A Six ppb Drift at the Roadside NO2 Station
A roadside air-quality station measures NO2 by chemiluminescence and logs 1-hour averages in ppb. On a winter morning the analyser recorded 118, 96 and 84 ppb in three consecutive hours; the 24-hour average was 46 ppb. The local standard for NO2 is 80 ppb as a 1-hour average, not to be exceeded more than once per year at any site. At the end of the week the station's zero reading was found to be 6 ppb high, because the analyser had drifted since its last calibration; the drift is assumed constant across the week. The inlet is 3 m from the kerb of a six-lane road and 1.5 m above ground. Calibration gas concentrations are quoted at 25 °C and 101.3 kPa (molar volume 24.5 L/mol; M(NO2) = 46.0 g/mol).
1. Correct the data for the drift and convert the peak values to µg/m³. Comment on whether the correction is chemically significant.
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Conversion
µg/m³ = ppb × (M / molar volume) = ppb × (46.0 / 24.5) = ppb × 1.878
Drift correction
A zero 6 ppb high means every reading is 6 ppb too large, so subtract 6:
- 118 → 112 ppb
- 96 → 90 ppb
- 84 → 78 ppb
- 24-hour average 46 → 40 ppb
Converted
- 112 ppb = 210 µg/m³
- 90 ppb = 169 µg/m³
- 78 ppb = 147 µg/m³
- 40 ppb = 75 µg/m³
Is 6 ppb significant?
In relative terms it is about 5% of the peak, which sounds minor. But 6 ppb = 11 µg/m³, which is larger than the 4 ppb gap between the 84 ppb reading and the 80 ppb standard. So the correction does not just change the numbers, it changes which hours count as exceedances - the drift is small but decisive.
2. Using the corrected data, decide whether the site breached the 1-hour standard. Discuss how the answer depends on the drift model.
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Exceedances after correction
112 ppb and 90 ppb both exceed 80 ppb; 78 ppb does not. So two hours breach, not three.
Against the standard
The standard allows one exceedance per year at a site. Two exceedances in a single morning already uses up that allowance and then some, so the site is non-compliant. The breach is real but not dramatic: the margin above 80 ppb is 32 ppb and 10 ppb respectively.
Sensitivity to the drift model
- If the drift was constant all week, subtracting 6 ppb from every value is defensible and the answer above holds.
- If the drift occurred only at the calibration event, the corrected values could be quite different again, and the number of exceedances might be one or three.
- Had the drift gone unnoticed, three exceedances would have been reported - the same compliance verdict, but a wrong magnitude, wrong exceedance-day count and a misleading trend.
Judgement
Report 112 and 90 ppb as exceedances, flag the calibration uncertainty explicitly, and confirm with a freshly calibrated analyser and a co-located passive sampler before any enforcement action. The decision is about the reliability of the evidence as much as about the concentration.
3. The council wants to cite this station's 112 ppb as proof that the whole suburb is polluted. Evaluate that use of the data and recommend how the network should be changed.
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What this site measures
A kerbside inlet 3 m from a six-lane road at 1.5 m height is a hot-spot monitor. It measures what a pedestrian at the kerb breathes - the worst case, and exactly the right place to test a 1-hour hot-spot standard. It is not a neighbourhood monitor.
Why citing it for the suburb is wrong
NO2 is removed by reaction with O3 and by turbulent dilution, so concentrations fall steeply within tens of metres of the carriageway and with height. Reporting 112 ppb as 'the suburb's air quality' overstates exposure for most residents, who live well back from the road.
Why dismissing it is also wrong
The kerb is where people actually stand - at the crossing, the bus stop, the school gate. A 210 µg/m³ peak is a genuine local exposure event even if the suburb average is far lower.
Recommendation
Keep the kerbside site for the hot-spot test. Add a second site 30-50 m back and 4 m up to measure the gradient, and report both alongside the 24-hour average. Co-locate a passive NO2 sampler as an independent drift check - the 6 ppb zero error found this week is exactly the fault a co-located sampler would have caught within days rather than at the next calibration.