What drives isoprene in tropical Australia?

Abstract

Isoprene is the dominant biogenic svolatile organic compound (BVOC) emitted from plants across the globe, with a mass of 400-600 Tg emitted annually. Its emission and chemical degradation plays a central role in the atmosphere, contributing to the formation of ground-level ozone and secondary organic aerosol. Tropical ecosystems contribute up to 75% of the global isoprene budget. Seasonal isoprene emission patterns in tropical regions remains unclear, particularly when compared to the mid-latitudes. It was hypothesised that in tropical regions, isoprene would be consistent throughout the year. However, a 12-year record of satellite observations of formaldehyde (HCHO) over the Amazon basin showed that HCHO columns reduced by 20-40% each year during the wet-to-dry transition. This thesis verifies these observations and investigates the hypothesis with a long-term, ground-based measurement study in a rainforest environment, paying particular attention to this transition period. To improve understanding of seasonal isoprene emissions patterns I conducted a measurement and modeling campaign in Far North Queensland to understand the drivers of isoprene emission in tropical Australia. A Fast Isoprene Sensor was installed in the Daintree Rainforest and I measured canopy-level isoprene concentrations over three years. They show that isoprene emissions follow a seasonal cycle, which differs from the Amazon. The measurements are compared against GOME-2B satellite observations and MEGAN and MLC-CHEM models to investigate factors driving emission patterns across several timescales. Findings show that model bias decreases by over 10% when leaf area index varies in response to the growing season and the emission factor is optimised by time and season. This is the first major BVOC study to be conducted in the Daintree, and is the first isoprene study in tropical Australia in over 20 years. The results presented in this thesis represent the first observations of seasonal isoprene emission patterns in Australia and provide an important contrast to other tropical ecosystems.