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Discussion papers
https://doi.org/10.5194/acp-2019-1166
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-1166
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 04 Feb 2020

Submitted as: research article | 04 Feb 2020

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This preprint is currently under review for the journal ACP.

Large difference in aerosol radiative effects from BVOC-SOA treatment in three ESMs

Moa K. Sporre1,a, Sara M. Blichner1, Roland Schrödner2, Inger H. H. Karset1, Terje K. Berntsen1,3, Twan van Noije4, Tommi Bergman4,5, Declan O'Donnell5, and Risto Makkonen5,6 Moa K. Sporre et al.
  • 1Department of Geosciences, University of Oslo, Postboks 1022 Blindern, 0315 Oslo, Norway
  • 2Institute for Tropospheric Research, Permoserstr. 15, 04318 Leipzig, Germany
  • 3CICERO Center for International Climate Research, Postboks 1129 Blindern, 0318 Oslo, Norway
  • 4Royal Netherlands Meteorological Institute (KNMI), P.O. Box 201, 3730 AE De Bilt, the Netherlands
  • 5Climate System Research, Finnish Meteorological Institute, P.O. Box 503, FI-00101, Helsinki, Finland
  • 6Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, University of Helsinki, P.O. Box 64, FI-00014, Finland
  • anow at: Department of Physics, Lund University, Box 118, 22100 Lund, Sweden

Abstract. Biogenic volatile organic compounds (BVOCs) emitted from vegetation are oxidized in the atmosphere and can form aerosol particles either by contributing to new particle formation or by condensing onto existing aerosol particles. As the understanding of the importance of BVOCs for aerosol formation has increased over the past 10 years these processes have made their way into Earth System Models (ESMs). In this study, sensitivity experiments are run with three different ESMs, (the Norwegian Earth System Model (NorESM), EC-Earth and ECHAM) to investigate how the direct and indirect aerosol radiative effects are affected by changes in the formation of secondary organic aerosol (SOA) from BVOCs. In the first two sensitivity model experiments, the yields of SOA precursors from oxidation of BVOCs are changed by ± 50 %. For the third sensitivity test, the formed oxidation products do not participate in the formation of new particles, but are only allowed to condense onto existing aerosols. In the last two sensitivity experiments, the emissions of BVOC compounds (isoprene and monoterpenes) are turned off, one at a time.

The results show that the impact on the direct radiative effect (DRE) are linked to the changes in the SOA production in the models, where more SOA leads to a stronger DRE and vice versa. The magnitude by which the DRE changes (maximally 0.15 W m−2 globally averaged) in response to the SOA changes however varies between the models, with EC-Earth displaying the largest changes. The results for the cloud radiative effects (CRE) are more complicated than for the DRE. The changes in CRE differ more among the ESMs and for some sensitivity experiments they even have different signs. The most sensitive models are NorESM and EC-Earth, which has CRE changes of up to 0.82 W m−2. The varying responses in the different models are connected to where in the aerosol size distributions the changes in mass and number due to SOA formation occur, in combination with the aerosol number concentration levels in the models. We also find that interactive gas-phase chemistry as well as the new particle formation parameterization have important implications for the DRE and CRE in some of the sensitivity experiments. The results from this study indicate that BVOC-SOA treatment in ESMs can have a substantial impact on the modelled climate but that the sensitivity varies greatly between the models. Since BVOC emissions have changed historically and will continue to change in the future, the spread in model results found in this study introduces uncertainty into ESM estimates of aerosol forcing from land-use change and BVOC feedback strengths.

Moa K. Sporre et al.

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Short summary
In this paper we investigate how emissions/parameters in current SOA-parameterizations in three ESMs affect both the resulting SOA in the models and the impact this has on climate trough the direct and indirect aerosol effects. The SOA changes induce very different response in the models, in particular in terms of the indirect aerosol effects. This introduces uncertainties in ESMs estimates of SOA climate impact, through feedbacks in a warming climate and through anthropogenic land use change.
In this paper we investigate how emissions/parameters in current SOA-parameterizations in...
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