Ultrafine Particles and Black Carbon in Two Roadside Sites in Europe: Long-term Data Analysis

Road vehicles are typically the main source of ultrafine particles (UFP, Dp <100nm) in urban areas. A high spatio-temporal variation of UFP is found even in similar types of sites e.g roadside. We analyzed long-term particle datasets (number count or PNC, and number size distribution or NSD), and Black Carbon (BC) from two roadside sites in Europe, London Marylebone Road (LMR) in the UK, and Mäkelänkatu Street in Finland (Mäk). We evaluated 11 years (2010-2021) and 5 years (2015-2020) of particle data derived from SMPS (16.55-604.3nm) and DMPS (6-798.42nm) for LMR and Mäk, respectively.

The initial analysis shows that at LMR, PNC reduced by 64% over 2010-2021 (~20000#/cm3 to less than 10000#/cm3), while the BC declined by 86% (~9 µg/m3 to 1.2 µg/m3). Meanwhile, PNC at Mäk remained constant at ~15000#/cm3 during 2015-2018, then decreased to ~10000#/cm3 in 2020. BC decreased by 58% (1.3 to 0.6 µg/m3) during 2015-2020.

Using the same size range (16.5–604nm) from both sites during 2015-2019, the TNC at LMR were 1.4-1.6 times higher than that at Mäk, and 2.4-3.9 higher for BC. Particles less than 16.39 nm at Mäk account for approximately 50% of TNC over the full measured size range. When particles are classified into nucleation, Aitken and accumulation ranges, the nucleation mode contributes most at Mäk (68% of the full size range), while the Aitken mode is dominant at LMR (51%).

A downward trend was observed during 2015-2019 at both sites. However, the pollutant concentration reduced faster at LMR, apart from the Nucleation mode which slightly increased. BC showed the fastest decline (9.6%/yr and 14.2%/yr in Mäk and LMR, respectively). Further investigation using wind direction data shows that in LMR, the most significant reduction of pollutants mainly occurred from the southerly wind sector, which is associated with the emissions from road vehicles on the adjacent road, suggesting that interventions applied to the road vehicle fleet have effectively decreased pollutants concentration. In Mäk, pollutant concentrations decreased much faster on winds from the NW and SE sectors, corresponding to the alignment of the road. The Nucleation mode proportion has increased since 2014 from the southern and westerly sectors at LMR, and NW and west at the Mäk due to the rapid reduction of the Aitken or Accumulation mode, rather than an increase of Nucleation mode concentration, which has changed little.

The annual mean PNSD shows a bimodal pattern at both sites. A change was observed from a minor peak at ~60-70nm that gradually disappeared over the period seen more clearly at LMR.

We interpret the rapid reduction in BC and PNC as being largely attributable to the progressive uptake of diesel particle filters as Euro 5 and 6 standard vehicles have entered the fleet since 2011.  However, the greatest impact has been upon the BC and Aitken and Accumulation mode particles, with little change seen in the Nucleation mode particles, which are comprised largely of condensed lubricating oil, and form in the cooling exhaust after passage through the particle filter (Harrison et al., 2015).