Abstract
Abstract. One of the key challenges in atmospheric chemistry is to reduce the uncertainty of biogenic volatile organic compound (BVOC) 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. The Model of Emissions of Gases and Aerosols from Nature (MEGAN) has performed poorly against Australian isoprene and monoterpene observations. Finding reasons for the MEGAN discrepancies and strengthening our understanding of biogenic emissions in this region is our focus. We compare MEGAN to the locally produced Australian Biogenic Canopy and Grass Emissions Model (ABCGEM), to identify the uncertainties associated with the emission estimates and the data requirements necessary to improve isoprene and monoterpene emissions estimates for the application of MEGAN in Australia. Previously unpublished, ABCGEM is applied as an online biogenic emissions inventory to model BVOCs in the air shed overlaying Sydney, Australia. The two models use the same meteorological inputs and chemical mechanism, but 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. When the light dependence of monoterpenes is switched off in MEGAN, night-time emissions increase by 90–100 % improving the comparison with observations, suggesting the possibility that monoterpenes emitted from Australian vegetation may not be as light dependent as vegetation globally. Targeted measurements of emissions from in situ Australian vegetation, particularly of the light dependence issue are critical to improving MEGAN for one of the world's major biogenic emitting regions.
Highlights
The emission of biogenic volatile organic compounds (BVOCs) by vegetation and their impact on air quality was first noted by Went (1960), who proposed that their oxidation produced the “blue haze” often seen over forested areas
ABCGEM uses constant emission factors described in Sect. 2.2.1, which are converted to area units using the Bm weighted by leaf area index (LAI), in 1 m2 m−2 bins
The purpose of this work was to uncover reasons for the discrepancies produced by MEGAN in modelling BVOCs in the south-east Australian region identified by Emmerson et al (2016)
Summary
The emission of biogenic volatile organic compounds (BVOCs) by vegetation and their impact on air quality was first noted by Went (1960), who proposed that their oxidation produced the “blue haze” often seen over forested areas. The two most important BVOCs in terms of emissions are isoprene, and the group of C10H16 monoterpene species. Emmerson et al.: Isoprene and monoterpene emissions in south-east Australia can significantly affect atmospheric chemistry by increasing ground level ozone production. The interaction of BVOCs with anthropogenic pollutants (e.g. NOx, SO2, NH3 and organic carbon) can lead to the production of low volatility organic compounds that can condense to form secondary organic aerosols (SOA) (Hallquist et al, 2009; Xu et al, 2015; Lin et al, 2013). Biogenic SOA contributes to the total atmospheric fine particle burden and exposure to these particles can have deleterious impacts on human health (Schwartz et al, 1996)
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