Atmospheric mercury in Australia: Recent findings and future research needs

Abstract

Mercury is a toxic bioaccumulative pollutant, with the atmosphere being the primary pathway for global distribution. Although atmospheric mercury cycling has been extensively monitored and modeled across the Northern Hemisphere, there has long been a dearth of mercury data for the Southern Hemisphere. Recent efforts in Australia are helping to fill this gap, with new observational records that span environments ranging from cool temperate to warm tropical climates and near-source to background conditions. Here, we review recent research on atmospheric mercury in Australia, highlighting new observational constraints on atmospheric concentrations, emissions, and deposition and, where possible, comparing these to model estimates. We also provide our best estimate of the current Australian atmospheric mercury budget. Ambient mercury observations collected to date show unique features not captured at other observing sites across the Southern Hemisphere, including very low concentrations at inland sites and a monsoon season drawdown in the tropical north. Previously compiled estimates of Australian anthropogenic mercury emissions differ substantially due to both methodological differences (e.g., assumptions about mercury control technology in coal-fired power plants) and recent closures of major Australian mercury sources, and none are appropriate for modern-day Australia. For mercury emissions from biomass burning, new measurements from Australian smoke plumes show emission factors for both savanna and temperate forest fires are significantly lower than measured elsewhere in the world, and prior estimates based on non-Australian data are likely too high. Although significant uncertainties remain, our analysis suggests that emissions from terrestrial sources (both newly released and legacy) significantly exceed those from anthropogenic sources. However, recent bidirectional air-surface flux observations suggest this source is likely balanced by deposition and surface uptake at local scales. Throughout, we highlight lingering uncertainties and identify critical future research needs for understanding Australian atmospheric mercury and its role in Southern Hemisphere mercury cycling.