The COVID-19 pandemic response elicited several short and long-term effects on the environment and the economy. Most notably, the scale of these impacts has been an unprecedented reduction in both environmental and economic activity, with subsequent effects on air pollution.
Major sources of outdoor absolution include power generation, traffic, residential energy use, and industrial power production. With the rapid emergence of COVID-19, and along with it, government-enforced lockdowns aimed at containing the spread of the virus, both economic activity associated with transport, mobility, and commerce, approached near-complete stasis in several countries across the world.
The extent of the lockdown measures was varied but included partial or complete closure off international borders, nonessential businesses, schools, and in many places across the world, restricted the movement of local populations within cities. These measures are still in effect to lesser degrees as the pandemic continues.
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Air pollution: The COVID-19 impact on carbon dioxide, nitrogen dioxide, and particulate matter
Associated with lockdown measures, are estimates of the short-term effects of economic activity on global application and its relation to human health. A research article published in the proceedings of the National Academy of Sciences of the United States of America demonstrated that lockdown events had reduced the population-weighted concentration of both nitrogen dioxide and particulate matter by 60% and 31% respectively across 34 countries.
These effects on the ozone were mixed, and declining ambient nitrogen dioxide concentrations could be directly coupled to vehicle transport reduction. Furthermore, the group demonstrated that maintaining a ‘business as usual’ approach to daily living is an exacerbator of global pollutant emissions and ambient exposure levels.
This study serves to highlight a dramatic short-term effect of global reductions in transport and economic activity on reducing absolution as indicated by ground-level nitrogen dioxide and particulate matter. Moreover, the study suggests that city-level initiatives that promote the use of public transport as well as encourage cycling and pedestrian activities are justified going forward.
Lockdown measures have also brought down greenhouse gases at large. In countries such as China, both carbon monoxide and N2O were reduced by 50%; this 50% reduction was also seen in cities such as New York. Taken together, the emission of nitrogen dioxide indicates a sign of a reduction in economic activity, which reflects the nature of lockdown restrictions.
Typically, nitrogen dioxide is produced as a result of burning fossil fuels and 80% of the nitrogen dioxide generated arises from more motor vehicle exhaust. The European Environmental Agency has predicted that overall, the emission has dropped from between 30 to 60% across several European cities. In the US, nitrogen dioxide declined by 25.5% compared to previous years. Globally, nitrogen dioxide levels were found to fall.
The key reason for the dramatic decrease in greenhouse gas emissions is due to the sizeable effect that vehicles and aviation play across the world. For example, vehicles contribute approximately 72% and aviation 11% of the transport sector's greenhouse gas emissions.
The reduced use of motor vehicles was also accompanied by decreased flights as a consequence of restricted international travel. For instance, China saw reductions of between 50 and 90% capacity for international departures and a decrease of 70% in domestic flights due to the pandemic compared to the previous year. This ultimately reduced carbon dioxide emissions by approximately 17%. Overall, air travel dropped by 96%, which ultimately will have had positive effects on the environment.
In addition to emission, there was a subsequent decrease in consumption of fossil fuels which further lessened the emission of greenhouse gases. This two-pronged effect helps to combat global climate change. The International Energy Agency reported a drop in 435,000 barrels of oil globally during the first three months of the year 2020 relative to the previous year.
Concomitantly, global coal consumption was reduced due to decreased energy demand produced by lockdown measures. In India for example, coal-based power generation declined by 26%; the total power generation after the lockdown was also reduced, by 19%. Similarly, coal-based power generation in China dropped 36% over the same period compared to that of the preceding year.
Preliminary results for cities across the world have demonstrated that the scale of the drop is small relative to the amount of carbon dioxide in the atmosphere.
WHO’s Science in 5 : Air pollution & COVID-19 - 1 October 2021
When viewed across the globe, air pollution unequivocally dropped during the lockdown. Satellite data along with more than 10,000 ground-based monitoring stations globally have demonstrated that the average global air quality during the lockdown was improved in early 2020 compared to the same period of the previous year.
While global averages are promising, they do not report on the secondary effects i.e., reactions between different molecules in the atmosphere. This same study demonstrated that global average ozone was found to increase despite the decrease in nitrogen dioxide and particulate matter. Moreover, another study has demonstrated that particulate matter was not found to drop consistently across the United States.
This loss of ozone is associated with declining levels of nitrogen oxides. Generally, nitrogen oxides can terminate hydroxyl radicals, which prevent them from reacting with volatile organic compounds to form ozone.
In addition, when nitrogen oxides reach a critical concentration, they begin to react with the ozone itself, consequently removing it from the atmosphere. Therefore, in urban areas, where this type of chemistry dominates, a reduction in nitrogen dioxide can cause ozone levels to rebound. The relationship between nitrogen dioxide and ozone has been known since the late 1980s, however the emission differences during the pandemic or artificially inflated, resulting in unexpected non-linear chemistry.
In turn, nitrogen dioxide and ozone have nonlinear relationships with particulate matter. Particulate matter can be produced as the consequence of nitrogen dioxide reactions with several gases in the atmosphere - these pathways involve ammonia produced in agricultural activities and interactions with organic compounds.
Analysis of the European Commission’s Copernicus Atmosphere Monitoring Service (CAMS) data showed that even though nitrogen dioxide emissions dropped by as much as 93%, there were still particulate matter hotspots observed during the lockdown. Models that decipher the pollution chain reaction that causes periods of high particulate matter suggest it is due to the production of NO3 radicals, which in turn form particulates made of nitrates.
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