LONG TERM VARIATION IN ULTRAFINE PARTICLES (UFPs) SOURCES: RISING EVIDENCE FOR INCREASING NUCLEATION SOURCES CONTRIBUTION

Ultrafine particles (UFPs, with diameters below 100 nm) pose greater health risks, as they can penetrate deep into the pulmonary alveoli and reach the bloodstream (Ohlwein et al., 2019). Understanding the sources of UFPs and their relative contributions to particle number concentration (PNC) through source apportionment is essential for developing effective emission regulation policies. This study aims to develop a newly implemented approach based on Non-Negative Matrix Factorization (NMF) receptor modeling to identify and quantify the sources of UFPs at the ATmospheric Observatory in LiLLe (ATOLL).
To ensure robust source resolution, three temporal scales approaches to source apportionment were applied in this study: (i) a focus analysis of summer months to resolve expected nucleation sources enhance by a strong photochemistry activity (ii) seasonal source apportionment over a full year to quantify intra-annual variability and (iii) a four year long term source apportionment to assess temporal trends of the sources.
The model was first applied to summer data (June – August) for all the year revealing a strong nucleation factor (~26% of PNC). This finding is consistent with previous observations of summer photochemical strong activities and New Particle Formation (NPF) events (Crumeyrolle et al., 2023). As expected, seasonal analysis then showed a lower nucleation contribution on winter (10.6% vs. 37% on summer). Together, these two approaches demonstrate the robustness of NMF to separate the sources of UFPs.
Long-term variations of sources were also investigated using a single source apportionment on a four-year dataset (2020–2024) , and a linear regression model was applied to the results to assess temporal trends. Traffic-related sources showed a decreasing trend with average annual reductions of -7.75 % (gasoline emissions) and -12.68 % (diesel emissions) likely following the impact of European Union regulation on PM (EC, 2023). In contrast, nucleation-related sources exhibit a significant increase of 9.28 % yr-1, consistent with recent observations of rising UFP PNC on ATOLL (Suchánková et al., 2025) but not with other studies on suburban sites (Garcia-Marlès et al., 2024). This observed increase in nucleation sources shows strong evidence on the growing role of secondary formation processes which might be enhance by the emission of gaseous precursors such as SO2 and environmental conditions.
Overall, traffic emissions remained the dominant contributor (~69.20 %) to total PNC with a contribution decreasing trend of -3 %yr-1, in contrast to nucleation contribution (~19.72 %) with an increasing trend of +12.48 %yr-1. These findings highlight the predominantly anthropogenic origin of UFPs on ATOLL and the rising importance of nucleation factor on PNC, emphasizing the need for specific emission policies targeting UFPs alongside existing PM2.5 regulations.