Combination of aerosol sectional scheme and modal scheme in NorESM: Sensitivities to emissions

Cloud-aerosol interactions give rise to much of the uncertainty in estimates of climate forcing, climate sensitivity and thus also future climate predictions. Furthermore, the modelled concentration of cloud condensation nuclei (CCN) in the past, present and future are highly dependent on the how the models represent new particle formation (NPF) – a process which is both poorly understood theoretically and difficult to model due to its complex nature. Global modellers in particular have to prioritize between theoretical accuracy and keeping computational costs low. A common approach in these models is to use a modal scheme to parameterize the sizedistribution of the aerosols, while sectional schemes are in general considered closer to first principals.

To better capture the dynamics of early growth in the Norwegian Earth System Model (NorESM), we have implemented a sectional scheme for the smallest particles (currently 5 - 39 nm), which proceeds to feed particles into the original modal scheme (Kirkevåg et al, 2018) after growth. The sectional scheme includes two species, H₂SO₄ and low volatile organics and has 5 bins. The motivation is: (1) In the original scheme in NorESM, newly formed particles are added to the smallest mode which has a number median diameter of 23.6 nm. The survival of particles from NPF (formed at ~4 nm diameter) to this mode is calculated based on Lehtinen et al (2007). Thus it does not take into account dynamics within this size range, i.e. competition for condensing vapours and growth of particles over more than one time step. (2) Including a sectional scheme in this range adds precision for this crucial stage of growth while keeping the computational cost low due to the limited number of species involved (currently 2 in the model). (3) A sectional scheme within this size range is an interesting alternative to a nucleation mode, which is known to have problems with moving particles to larger sizes at the same time as adding newly formed particles.

We present several sensitivity tests which investigate the response of the model to changes in emissions of SO₂ and biogenic volatile organic compounds and nucleation parameterizations, with and without the sectional scheme. Our results in particular show that in the globally averaged boundary layer, the sectional scheme drastically reduces the number of particles that survive to the modal scheme compared the original model, while more particles survive in remote regions. On the other hand, the sectional scheme is less sensitive to the choice in NPF/nucleation parameterization. 

References: 

Lehtinen, Kari E. J., Miikka Dal Maso, Markku Kulmala, and Veli-Matti Kerminen. "Estimating Nucleation Rates from Apparent Particle Formation Rates and Vice Versa: Revised Formulation of the Kerminen–Kulmala Equation." Journal of Aerosol Science 38, no. 9 (September 1, 2007): 988–94. https://doi.org/10.1016/j.jaerosci.2007.06.009.

Kirkevåg, A., A. Grini, D. Olivié, Ø. Seland, K. Alterskjær, M. Hummel, I.H.H Karset, A. Lewinschal, X. Liu, R. Makkonen, I. Bethke, J. Griesfeller, M. Schulz and T. Iversen. "A production-tagged aerosol module for Earth system models, OsloAero5.3 – extensions and updates for CAM5.3-Oslo." Geoscientific Model Development 11. no. 10 (October, 2018): 3945--3982. https://doi.org/10.5194/gmd-11-3945-2018