Articles | Volume 16, issue 21
Atmos. Chem. Phys., 16, 13579–13599, 2016

Special issue: Global and regional assessment of intercontinental transport...

Atmos. Chem. Phys., 16, 13579–13599, 2016

Research article 01 Nov 2016

Research article | 01 Nov 2016

Global and regional radiative forcing from 20 % reductions in BC, OC and SO4 – an HTAP2 multi-model study

Camilla Weum Stjern1, Bjørn Hallvard Samset1, Gunnar Myhre1, Huisheng Bian2, Mian Chin3, Yanko Davila4, Frank Dentener5, Louisa Emmons6, Johannes Flemming8, Amund Søvde Haslerud1, Daven Henze4, Jan Eiof Jonson7, Tom Kucsera9, Marianne Tronstad Lund1, Michael Schulz7, Kengo Sudo10, Toshihiko Takemura11, and Simone Tilmes6 Camilla Weum Stjern et al.
  • 1CICERO Center for International Climate and Environmental Research, Oslo, Norway
  • 2Goddard Earth Sciences and Technology Center, University of Maryland, Baltimore, MD, USA
  • 3Earth Sciences Division, NASA Goddard Space Flight Center, Greenbelt, MD, USA
  • 4Department of Mechanical Engineering, University of Colorado, Boulder, CO, USA
  • 5European Commission, Joint Research Centre, Institute for Environment and Sustainability, Ispra (VA), Italy
  • 6Atmospheric Chemistry Division, National Center for Atmospheric Research (NCAR), CO, USA
  • 7Norwegian Meteorological Institute, Oslo, Norway
  • 8European Centre for Medium Range Weather Forecast (ECMWF), Reading, UK
  • 9Universities Space Research Association, Greenbelt, MD, USA
  • 10Nagoya University, Furocho, Chigusa-ku, Nagoya, Japan
  • 11Research Institute for Applied Mechanics, Kyushu University, Fukuoka, Japan

Abstract. In the Hemispheric Transport of Air Pollution Phase 2 (HTAP2) exercise, a range of global atmospheric general circulation and chemical transport models performed coordinated perturbation experiments with 20 % reductions in emissions of anthropogenic aerosols, or aerosol precursors, in a number of source regions. Here, we compare the resulting changes in the atmospheric load and vertically resolved profiles of black carbon (BC), organic aerosols (OA) and sulfate (SO4) from 10 models that include treatment of aerosols. We use a set of temporally, horizontally and vertically resolved profiles of aerosol forcing efficiency (AFE) to estimate the impact of emission changes in six major source regions on global radiative forcing (RF) pertaining to the direct aerosol effect, finding values between. 51.9 and 210.8 mW m−2 Tg−1 for BC, between −2.4 and −17.9 mW m−2 Tg−1 for OA and between −3.6 and −10.3 W m−2 Tg−1 for SO4. In most cases, the local influence dominates, but results show that mitigations in south and east Asia have substantial impacts on the radiative budget in all investigated receptor regions, especially for BC. In Russia and the Middle East, more than 80 % of the forcing for BC and OA is due to extra-regional emission reductions. Similarly, for North America, BC emissions control in east Asia is found to be more important than domestic mitigations, which is consistent with previous findings. Comparing fully resolved RF calculations to RF estimates based on vertically averaged AFE profiles allows us to quantify the importance of vertical resolution to RF estimates. We find that locally in the source regions, a 20 % emission reduction strengthens the radiative forcing associated with SO4 by 25 % when including the vertical dimension, as the AFE for SO4 is strongest near the surface. Conversely, the local RF from BC weakens by 37 % since BC AFE is low close to the ground. The fraction of BC direct effect forcing attributable to intercontinental transport, on the other hand, is enhanced by one-third when accounting for the vertical aspect, because long-range transport primarily leads to aerosol changes at high altitudes, where the BC AFE is strong. While the surface temperature response may vary with the altitude of aerosol change, the analysis in the present study is not extended to estimates of temperature or precipitation changes.

Short summary
Air pollution can reach distant regions through intercontinental transport. Here we first present results from the Hemispheric Transport of Air Pollution Phase 2 exercise, where many models performed the same set of coordinated emission-reduction experiments. We find that mitigations have considerable extra-regional effects, and show that this is particularly true for black carbon emissions, as long-range transport elevates aerosols to higher levels where their radiative influence is stronger.
Final-revised paper