2- Even across city. 3- Dominant to the right of Skytower. â« Duration;. Time of the next photograph when there is no visible pollution. Nephelometer readings.
Visual Air Pollution in Auckland: A preliminary assessment
Prepared by
Lorna Pelly & Gavin Fisher, NIWA
Discussion Paper
March 2003
Visibility in Auckland
NIWA March 2003
Executive Summary A screening analysis of a large dataset of digital photographs of Auckland city was carried out to assess how often air pollution was visible over the central business district of the city. A visibility monitoring programme was established by the Auckland Regional Council, comprising digital images acquired every half-hour during daylight hours for over 2 years, a nephelometer (a specialised instrument for measuring visual range) and information from the related air quality and meteorological monitoring network covering the region. Basic criteria have been established to measure the intensity, extent and duration of the pollution seen in the photographs. In addition to this, visibility data from the nephelometer, as well as local winds were analysed over the same time period. The technical data from the nephelometer suggested that visibility was low, with visual ranges less than 50km for 40% of the year-long test period. The photographs however, only showed visible pollution for 20% of the time. The discrepancy can be explained by weather conditions, such as low clouds and rain, and time-of-day. The photographic record cannot be properly analysed in these weather conditions, nor at night. A clear connection was established between low wind speeds and the occurrence of visible air pollution. On all occasions when visible pollution resided after 1pm in the afternoon, the wind speed was below 5 m/s. The analysis was compared with a set of proposed visibility performance indicators. This showed that for at least 8% of the time, visibility in central Auckland was at ‘action’ and ‘alert’ levels, whilst the remaining 92% it was ‘acceptable’ or better. The Auckland Regional Plan target of having visibility constantly better than 20km and with no discernable discolouration was only valid for 77% of the time.
Visibility in Auckland
NIWA March 2003
Methods A test period of January 2002 to January 2003 (11/01/02 to 15/01/03) was selected for all datasets.
Photographic data Photographs of Auckland city were taken from Takapuna every half hour beginning at 0430 and ending at 2000 on each day of the year long period. An initial screening test was carried out whereby the 0830 photograph for each day was examined, being the time when it may be most likely to see air pollution. For each image a comment was made on the general outlook of the weather and the presence of any visible pollution was noted. On these occasions, further details about the pollution were then taken. !
Colour intensity; 1- Just noticeable 2- Distinctly visible 3- Clearly dominant
!
Pollution extent; 1- Just along city skyline 2- Half way up Skytower 3- Reaches top or above Skytower
!
Location; 1- Dominant to the left of Skytower 2- Even across city 3- Dominant to the right of Skytower
!
Duration; Time of the next photograph when there is no visible pollution.
Nephelometer readings A nephelometer is positioned on Great South Road in Penrose, Auckland. This takes continuous readings throughout the day and night and gives an average value every 10 minutes of the visual range in kilometres (km). This is a measure of the light scattering capacity of the atmosphere, and is strongly related to visual appearance. From this data set, the lowest value recorded each day was noted.
Anemometer readings The wind speed is continuously recorded from the top of the Skytower, at 318 meters height. This dataset, recorded in meters per second (m/s), was analysed and the reading was noted at 0830 for each day. Periods of calm during the daylight hours were also recorded. These were set as occasions when the wind speed was continuously below 5 m/s for over 6 hours, noted in the morning (0600-1200) or the afternoon (1200-1800). 1
Visibility in Auckland
NIWA March 2003
Location of apparatus The map below shows the location of the camera and monitoring locations used in the study, as well as the approximate field of view of the photographic images. The camera is located near the top of a multi-story building in Takapuna.
Camera site
Anemometer site
Nephelometer site
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Visibility in Auckland
NIWA March 2003
Results Photographic data Data at 0830 was available on 311 days throughout the test period, on 62 of these pollution was visible, a 20% rate of occurrence. However, for many of these days the pollution was ‘just noticeable’ with colour intensity rated 1. If these days are excluded to leave only those when pollution was ‘distinctly visible’ or ‘clearly dominant’, the rate of occurrence drops to 11%. Occasions when pollution is at least ‘distinctly visible’, at least half way up the Skytower and evenly spread across the city could be seen in 6% of the photographs analysed. For 50% of the days when pollution was visible, it had not cleared away by 1000. April 2002 was the worse month for occurrence rate, with 13 days of visible pollution. November and December 2002 both were free of any such days. Pollution was noted at the weekend on 9 occasions. Of these, 3 registered above being ‘just noticeable’, all being Saturdays. Of all the days when pollution was visible, 27% of these were in clear weather. Throughout November and December, when no pollution was seen, there were only 7 clear days at 0830.
Nephelometer readings Data was available for 296 days throughout the test period. Table 1:
Nephelometer readings during test period.
Visibility (km) 0-9 2
10-19 20-29 30-39 40-49 50-59 60-69 70+
Total
14
296
24
46
68
67
44
31
Visibility below 20 km was recorded on 16 days, giving a 5% occurrence rate of days of bad visibility. There were 154 days with visibility below 50 km. Of these, 51 also had visible pollution as indicated in the photographs. On only 9 occasions did the photographs show pollution and the nephelometer readings were above 50 km.
Anemometer readings Data was available for 328 of the days throughout the test period. The wind speeds recorded at 0830 ranged from 2 – 21 m/s. The days on which pollution was visible had wind speeds ranging from 2 – 14 m/s.
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Visibility in Auckland
NIWA March 2003
Results Analysis Pollution was visible, at 0830, in 20% of the photographs.
