Abstract
Abstract. Aerosol–cloud interactions contribute to a large portion of the spread in estimates of climate forcing, climate sensitivity and future projections. An important part of this uncertainty is how much new particle formation (NPF) contributes to cloud condensation nuclei (CCN) and, furthermore, how this changes with changes in anthropogenic emissions. Incorporating NPF and early growth in Earth system models (ESMs) is, however, challenging due to uncertain parameters (e.g. participating vapours), structural issues (numerical description of growth from ∼1 to ∼100 nm) and the large scale of an ESM grid compared to the NPF scale. A common approach in ESMs is to represent the particle size distribution by a certain number of log-normal modes. Sectional schemes, on the other hand, in which the size distribution is represented by bins, are considered closer to first principles because they do not make an a priori assumption about the size distribution. In order to improve the representation of early growth, we have implemented a sectional scheme for the smallest particles (5–39.6 nm diameter) in the Norwegian Earth System Model (NorESM), feeding particles into the original aerosol scheme. This is, to our knowledge, the first time such an approach has been tried. We find that including the sectional scheme for early growth improves the aerosol number concentration in the model when comparing against observations, particularly in the 50–100 nm diameter range. Furthermore, we find that the model with the sectional scheme produces much fewer particles than the original scheme in polluted regions, while it produces more in remote regions and the free troposphere, indicating a potential impact on the estimated aerosol forcing. Finally, we analyse the effect on cloud–aerosol interactions and find that the effect of changes in NPF efficiency on clouds is highly heterogeneous in space. While in remote regions, more efficient NPF leads to higher cloud droplet number concentration (CDNC), in polluted regions the opposite is in fact the case.
Highlights
The formation of new particles in the atmosphere, known as new particle formation (NPF), occurs through the clustering and nucleation of low-volatility vapours
The ocean model in CESM2 is replaced by the Bergen Layered Ocean Model (BLOM) (Seland et al, 2020b), though this is not used in this study as all simulations are run with prescribed sea surface temperature (SST) and sea ice concentrations
Production of methanesulfonic acid (MSA) by oxidation of dimethyl sulfate (DMS) is taken into account, but since the model lacks a tracer for MSA, 20 % of the MSA is put in the SOAGLV tracer and 80 % in the SOAGSV
Summary
The formation of new particles in the atmosphere, known as new particle formation (NPF), occurs through the clustering and nucleation of low-volatility vapours. Carslaw et al (2013) show that the Global Model of Aerosol Processes (GLOMAP) has low sensitivity for particles larger than 50 nm to nucleation rate parameterizations but high sensitivity to processes affecting the coagulation loss of newly formed particles This underlines the importance of adequately representing the processes that constrain the formation of new particles. They find that the 10 nm cut-off is sensitive to the time step Another drawback of a high cut-off diameter is that most of these parameterizations neglect self-coagulation within the sub-cut-off size range, which can be an important growth mechanism during intense new particle formation events. We present the global changes in the state of aerosols and following cloud properties in the model with the new scheme (OsloAeroSec) compared to the original model (Sect. 4.2)
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