Air quality update 5. Week three of level 4 restrictions.

Changes in NZ air quality due to COVID-19 level 4 lockdown

Update #5 – third full week of level 4 restrictions

Thursday 16th April 2020

Headlines

  • In week 3 levels of oxides of nitrogen (or NOx, representing mainly vehicle exhaust pollutants) in our major cities fell even further, with reductions of 83 – 91 % relative to normal at most sites.
  • In week 3 levels of PM10 (particulate matter) in Auckland during week 3 were still below normal at most sites, but – in contrast to NOx – were higher than in weeks 1 and 2 of lockdown.
  • These changes were most likely due to changes in the weather, with stronger winds dispersing exhaust pollutants more effectively, but lifting more particles in the form of dust and sea salt into the air.
  • It is too early to tell if changes in traffic volumes within the lockdown period, such as during the Easter holidays, or increases in home heating are also impacting air quality.
  • Levels of PM10 across South Island are normal for the time of year.

What are our findings?

1. Nitrogen oxides (mainly traffic exhaust pollutants)

The reductions in NOx concentrations, averaged over each week, are presented in table 1 and in figure 1 at the end of this update. Reductions in week 3 have been greater than weeks 1 and 2 at all sites except Queen Street (Auckland) and Willis Street (Wellington), with reductions of 80 – 90 % below normal levels.

Table 1: Changes in week-average NOx concentrations compared to the norm for this time of year (based on 2015-2019).

 

Week 1

(26th Mar – 1st Apr)

Week 2

(2nd – 8th Apr)

Week 3

(9th – 15th Apr)

Week 1 - 3 combined

Queen Street, AKL

-61 %

-59 %

-59 %

-60 %

Henderson, AKL

-65 %

-67 %

-89 %

-74 %

Takapuna, AKL

-68 %

-73 %

-88 %

-76 %

Penrose, AKL

-49 %

-68 %

-83 %

-67 %

Willis Str, WTN

-76 %

-74 %

-54 %

-68 %

Riccarton Rd, CHC

-79 %

-78 %

-91 %

-83 %

 

2. Particulate matter (traffic, heating, industrial and natural sources) in Auckland

The reductions in PM10 concentrations, averaged over each week, are presented in table 2 and figure 2. Whereas concentrations in week 3 are still lower than normal (at all sites but two), they are higher than in weeks 1 and 2.

Table 2: Changes in week-average PM10 concentrations compared to the norm for this time of year (based on 2015-2019).

 

Week 1

(26th Mar – 1st Apr)

Week 2

(2nd – 8th Apr)

Week 3

(9th – 15th Apr)

Week 1 - 3 combined 

Queen Street, AKL

-32 %

-44 %

-25 %

-34 %

Henderson, AKL

-31 %

-38 %

-10 %

-26 %

Takapuna, AKL

-45 %

-42 %

-10 %

-32 %

Penrose, AKL

-26 %

-39 %

-13 %

-26 %

Glen Eden, AKL

-26 %

-30 %

+14 %

-13 %

Papatoetoe, AKL*

-36 %

-37 %

-7 %

-27 %

Patumahoe, AKL

-18 %

+1 %

+8 %

-3 %

*based on one year of historic data only (2018)

3. Why are week 3 results for Auckland different to weeks 1 and 2?

Compared to weeks 1 and 2, concentrations of NOx across Auckland were further reduced during week 3, but concentrations of PM10 were increased.

The most likely reason is the marked change in weather with stronger winds in week 3 compared to weeks 1 or 2. Hourly air quality data clearly shows that when winds rose late in the week, NOx concentrations fell while PM10 concentrations rose. This is because the wind more effectively disperses emissions from combustion sources - like vehicle exhausts, shipping and industry – which are represented by NOx. Whereas this will impact PM10 as well, this is more than compensated for by an increase in dust and sea salt emissions lifted into the air by stronger winds, leading to an overall increase in particles in the air, and hence PM10 levels.

Another possible factors could be an additional reduction in traffic during the Easter holiday. Traffic data is not yet available to show whether this was also true this year. However, this would not explain the increase in PM10.

