Lidar-derived PM₁₀ and comparison with regional modeling in the frame of the MEGAPOLI Paris summer campaign

Abstract

Abstract. An original approach using mobile lidar measurements was implemented to validate mass concentrations (PM10) predicted by chemistry-transport models. A ground-based mobile lidar (GBML) was deployed around Paris onboard a van during the MEGAPOLI (Megacities: Emissions, urban, regional and Global Atmospheric POLlution and climate effects, and Integrated tools for assessment and mitigation) summer experiment in July 2009. The measurements performed with this Rayleigh-Mie lidar are converted into PM10 profiles using optical-to-mass relationships previously established from in situ measurements performed around Paris for urban and peri-urban aerosols. The method is described here and applied to the 10 measurements days (MD). MD of 1, 15, 16 and 26 July 2009 correspond to contrasted levels of pollution and atmospheric conditions. They are analyzed here in more details. Lidar-derived PM10 are compared with results of simulations from POLYPHEMUS and CHIMERE chemistry-transport models (CTM) and with ground-based observations from AIRPARIF network. GBML-derived and AIRPARIF in situ measurements have been found to be in good agreement with a mean Root Mean Square Error RMSE (and a Mean Absolute Percentage Error MAPE) of 5.9 μg m−3 (21.0%) with peri-urban and 8.7 μg m−3 (25.4%) with urban relationships, respectively. The comparisons between CTMs and lidar have shown that CTMs tend to underestimate wet PM10 concentrations as revealed by the mean wet PM10 observed during the 10 MD of 22.7, 20.0 and 17.5 μg m−3 for lidar with peri-urban relationship, POLYPHEMUS and CHIMERE models, respectively. This leads to a RMSE (and a MAPE) of 7.2 μg m−3 (33.4%) and 7.4 μg m−3 (32.0%) when considering POLYPHEMUS and CHIMERE CTMs, respectively. Wet integrated PM10 computed (between the ground and 1 km above the ground level) from lidar, POLYPHEMUS and CHIMERE results have been compared and have shown similar results with a RMSE (and MAPE) of 6.7 μg m−2 (30.7%) and 7.1 μg m−2 (28.4%) with POLYPHEMUS and CHIMERE when comparing with lidar-periu-urban parametrization. The values are of the same order of magnitude than other comparisons realized in previous studies. The discrepancies observed between models and measured PM10 can be explained by difficulties to accurately model the background conditions, the positions and strengths of the plume, the vertical diffusion (as well as the limited vertical model resolutions) and the chemical modeling such as the formation of secondary aerosols.