfate
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Black carbon physical and optical properties across northern India during pre-monsoon and monsoon seasons
Black carbon (BC) is known to have major impacts on both climate and human health and is therefore of global importance, particularly in regions close to large populations that have strong sources. The size-resolved mixing state of BC-containing particles was characterised using a single-particle soot photometer (SP2). The study focusses on the Indo-Gangetic Plain (IGP) during the pre-monsoon and monsoon seasons. Data presented are from the UK Facility for Airborne Atmospheric Measurements BAe-146 research aircraft that performed flights during the pre-monsoon (11 and 12 June) and monsoon (30 June to 11 July) seasons of 2016.
Over the IGP, BC mass concentrations were greater (1.95 µg m−3) compared to north-west India (1.50 µg m−3) and north-east India (0.70 µg m−3) during the pre-monsoon season. Across northern India, two distinct BC modes were recorded; a mode of small BC particles (core diameter <0.16 µm and coating thickness <50 nm) and a mode of moderately coated BC (core diameter <0.22 µm and coating thickness of 50–200 nm). The IGP and north-east India locations exhibited moderately coated black carbon particles with enhanced coating thicknesses, core sizes, mass absorption cross sections, and scattering enhancement values compared to much lower values present in the north-west. The coating thickness and mass absorption cross section increased with altitude (13 %) compared to those in the boundary layer. As the monsoon arrived across the region, mass concentration of BC decreased over the central IGP and north-east locations (38 % and 28 % respectively), whereas for the north-west location BC properties remained relatively consistent. Post-monsoon onset, the coating thickness, core size, mass absorption cross section, and scattering enhancement values were all greatest over the central IGP much like the pre-monsoon season but were considerably reduced over both north-east and north-west India. Increases in mass absorption cross section through the atmospheric column were still present during the monsoon for the north-west and central IGP locations, but less so over the north-east due to lack of long-range transport aerosol aloft. Across the Indo-Gangetic Plain and north-east India during the pre-monsoon and monsoon seasons, solid-fuel (wood burning) emissions form the greatest proportion of BC with moderately coated particles. However, as the monsoon develops in the north-east there was a switch to small uncoated BC particles indicative of traffic emissions, but the solid-fuel emissions remained in the IGP into the monsoon. For both seasons in the north-west, traffic emissions form the greatest proportion of BC particles.
Our findings will prove important for greater understanding of the BC physical and optical properties, with important consequences for the atmospheric radiative forcing of BC-containing particles. The findings will also help constrain the regional aerosol models for a variety of applications such as space-based remote sensing, chemistry transport modelling, air quality, and BC source and emission inventories
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Nonlinear response of Asian summer monsoon precipitation to emission reductions in South and East Asia
Anthropogenic aerosols over South and East Asia currently have a stronger impact on the Asian Summer Monsoon (ASM) than greenhouse gas emissions, yet projected aerosol emission changes in these regions are subject to considerable uncertainties such as timescale, location, or emission type. We use a circulation/climate model with idealised aerosol distributions to demonstrate that the sum of ASM responses to aerosol emission reductions in each region is very different to the response to simultaneous reductions in both regions, implying the ASM response to aerosol emissions reductions is highly nonlinear. The phenomenon is independent of whether aerosols are scattering or absorbing, and results from interaction of induced atmospheric circulation changes. The nonlinearity from interactions between aerosol forcing from different regions represents a new source of uncertainty in projections of ASM changes over the next 30-40 years, and may limit the utility of country-dependent aerosol trajectories when considering their Asia-wide effects, though we recommend further work to establish whether the nonlinearity is buffered by other drivers. To understand likely changes in the ASM due to aerosol reductions, countries will need to accurately take account of emissions reductions from across the wider region, rather than approximating them using simple scenarios and emulators. The nonlinearity in the response to forcing therefore presents a regional public goods issue for countries affected by the ASM, as the costs and benefits of aerosol emissions reductions are not internalised; in fact, forcings from different countries such as India and China work jointly to determine outcomes across the region