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Impact of COVID-19 Lockdowns and Australian Bushfires on Aerosol Loading over the Downwind Oceanic Regions

Nearly 40 years of aerosol optical thickness (AOT) climate data record (CDR) from AVHRR satellite observations over the global ocean is used to study the impact of COVID-19 lockdowns and Australian bushfires on aerosol loading over downwind oceanic regions. Recent similar impact studies, by contrast, were mainly focused on the urban and suburban areas near the emission sources. AOT is used as a proxy for aerosol loading in our study. We found that AOT variations due to anthropogenic emission reduction during the lockdown period are complex and non-linear. AOT reduction was observed over China’s east coastal oceans due to emissions reduction during the stringent lockdown period in China. However, an unexpected surge of AOT occurred over the coastal oceans of India during the tight lockdown period in the pre-monsoon season. Moreover, inter-annual variations of AOT, along with a long-term decreasing trend, may conceal the lockdown-related AOT changes over the US east coastal ocean and the Mediterranean Sea due to relatively relaxed lockdown measures. By contrast, AOT increases in a monotonic way due to the emissions from severe Australian bushfires in the 2019/2020 fire season. The AOT surge is evident not only over the Australian coastal ocean but also extends to the entire southern ocean. This comparative study of two opposite extreme emission scenarios in natural conditions could be a benefit in creating effective mitigation strategies for future anthropogenic emissions and air pollution.