A further possible explanation would be an increase in home heating emissions due to either colder temperatures or more people being home during the day. More heating emissions would increase PM10 but not NOx. It is possible that this could explain the greater rise in PM10 at Glen Eden which is a more residential site than the other monitoring sites. On the other hand, the change in weather has led to only slightly reduced temperatures. We will be maintaining a watch on this issue as temperatures drop further.

4. Why are results different for Queen Street?

Queen Street, and most of the similar streets in Auckland’s city centre, is likely to be less affected by both additional Easter reductions in traffic and increased windiness. This is due to the continued operation of bus services in central Auckland and due to the relative sheltering from the wind that is provided by dense clusters of tall buildings. This may also be true to some degree for downtown Wellington. 

 5. How long is it since air quality was this good?

We often see air quality at these low levels on Christmas Day or New Years Day, especially in the morning. However, we have not seen these levels sustained over days and weeks since monitoring began. In most locations NOx monitoring began within the last 20 years. The longest record is at Penrose where monitoring began in 1997. Figure 3 below shows the weekly-average NOx concentration for week 15 of each year (which corresponds to lockdown week 2 in 2020) at Penrose. This shows a steady decline over the last two and a half decades. This indicates how our vehicle fleet has become gradually cleaner over the years, despite some fluctuations due to differences in weather.

6. Particulate matter elsewhere

Whereas we would normally expect PM10 to start rising in April due to the beginning of the home heating season, with more people at home for longer during lockdown there is a possibility of heating emissions being higher than normal this year.

The impact of home heating on air quality is usually seen first in South Island due to colder temperatures. However, to date, PM10 levels are approximately normal for the time of year across the whole of South Island.

We’ll be keeping a watch on this and providing further updates soon.

Future topics planned for these updates:

  • How do the results compare to findings overseas?
  • Significance of the changes in air quality for health
  • Air quality in our homes
  • Impact of move to level-3.

Technical notes

We’re comparing air quality now with ‘normal’, but how do we define ‘normal’?

As well as fluctuating daily, air quality changes on a seasonal basis, being generally worse in winter and better in summer. However, for most pollutants, the long-term trend is very slow so that concentrations have changed relatively little from year to year over the past 5 years. To account for these effects, this analysis compares hourly data from the first and second week of level-4 lockdown with average air quality in the same weeks (14, 15 and 16), and one week either side, for the previous 5 years, which we take to represent ‘normal’ air quality at each site (unless there is evidence of a significant change in the ‘normal’ in the 5 years preceding the lockdown).

Since the week 1 update, the method has been slightly refined and the week 1 figures have been recalculated leading to very small differences compared to what we previously reported.

Data sources:

Routine automated air quality monitoring is conducted by regional councils. This analysis is based on unverified data at the moment and conducted by NIWA Principal Air Quality Scientist Dr Ian Longley.

What pollutants does the analysis focus on

This analysis updates our previous report on concentrations of oxides of nitrogen (NOx). NOx consists of two pollutants (nitric oxide and nitrogen dioxide). They are known to exacerbate asthma, stunt childhood lunge development and are linked to numerous other health impacts for the young and vulnerable. In our cities road traffic exhaust is overwhelmingly the main source, especially diesel vehicles, and so NOx can also be used to indicate levels of all traffic exhaust pollutants.

This analysis is also our first to look at levels of particulate matter (PM).  As well as traffic exhaust, PM also comes from road dust, natural sources like sea salt, and (during winter) smoke from the burning of wood and coal especially for home heating. PM10 (particles smaller than 10 microns and likely to be inhaled deep into the lungs) is monitored at more than 50 sites across the country. Levels in some towns, mostly in South Island, can breech National Environmental Standards on cold winter days.

Figures


Figure 1: Week-average NOx concentrations compared to the norm for this time of year (based on 2015-2019).

 

Figure 2: Week-average PM10 concentrations compared to the norm for this time of year (based on 2015-2019).

 

Figure 3: Average NOx concentrations at Penrose (Auckland) for the 15th week (which is 2nd – 8th April for 2020) of every year since records began

 

Read more updates on the changes in NZ air quality due to COVID-19 level 4 lockdown.

Contact

Principal Scientist - Air Quality
Research subject: Air QualityAtmospherePollution