Regional and seasonal radiative forcing by perturbations to aerosol and ozone precursor emissions

Abstract

<p><strong>Abstract.</strong> Dedicated model simulations by four general circulation and chemistry-transport models are used to establish a matrix of specific radiative forcing, defined as the radiative forcing per unit change in mass emitted, as a function of the near-term climate forcer emitted, its source region, and the season of emission. Emissions of eight near-term climate forcers are reduced: sulphur dioxide, the precursor to sulphate aerosols; black carbon aerosols; organic carbon aerosols; ammonia, a precursor to nitrate aerosols; methane; and nitrogen oxides, carbon monoxide, and volatile organic compounds, the precursors to ozone and to secondary organic aerosols. The focus is on two source regions, Europe and East Asia, but the shipping sector and global averages are also included. Emission reductions are applied over two time periods: May–Oct and Nov–Apr. Models generally agree on the sign and ranking of specific radiative forcing for different emitted species, but disagree quantitatively. Black carbon aerosols, methane, and carbon monoxide exert positive specific radiative forcings. Black carbon exerts the strongest specific radiative forcing, even after accounting for rapid adjustments from the semi-direct effect, and is most efficient in local summer. However, although methane and carbon monoxide are less efficient in a specific sense, the potential for decreasing the mass emitted is larger. Organic carbon aerosols, sulphur dioxide, ammonia, and emissions by the shipping sector exert negative specific radiative forcings, with local summer emission changes being again more efficient. Ammonia is notable for its weak specific radiative forcing. For aerosols, specific radiative forcing exerted by European emissions is stronger than for East Asia, because the European baseline is less polluted. Radiative forcing of European and East Asian emission reductions is mainly exerted in the mid-latitudes of the Northern Hemisphere, but atmospheric transport yields sizeable radiative forcings in neighbouring regions, such as the Arctic. Models disagree on the sign of the net radiative forcing exerted by reductions in the emissions of nitrogen oxides and volatile organic compounds, because those reductions trigger complex changes in the oxidising capacity of the atmosphere, translating into radiative forcings by aerosols, methane, and ozone of different signs. The response of nitrate aerosols to nitrogen oxide reductions is particularly important in determining the sign of the corresponding radiative forcing. Model diversity comes from different modelled lifetimes, different unperturbed baselines, and different numbers of species and radiative forcing mechanisms represented. The strength of the aerosol-chemistry coupling is also diverse, translating into aerosol responses to perturbations of ozone precursors of different magnitudes.</p>

Similar spatial patterns of global climate response to aerosols from different regions

“…Despite this, the global radiative forcing, temperature and precipitation changes are all significantly larger in the cases of North American or European emission perturbations. This is also consistent with the stronger response per unit SO 2 emission from Europe compared with East Asia reported in a previous multi-model study, 19,20 which diagnosed radiative forcings and estimated global temperature change metrics from a range of emission species in these two regions.…”

Regional temperature change potentials for short-lived climate forcers based on radiative forcing from multiple models

“…In addition, OsloCTM2 estimated the RF from BC deposition on snow and the semi-direct effect. The semi-direct effect is quantified in Bellouin et al (2016) by prescribing control and perturbed distributions of BC mass-mixing ratios based on OsloCTM2 in 30-year, fixed sea-surface simulations with the Community Earth System Model (CESM). The RF from aerosol-radiation interactions was quantified with multiple calls to the radiation scheme.…”

“…The quantification of these effects are given by the RF data from Bellouin et al (2016). For the general circulation models, the RFs of the aerosol perturbations are calculated online using two calls to the radiation scheme.…”

“…The emission regions are defined according to tier1 Hemispheric Transport of Air Pollution (HTAP) regions (see Bellouin et al, 2016). Europe is defined as western and eastern Europe up to 66 • N including Turkey.…”

“…The RFs that are the basis for the ARTP calculations of the SLCFs are calculated using four different chemistry climate models or chemical-transport models presented by Bellouin et al (2016); see details about the models in Table 1. RFs are produced based on a control simulation and numerous perturbation simulations that consider a 20 % emission reduction in one type of species and one region in NH summer or winter.…”