How well do the latest Earth System Models capture the behaviour of biogenic secondary organic aerosol in the atmosphere?

Biogenic secondary organic aerosol (SOA) is formed as a result of the atmospheric oxidation of gas-phase biogenic volatile organic compounds (BVOCs). Here, we evaluate the ability of five European Earth System Models (CNRM-ESM2-1, EC-Earth3, IPSL-CM6, NorESM1.2, UKESM1) to capture the amount, and behaviour, of biogenic SOA in the atmosphere.

The ESMs cover a range of complexity in terms of their representation of the sources and processing of biogenic SOA (i.e., from a fixed climatology of SOA amount to an interactive BVOC emission scheme followed by atmospheric processing).

We combine station measurements of BVOC emission and atmospheric BVOC concentrations with remotely sensed isoprene emission estimates to evaluate the models’ representation of the sources of biogenic SOA. We use organic aerosol mass and particle number concentration measurements from a number of forested sites to evaluate the ability of the models to capture the seasonal cycle in the amount of biogenic SOA present, as well as its impact on the aerosol size distribution. Whilst the models appear to capture the seasonal cycle in organic aerosol well for a boreal forest site, the ESMs consistently over-predict the amount of organic aerosol present at a tropical forest location. 

Finally, we explore the ability of these models to capture the observed relationships between organic aerosol mass, or particle number, and temperature. We find that the ESMs equipped with vegetation models that generate BVOC emissions interactively are able to capture well the strength of the observed relationship between temperature and organic aerosol mass. This lends confidence to the ability of these ESMs to accurately represent changes in atmospheric composition driven by climate.