Abstract
Abstract. The spatiotemporal distributions of aerosol optical properties and major aerosol types, along with the vertical distribution of major aerosol types over Australia, are investigated based on multi-year Aerosol Robotic Network (AERONET) observations at nine sites, the Moderate Resolution Imaging Spectroradiometer (MODIS), Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2), Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), and back-trajectory analysis from the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT). During the observation period from 2001–2020, the annual aerosol optical depth (AOD) at most sites showed increasing trends (0.002–0.029 yr−1), except for that at three sites, Canberra, Jabiru, and Lake Argyle, which showed decreasing trends (−0.004 to −0.014 yr−1). In contrast, the annual Ångström exponent (AE) showed decreasing tendencies at most sites (−0.045 to −0.005 yr−1). The results showed strong seasonal variations in AOD, with high values in the austral spring and summer and relatively low values in the austral fall and winter, and weak seasonal variations in AE, with the highest mean values in the austral spring at most sites. Monthly average AOD increases from August to December or the following January and decreases during March–July. Spatially, the MODIS AOD showed obvious spatial heterogeneity, with high values appearing over the Australian tropical savanna regions, Lake Eyre Basin, and southeastern regions of Australia, while low values appeared over the arid regions in western Australia. MERRA-2 showed that carbonaceous aerosol over northern Australia, dust over central Australia, sulfate over densely populated northwestern and southeastern Australia, and sea salt over Australian coastal regions are the major types of atmospheric aerosols. The nine ground-based AERONET sites over Australia showed that the mixed type of aerosols (biomass burning and dust) is dominant in all seasons. Moreover, Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) showed that polluted dust is the dominant aerosol type detected at heights 0.5–5 km over the Australian continent during all seasons. The results suggested that Australian aerosol has similar source characteristics due to the regional transport over Australia, especially for biomass burning and dust aerosols. However, the dust-prone characteristic of aerosol is more prominent over central Australia, while the biomass-burning-prone characteristic of aerosol is more prominent in northern Australia.
Highlights
Aerosols play a crucial role in the Earth’s radiation budget and climate change, both through direct interaction with the solar radiation and through indirectly modifying the optical properties and lifespan of clouds (Albrecht, 1989; Charlson et al, 1992; Garrett and Zhao, 2006; Ramanathan et al, 2001; Twomey, 1977; Zhao and Garrett, 2015; Zhao et al, 2018)
The general aerosol optical depth (AOD) decrease at sites over northern Australia and the general increase at sites over central Australia are consistent with the findings obtained by Mehta et al (2016) using the Moderate Resolution Imaging Spectroradiometer (MODIS) and MISR AOD dataset during the period 2001–2014 and Mitchell et al (2010) using site observations over the decade 1997–2007
The highest AOD (0.20) at Canberra was observed in 2003 followed by a decrease to 0.06 in 2004. This was mostly related to the wildfires of southeastern Australia in January 2003, which generated large amounts of smoke aerosols, leading to a maximum in AOD during that observation period (Mitchell et al, 2006)
Summary
Aerosols play a crucial role in the Earth’s radiation budget and climate change, both through direct interaction with the solar radiation and through indirectly modifying the optical properties and lifespan of clouds (Albrecht, 1989; Charlson et al, 1992; Garrett and Zhao, 2006; Ramanathan et al, 2001; Twomey, 1977; Zhao and Garrett, 2015; Zhao et al, 2018). Frequent fires that occur during the dry season (typically April–November) each year in this region have released large quantities of biomass burning aerosols into the atmosphere (Radhi et al, 2012; Paton-Walsh et al, 2004). Mitchell et al (2013) demonstrated that Australia is a globally significant source of biomass burning aerosol from savanna burning. To the south of the savanna are huge desert regions (including Great Victoria, Simpson, Gibson, and Sturt deserts) and agricultural lands that have been under the control of prolonged drought, which makes Australia a dominant dust source in the Southern Hemisphere (Mitchell et al, 2010). Australian aerosol constitutes a significant component of the global aerosol budget, with great impacts on regional and global climate and the radiation budget
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