Changes in NZ air quality during COVID-19 level 3 restrictions.
Air quality update for Friday 15th May 2020. End of level 3 restrictions.
Headlines
- Concentrations of road traffic exhaust pollutants (nitrogen oxides) rebounded from 25–35 % of normal levels during level-4 to 47–63 % of normal during level-3 at representative sites in Auckland and Christchurch.
- At Queen Street (Auckland) NOx concentrations remained unchanged at 45 % of normal levels.
- At Willis Street (Wellington) NOx concentrations rebounded further from 31 % of normal in level-4 to 75 % of normal in level-3.
- Concentrations of particulate matter (PM10)—which includes emissions from transport, industry, heating, dust and sea spray—were largely unchanged between levels 4 and 3, and around 85–96 % of normal levels at selected sites for this time of year.
- It is too early to assess air quality under level-2 restrictions.
- Reductions in human exposure to traffic pollution would have varied depending on where you live, work and travel, but will have been greatest (perhaps 90 %) for people who would normally have commuted from the suburbs to city centres, but instead remained at home.
- More people could have benefitted from these larger reductions in exposure to traffic pollution if vehicles, and especially diesel buses and trucks, had been removed from city centres.
Background
This is the latest in a series on air quality in New Zealand during the COVID-19 lockdown. The update is created by NIWA’s Principal Air Quality Scientist, Dr Ian Longley, and is based on unverified[1] data provided by Auckland Council, Environment Canterbury and Greater Wellington Regional Council. [N.B. “Unverified” means that the data has yet to go through the Council’s quality control process. This means a small error might be present. Verified data is usually available a few months after the original data was recorded. Lockdown conditions may introduce additional delays in verification of the data presented here.]
Updates are not intended to provide comprehensive national coverage, but rather focus on emerging themes at indicative locations.
This review covers the whole of level-3 (28th April–13th May) compared to level-4 (26th March–27th April).
What are our findings?
1. Oxides of nitrogen (traffic exhaust pollutants)
Nitrogen oxides (NOx) consists of two pollutant gases: nitric oxide (NO) and nitrogen dioxide (NO2). Levels of NOx in the air most closely represent the impact of emissions from vehicle tailpipes, especially diesels, although industry and shipping emissions may also contribute, whereas high levels of NO2 are associated with increased risk of stunted lung development in children, and a wide range of poor health outcomes.
During level-4 concentrations of NOx were reduced by 65–75 % of normal levels at most monitoring sites, i.e. to 25–35 % of normal levels. Over level-3 NOx concentrations rose to 13–17 percentage points to 47–63 % of normal.
The exception was at Queen Street in central Auckland, where the 55 % reduction in NOx during level-4 was unchanged in level-3.
Table 1: Average NOx concentrations compared to the norm for this time of year (based on 2015-2019).
|
Level-4 |
Level-3 |
|
|
Queen Street, AKL |
46 % |
45 % |
|
Henderson, AKL |
26 % |
39 % |
|
Takapuna, AKL |
27 % |
41 % |
|
Penrose, AKL |
34 % |
51 % |
|
Riccarton Rd, CHC |
32 % |
48 % |
These changes in traffic pollution are consistent with the changes in traffic volumes, and especially heavy duty vehicle volumes as reported by the NZ Transport Agency. Although we have not seen data to verify it, volumes of heavy duty vehicles in the Auckland CBD have probably changed less than elsewhere.
2. Particulate matter (traffic, heating, industrial and natural sources)
During level-4 concentrations of PM10 were reduced by 9 - 15 % of normal levels at most of the monitoring sites we are tracking, i.e. to 85 - 91 % of normal levels. Relative to level-4, concentrations during level-3 rose slightly at some sites and fell at others, with no clear pattern. Across level-3 overall, PM10 concentrations were at 84 – 96 % of normal levels at urban sites.
Table 1: Average PM10 concentrations compared to the norm for this time of year (based on 2015-2019).
|
Level-4 |
Level-3 |
|
|
Queen Street, AKL |
86 % |
95 % |
|
Henderson, AKL |
91% |
86 % |
|
Takapuna, AKL |
69 % |
84 % |
|
Penrose, AKL |
85 % |
96 % |
|
Patumahoe, AKL |
107 % |
105 % |
|
St Albans, CHC |
91 % |
95 % |
The much smaller reductions for PM10 compared to NOx is to be expected. Road traffic exhaust contributes only up to third of PM10 in the air. The rest comes from sources that were much less likely to be impacted by lockdown, especially sea spray but also home heating emissions. The increase in PM10 at the rural site of Patumahoe suggests that weather conditions, or other natural phenomena, have also raised slightly PM10 levels across the region and masked some of the reduction in PM10 from road traffic. The cause of the larger reduction at Takapuna during level-4 is unknown, but is highly unlikely to be related to reductions in road traffic emissions.
An “eyeball” review of PM10 levels at other sites across the country suggests similar results, with no significant changes in PM10 compared to the norm for this time of year.
