Air Quality in Dunedin

Transcription

1 Air Quality in Dunedin Summary report December 2010 Contents patterns Emission patterns Spatial variability of 2009 Spatial study Future work Summary Glossary Further information Small particles in the air are commonly referred to as PM (particulate matter). Those PM with a diameter of ten microns and less are called and are associated with adverse health effects. Introduction Air pollution, particularly in winter, is of concern to communities throughout Otago. Poor air quality is due to elevated levels of which are small particles suspended in the air. Three factors affect air quality how much pollution is put into the atmosphere, the weather, and the topography of an area. How these factors relate to one another determines whether air quality is good, fair, or poor. A major source of in Dunedin, similar to other towns in Otago, is smoke from domestic solid fuel burners. That s why winter-time levels are generally higher than those measured during summer. In addition, the temperature inversions that commonly form in winter create conditions that discourage dispersion, allowing to accumulate. Cold night-time air sinking down the sides of hills also serves to keep smoky air close to ground level. While Dunedin is similar to other Otago towns, there are some important differences that make the city s patterns different and more complex. ORC has been studying various aspects of the city s levels. Some interesting patterns are unfolding. This summary report describes the work that s been done so far and what we ve learned. is regulated by the Ministry for the Environment (MfE). The Otago Regional Council s Air Plan sets guideline values for and presents the structure for managing the air resource. is measured in micrograms per cubic metre of air (µg/m 3 ). Figure 1. Smoke can hang over Central Dunedin on a calm day.

2 Daily PM10 (ug/m 3 ) patterns in central Dunedin ORC has been monitoring air quality at various locations around Dunedin over the last 12 years. In that time, air samplers have evolved from low-resolution, labour-intensive equipment, to state-of-the-art automated monitors. Given the dataset, we can look at patterns and trends at various timescales. Figure 2. Location of monitoring sites in Dunedin. Currently, only the Albany Street site is active. During a typical year, central Dunedin patterns (in red, measured at Albany Street) show that values range between 15 and 40µg/m 3, and that higher values can occur any time of the year. This pattern reflects the contribution of background sources such as sea salt and dust, as well as industrial emissions in the city. By comparison, this is a very different trend to that shown for Alexandra (in blue) where domestic heating emissions during winter make up the vast majority of Alexandra Dunedin Figure 3. Averaging five years of data, these graphs represent a typical air quality year in Alexandra and central Dunedin.

3 PM10 (ug/m 3 ) Number of Days Compared to several other air quality sites in the region, Dunedin enjoys relatively good air quality throughout the year. Overall, the vast majority of days (83%) have relatively low levels of, with only a small percentage of days (3%) with greater than 50µg/m 3. On a weekly scale, the data show that of the 35 high-pollution days (days when the average level was greater than 50µg/m 3 ) that have occurred at the central Dunedin site since 1997, half have fallen on either a Monday or a Tuesday. Only one has been reported on a Sunday. Seasonally, 80% have occurred during winter months (May-August) Figure 4. Most high-pollution days in central Dunedin have occurred early in the week. On a daily scale, winter weekday levels increase in the morning and remain elevated throughout the day, peaking from 3pm to 4pm. After that, they gradually taper off to a minimum during the early morning hours. Winter weekends have their maximums in the evening. All of these patterns point to a combined contribution of emissions to the unique signature of central Dunedin hour of the day Weekends Weekdays Figure 5. Representative winter weekday and weekend days in central Dunedin.

4 Spatial variability of across Dunedin Concentration patterns Dunedin s complex terrain, multiple emission sources, and variable wind patterns make hard to predict across the city and from day to day. Over the past 10 years, ORC has monitored air quality in several areas of Dunedin, including North East Valley, central Dunedin, South Dunedin, Kaikorai Valley, and Green Island. An examination of the monitoring results shows that levels in one area are not always indicative of air quality throughout the city. It is quite possible to have poor air quality in one area while over the hill the air quality may be quite good. The graphs below show how similar or how different levels can be on any given day around the city. Figure 6. On a given day, air quality can be quite different around the city. These graphs show two days levels at three different monitors during winter Emission patterns These results show that no single monitor can represent the air quality of the entire city. The variation is in part due to the different emission types and rates; one of the ways we can see this is by looking at land use. The monitor used to report levels to MfE is located in the campus zone, with industrial and residential zones nearby. The map below outlines the different land use designations in Dunedin.

