Measurement-based assessment of the regional contribution and drivers of reduction in annual and daily fine particulate matter impact metrics in Paris, France (2009–2018)

Health effects from long- and short-term exposure to fine particulate matter (PM2.5) have resulted in an annual average PM2.5 standard across Europe and World Health Organisation guidelines for annual (10 μg m−3) and 24 h PM2.5 concentrations (25 μg m−3). Developing strategies to reduce both annual and 24 h average PM2.5 requires that the conditions that produce the magnitude of these metrics are understood. This paper presents a standard and replicable set of statistics that link the magnitude of annual and daily PM2.5 metrics to variation in i) hourly PM2.5 concentrations, ii) geographic regions traversed by air mass back trajectories, and iii) the ‘urban increment’ and ‘regional contribution’ to urban PM2.5 concentrations. These statistics are calculated between 2009 and 2018 at monitoring sites across Paris and the Île-de-France region, France, where there is a national objective to achieve the WHO annual PM2.5 guideline, and where short-term PM2.5 episodes still occur. The aim is to investigate changes in the conditions producing annual average, and 24 h PM2.5 concentrations exceeding 25 μg m−3, and how these long- and short-term metrics could be reduced further. The statistics indicate that reductions between 2009 and 2018 in both annual PM2.5 concentrations (PM2.5AA, −0.79 μg m−3 y−1 averaged across 3 urban background sites (33% average 2009–2018 reduction)) and the number of days with 24 h PM2.5 concentrations above 25 μg m−3 (D24h25, -6 days y−1 (62% average 2009–2018 reduction)), were driven by reductions in local emissions in Paris and the Île-de-France region. For example, reduction in PM2.5AA and D24h25 were greater at urban traffic sites, and between 2009 and 2018 the highest hourly PM2.5 concentrations occurred less frequently during rush hour periods, while the lowest hourly PM2.5 concentrations occurred more frequently during the day. In addition, when relatively moderate and high hourly PM2.5 concentrations were measured, air mass back trajectories spent more time (during the 4 preceding days) over European geographic regions, compared to the ocean indicating an increased relative contribution from regional transport to these hourly PM2.5 concentrations. Consequently, there is now a greater difference in the contribution of different hourly PM2.5 concentrations to annual and 24 h PM2.5 compared with 2009, with relatively high hourly PM2.5 concentrations having a larger contribution to D24h25, and moderate hourly PM2.5 concentrations having a larger contribution to PM2.5AA. Strategies to reduce PM2.5 concentrations in Paris should consider how mitigation measures will affect different ranges of hourly PM2.5 concentrations to understand the (potentially differing) effect on long- and short-term PM2.5 impact metrics. Comparison of hourly PM2.5 concentrations at urban sites and upwind rural sites showed regional contributions to PM2.5AA of approximately 50% and 70% at urban traffic and urban background sites, respectively. The largest regional contributions were also estimated for the highest hourly PM2.5 concentrations, compared to moderate hourly PM2.5 concentrations. Regional emission reductions could therefore make a substantial contribution to achieving the WHO air quality guidelines in Paris.