Atmospheric concentrations and sources of black carbon over tropical Australian waters

Black carbon (BC) aerosols significantly contribute to radiative budgets globally, however their actual contributions remain poorly constrained in many under-sampled ocean regions. The tropical waters north of Australia are a part of the Indo-Pacific warm pool, regarded as a heat engine of global climate, and are in proximity to large terrestrial sources of BC aerosols such as fossil fuel emissions, and biomass burning emissions from northern Australia. Despite this, measurements of marine aerosols, especially BC remain elusive, leading to large uncertainties and discrepancies in current chemistry-climate models for this region. Here, we report the first comprehensive measurements of aerosol properties collected over the tropical warm pool in Australian waters during a voyage in late 2019. The non-marine related aerosol emissions observed in the Arafura Sea region were more intense than in the Timor Sea marine region, as the Arafura Sea was subject to greater continental outflows. The median equivalent BC (eBC) concentration in the Arafura Sea (0.66 μg m^-3) was slightly higher than that in the Timor Sea (0.49 μg m^-3). Source apportionment modelling and back trajectory analysis and tracer studies consistently suggest fossil fuel combustion eBC (eBC_ff) was the dominant contributor to eBC across the entire voyage region, with biomass burning eBC (eBC_bb) making significant additional contributions to eBC in the Arafura Sea. eBC_ff (possibly from ship emissions or oil and gas rigs and their associated activities) and cloud condensation nuclei (CCN) were robustly correlated in the Timor Sea data, whereas eBC_bb positively correlated to CCN in the Arafura Sea, suggesting different sources and atmospheric processing pathways occurred in these two regions. This work demonstrates the substantial impact that fossil fuel and biomass burning emissions can have on the composition of aerosols and cloud processes in the remote tropical marine atmosphere, and their potentially significant contribution to the radiative balance of the rapidly warming Indo-Pacific warm pool.