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2 Visibility Degradation

One of the most noticed impacts of concentrations of particles in the air is visibility degradation or haze. Haze typically refers to the ambient air quality visual impact of concentrations of particles, which are either uniformly distributed both horizontally and vertically to a height well above the lowest terrain or stratified in a layer near to the surface. The latter type of haze is common when temperature inversion conditions restrict the dispersion of pollution. This is also referred to as a surface layer haze, and is characterised by a distinct line at the top edge of the pollution layer. An elevated haze layer is another type of stratified ambient air haze that occurs when the pollution distribution is not in contact with the ground.

Figure 2.1 illustrates both uniform and stratified haze in Christchurch. The stratified surface layer haze is very common in Christchurch during winter mornings because of the frequency of temperature inversion conditions. These inversions typically last until around 10 am by which time the heat from the sun has typically warmed the air sufficiently to allow vertical mixing of air contaminants. This can result in the appearance of a uniform haze, although this is typically of short duration.

Figure 2.1: Uniform (left) and stratified (right) haze in Christchurch

Photo showing uniform haze in Christchurch.

Photo showing stratified haze in Christchurh.

Photos supplied by Environment Canterbury.

As well as the ambient air quality impact of haze, adverse amenity effects are associated with localised issues such as a visible smoke or dust plume. Examples of situations where a smoke plume may cause localised visibility degradation include tailpipe exhaust emissions from a smoky motor vehicle, boiler emissions from a train or boat, industrial combustion processes and backyard burning. Examples of localised dust sources that may result in visibility degradation include industrial activities (e.g. sandblasting, unsealed roads, construction sites, land tilling and stockpiling of materials). In the case of dust related sources, a large proportion of the particulate is likely to be in the larger TSP size fraction.

Figures 2.2 to 2.6 show some examples of the impacts of smoke plumes from different sources within New Zealand. The impact of the smoke plume in the Milford Sound (Figure 2.2) is particularly noteworthy as it illustrates the adverse impact of human activities on an otherwise pristine and highly valued area.

Figure 2.2: Smoke plume from a boat in the Milford Sound

Photo of showing the smoke plume from a boat in the Milford Sound.

Photo by Russell Winter, SRC.

Figure 2.3 shows the effect of the visible discharge of smoke from an industrial boiler on the West Coast of the South Island. Visible emissions of smoke from industrial processes are generally controlled within New Zealand for industrial processes that require resource consents. Typically, consent conditions might relate to the presence of visible smoke for a specified duration.

Figure 2.3: Smoke plume from a boiler on the West Coast

Photo showing the smoke plume from a boiler on the West Coast.

Photo supplied by Chris Pullen, WCRC.

Backyard burning emissions in New Zealand typically result in both smoke nuisance and visibility impacts. In Figure 2.4 however, the extreme vertical and minimal horizontal dispersion of the plume means that the most significant impact of this burning episode is visual.

While outdoor burning practices are very common in rural New Zealand, overseas studies indicate a higher value is placed on visibility in rural scenes than in urban areas, suggesting a lower tolerance for visibility degradation in these areas. [Pryor S (2002)Particles and Visibility. Keynote paper presented at the 16th International Clean Air and Environment Conference of the Clean Air Society of Australia and New Zealand.]

The amenity impacts of outdoor burning or other sources of particles in any location will depend on meteorological conditions as well as the value placed on the scene by those observing the discharge. Even a relatively small outdoor rubbish fire can have a significant impact on visibility if meteorological conditions are conducive to elevated pollution. Figure 2.5 shows the visual impact of a relatively small rubbish fire in the Lyttelton Harbour.

Figure 2.4: Smoke plume from backyard burning in Marlborough

Photo showing smoke plume from backyard burning in Marlborough.

Photo by Peter Hamill.

Figure 2.5: Impacts of meteorology on outdoor burning emissions in the Lyttelton Harbour

Photo showing outdoor burning emissions in the Lytttelton Harbour.

Emissions of particles from domestic chimneys can also result in a visible plume, particularly if the burner is being poorly operated or inappropriate fuel is burnt (Figure 2.6). The extent to which the visual impact of a domestic home fire is regarded as an adverse amenity impact in New Zealand is unknown, although regional councils receive numerous complaints about them, both in terms of nuisance and amenity.

Figure 2.6: Smoke emissions from a solid fuel burner in Lyttelton Harbour

Photo showing smoke emissions from a solid fuel burner in Lyttelton Harbour.

2.1 How do particles affect visibility?

Particles and gases in the air degrade visibility by scattering and absorbing light. These processes impact on the visibility of an object in the distance by reducing the amount of light transmitted from a source (e.g. the sun and reflected off the object). Other factors that impact on how the object is viewed include characteristics of the observer, optical illumination such as sun angle and cloud cover and characteristics of the object (e.g. colour, texture and contrast).

Of the impact of particles and gases in the intervening atmosphere, the impact of gases in visibility degradation is generally minimal. Light scattering by gases is a constant referred to as Rayleigh Scattering and doesn't vary with increasing concentrations. Light absorption by gases in the air is effectively limited to NO2. Measurements of this component in Christchurch indicate it is not a significant contributor to daytime haze episodes.

