Isoprene and monoterpene emissions in Australia: comparison of a multi-layer canopy model with MEGAN and with atmospheric concentration observations

Abstract. One of the key challenges in atmospheric chemistry is to reduce the uncertainty of biogenic emission estimates from vegetation to the atmosphere. In Australia, eucalypt trees are a primary source of biogenic emissions, but their contribution to Australian air sheds is poorly quantified. CSIRO developed the Australian Biogenic Canopy and Grass Emissions Model (ABCGEM) 15 years ago to investigate this issue. Previously unpublished, ABCGEM is applied as an inline biogenic emissions inventory to model volatile organic compounds in the air shed overlaying Sydney, Australia. For comparison, biogenic emissions are calculated by the well-accepted Model of Emissions of Gases and Aerosols from Nature (MEGAN) for the same region using the same meteorological inputs. The two models use independent inputs of Leaf Area Index (LAI), Plant Functional Type (PFT) and emission factors. We find that LAI, a proxy for leaf biomass, has a small role in spatial, temporal and inter-model biogenic emission variability, particularly in urban areas for ABCGEM. After removing LAI as the source of the differences, we found large differences in the emission activity function for monoterpenes. In MEGAN monoterpenes are partially light dependent, reducing their dependence on temperature. In ABCGEM monoterpenes are not light dependent, meaning they continue to be emitted at high rates during hot summer days, and at night. Comparison with observations suggests that monoterpenes emitted from Australian vegetation may not be as light dependent as vegetation globally, as assumed in MEGAN. The simplified ABCGEM model is comparable with the state-of-the-art MEGAN model when measured by normalised mean bias values between the models and observed atmospheric isoprene and monoterpene observations. Observed ratios of isoprene to monoterpene carbon in south east Australia are approximately unity. ABCGEM replicates this ratio for both emission rates and predicted concentrations, while MEGAN over-estimates by a factor of 4. This suggests the correct balance between isoprene and monoterpene emissions in ABCGEM, but their magnitudes need further assessment. We estimate the uncertainty in Australian BVOC emissions to be a factor of 2 for isoprene and 3 for monoterpenes. This study identifies the uncertainties associated with the ABCGEM emission estimates, and data requirements necessary to improve isoprene and monoterpene emissions estimates for the application of both ABCGEM and MEGAN in Australia.