Air quality update 9. Level 3 to Level 2
Changes in NZ air quality during COVID-19 level 3 restrictions.
Update: Thursday 10th September 2020
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 data provided by Auckland Council. Updates are not intended to provide comprehensive national coverage, but rather focus on emerging themes at indicative locations. [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.]
This review covers the period 13th May 2020 (transition from level-3 to level-2) to 30th August 2020 (end of second period of level-3 in Auckland).
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.
What are our findings?
Air quality is very sensitive to weather conditions. Winter 2020 was New Zealand’s warmest winter on record with more frequent warm northeasterly winds than normal, particularly to the North Island. This makes it even more challenging than normal to compare air quality in winter 2020 with previous winters on a like-for-like basis.
During level-4 reductions in NOx concentrations (compared to the average for the same time of year over 2016-2019) of 73-74 % were recorded at Auckland Council air monitoring sites in Takapuna and Henderson. In the level-3 period that followed concentrations rose slightly, but with levels still down by 59 - 61% compared to recent years.
Since moving to level-2 on 13th May, however, results from these two sites started to diverge. During level-2 and level-1 combined (13th May – 11th August) NOx levels at Henderson have been gradually returning to normal, whereas at Takapuna they remained around 40 % lower than normal on average. Explaining this different response between sites is complex. Possible explanations include:
- Differences in the degree to which traffic volumes had, or had not, returned to normal on the roads most affecting the monitoring sites (Lincoln Road in Henderson and the Northern Motorway in Takapuna),
- Differences in congestion (compared to historic norms) on the same roads (congested driving can increase emissions per vehicle)
- Winter 2020 was New Zealand’s warmest winter on record with more frequent warm northeasterly winds than normal, particularly to the North Island. The Takapuna site is on the east side of the Northern Motorway and is therefore less affected by motorway emissions when there are more easterly winds, bringing average concentrations down. Conversely the Henderson site is on the west side of Lincoln Road meaning the opposite occurs – average concentrations increase when there are more easterly winds.
NOx levels are also monitored on Queen Street. The traffic profile on Queen Street is quite different to suburban roads and motorways, both pre-COVID, during lockdown and post-lockdown. It is therefore not unexpected that air quality would respond differently also. During levels 4 and 3 NOx levels fell at Queen Street but not as dramatically as at other sites (by around 55 %). It was not until level-1 (from 8th June that NOx levels at Queen Street began rising again, although still remaining 26 % down on normal levels on average.
The recent return to level-3 in Auckland lasted only 19 days. This is generally too short a period to assess changes in air quality, and particularly to distinguish the short-term impacts of fluctuations in the weather. However, no large changes in air quality have been apparent.