Since the detection of the first case of COVID-19 disease in Wuhan, China, at the end of 2019, the novel coronavirus has evolved into a global pandemic, with over 31.2 million confirmed cases and over 963,000 deaths across the globe.

In an attempt to contain the spread of the disease, many countries worldwide, including Malaysia and other Southeast Asian (SEA) countries, have enforced lockdowns of varying degrees, which has inadvertently caused some positive changes to the environment.

The evaluation of the decrease in anthropogenic emissions because of COVID-19-related restrictions imposed by governments is critical to determine the impact of the pandemic on pollution and economic growth. A recent study published in the journal Science of The Total Environment analyzes the impact of COVID-19-related lockdown on aerosols and pollutants over the Southeast Asian atmospheric environment.

This study used aerosol optical depth (AOD) measurements from the Himawari-8 satellite, along with tropospheric NO2 column density measurements from Aura-OMI over SEA countries, as well as ground-based measurements of pollution at various stations across Malaysia, so as determine the changes in aerosol and pollutants linked to the shutdown of human and industrial activities.

Aerosol optical depth (AOD) is a measure of the extinction of solar radiation due to light absorption or scattering by atmospheric dust and haze. The aerosols over the SEA region are mainly from urban and industrial emissions such as organics, nitrate, sulfate, and ammonium; black carbon from fossil fuel and biofuel burning; volcanic ash and dust transported from long distances during pre-monsoon forest and vegetation fires. In addition to these, trace amounts of sulfur dioxide (SO2), ammonia (NH3), nitrogen oxides (NOx), and carbon monoxide (CO) gases undergo complex homogeneous or heterogeneous chemical reactions in the atmosphere and release organic and inorganic aerosols.

Lockdown has helped reduce pollution over SEA regions

The study found that the lockdown has caused a notable decrease in AOD over SEA regions and in the pollution outflow over the oceanic regions. It also detected large reductions (~27 - 34%) of tropospheric NO2 over urban agglomerations and nearly 26-31%, 23-32%, 63-64%, 9-20%, and 25-31% reductions, respectively, in PM10, PM2.5, NO2, SO2, and CO in urban Malaysia during the lockdown phase compared to the same periods in years 2018 and 2019. NO2 levels were reduced even more by 64% in the urban areas and by 33–46% in the industrial sites. Lower reductions were recorded for SO2 and CO, while O3 changes were not significant compared to the previous years.

The results showed a significant reduction in AOD over the southern SEA region, including Singapore, Brunei, Malaysia, and the Philippines. In Malaysia, the AOD values over urban and industrial sites showed a notable decrease (~70% and ~40%) between March to April 2020 compared to the same period in 2018 and 2019. In contrast, in the northern SEA region, AODs stayed at very high levels even during the lockdown, because of extensive agricultural burning and forest fires in this area. This was also in agreement with the highest NO2 levels detected in this area.

There were large reductions in NO2 levels (~27%–34%) during the shutdown period in most SEA cities, except for Yangon and Ho Chi Minh. This NO2 reduction was strongly linked with the efforts taken by these countries to restrict the people's movement within and across countries and to control the industrial and business activities.

Some SEA countries, including Malaysia, Brunei, and Singapore, imposed aggressive restrictions such as the prohibition of mass gatherings, border closures, restriction of religious activities, and partial to complete lockdowns by the army, compared to other countries such as Indonesia, Cambodia, Laos, Thailand, Myanmar, and the Philippines that only enacted mild restrictions.

The results of this study indicate that the degree of restriction and the regional lockdowns enacted to contain COVID-19 has affected the air pollution over regions with high aerosols and pollutant levels. The apparent benefits of COVID-19-related restriction measures highlight a unique opportunity for changing pollution control policies and climate change mitigation strategies in SEA countries. Also, the assessment of the reductions in major, harmful air pollutants is very important for air quality and climate change studies and health research. These findings also call for a more detailed analysis of the impact of lockdowns on atmospheric pollution in the future.