Visible pollution in photographs
7%
5%
9% No pollution visible Just noticeable Clearly visible Dominant 79%
There is a clear link between the weather conditions and times when pollution is visible in the photographs. Of the days when pollution could be seen, 74% took place in fine weather (i.e. not cloudy or raining).
Weather when pollution is visible in photographs
26% 42%
15% 2%
15%
4
Clear Low clouds Haze Light clouds Cloudy
Visibility in Auckland
NIWA March 2003
This would suggest that although the 20% chance of seeing pollution is valid for this dataset, it is unlikely that this is a totally accurate reflection of how often there is an accumulation of pollution, but rather, how often the weather allows for it to be seen. For 65% of all the photographs, cloudy and rainy weather would not allow any pollution to be seen, regardless of whether it was present. Perhaps of more concern is the fact that there were only 10 clear days throughout the year when pollution was not seen.
General outlook at 0830 throughout the test period
11%
1%
8% 7% 54% 8% 0% 11%
5
Cloudy Rain Stormy Low cloud Light clouds Cloudy / sunny Clear Haze
Visibility in Auckland
NIWA March 2003
Taking this into account, from the days in which pollution was visible, for 43% of these the pollution was only ‘just noticeable’. However, for almost a quarter of these days, pollution clearly dominant across the skyline.
Colour intensity of pollution
23% Just notieable Clearly visible Strongly dominant
43%
34%
Additionally, for 45% of the days when pollution was visible, this was only along the city skyline. For the remaining 55% the pollution extended noticeably higher.
Vertical extent of visible pollution
16% Just along city skyline 45%
39%
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Reaches half way up Skytower Reaches top or above Skytower
Visibility in Auckland
NIWA March 2003
Generally, the pollution could be seen evenly across the city, although on some occasions, it would seem to dominate to one side.
Location of visible pollution 50 45 40 35 30 25 20 15 10 5 0
43
13 6
Dominant to the left of Skytower
Even across the city
Dominant to the right of Skytower
The nephelometer readings indicated that visibility in Auckland predominantly ranges from 30-70 km.
Nephelometer readings 80 70 60 50 40 30 20 10 0 0-9
10-19
20-29
30-39
40-49
Visibility (km)
7
50-59
60-69
70+
Visibility in Auckland
NIWA March 2003
The correlation between the photographic data and the nephelometer data was strongly positive. On days when pollution was visible, 85% of these also had nephelometer readings less than 50 km.
Correlation between nephelometer and photographs
15% Positive (ie. Photograph and nephelometer both show poor visibility) Negative
85%
However, there were 77 days when the nephelometer gave readings less than 50 km but there was no pollution registered in the photographs. This discrepancy appeared to be mainly due to the weather conditions.
Weather conditions when nephelometer readings were below 50km, but no pollution was visible
16% 1% 5% 6% 59% 13%
Cloudy Rain Low clouds Light clouds Clear No photo available
There were 6 occasions when the nephelometer gave readings below 20 km but no pollution was visible. These days all experienced low clouds and poor lighting.
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Visibility in Auckland
NIWA March 2003
The 16 occasions when visibility fell below 20 km all occurred in the winter months.
Days when visibility fell below 20 km
Au gu Se st pt em be r O ct ob er N ov em be D r ec em be r
Ju ly
Ju ne
ay M
Ap ril
Ja nu ar y Fe br ua ry M ar ch
8 7 6 5 4 3 2 1 0
When pollution was visible the wind speed was generally low. No pollution was seen when the wind was greater than 14 m/s.
Wind speed at 0830 45 40 35 30 25 20 15 10 5 0
Pollution visible No pollution seen
1
3
5
7
9
11
13
15
Wind speed (m/s)
9
17
19
21
Visibility in Auckland
NIWA March 2003
On most occasions the pollution would clear by 1100.
Time when pollution has cleared
9: 00 9: 30 10 :0 0 10 :3 0 11 :0 0 11 :3 0 12 :0 0 12 :3 0 13 :0 0 13 :3 0 14 :0 0 14 :3 0 15 :0 0 15 :3 0 16 :0 0 16 :3 0 17 :0 0 17 :3 0 18 :0 0
18 16 14 12 10 8 6 4 2 0
The duration of the visible pollution can be linked to the wind speed. On all occasions when pollution was visible after 1300, the wind speed was below 5 m/s. Furthermore, on days on which pollution was visible in the morning and the wind speed was above 10 m/s, all pollution had cleared by 1030. During mornings when a ‘period of calm’ was noted, for 62% of these, pollution was visible.
During a period of calm in the morning, is pollution seen?
38% Yes No 62%
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Visibility in Auckland
NIWA March 2003
Furthermore, on some occasions when the wind speeds were continuously low, pollution would accumulate and the intensity would increase. For instance, on the 16th July 2002, the wind speed never rose above 5 m/s and the situation at 1200 is considerably worse than that at 0830. This is illustrated below. 16/07/2002. 0830
16/07/2002. 1200.
(NB These photos may not show features in black and white – originals are in colour)
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Visibility in Auckland
NIWA March 2003
Discussion The results from each dataset were combined and analysed against visibility performance indicators. Corresponding data was available for this on 273 days. Discernable ‘off’ colour was noted for any day on which pollution was visible and distinct ‘off’ colour days were counted for all those that scored ‘clearly dominant’ in the initial screening. (The visibility indicators are those proposed by the Ministry for the Environment, but not yet adopted). Table 2: Suggested visibility indicators. Category
Visual range +/or appearance
Results
Excellent
70 km + and no ‘off’ colour
25
Good
20-70 km and no ‘off’ colour
184
Acceptable
20-70 km and discernable ‘off’ colour
48
Alert
