Estimation of the Paris NO<sub>x</sub> Emissions from mobile MAX-DOAS observations and CHIMERE model simulations using the closed integral method

Abstract. We determine NOx emissions for Paris in summer 2009 and winter 2009/2010 by applying the closed integral method (CIM) to a large set of car Multi-AXis (MAX)-DOAS measurements performed in the frame of the MEGAPOLI project. MAX-DOAS measurements of the tropospheric NO2 vertical column density (VCD) are performed at large circles around Paris. From the combination of the observed NO2 VCDs with wind fields the influx into and the outflux from the encircled area is determined. The difference of both fluxes represents the total emission. Compared to previous applications of the CIM, the large number of measurements during the MEGAPOLI campaign allowed the investigation of important aspects of the CIM. In particular the applicability of the CIM under various atmospheric conditions could be tested. Another important advantage of the measurements during MEGAPOLI is that simultaneous atmospheric model simulations with high spatial resolution (3 × 3 km2) are available for all days. Based on these model data it is possible to test the consistency of the CIM and to derive information about favorable or non-favorable conditions for the application of the CIM. We find that in most situations the uncertainties and the variability of the wind data dominate the total error budget. Also measurement gaps and uncertainties in the partitioning ratio between NO and NO2 are important error sources. Based on a consistency check, we deduced a set of criteria on whether measurement conditions are suitable or not for the application of the CIM. We also developped a method for the calculation of the total error budget of the derived NOx emissions. Typical errors are between ±30 % and ±50 % for individual days (with one full circle around Paris). From the application of the CIM to car MAX-DOAS observations we derive daily average NOx emissions for Paris of 4.2 × 1025 molecules/s for summer and of 7.8 × 1025 molecules/s in winter. These values are by a factor of about 1.4 and 2.0 larger than the corresponding emissions derived from the application of the CIM to the model data, using the TNO-MEGAPOLI emission inventory, in summer and winter, respectively. Similar ratios (1.5 and 2.3 for summer and winter, respectively) were found for the comparison with the MACC-III emission inventory.