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Direct Answer: The EU Ambient Air Quality Directive (AAQD 2024/2881) requires all EU member states to measure ultrafine particle (UFP) number concentrations and size distributions by December 2026, in accordance with EN 16976 and CEN/TS 17434. Environmental agencies are increasingly engaging aerosol researchers to support or lead this measurement work. TSI's butanol-based Scanning Mobility Particle Sizer (SMPS) system provides a complete, single-supplier solution covering every component required for compliance — from inlet sampling and aerosol conditioning through particle counting, size distribution measurement, and data export.
Table of Contents
1. Why UFP monitoring is now a regulatory requirement
In October 2024, the European Union approved a revised Ambient Air Quality Directive — a major step toward aligning EU air quality regulation more closely with WHO guidelines. One of the most significant updates is the mandatory inclusion of ultrafine particle monitoring: for the first time, all EU member states must include particle number concentration and particle size distribution measurements as part of their ambient air quality monitoring strategies.
Member states are required to apply these new rules — including the measurement of particle number concentration and particle size distribution — by December 2026. The first assessment to determine whether member states are on track to meet the requirements will take place by 2030.
This regulatory shift is directly relevant to the aerosol research community. Environmental Protection Agencies across Europe face a hard implementation deadline with demanding technical requirements — and many do not have the in-house instrumentation expertise or measurement infrastructure to meet them alone. Aerosol researchers are increasingly being called upon to support agencies in designing monitoring approaches, selecting and validating instrumentation, and in some cases leading the measurement programs directly. Whether you are building a compliance-ready system for an EPA partner or establishing your own ambient monitoring capability, the technical requirements are the same.
Those requirements exist because UFPs — particles 100 nm and below — are effectively invisible to conventional mass-based monitoring methods such as PM2.5 and PM10. They are present in high numbers near traffic, airports, seaports, and industrial sources, and their small size allows them to penetrate deep into the respiratory tract and potentially enter the bloodstream. UFPs have been linked to respiratory, cardiovascular, and neurological disease — health impacts that mass concentration metrics are structurally unable to capture. The AAQD addresses this by mandating number-based measurement for the first time.
To ensure consistency across member states, the directive requires that particle number concentration and size distribution monitoring be conducted in accordance with EN 16976 — which defines standardized methods for measuring ambient particle number concentrations, including sampling, conditioning, and data processing — and CEN/TS 17434, which outlines technical specifications for measuring ambient particle size distributions. Note that CEN/TS 17434 is currently under revision for transposition into a full European Standard.
2. What EN 16976 and CEN/TS 17434 require
Compliance with the AAQD's UFP provisions is not simply a matter of selecting an instrument — it requires a coordinated measurement system meeting specific standards at every stage.
EN 16976 governs the measurement of atmospheric particle number concentration using condensation particle counters (CPCs). It sets requirements for instrument design, performance, operation, and calibration, and specifies that the relative humidity of the sample entering the CPC inlet must not exceed 40% (±3%).
CEN/TS 17434 governs the measurement of ambient particle size distribution using Mobility Particle Size Spectrometer (MPSS) systems — more commonly known as SMPS instruments. It sets requirements for instrument design, performance, and annual calibration, and also requires sheath flow drying to ensure measurement quality across varying ambient humidity conditions.
Together, these two standards define what a compliant UFP monitoring system must deliver. TSI's butanol-based solution is designed to meet both.
3. Instrumentation for compliant UFP monitoring
Meeting the AAQD's requirements calls for advanced instrumentation capable of accurately and continuously measuring both particle number concentrations and size distributions across the ultrafine range.
Condensation particle counters (CPCs) are the most widely used instruments for continuous ambient particle number concentration measurement. They are highly sensitive, capable of detecting particles as small as 10 nm, and well-suited to long-term unattended deployment in monitoring networks. For size distribution data, CPCs can be integrated into SMPS systems, which classify particles by electrical mobility to generate detailed size distributions alongside integrated total number concentrations — providing a comprehensive picture of ambient aerosol composition.
TSI's butanol-based SMPS system brings these capabilities together in a fully CEN-compliant configuration, designed for integration into both new and existing monitoring networks.
4. The complete TSI butanol-based CEN-compliant UFP monitoring system
TSI supplies every component of a fully compliant UFP monitoring system — allowing you to specify, procure, and integrate the complete solution from a single supplier.
