Three-Station Manual Network, One Bad AQI Day: What Can the Board Validly Publish?
An industrial town of about 1.1 million people operates three manual NAAQS stations. On 12 January the 24-hour values were: Station A, 2 m from the kerb of a four-lane junction - PM2.5 118 µg/m³, PM10 265 µg/m³, NO2 92 µg/m³, SO2 18 µg/m³; Station B, a residential colony 400 m from any road - PM2.5 74, PM10 168, NO2 44; Station C, an industrial estate - PM2.5 96, PM10 310, NO2 61. The network measures only these four pollutants; CO, ozone and ammonia analysers were never installed. At Station A the sampler inlets stand 2 m above ground and 2.5 m from the traffic lane, filters are changed and weighed once a month, and the weighing room has no temperature or humidity control. The State Board must issue a press note the same evening, decide whether any exceedance can be formally declared, and whether the town qualifies as a non-attainment city.
1. Compute the AQI for Station A, showing every sub-index and the category. What can the Board validly publish that evening, and what caveat must accompany it?
Read the worked answer
Sub-index formula. For 24-hour concentration C:
I = I_lo + (I_hi − I_lo) × (C − B_lo)/(B_hi − B_lo)
| Pollutant | C (µg/m³) | Breakpoint band used | Sub-index |
|---|---|---|---|
| PM2.5 | 118 | 91–120 → 301–400 | 301 + 99 × (27/29) ≈ 393 |
| PM10 | 265 | 251–350 → 301–400 | 301 + 99 × (14/99) = 315 |
| NO2 | 92 | 81–180 → 101–200 | 101 + 99 × (11/99) = 112 |
| SO2 | 18 | 0–40 → 0–50 | 50 × (18/40) ≈ 23 |
AQI = maximum sub-index = 393 → 'Very Poor'.
Reasoning points the examiner wants:
- The AQI is the worst sub-index, not an average; PM2.5 alone drives the category, so quoting 'moderate' from PM10 or NO2 would be wrong.
- An AQI can be computed with a subset of pollutants provided at least three sub-indices exist and one of them is PM2.5 or PM10 - that condition is met (four pollutants). The published number must therefore be labelled as based on four of the eight AQI pollutants.
- Because the missing pollutants (CO, O3, NH3) can only add sub-indices, 393 is a floor, not a ceiling - the true AQI could be worse but not better.
- The city AQI for the press note is the worst station value that day (Station A, 393), with Station C (PM10 310 → 315) reported alongside as the industrial hotspot.
- The station must be flagged as a kerbside exposure site, not a neighbourhood representative site - that is a siting question, not an AQI question.
2. Can the Board declare a NAAQS exceedance and treat the town as a non-attainment city on this data? Explain the averaging rules, the number of valid values needed, and the effect of the siting and weighing defects at Station A.
Read the worked answer
Step 1 - compare with the correct standard. The 24-hour NAAQS are PM2.5 60, PM10 100, NO2 80 and SO2 80 µg/m³. On 12 January, Station A exceeds all three of PM2.5, PM10 and NO2; Station B exceeds PM2.5 and PM10; Station C exceeds PM10. So an exceedance on that day can be stated, and nothing more.
Step 2 - compliance is not judged on one day. The annual standards (PM2.5 40, PM10 60, NO2 40 µg/m³) need a full year of data, and the short-term standards are met if the 24-hour value is not exceeded on more than 2% of days - i.e. compliance is tested against the 98th percentile of a complete year's 24-hour values, not the maximum. A year's annual average also needs a minimum number of valid values (CPCB practice: at least 104 24-hour values, with no month unrepresented). One January day is therefore evidence, not proof.
Step 3 - the data are challengeable. Inlet height of 2 m and a 2.5 m setback from a busy kerb fall outside CPCB siting norms (inlet typically 3–10 m above ground, set back from traffic and obstructions), so Station A measures roadside exposure, not ambient air. Monthly filter change and an unconditioned weighing room invalidate the gravimetric mass: filters must be equilibrated (around 20 ± 5 °C, 45 ± 5% RH) and weighed before and after each 24-hour run. These defects affect A more than the others.
Step 4 - non-attainment designation. A city is a non-attainment city when it has failed the NAAQS for PM10 for five consecutive years (the basis of the National Clean Air Programme list). That determination needs the multi-year, quality-assured record, so the Board can open the case but cannot declare status from this dataset.
3. Draft the immediate action set and state the legal basis for each item, distinguishing hotspot-level measures from the city action plan.
Read the worked answer
Immediate, hotspot-level (Station A/C, enforceable within days):
- Mechanised road sweeping and water sprinkling on the junction and the industrial approach road - source: road and traffic dust, the dominant contributor to a 265 µg/m³ PM10 at a kerbside site.
- Stop-work and penalty directions on uncovered C&D sites and open burning of waste within 1 km - Section 5, Environment (Protection) Act 1986 read with the C&D Waste Rules 2016 and SWM Rules 2016.
- Inspection and shut-down of uncontrolled DG sets and check on fuel quality at the junction's idling traffic; diversion or signal re-phasing to cut idling.
- Health advisory for the Very Poor category: avoid outdoor exertion, schools curtail outdoor activity, hospitals alerted - this is the standard graded-response (GRAP-type) trigger for the 301–400 band.
City-level (planning, months):
- Fresh source apportionment and a non-attainment city action plan under NCAP with sector-wise targets (transport, road dust, industry, biomass), because the three stations show three different source signatures.
- Repair the monitoring network first: install CO/O3/NH3, correct Station A siting, restore daily filter change and conditioned weighing. An action plan built on non-representative data cannot be audited.
Reasoning: enforcement attaches to identifiable sources and has to be defensible in appeal; the action plan attaches to the city. Mixing them produces directions that are either too slow or too weak.