Potentially damaging ozone increased due to less traffic in first lockdown

Less traffic on the roads during the first lockdown led to a reduction in air pollution, but may have caused potentially damaging surface ozone levels to rise.

The COVID-19 pandemic saw a 50% decrease in traffic counts at the start of the soft lockdown in April 2020, compared to the previous year, leading to an instant decrease in air pollution.

But a University of Malta found that while levels of nitrogen dioxide (NO2) were down due to the absnece of traffic, surface ozone (O3) increased on average.

Surface, or ground-level ozone, can trigger a variety of health problems, particularly for children, the elderly, and people of all ages who have lung diseases such as asthma.

The report suggests how air quality will respond to future reductions in vehicle emissions, which means the problem of ozone gases must also be considered.

Using traffic data collected by Transport Malta and the Environment and Resources Authority, scientists found a decrease in monthly nitrogen dioxide (NO2) levels in hotspots like Msida of up to 54% during April, and of 57% in Zejtun during June.

On easing the restrictions on the 15 June 2020, traffic started to increase steadily and reached the 2019 levels by 7 October 2020 when schools were reopened.

A reduction in NO2 concentrations across the Maltese Islands was also observed from satellite measurements.

However, photochemical activity determines the formation of ozone gas (O3) in cities, which increase in polluted urban areas due to high nitrogen oxides and traffic exhaust. In Msida, during the pandemic months, an increase of up to 61% in Ozone was noted, compared to the business-as-usual scenario.

In the urban background site of Żejtun,  a 20% increase of Ozone concentration was observed in September 2020. In Għarb, Gozo, which has extremely low traffic, Ozone decreased by up to 11% compared to a business-as-usual scenario.

The study also estimated that the all-cause mortality from exposure to a daily one-hour nitrogen dioxide concentration was 0.9% lower in April and May 2020, compared to a scenario without the pandemic.

“Although the AF, expressed as a percentage may seem small, this represents the percentage of all-cause daily mortality associated with short-term air pollution exposure,” scientists Dr Noel Aquilina, from the Department of Chemistry, Dr Sara Fenech, from the Faculty of Science and Mr Ryan Vella from the European Space Agency, said in their research.

The results also highlight that while policies to reduce one pollutant such as NO2 could be beneficial, this will inevitable alter the atmospheric chemical balance and may lead to increases in other pollutants such as ozone.

“Thus, in a future where [these] emissions are targeted to decrease by similar levels to those observed during the pandemic, the burden to control ozone  levels will be enhanced. Therefore, policies should focus on limiting Ozone precursors which are not routinely measured across Europe.”