Aerosol sampling
A measurement is only as representative as the sample delivered to the instrument. The TSI sampling system preserves the aerosol size distribution and number concentration from ambient air through to the instrument inlet. A CEN-compliant sampling system includes a PM10 sampling head, sampling tube, cyclone (optional, depending on the expected aerosol size distribution), and dryer.
Particle number concentration
EN 16976 specifies a condensation particle counter as the required instrument for ambient particle number concentration measurement. CPCs detect particles down to a few nanometres in diameter, respond rapidly to concentration changes, and have underpinned ambient monitoring networks globally for decades.
TSI's EN-compliant CPC meets EN 16976 requirements for design, performance, operation, and calibration in full.
Particle size distribution
A Scanning Mobility Particle Sizer (SMPS) measures how particles are distributed across the ultrafine size range — data essential for source apportionment, health effects research, and atmospheric science. The instrument coordinates four simultaneous processes: inlet impaction, aerosol neutralisation, electrical mobility classification, and particle counting.
TSI's butanol-based SMPS meets CEN/TS 17434 requirements for design, performance, and annual calibration, and is the instrument of choice for regulatory and compliance-focused ambient monitoring.
5. Supporting components
The following components are required or strongly recommended to achieve and maintain compliance:
Compact Catalytic Vapor Filter (CCVF100) — Butanol-based CPCs produce exhaust containing residual hydrocarbon vapours. The CCVF100 reduces these by more than 99.99%, preventing interference with co-located measurements and ensuring a clean operating environment.
Aerosol Diluter 3333-10 — At high-concentration locations such as traffic junctions, airports, and seaports — precisely the environments highlighted in the AAQD — aerosol concentrations can exceed the single-particle counting range of the CPC. The diluter reduces the sample concentration by a factor of 10 (±5%) using a controlled supply of particle-free air. Dilution is optional under EN 16976 and CEN/TS 17434, and should only be applied when concentrations exceed instrument limits.
RHT3000 Humidity and Temperature Sensor — EN 16976 requires that the aerosol sample entering the CPC inlet has a relative humidity below 40% (±3%). The RHT3000 monitors temperature and RH in the sampled flow continuously, verifying that this condition is met and providing additional data for quality assurance.
Sheath Flow Dryer — CEN/TS 17434 requires sheath flow drying to guarantee measurement quality for particle size distribution regardless of ambient humidity. This component is essential for continuous ambient monitoring applications under the standard.
6. Data management and system integration
Adding particle number and size distribution measurement to an existing monitoring network raises real questions around data handling, workflow integration, and day-to-day operational capacity. TSI instruments are designed for continuous 24/7/365 unattended operation, with automation and real-time remote diagnostics that minimise manual intervention — important for both research groups managing multiple deployments and EPA teams running distributed networks. Data export covers full instrument status, aerosol humidity and temperature, particle number concentrations, and size distributions (raw, inverted, or both), with compatibility with common data formats reducing the complexity of integrating into existing infrastructure. Raw data outputs ensure complete traceability; detailed diagnostic data provides ongoing confidence in measurement quality.
Where further support is needed, TSI offers dedicated software, expert operator training, and consultation services to assist with data integration and alignment with existing reporting frameworks — ensuring PN and PSD data sits coherently alongside legacy datasets rather than creating a separate data stream. TSI systems are compliant with EN 16976 and CEN/TS 17434 from the point of installation, helping to ensure accuracy and regulatory alignment from day one.
7. Why choose TSI for AAQD-compliant UFP monitoring?
TSI has supported ambient UFP monitoring programmes globally for decades — across research institutions, national environmental agencies, and regulatory monitoring networks. For organisations now preparing for the December 2026 AAQD implementation deadline, this experience translates into a clear advantage:
- Complete compliance-ready system — every component from inlet sampling to data export, designed to meet EN 16976 and CEN/TS 17434
- Single supplier — simplifies procurement, integration, and ongoing support
- Continuous, unattended operation — designed for long-term deployment with minimal unscheduled maintenance
- Remote data access — retrieve and monitor data from any location
- Proven in regulatory networks — TSI SMPS instruments are used in national and supranational monitoring programmes worldwide
- End-to-end support — from instrument selection and installation through calibration, maintenance guidance, and beyond