Quantifying CO and NO₂ transport fluxes over southeastern Australia: implications from the 2019-2020 Black Summer and multi-year variability

Abstract

The 2019–2020 Black Summer (BS) wildfires provide a representative case for quantifying how extreme biomass burning affects regional pollutant transport. In this study, we combine satellite-derived column concentrations, ground-based observations, flux diagnostics, and forward trajectory analysis to evaluate the cross-boundary transport responses of NO₂ and CO over southeastern Australia. During the peak fire period, both CO and NO₂ fluxes increased markedly, indicating enhanced cross-boundary export of fire-related pollutants. However, the enhancement of CO net flux was more sustained over time than that of NO₂, and remained elevated during the later fire stage, indicating a more persistent cross-boundary transport. Combined analysis of forward trajectories and spatial concentration fields indicates that large-scale atmospheric circulation provides transport pathways extending from near-source regions toward the South Pacific and beyond, where CO can maintain a detectable signal during transport, whereas NO₂ decays rapidly along the transport pathway. Overall, this study highlights the value of cross-boundary fluxes as a physically constrained metric for extreme wildfire transport, providing a quantitative framework for assessing the atmospheric impacts of large-scale biomass burning across regional to broader scales.