Thus the primary cause of haze is light scattering and absorption by particles. Typically light scattering is the dominant cause with particles in the size range 0.3-0.7 microns scattering light most effectively. Light absorption by particles depends on composition and tends to be governed by elemental carbon in most urban environments.

Sources of visibility degradation in New Zealand also include fog and low cloud (although these meteorological events are natural and are unlikely to constitute an adverse amenity impact), sea spray, dusts and anthropogenic sources such as combustion emissions and industrial processes. It is also likely that emissions from vegetation could contribute to visibility degradation in some locations.

2.2 Visibility guidelines in New Zealand

While no visibility standards currently exist for New Zealand, the Ministry for the Environment has signalled an interest in establishing a criterion to protect visibility and has developed visibility degradation categories. A series of reports were prepared [Ministry for the Environment (1999)Visibility in New Zealand: Guidance on measurement methods. Wellington: Ministry for the Environment.][Ministry for the Environment (1999)Visibility in New Zealand: Amenity value and management. Wellington: Ministry for the Environment.][Ministry for the Environment (1999)Visibility in New Zealand: National risk assessment. Wellington: Ministry for the Environment.] which provide guidance on visibility measurement methods, amenity values and management and a risk assessment of visibility in New Zealand. Actions required to protect and enhance visibility in New Zealand were recommended as follows:

  • Develop and implement guidelines and indicators for visibility protection.
  • Fully integrate visibility as an objective in Air Plans.
  • Raise awareness in the public, educational, industry and political sectors.
  • Develop and recommend monitoring methods.
  • Define national goals for visibility, relevant for different types of regions within New Zealand.
  • Continue research on air shed modelling, with an emphasis on understanding key causes of visibility degradation.

While no specific guideline value for visibility is recommended, a proposed visibility indicator is included with which to assess visibility degradation. This is identified as preliminary with the intention that it be refined in time with regard to user comment and use. There is some flexibility in the target categories selected by councils to allow for community specific visibility objectives. A combination of both visual range and colour are proposed (Table 2.1).

Table 2.1: Proposed visibility indicators

Category

Visual range and/or appearance

Excellent

>70 km and/or no 'off' colour

Good

>20-70 km and/or no 'off' colour

Acceptable

>20-70 km and/or discernable 'off' colour

Poor

<20 km and/or discernable 'off' colour

Alert

<20 km and/or distinct 'off' colour

Action

<8 km and/or distinct 'off' colour

Source: Ministry for the Environment (1999)

2.3 Visibility perception in New Zealand

A study of visibility perception was carried out in New Zealand by NIWA in 1998. [National Institute of Water and Atmospheric Research (1998)Air Visibility Telephone Survey Results. NIWA Report AK97071.] The study involved household surveys in the areas of Auckland, Hawkes Bay, Hamilton City, Dunedin and Christchurch. The results of these surveys can be summarised as follows:

  • In Auckland the majority of the participants thought that visibility had deteriorated and that motor vehicles were the primary cause of reduced visibility.
  • Most respondents rated visibility in Hawkes Bay as excellent and there was little indication of perceived deterioration.
  • In Hamilton visibility degradation was mostly attributed to natural causes such as fog and rain.
  • Dunedin residents considered the standard of visibility in the area was very high and predicted that weather had the main effect on visibility. Of the anthropogenic sources domestic heating was thought to be the main contributor.
  • Although Christchurch was initially included in the survey, households in Christchurch were reluctant to respond to the questionnaire. Consequently results were unable to be reported due to the small sample size (31).
  • In all areas visibility was identified as an issue of importance.

Figure 2.7 shows brown haze in Auckland around the time the Auckland visibility perception survey was conducted.

Figure 2.7: Illustration of brown haze in Auckland

Photo showing brown haze in Auckland.

Photo supplied by Jayne Metcalfe, ARC.

2.4 Visibility risk assessment for New Zealand

An estimate of the relative risk of poor visibility in New Zealand was assessed based on concentrations of particles and nitrogen dioxide (MfE, 1999). Table 2.2 shows the risk index that was developed for the 1999 risk assessment based on the air quality categories. An illustration of the risk of visibility degradation across New Zealand based on these data is shown in Figure 2.8. Results highlight the areas of Christchurch and Auckland as high risk areas. This is consistent with visibility observations in these areas. However, the spatial resolution of the analysis and presentation, which is based on territorial local authorities, does lead to some inconsistencies in other areas. For example, the size of the TLA areas may be too large to show the extent of visibility degradation in some small towns (e.g. Reefton). Other inconsistencies are also noted, for example, Nelson, which is known for it's visible air pollution only features as low risk.

Table 1.2: Classification of risk index into air quality categories

Risk index

Category

>125

Very high risk (action)

25-125

High risk (alert)

5-25

Medium risk (acceptable)

1-5

Low risk (good)

0-1

Very low risk (excellent)

Figure 2.8: Visibility risk index for New Zealand