Diverse changes in shipping emissions around the Western Pacific ports under the coeffect of the epidemic and fuel oil policy.

The Western Pacific Ocean (the WPO), as one of the busiest shipping areas in the world, holds a complex water traffic network. In 2020, the International Maritime Organization (IMO) low-sulfur fuel regulations were implemented globally, while the COVID-19 outbreak influenced shipping activities together. This study aimed to assess the combined impact of epidemics and low-sulfur fuel policies on ship emissions, as well as their environmental effects on the WPO. The ship emission model based on the Automatic Identification System (AIS) data was applied to analyze the monthly emission variations during 2018-2020. It was found that the epidemic had obvious diverse influences on the coastal ports in the WPO. Overall, shipping emissions declined by 15 %-30 % in the first half of 2020 compared with those in 2019 due to the COVID-19 lockdown, whereas they rebounded in the second half as a result of trade recovery. The pollutants discharged per unit of cargo by ships rose after the large-range lockdown. China's multiphase domestic emission control areas (DECAs) and the IMO global low-sulfur fuel regulation have greatly reduced SO emissions from ships and caused them to "bypass and come back" to save fuel costs around emission control areas from 2018 to 2020. Based on satellite data and land-based measurements, it was found that the air quality over sea water and coastal cities has shown a positive response to changes in ship-emitted NO and SO. Our results reveal that changes in shipping emissions during typical periods, depending on their niches in the complex port traffic network, call for further efforts for cleaner fuel oils, optimized ECA and ship lane coordination in the future. Shipping related air pollutions during the later economic recovery also needs to be addressed after international scale standing-by events.