5 Figure 7. Dunedin land use map. An Air Emission Inventory (2005) identified that 8% of the total Dunedin emissions were from industrial and commercial air discharges, with 92% of the total from solid fuel domestic heating appliances. Of course, the location and distribution of those houses using solid fuel burners will influence emissions around the city. Mapping from the latest census data shows the distribution of wood burners in and around the city. The map suggests that the highest emissions from domestic chimneys may be found in parts of South Dunedin and the hill suburbs. Figure 8. Density of Dunedin households using wood fuel for domestic heating purposes.

6 Spatial study of distribution In 2009, ORC studied the real-time patterns of around the city. NIWA was contracted to use their mobile air quality monitoring system and travel the city streets for several days and nights during May and June. While there was definite variation from day to day, the graphic below shows the composite of the total monitoring campaign. South Dunedin, Green Island, and north Dunedin all show up as having some of the highest values during this time Figure 9. Composite of all monitoring runs done in Dunedin May and June Using state-of-the-art monitoring equipment, NIWA was able to collect information not only about the spatial distribution of, but also of the likely sources of in various areas. For instance, there was evidence that most of South Dunedin s PM came from wood sources, whereas most of central Dunedin s was from fossil fuels (oil and coal). Whatever the source, results did show that over a day, values recorded at Albany Street are generally representative of the centre city s air quality. But because of the complex nature of the city s terrain and weather, Albany Street values may not make good predictors for other areas.

7 Future work The ORC will continue to monitor at the Albany Street site, recording hourly, air temperature, and wind data continuously. These data can be viewed on our website at: ORC is conducting a study on the composition and sources of the collected at the Albany Street monitor in 2010/11. Samples are being taken every other day throughout the year in two different sizes: PM 2.5 (everything less than 2.5 microns in diameter) and those PM from microns. The ORC has contracted GNS to analyse these samples using an x-ray technique that will differentiate between the various elements that are in the PM. Then, by means of a statistical technique called factor analysis, the elements are grouped into their likely sources such as wood smoke, sea salt, diesel, etc. Once that is done, the proportion of each emissions source in the air can be determined at various time intervals. The sampling site with the two monitors is shown below. Figure 10. The air quality monitoring site in central Dunedin.

8 Summary Over the past ten years, the ORC has gained a better understanding of the multi-faceted patterns of particulate pollution in Dunedin. emission sources include domestic and commercial solid fuel burners, industrial discharges (including schools), vehicles, and even natural particles such as sea salt and pollen. The residents of the city usually experience from three to nine high-pollution days a year. The monitor at the central Dunedin site is used for reporting purposes but on a given day, due to the complex topography and wind patterns in and around the city, it may not represent other areas. To achieve clean air in Dunedin, both domestic and industrial emissions will need to be reduced. Glossary Particles having an aerodynamic diameter of 10 microns and less that are suspended in air. High-pollution day A day when the average concentration is greater than 50 micrograms per cubic metre of air. Temperature inversion An atmospheric condition that occurs when colder, heavier air is near the surface with warmer air above. This keeps the atmosphere in a stable state with little to no air movement, thus trapping emissions near the surface. Fossil fuel A hydrocarbon deposit, such as petroleum, coal, or natural gas, derived from living matter of a previous geologic time and used for fuel. Further information ORC (2005) Ambient Air Quality in Otago Particulate Matter ORC (2009) Ambient Air Quality in Otago Particulates ORC (2005) Air Emission Inventory Dunedin, Mosgiel and Alexandra, 2005 Prepared for the ORC by Environet Ltd ORC (2010) Air Quality in Dunedin: Patterns and characteristics of ORC website: