How external trade reshapes air pollutants emission profile of an urban economy: A case study of Macao

External trade has been approved to greatly affect environmental emissions by typical heterotrophic urban economies. However, the answers to questions such as how the trend of air pollutant embodied in external trade changes and what are the key influential factors need further exploration. Given that, using Macao as a case study, this paper first investigates the temporal evolution of 5 important air pollutants (i.e., SO2, PM10, NMVOC, NOX, and CO) emissions embodied in external trade based on the multi-regional input-output analysis, and then dissects the effects of key trade factors on air pollutant emissions by using the dynamic regression model. The results show that large amounts of air pollutants emissions have been avoided in Macao via increasing net imports, indicating that the direct emission reduction is achieved by transferring the emission-intensive industries to other regions. Specifically, the air pollutants emissions embodied in imports increased from 1.61E+05 t in 2000 to 2.77E+05 t in 2013. Besides, the air pollutants emissions embodied in exports increase from 1.60E+05 t in 2000 to 2.53E+05 t in 2013, dominated by gaming industry. On the other hand, the dynamic regression results reveal that there is a positive correlation between the total embodied emissions and both the growth of GDP per capita and imports, while factors such as exports growth can help decrease Macao's air pollutants emissions. Hence, a set of relevant air-pollution abatement strategies from both the local and global perspectives are proposed in this study.