Measurement report: Aerosol vertical profiling over the Southern Great Barrier Reef using lidar and MAX-DOAS measurements

Abstract. Aerosol vertical profile measurements were made using multi-axis differential optical absorption spectroscopy (MAX-DOAS) and mini-Micropulse LiDAR (MPL) at One Tree Island in the Southern Great Barrier Reef from February to April 2023. This is an understudied location in terms of atmospheric aerosols and chemistry but is growing in importance as multiple research streams examine the influence of aerosols on radiation over the Great Barrier Reef. Solar radiation management proposals require regional-scale aerosol modelling, which is evaluated against aerosol extinction and optical depth measurements, necessitating a thorough understanding of measurements of these quantities. MPL aerosol retrieval showed extinction-to-backscatter ratios (0.031 on average) and depolarization ratios (0.015 on average) consistent with clean, unpolluted Southern hemispheric marine aerosol. The maximum depolarization ratio tended to be above the layer of maximum MPL backscatter, which is attributed to dried sea-salt layers above the boundary layer. MAX-DOAS and MPL extinction profiles show aerosol layers extending beyond 2 km altitude in the middle of the day, but predominantly below 1 km at other times. We also compared aerosol optical depth measurements from integrating the MAX-DOAS and MPL extinction profiles, with observations from a hand-held Microtops sun photometer. Mean aerosol optical depth (AOD) values across the campaign compare well, being 0.083 ± 0.002 for the Microtops, 0.090 ± 0.032 for the MAX-DOAS and 0.104 ± 0.028 for the MPL. However, AOD observations at a given time, and the AOD diurnal cycle, often varied between instruments. This likely indicates strong horizontal inhomogeneity in aerosol in this environment, a factor which makes it challenging to accurately compare AOD estimates from different viewing geometries, but which is important for future aerosol modelling studies in this region to consider.