Biosphere–atmosphere related processes influence trace-gas and aerosol satellite–model biases

Biogenic volatile organic compounds (BVOCs), such as isoprene, impact aerosols, ozone and methane, adding uncertainty to assessments of the climate impacts of land cover change. Recent United Kingdom Earth System Model (UKESM) developments allow us to study how various processes impact biosphere–atmosphere interactions and their implications for atmospheric chemistry, while advances in remote sensing provide new opportunities for assessing biases in isoprene alongside formaldehyde and aerosol optical depth (AOD).

The standard setup of UKESM1.1 underestimates the regional formaldehyde column by up to 80 %, despite positive isoprene biases of over 500 %. Seasonal average AOD values are underestimated by over 60 % in parts of the Northern Hemisphere but overestimated (> 180 %) in the Congo.

The effects of several processes are studied to understand their impacts on satellite–model biases. Of these, changing from the default to a more detailed chemistry mechanism has the greatest impact on the simulated trace gases. Here, the isoprene lifetime decreases by 50 %, the formaldehyde column increases by > 20 %, whilst reductions in upper-tropospheric oxidant mixing ratios decrease sulfate nucleation (−32 %). Organically mediated boundary layer nucleation and contributions to aerosol mass from isoprene oxidation decrease AOD values in the Northern Hemisphere, while revised BVOC emission factors and land cover representation affect the emissions of BVOCs and dust.

The combination of processes substantially affects regional model–satellite biases, typically decreasing isoprene and AOD and increasing formaldehyde. We find significant differences in the aerosol direct radiative effects (+0.17 W m−2), highlighting that these processes may have substantial ramifications for impact assessments of land use change.