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9 Reporting Monitoring Results

9.1 Time formats

In September (last Sunday of the month) each year, New Zealand switches to New Zealand Daylight Time, moving clocks forward one hour. This reverts to New Zealand Standard Time (NZST) in April (first Sunday of the month), when clocks move one hour back (Department of Internal Affairs, 2009).

For all air quality data, it is recommended that time and date always be reported in NZST. Converting to daylight time would upset the averaging processes. It is also recommended that the midnight hour be labelled as 24.00 instead of 00.00 to avoid time averaging issues.

Recommendation 27: Time format

Times and dates should always be reported in New Zealand Standard Time.

Labelling of the midnight hour should be 24.00 instead of 00.00.

9.2 Reporting data formats

Instruments have different precision levels and those undertaking air quality monitoring use different instruments. This leads to inconsistencies in reporting data at the national level. To address this, it is recommended that a uniform format be used for reporting air quality data.

 

Recommendation 28: Reporting data formats

It is recommended that data from all monitoring sites be reported in the following format:
Contaminant Data format Nominal precision
Carbon monoxide X.X mg/m3 tenth of a mg/m3
Nitrogen dioxide X.X µg/m3 tenth of a µg/m3
Ozone X.X µg/m3 tenth of a µg/m3
Particulate matter (PM10) XX µg/m3 a whole µg/m3
Sulphur dioxide X.X µg/m3 tenth of a µg/m3
Lead X.X µg/m3 tenth of a µg/m3

9.3 Significant digits and rounding protocols

Significant digits are specified in the recommended data format in section 9.2. As an example, PM10 should be reported as a whole number, while NO2 should be reported up to the first decimal place. If instrument precision is higher than the recommended data format, it would be necessary to round off to the significant digit when reporting results.

When the value following the significant digit is equal to or greater than 5, the digit should be rounded up; otherwise, the digit is retained (National Environment Protection Council, 2001).

Examples:

PM10

  49.9 is rounded-off to 50 µg/m3

  50.3 is rounded-off to 50 µg/m3

  50.5 is rounded-off to 51 µg/m3

  50.7 is rounded-off to 51 µg/m3

NO2

  199.99 is rounded-off to 200.0 µg/m3

  200.04 is rounded-off to 200.0 µg/m3

  200.05 is rounded-off to 200.1 µg/m3

  200.08 is rounded-off to 200.1 µg/m3

The recommended data format in section 9.2, and the rounding-off protocols above also provide guidance for the reporting of all exceedences. For all monitoring sites, an exceedence occurs when the reported concentration is above the standard, after rounding to the significant digit (see Recommendation 28). For example, a PM10 exceedence is counted once a concentration of 50.5 µg/m3 is obtained.

Recommendation 29: Significant digits and rounding-off protocols

The recommended data format in section 9.2 specifies the significant digits for the reporting of a contaminant.

When the value following the significant digit is equal to or greater than five, the digit should be rounded up; otherwise, the digit is retained.

For all monitoring sites, an exceedence occurs when the reported concentration is above the standard, after rounding to the significant digit.

9.4 Summary statistics

Air quality monitoring can generate substantial amounts of data. Different audiences will be interested in different levels of detail. Most users, however, like to get high-level information that describes air quality conditions during a particular period. This is why summary statistics are useful in reporting air quality data.

It is recommended that the following summary statistics be prepared for each reporting period:

  • maximum and minimum concentrations based on the relevant averaging period

  • mean (arithmetic) and median

  • number of exceedences

  • percentiles (eg, 10th, 25th, 50th, 75th, 99.7th)

  • proportion of results belonging to particular bands (eg, air quality categories).

Table 4 is an example showing summary statistics of 24-hour average concentrations for PM10 in a year. Figure 9.1 shows an example of a graphical representation of summary statistics using box plots, while Figure 9.2 shows the proportion of results belonging to different air quality categories.

Table 4: Example of table showing PM10 summary statistics

PM10 (µg/m3) at (airshed name)

Monitoring site: (physical address of site)
Classification: Residential – neighbourhood
From 1 January 2008 to 31 December 2008
Method: BAM
24-hour concentrations (ending at midnight each day)
Per cent valid data: 96%            Data capture rate: 100%
Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec
Minimum 4 3 4 5 8 10 12 11 9 5 4 5
Mean 9 12 10 11 17 19 22 27 25 12 10 8
Maximum 20 18 16 18 31 54 60 67 55 22 23 22
Lowest 3
Highest 67
Number of exceedences 0 0 0 0 0 2 5 8 3 0 0 0
Annual mean 15

Figure 9.1: Example of box plot showing contaminant distribution at different monitoring sites

Figure 9.1: Example of box plot showing contaminant distribution at different monitoring sites

Source: Auckland Regional Council.

See text description of figure 9.1

Figure 9.2: Example of figure showing the proportion of PM10 samples belonging to different air quality categories

Figure 9.2: Example of figure showing the proportion of PM10 samples belonging to different air quality categories

See text description of figure 9.2

 

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