New Zealand’s unhealthy home-heating in the age of COVID-19: Part 2: What's happened to air quality during level-4 lockdown?

You may have heard the news that the air around us has been sparklingly clean during lockdown – or level-4 at least. Dramatic images have been circling the world showing empty highways coinciding with clear views of distant mountains. Levels of PM2.5 – the tiny particulates that penetrate deep into our lungs with every breath, and have been linked to not just respiratory illnesses, but also heart disease, strokes and premature death – have fallen 50 – 60 % in Delhi, Los Angeles and Wuhan. And in New Zealand? Somewhere between 10 % and zero.

This has surprised some. Traffic volumes plummeted all over New Zealand almost immediately when level-4 was declared and pretty much stayed that way, despite a slight up-creep as level-3 approached. For example, data released by the NZ Transport Agency shows that, averaged over the whole of level-4, volumes of light vehicles measured at 6 sites in 4 cities fell by 73 - 81 %  and heavy duty vehicles fell by 64 – 84 %. Yet, PM2.5 fell by only 10 % at Queen Street in Auckland and 2 % in St Albans in Christchurch.

Data for other locations is scarce, as PM2.5 is not yet routinely monitored in New Zealand. We do, however, monitor PM10 quite widely. PM10, which includes particles up to 10 microns in size, rather than just 2.5 microns, clearly includes PM2.5, but also includes those additional particles between 2.5 and 10 microns, which are mainly mineral dusts and sea salt. PM10 is measured in most of our major towns (and some smaller ones too) across all of our regions. And across the country, the results are pretty much the same. Levels of PM10 during lockdown were only very slightly down on the norm for this time of year.

However, this is not just a New Zealand phenomenon. Other cities across the world, including some very large ones, have experienced huge drops in traffic but only very small reductions, or even increases in PM levels. This includes London and Madrid, for instance.

This matters because we have very robust evidence, collected from thousands of research studies from across the world over the last four decades, that higher PM levels means more ill-health, more GP visits, more inhaler use, more heart attacks and more deaths. This is why we have a National Environmental Standard for PM10 and why Regional Councils are required to monitor PM10 continuously. A recent analysis has estimated that at the normal levels over the last few years, those components of PM caused by human activities cause around 600 hospital admissions, 1.6 million restricted activity days and 600 extra premature deaths each year. Any reduction in this burden should be an unintended benefit of the lockdown.

So why might such a big reduction in traffic have such little effect on PM levels?

Firstly, let’s just check that emissions from road traffic really have gone down. This is relatively easy to verify. As well as monitoring PM, some councils also monitor NOx – or oxides of nitrogen. They do this because one of those oxides (nitrogen dioxide), like PM, is also associated with a wide range of serious health effects, and is also subject to a National Environmental Standard. Although NOx can come from ships, plus industrial and power plants, in the places where most New Zealanders live it almost entirely comes from vehicle exhausts, and especially diesels. In the few places it is monitored, levels of NOx have fallen pretty much in step with traffic volumes, for example 74 % in Takapuna, Auckland.

So there has definitely been less traffic pollution in the air. The smaller reductions in PM, compared to NO_x, are therefore due to the fact that many emissions sources other than traffic emit particles and contribute to PM₁₀ levels in the air. This is shown in the chart below for three sites - Queen Street in the city centre, Takapuna, near the Northern Motorway and a roadside suburban site - Henderson. In Auckland during autumn the most significant source is sea salt (orange in the chart) which is left in the air after sea spray evaporates. Samples of particulates have been analysed by GNS Science over many years showing that, at this time of year, sea salt contributes around 5 mg m^-3, or a third to a half of all PM₁₀, and this will not have been affected by the lockdown.

The same analyses have shown that road traffic (the green blocks) probably contributes only a third of PM₁₀ at the most impacted site (Queen Street), a sixth at less impacted sites (like Lincoln Road, Henderson) and even less at low-traffic suburban sites. This is a lot less than many people expect. However, the lockdown has provided a perfect opportunity to validate these previous analyses. If these relative contributions from different sources are correct then it makes perfect sense that if particles from traffic are reduced by 75 %, but all other sources are unaffected, then the total level of all particles from all sources (i.e. PM₁₀) will only fall by a few percent.

Coming soon... What’s happening to lockdown air quality in our wood-burning towns?