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 (PM_2.5) have resulted in an annual average PM_2.5 standard across Europe and World Health Organisation guidelines for annual (10 μgm⁻³) and 24 h PM_2.5 concentrations (25 μgm⁻³). Developing strategies to reduce both annual and 24 h average PM_2.5 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 PM_2.5 metrics to variation in i) hourly PM_2.5 concentrations, ii) geographic regions traversed by air mass back trajectories, and iii) the ‘urban increment’ and ‘regional contribution’ to urban PM_2.5 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 PM_2.5 guideline, and where short-term PM_2.5 episodes still occur. The aim is to investigate changes in the conditions producing annual average, and 24 h PM_2.5 concentrations exceeding 25 μgm⁻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 PM_2.5 concentrations (PM_2.5AA, −0.79 μgm⁻³ y⁻¹ averaged across 3 urban background sites (33% average 2009–2018 reduction)) and the number of days with 24 h PM_2.5 concentrations above 25 μgm⁻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 PM_2.5AA and D24h25 were greater at urban traffic sites, and between 2009 and 2018 the highest hourly PM_2.5 concentrations occurred less frequently during rush hour periods, while the lowest hourly PM_2.5 concentrations occurred more frequently during the day. In addition, when relatively moderate and high hourly PM_2.5 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 PM_2.5 concentrations. Consequently, there is now a greater difference in the contribution of different hourly PM_2.5 concentrations to annual and 24 h PM_2.5 compared with 2009, with relatively high hourly PM2.5 concentrations having a larger contribution to D24h25, and moderate hourly PM_2.5 concentrations having a larger contribution to PM_2.5AA. Strategies to reduce PM_2.5 concentrations in Paris should consider how mitigation measures will affect different ranges of hourly PM_2.5 concentrations to understand the (potentially differing) effect on long- and short-term PM_2.5 impact metrics. Comparison of hourly PM_2.5 concentrations at urban sites and upwind rural sites showed regional contributions to PM_2.5AA of approximately 50% and 70% at urban traffic and urban background sites, respectively. The largest regional contributions were also estimated for the highest hourly PM_2.5 concentrations, compared to moderate hourly PM_2.5 concentrations. Regional emission reductions could therefore make a substantial contribution to achieving the WHO air quality guidelines in Paris.