The Impact of COVID-19 on NO2 and PM2.5 Levels and Their Associations with Human Mobility Patterns in Singapore (preprint)

ABSTRACT

The decline in NO 2 and PM 2.5 pollutant levels were observed during COVID-19 around the world, especially during lockdowns. Previous studies explained such observed decline with the decrease in human mobility, whilst overlooking the meteorological changes (e.g., rainfall, wind speed) that could mediate air pollution level simultaneously. This pitfall could potentially lead to over-or under-estimation of the effect of COVID-19 on air pollution. Consequently, this study aims to re-evaluate the impact of COVID-19 on NO 2 and PM 2.5 pollutant level in Singapore, by incorporating the effect of meteorological parameters in predicting NO 2 and PM 2.5 baseline in 2020 using machine learning methods. The results found that NO 2 and PM 2.5 declined by a maximum of 38% and 36%, respectively, during lockdown period. As two proxies for change in human mobility, taxi availability and carpark availability were found to increase and decrease by a maximum of 12.6% and 9.8%, respectively, in 2020 from 2019 during lockdown. To investigate how human mobility influenced air pollutant level, two correlation analyses were conducted one between PM 2.5 and carpark availability changes at regional scale and the other between NO 2 and taxi availability changes at a spatial resolution of 0.01°. The NO 2 variation was found to be more associated with the change in human mobility, with the correlation coefficients vary spatially across Singapore. A cluster of stronger correlations were found in the South and East Coast of Singapore. Contrarily, PM 2.5 and carpark availability had a weak correlation, which could be due to the limit of regional analyses. Drawing to the wider context, the high association between human mobility and NO 2 in the South and East Coast area can provide insights into future NO 2 reduction policy in Singapore. Graphical Abstract