Comment on egusphere-2022-748

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Emissions of biogenic volatile organic compounds (BVOCs) influence atmospheric composition and climate and will be influenced by future changes in land use and land cover (LULC) and change. Climate and Earth System Models typically calculate emissions using parameterisations involving surface temperature, photosynthetic activity, CO₂ and the type of vegetation present. The influence of vegetation is described by assigning emission factors (EF) to different types of vegetation simulated in the model. We detail calculations of new EF for the Interactive BVOC Emission Scheme (iBVOC) used in the United Kingdom Earth System Model (UKESM). These EFs are based on those used by the Model of Emissions of Gases and Aerosols from Nature (MEGAN) v2.1 scheme. We present these EFs as alternatives to the current EFs used in iBVOC which are derived from older versions of MEGAN and the Organizing Carbon and Hydrology in Dynamic Ecosystem (ORCHIDEE) emission scheme. The EFs currently used by iBVOC include an oversimplification which incorporates the EF of shrubs (high isoprene emitters) into the EFs for C3 and C4 grasses (low isoprene emitters) despite UKESM1 treating grasses and shrubs separately. Thus, the current approach significantly overestimates the isoprene emissions from grasses, particularly C4 grass which is responsible for 40 % of total simulated isoprene emissions in the present day, much higher than other estimates of ~0.3&ndash;10 %. The new isoprene EF calculated in this work substantially reduce the amount of isoprene emitted by C4 grassland, in line with observational studies and other modelling approaches, while also increasing the emissions from known sources such as tropical broadleaf trees. Similar results are found from the change to terpene EF. The total global isoprene and terpene emissions with the new EF are in the range suggested by literature. The existing model biases in isoprene column are slightly exacerbated with the new EFs although other drivers of this bias are also noted. The disaggregation of shrub and grass EFs should lead to a more faithful description of the contribution to BVOC emissions from different vegetation types, critical for understanding BVOC emissions in the pre-industrial and under different future LULC scenarios such as those including wide scale reforestation or deforestation.