Do regional background sites capture changes in primary PM2.5 emissions at the national scale? Recent trends in PM2.5 in rural environments in metropolitan France

Trends in daily PM 2.5 concentrations were assessed at 5 rural background sites in France in 2014-2021, together with major particulate components which concentrations were attributed to (i) anthropogenic emissions including fossil fuels (FF) and biomass burning (BB) from primary emissions; and (ii) secondary particulate constituents including non-sea-salt sulphate, nitrate and ammonium. Annual PM 2.5 concentrations correlated to annual emissions in both SO 2 and NO X in France at all sites; correlations to primary PM 2.5 emissions varied depending on the site. To disentangle the influence of weather, long-range transport, and the conditions controlling for PM secondary formation on the PM 2.5 time series, boosted regression tree (BRT) models were built at each site for PM 2.5 ; and normalised time series calculated by randomising the value of the explanatory variables at a given time. Two BRT models with their respective normalised PM 2.5 time series were calculated: de-weathered time series (without the influence of the meteorological and long-range transport) and de-weathered & de-oxidised time series (randomisation of meteorology, transport and O X (NO 2 + O 3 ) concentrations). In 2014-2021, PM 2.5 decreased at -5.6% year -1 , almost twice as fast as changes in primary emissions in France and neighbouring countries (about -3% year -1 ). Overall trends in de-weathered, and de-weathered and de-oxidised PM 2.5 were lower than that in PM 2.5 observations, at -3.9% year -1 and -3.2 % year -1 , respectively. Trends in de-weathered & de-oxidised PM 2.5 were close to those in emissions, demonstrating the role of including variables capturing the oxidative capacity of the atmosphere in the normalising techniques to compare trends in PM 2.5 with primary emissions. Faster downward rates in PM 2.5 aerosols in rural background in mainland France were mostly associated with the decrease in gas emissions responsible for secondary particles and less sensitive to changes in primary emissions. Control of NH 3 emissions is suggested to be crucial to ensure downward trends in PM 2.5 at the regional background environments to reduce concentrations closer to WHO limits. • Decreasing trends in daily PM 2.5 at French rural sites at -5.6% year -1 in 2014-2021 • Decrease in PM 2.5 emissions in the footprint aligned with that in PM 2.5 observations • Trends in de-weathered, de-weathered & deoxidises PM 2.5 closer to that in emissions • Lack of linearity between NO X emissions and nitrate at the north sites of France