High-Emitter Identification Service (HEIS)

Concept

An auditing service for municipal transport authorities and fleet operators that employs automated Plume Chasing to screen vehicle fleets in real-world driving conditions. Instead of testing every vehicle in a lab, the service identifies the 'high emitters' — the 3–28% of vehicles contributing over 50% of total emissions — using a rolling minimum for background determination and following vehicles through varied engine states. The output is a prioritized 'hit list' of vehicles needing maintenance or decommissioning.

The new evidence refines this into a tiered workflow rather than a one-step verdict: plume-chase screening flags suspects; low-cost on-board NOx sensors confirm the flags on suspect vehicles [2]; PEMS provides the definitive, legally robust measurement only where mandatory action is contemplated [1]. This reserves the expensive reference method for the few genuine super-emitters and answers the operator-acceptance question — verification does not require a PEMS campaign for every flagged vehicle.

Why now

• A review of 228 publications concludes that internal-combustion vehicles will dominate global fleets well past 2050 and identifies four persistent obstacles to air-quality goals: manufacturer cheating, owner tampering, malfunctioning emission-control systems, and inadequate in-service emission programs [4]. HEIS is a direct instrument for the malfunction-detection and in-service-screening gaps.
• Real-world emissions routinely exceed certification values: among 72 SCR-equipped heavy-duty diesel trucks in California, 12 emitted more than three times the standard, and SCR conversion efficiencies stayed below the expected 90% even above the 250 °C optimum operating temperature — consistent with catalyst deterioration [2]. Certification tests and periodic lab visits miss these vehicles.
• Current in-use compliance instruments are structurally narrow: valid Not-to-Exceed events covered only 4.2–16.4% of engine operation and 6.6–34.6% of estimated NOx emissions [2]. Plume chasing's longer measurement interval across different road sections — the reason it deviates least from PEMS results [0] — is exactly the broad-coverage, real-world sampling this gap calls for.
• Deterioration is measurable at scale: analysis of 210,000 remote-sensing measurements shows NH3 emissions rising with vehicle mileage [3], supporting the premise that a mileage- and age-stratified screening program will find concentrated excess emissions rather than uniformly distributed noise.

How the evidence bounds the design

• Legal admissibility. PEMS is being embedded in current and future EU emissions legislation as the in-use verification method for light-duty, heavy-duty, and non-road machinery [1]. Plume-chasing measurements alone are therefore unlikely to support mandatory decommissioning or fines in EU jurisdictions. HEIS adapts by positioning its hit list as a prioritization and maintenance trigger, routing enforcement-grade decisions through PEMS verification, and aligning its data-evaluation method with the legislative principles under development [1].
• False positives and minimum sample size. Cold-start emissions average 1.7 times hot-exhaust levels [3], so a single encounter with a cold vehicle can mimic a super-emitter. HEIS therefore requires multiple encounters across warm engine states and road sections before flagging, records cold-start status, and uses mileage-based deterioration expectations [3] to separate genuinely deteriorated vehicles from transient high-load operation.
• Pollutant scope. NH3 is unregulated yet contributes to urban fine-particulate formation [3]. HEIS reports NH3 — and hybrid CO2-measurement anomalies, which reveal real-world battery usage [3] — as advisory fleet intelligence, not as enforcement triggers.
• On-board sensors as complement, not competitor. Low-cost on-board NOx sensors are a convenient real-time monitoring tool [2]. HEIS integrates them as its second verification tier for flagged vehicles; plume chasing remains the only tier needing no on-board equipment, which is what lets the service screen entire fleets — including vehicles without sensors — before any hardware is installed.

Remaining open questions

• The false-positive rate of plume chasing in dense urban traffic, where plumes from multiple vehicles overlap, remains unquantified.
• Performance of the rolling-minimum background determination in high-ambient-pollution environments.
• Full cost-effectiveness of a mobile plume-chasing fleet versus periodic lab testing — partially mitigated by the tiered model, which reserves PEMS for confirmed suspects.
• Jurisdiction-by-jurisdiction acceptance of plume-chasing data as a screening trigger within EU in-use compliance frameworks.