Diagnostic Evaluations of the CHIMERE Model: Local Versus Advected Contributions of Fine Particles and Nitrate Formation Regime in the Paris Megacity

Chemistry transport models (CTMs) are a powerful tool to investigate various features of the aerosol pollution in megacities, including its geographical origin or its sensitivity to anthropogenic emissions changes (scenario analysis). However, due to the numerous uncertainties still at stake in CTMs, assessing the reliability of the results obtained in these two common exercises remains a challenging task that usually requires specific observations and methodologies. In our work, we have taken advantage of some recent campaigns in the Paris region—PARTICULES and FRANCIPOL—to run a diagnostic evaluation of the CHIMERE model regarding these two issues. The first substantive point is to assess in what extent the model is able to retrieve the correct share between local production and regional advection of aerosol pollution in the Paris agglomeration. During a whole year, daily measurements of the fine particulate matter (PM2.5) and its main chemical constituents (elemental and organic carbon, nitrate, sulfate and ammonium) are available at various stations both in and around Paris (PARTICULES project). Based on back-trajectory data, we can locate the upwind station, from which the concentration is identified as the import, the local production being deduced from the urban concentration by subtraction. Uncertainties on these contributions are quantified. Small biases in urban background PM2.5 simulations (+16 %) hide significant error compensations between local and advected contributions, as well as in PM2.5 chemical compounds. In particular, wintertime OM imports appear strongly underestimated (potentially explained by uncertain continental woodburning emissions and missing SOA pathways) while local OM and EC production are overestimated all along the year (likely to be related to uncertainties in emissions and dynamics). A statistically significant local formation of nitrate is also highlighted from observations, but missed by the model. Together with the overestimation of nitrate imports, it leads to a bias of +51 % on the local PM2.5 contribution. In parallel to inorganic aerosols measurements, gaseous nitrate precursors (nitric acid and ammonia) have also been measured (FRANCIPOL project), which offers the opportunity to investigate the regime of nitrate formation in Paris and its sensitivity to precursor changes and to assess, again, the ability of the CHIMERE model to retrieve the observed sensitivity. Experimental data clearly point to NH3-rich conditions in the city (as indicated by high gas ratio values), but a quite similar sensitivity of nitrate concentrations to changes in nitric acid and ammonia. However, simulation results indicate that the model highly overestimates the sensitivity of nitrate to ammonia changes. Thus, while overall particulate matter levels are well reproduced by the model, differences with observations are much larger for local versus advected contributions, and the sensitivity of nitrate formation with respect to gaseous precursors.