Who has benefitted and what does it mean for health?
Although air quality changed dramatically across the cities monitored (and probably all other towns and cities too), the benefits would not have been experienced equally. Air quality was improved in our home neighbourhoods, around our workplaces and on the roads we use to travel from one to the other, but to slightly different degrees. Our personal experience was therefore determined by where we live, work and how we experienced lockdown.
A rough estimate shows that the average reduction in exposure to traffic pollution for Aucklanders may have fallen by around 75 % during April 2020. Those benefitting more than this would be people who (normally) commute during normal peak hours from a less-polluted home location to a more polluted work location (and especially downtown Auckland), but instead were locked-down at home. Not only was air quality improved at home but they would experience additional benefit from avoiding the downtown area, and not spending time in busy traffic. A rough estimate suggests these people, who could number a third of a million in Auckland, could have reduced their exposure to traffic pollution by around 90 %.
This gain could have been extended to a few hundred thousand more people if through traffic, and especially diesel buses and trucks, had been removed from the city centre in a zero emissions zone, as proposed under Auckland Council’s “Access 4 Everyone” plan. This is due to the disproportionately high influence diesel vehicles in downtown areas can have on air pollution exposure because of the dense concentration of exposed people and buildings which inhibit dispersion.
Whether these reductions in exposure to air pollution will translate into improvements in health remains to be seen, especially considering the much smaller reductions in levels of particulate matter. In fact, the exposure of a large number of people (not just in New Zealand, but worldwide) to a large and sustained reduction in traffic pollution, but not particles from other sources, is a rare occurrence that provides a natural experiment to improve our understanding of the different health impacts of different forms of air pollution.
What happens next?
The lockdown experience so far has provided vivid confirmation of how in New Zealand’s windy cities, isolated from each other and international neighbours, and where heavy industry is largely absent, pollutants like NOx can be made to “vanish overnight”. Working from home can not only reduce emissions and improve air quality, but also reduce personal exposure and health risk by an even greater degree. In the short-term what happens next depends on how many people have work to return to, whether they are allowed, or encouraged to continue working from home (or not), and whether they chose to travel by car or public or active transport. Distancing provisions on public transport may force more people into cars. This may worsen congestion, emissions, air quality and exposure to pollution.
In the longer-term our previous analysis showed that “business as usual” improvements in vehicle emissions technology means that we may achieve similar air quality as during lockdown, sometime in the late 2030s (if at all). The data from Queen Street highlights the disproportionate impact that diesel buses can have on pollution exposure when operating in dense and busy downtown settings. A rapid transition to electric buses, and establishment of zero emission zones, will not only clean up our urban centres, but should incentivise the adoption of zero emissions transport across whole cities and may be one of the fastest ways to recreate some of the air quality gains of this lockdown period and deliver lasting benefits to the health of New Zealanders.
Future topics planned for these updates
- Impact of move to level-2
- Air quality in our homes
Methods
What pollutants are used to judge urban air quality?
Urban air quality is most commonly assessed using three different measures of air quality, each with its own strengths and weaknesses:
- Oxides of nitrogen (or NOx) – the sum of nitric oxide (NO) and nitrogen dioxide (NO2) – this measure is the most representative of vehicle exhaust emissions, but can also be impacted by industrial and shipping emissions
- Nitrogen dioxide (NO2) – the more toxic of the oxides of nitrogen, for which national standards and guidelines are set
- Particulate Matter smaller than 10 or 2.5 microns (PM10 or PM2.5) – the sum of all particles in the air both man-made and natural, including smoke from home heating, dust and sea salt. Health risk assessments are most frequently based on these measures.
2. When do we compare lockdown air quality with?
Assessing changes in air quality is not as simple as it might seem. Variation in the weather can introduce considerable variation in air quality which any comparison should ideally compensate for. In general, one needs to compare air quality during lockdown with air quality in some comparable period. Five methods of increasing sophistication are:
- Air quality before the lockdown
- Air quality for this period last year
- Air quality for this period averaged over the last few years
- Air quality for this period assuming it follows the annual trend over the last few years
- Air quality as it would have been during lockdown had emissions not changed.
Different methods have been used by different researchers and organisations to calculate reductions in air pollution. Method 1 is likely to understate improvements as air quality is normally better in summer than in autumn. Method 3 is preferable to method 2 if historic data is available. Method 4 is better still so long as there is an evident trend and one has confidence that the trend would have continued into this year. Method 5 is the most sophisticated but requires a credible model that can reliably simulate air quality on an hour-by-hour basis.
Given that this is a quick analysis on data that has only just been collected, and that annual trends are not consistent across sites, in this case we have taken adopted method 3 – i.e. calculated the average concentrations over the period 26th March–27th April inclusive in 2020 and compared the result to the average concentration over the same days over the period 2015–2019. This method may over-estimate reductions by up to 10 % by not accounting for long-term trends in improving air quality.