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 a Multi-Axis Differential Optical Absorption Spectrometer (MAX-DOAS) and Mini Micro Pulse 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, such as marine cloud brightening, 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, consistent with dried sea salt layers above the boundary layer. MAX-DOAS and MPL extinction profiles show aerosol layers extending beyond 2 km altitude in the morning and to around 1 km in the afternoon. We run the MAX-DOAS retrieval at 360 and 477 nm simultaneously, using the Retrieval of Atmospheric Parameters from Spectroscopic Observations using DOAS Instruments (RAPSODI) algorithm, finding that this gives much better agreement with the vertically resolved diurnal cycle of aerosol extinction from the MPL. We also compared aerosol optical depth measurements from integrated 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 0.084 ± 0.003 for the Microtops, 0.090 ± 0.040 for the MAX-DOAS and 0.091 ± 0.025 for the MPL (smoothed to match the MAX-DOAS vertical sensitivity). The diurnal cycles of the smoothed MPL and the MAX-DOAS AOD agreed within uncertainty in most hours of the day, showing a morning peak and afternoon minimum in boundary layer aerosol amount. These measurements show that even in this challenging environment with frequent occurrences of low marine clouds and high humidity, MAX-DOAS (with dual-wavelength retrieval) and MPL provide robust methods for probing aerosol vertical distributions and optical depth in the lower troposphere.