While periodic shutdowns due to electric blackouts, athletic events, and governmental decrees against air travel have allowed us to ascertain the effect of certain activities on air pollution, the shutdown due to COVID is the first economy-wide loss of activity. Carbon dioxide emissions were down about 7% during most of 2020. In many cities emissions of nitrogen dioxide (NO2), fine particulate matter (PM2.5) and volatile organic compounds (VOCs) were reduced by even larger percentages, especially with the loss of vehicular and air traffic. In cities, NO2 fell by 23-37%.
During the day, NO2 and VOCs are widely recognized to enhance the formation of ozone (O3) as an air pollutant. O3 is known as a secondary pollutant because direct human emissions are relatively minor. Broad-scale comparisons of ozone concentrations in the atmosphere before and after the arrival of COVID show reductions of about 7% during economic lockdowns. Lower emissions of the precursors to ozone formation, resulted in less ozone in the lower atmosphere—the troposphere.
But, in cities themselves, the situation was different. Around the world, concentrations of ozone in urban areas increased during the shutdown, despite lower emissions of some of its precursor compounds.
Ozone chemistry is complex. During the night, NO2 forms NO in the atmosphere, and when O3 concentrations are high NO can act to scrub it (aka titration) from the atmosphere. Thus, a loss of NO2 in urban areas reduced O3 formation, but also, and apparently to a greater extent, reduced the destruction of O3 by NO. So, ozone concentrations in cities were above average by as much as 20%, even though the average concentration in the atmosphere of the northern hemisphere was down 7%.
Worldwide ozone in the air is estimated to kill about one million people each year. Higher ozone concentrations in cities, where most people live, is likely to have exacerbated the respiratory difficulties of people suffering from COVID, and thus increased mortality. Meanwhile, lower concentrations of NO2 and fine particulate matter may have alleviated the stress on some respiratory patients.
We may never know the relative role of air pollution versus infection with COVID on mortality rates, but these observations show that our health and the health of the environment are deeply interrelated. You can’t expect to have healthy people in a dirty environment.
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