Fewer vehicles, less air pollution

As Auckland traffic volumes plummeted during New Zealand’s strictest lockdown, so did air pollution levels but a detailed analysis shows a more complex picture than might be expected.

Associate Professor Jennifer Salmond

Worldwide, the WHO estimates that air pollution kills around seven million people a year and in the world’s big cities, road traffic emissions have the biggest impact on air quality. But scientists have struggled to establish causal links between air pollution and vehicle emissions because things like weather, woodburners and sea salt also play a role.

New Zealand’s Level 4 lockdown, from 27 March to 26 April, provided a golden opportunity for scientists to test air pollution models and historic baseline data against a period of almost no traffic.

In a University of Auckland study on how lockdown impacted air quality in Auckland, Associate Professor Jenny Salmond and PhD candidate Hamesh Patel measured the top five pollutants associated with air quality: fine and coarse particulate (PM2.5 and PM10), black carbon (BC), ozone (O3) and nitrogen dioxide (NO2). Measurements were from three sites: central Auckland (Queen St and Customs St), a suburban area at Henderson and a rural background site at Patumahoe.

The study also compared meteorological conditions during the month to past years and found wind levels to be similar to previous years, although rainfall was historically low for Auckland.

Figures show that during level 4, light vehicle traffic volume dropped by an average of 80 percent per day compared to the previous year, with a lower reduction in the number of heavy vehicles – mainly diesel buses and trucks - of 60 percent.

Overall, there was a marked reduction in recorded levels of ambient air pollution during lockdown. NO2 levels, primarily associated with the combustion of fossil fuels from motor vehicles, decreased by 34.1 percent in central Auckland, 56.9 percent at Henderson and 35.6 percent at Patumahoe.

One reason for the smaller reduction in NO2, particularly at the Auckland central site, could be the continued operation of heavy vehicles such as buses for public transport and trucks travelling to and from the Port which continued to operate through lockdown.

Smaller reductions in coarse and fine particulate (PM2.5 and PM10) were observed at all three sites. BC is a constituent of PM and primarily associated with diesel vehicles and woodburner emissions which decreased by 75 percent in central Auckland and 56 percent at Henderson.

Reductions in PM at Patumahoe were less significant. This could possibly reflect the background sea salt concentrations or emissions from domestic heating. A small increase in ozone was observed, although this was expected due to a complex relationship between pollutants.

Associate Professor Salmond says the research shows that it matters which type of vehicles are removed from the roads not just the number. During lockdown, there was a greater reduction in the number of private cars than diesel vehicles such as buses and trucks.

“That meant that although we reduced traffic by up to 80 percent, pollution wasn’t cut by the same amount and that’s because buses and trucks account for a higher proportion of NO2 emissions.

“This is important when make decisions about how to manage air quality in urban areas. If lockdown showed us one thing it was that we can make significant improvements in air quality by reducing traffic flows. But these results suggest that gains in air quality can also be made by opting for electric buses and lower-emission buses and trucks.”

Mr Patel says the research also highlights the lack of detail in previous emissions inventories with some pollutant and emissions sources potentially misrepresented or under-reported.

“This meant that what was thought to be known about pollutant and emissions sources didn’t align with what was observed during lockdown. Increased monitoring of the ambient air is therefore crucial in improving our knowledge and understanding and so regulators can make better informed decisions to provide safer and healthier air for us all to breathe.”

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